the grain and strains cannot be greater than those due to the highest
heat, and this maximum heat can be watched and kept within limits. If
a piece be quenched from a falling temperature, the grain and strains
will be those due to the highest temperature, modified slightly by
the distance through which it has cooled, and always coarser and
more brittle than if quenched at the same heat produced by rising
temperature. If by accident a piece gets too hot to be quenched, it
should be allowed to go entirely cold, and then be heated again to the
right color.
After a piece of steel is hardened it is usually tempered to relieve
some of the strain, reduce brittleness, and increase the toughness.
This is done by heating; usually the piece is held over the fire, or
in contact with a large piece of steel or iron heated for the purpose,
until it takes on a certain color which indicates the degree of
tempering that is wanted.
Where great numbers of pieces are to be tempered, a bath is very
convenient. Boiling in water produces only a slight tempering
sufficient for some purposes. Steaming under given pressure will
produce even heating and uniform tempering.
When pieces are quenched in oil, they can be tempered easily and nicely
by watching the oil that adheres to them. When the oil is dried off
and begins to char, the tempering is good, about right for saw-teeth.
If the heat is run up until the oil flashes, the tempering is pretty
thorough and is about right for good springs. If the oil be all burned
off, there will be little temper left except in very high steel. High
steel becomes much harder when quenched than low steel; consequently
very high hardened steel may be heated until it begins to show color
and still retain considerable hardness or temper, whereas a milder
steel, under 90 or 100 carbon, when heated to such a degree will retain
no temper, it will be soft.
Saw-teeth, tap, reamer, and milling-cutter teeth, may be drawn, and
usually should be drawn, down until a file will barely catch them;
then they will do excellent work. Many inexperienced temperers are apt
to complain if such tools can be filed at all when drawn to the proper
color, forgetful or ignorant of the fact that a file should always
contain about twice as much carbon as a tap or reamer, and that if both
are drawn to the same color the file must necessarily be the harder.
Such men often destroy much good work by trying to get the tools too
hard. If a tap-tooth be left file hard, it will be pretty certain to
snip off when put to work.
TEMPER COLORS.
When a clean piece of iron or steel, hardened or unhardened, is exposed
to heat in the air, it will assume different colors as the heat
increases. First will be noticed a light, delicate straw color; then
in order a deep straw, light brown; darker brown; brown shaded with
purple, known as pigeon-wing; as the brown dies out a light bluish
cast; light brilliant blue; dark blue; black.
When black, the temper is gone. It is well established that these
colors are due to thin films of oxide that are formed as the heat
progresses.
These colors are very beautiful, and as useful as they are beautiful,
furnishing an unvarying guide to the condition of hardened steel.
The drawing of hardened steel to any of these colors is _tempering_.
So we have the different tempers:
Light straw For lathe-tools, files, etc.
Straw “ “ “ “ “
Light brown “ taps, reamers, drills, etc.
Darker brown “ “ “ “ “
Pigeon-wing “ axes, hatchets, and some drills
Light blue “ springs
Dark blue “ some springs; but seldom used
This is the unfortunate second use of the word temper, which must
be borne in mind if confusion is to be avoided in consulting with
steel-makers and steel-workers. The meanings may be tabulated thus:
------------------+------------------------+-------------------------
Temper. | Steel-maker’s Meaning. | Steel-worker’s Meaning.
------------------+------------------------+-------------------------
Very high | 150 carbon + | light straw
High | 100 to 120 C | straw
Medium | 70 to 80 C | brown to pigeon-wing
Mild | 40 to 60 C | light blue
Low | 20 to 30 C | dark blue
Soft or dead-soft | under 20 C | black
------------------+------------------------+-------------------------
The uses given for temper colors are not meant to be absolute; they
merely give a good general idea; experienced men are guided by results,
and temper in every case in the way that proves to be most satisfactory.
DIFFERENCE BETWEEN CRACKS AND SEAMS.
When temperers find that their tools are cracking under their
treatment, they are apt to assume that, as they are working in their
ordinary way, there must be something wrong with the steel. It is
either seamy, or harder than usual, or not uniform in temper, or it is
of inferior quality.
All or any of these conditions may exist and be the cause of the
trouble; but every man should bear in mind that he is also a variable
quantity; he may be unwell and not see and observe as closely as
usual; there may be a long spell of unusual weather giving him a light
differing from that to which he is accustomed; or, as is often the
case, he may simply have unconsciously departed from the even track by
not having his mind carefully intent upon the routine which has become
a sort of second nature to him, so that for a time he ceases to think,
makes of himself an animated machine, and the machine left to itself
does not run with perfect regularity.
If personal pride, egotism, or ill temper be set aside, it is always
easy to find out whether the fault is in the steel or in the man; that
once determined the remedy is easily applied, and the sooner the better
for all parties.
_How to Break a Tool._ Let an ordinary axe be considered.
[Illustration]
[Illustration]
[Illustration]
If the axe be cracked as shown in Fig. 1, the corners have been hotter
than the middle of the blade; probably by snipping the corners and the
middle and comparing the fractures the coarser grain at the corners
will tell the tale.
If the crack be as shown in No. 2, the middle of the blade has been
hotter than the corners: snipping and comparing the grains will tell
the story.
If the crack be more nearly a straight line, as shown in number 3, the
chances are that there is a seam there and the steel is at fault.
_How to Tell a Seam from a Water-crack._--A seam is caused by a
gas-bubble in the ingot which has not been closed up by hammering or
rolling; it always runs in the direction of the work; in bars it is
parallel to the axis.
The walls of a seam are always more or less smooth, the surfaces having
been rubbed together under heavy pressure during hammering or rolling,
and they are black usually, being coated with oxide.
The walls of a water-crack are never smooth, they are rough and gritty,
and they may have any of the temper colors caused by the action of
water and heat.
There need never be any question as to which is which.
If a long tool cracks down the middle, it may be from too much heat,
from seams, or from a lap.
A lap is caused by careless working under a hammer, or by bad draughts
in the rolls, folding part of the steel over on itself. Laps, like
seams, run parallel to the axis of a bar, and usually in very straight
lines.
Any long piece of steel may be split in hardening by too much heat.
In making the experiment of heating a piece continuously from
scintillating, or creamy color, down to black, to show the differences
of grain due to the different heats, the sample almost invariably
splits down the middle as far as the strong, refined grain, or nearly
that far.
As stated before, a round bar will be almost certain to split if it be
heated up to medium lemon, although a square bar may endure the same
heat without cracking.
An examination of the walls of a split will settle at once whether it
is a seam, a lap, or a water-crack.
A seam will not necessarily be long; its walls will be smooth.
A lap usually runs the whole length of the bar, and the walls are
smooth.
By smooth walls of seams and laps comparative smoothness is meant; they
are sometimes polished, but not always, and they are never granular
like the walls of water-cracks.
If the split be a water-crack, the walls will be rough and granular.
After a temperer has straightened himself out, and brought his work
to usual accuracy and uniformity, if his tools continue to crack and
indicate weakness in the steel, it is time for him to suspect the
character of his material and to require the steel-maker to either show
up the faults in tempering, or improve the quality of his product.
A WORD FOR THE WORKMAN.
_Give him a chance._ A steel-worker to be expert must have a
well-trained eye and know how to use it. He must work with delicate
tints, ranging in the yellows from creamy yellow to dark orange or
orange red as extremes, and most of his work must be done between
bright lemon and medium orange in forging, and between rather dark to
medium orange, or possibly nearly light orange, when hardening and
tempering.
Probably in no other business is there such ridiculous waste as is
often found in steel-working where the manufacturer economizes in his
blacksmiths.
A large, wealthy railroad condemns a brand of steel. The steel-maker
goes to the shop and is informed by a bright, intelligent blacksmith
that the steel will not make a track-chisel. It is a hot summer day;
the smith is working over a huge fire with a large piece of work in the
middle of the fire and a number of small pieces of steel stuck in the
edge of the fire.
He is welding large iron frog-points, and in the interval he is filling
a hurried order for four dozen track-chisels for which the trackmen
are waiting. He is not merely forging the chisels, he is hardening and
tempering them. The glare of the welding-work makes him color-blind,
the hurry gives him no time for manipulation, and the trackmen have no
chisels.
After a thorough expression of sympathy for the smith the steel-maker
turns upon the foreman and master mechanic, and gives them such a
tongue-lashing that they turn away silenced and ashamed.
Page after page of such cases could be written, but one should be
enough.
A steel-maker has a thoroughly skilled and expert steel-worker; he
rushes into the shop and says, “Mike, refine this right away, please; I
want to know what it is.”
Mike replies, “I will do that to-morrow; I am welding to-day.”
That is entirely satisfactory; those men understand one another, and
they know a little something about their business.
A temperer should do no other work when he is heating for hardening,
and he should always be allowed to use as much time about it as he
pleases, assuming that he is a decently honest man who prefers good
work to bad; and as a rule such honest men are in the majority, if they
are given a fair chance.
IX.
ON THE SURFACE.
The condition of the surface of steel has much to do with its
successful hardening and working.
A slight film adherent to the surface of steel will prevent its
hardening properly; the steel may harden under such a film and not be
hard upon the immediate surface, and, as in almost every case a hard,
strong surface is necessary to good work, it is important that a piece
of steel to harden well should have a clean surface of sound steel.
It has been stated already that all bars and forgings of steel have
upon the surface a coat of oxide of iron, and immediately beneath this
a thin film of decarbonized iron.
Neither of these substances will harden, and in every case where a
hard-bearing surface or a keen cutting-edge is desired these coatings
must be removed. Polished drill-wire and cold-rolled spring-steel for
watches, clocks, etc., should have perfect surfaces, and it is the duty
of steel-makers to turn them out in that condition. All black steel, or
hot-finished steel, contains these coatings.
In the manufacture of railroad, wagon, and carriage springs it is not
necessary or customary to pay any attention to these coatings; the
body of the steel hardens well, giving the required resilience and
elasticity, so that an unhardened coat of .01 to .001 inch thick does
no harm. To all bearing-surfaces and cutting-edges such coatings are
fatal.
The ordinary way of preparing steel is to cut the skin off, and
this is sufficient if enough be taken off; it happens often that a
purchaser, in pursuit of economy and unaware of the importance of this
skin, orders his bars or forgings so close to size that when they are
finished the decarbonized skin is not all removed, and the result is an
expensive tap, reamer, milling-cutter, or some tool of that sort with
the points of the teeth soft and worthless.
In small tools ¹/₁₆ inch, in medium-size tools, say up to two or three
inches in diameter, ⅛ inch cut off should be plenty; in large tools and
dies, especially in shaped forgings, it would be wiser to cut away ³/₁₆
inch.
In many cases sufficient hardness can be obtained by pickling off
the surface-scale, but this will not do where thorough hardening is
required, because the acid does not remove the thin decarbonized
surface. It seems to be impracticable to remove the decarbonized skin
by the action of acid, for if the steel be left in the acid long enough
to accomplish this the acid will penetrate deeper, oxidizing and
ruining the steel as it advances.
Grinding is frequently resorted to, being quicker and cheaper than
turning, planing, or milling.
When grinding is used, care must be taken not to glaze the surface
of the steel, or if it should be glazed the glaze must be removed by
filing or scraping.
In the manufacture of files it is customary to grind the blanks after
they are forged and before the teeth are cut.
After the blanks are ground they are held up to the light and examined
carefully for glaze. Every blank that shows by the flash of light that
it is glazed is put to one side; then these glazed blanks are taken
by other operatives and filed until all traces of glaze are removed.
The file-maker will explain that if this be not done the files when
hardened will be soft at the tips of the teeth over the whole of
the glazed surface. This inspection and filing of blanks involves
considerable expense, and it is certain that such an expense would not
be incurred if it were not necessary.
This glaze does not appear to be due to burning, at least the stones
are run in water; the blanks are handled by the bare hands of the
grinders, and do not appear to be hot.
After pieces are hardened and tempered they frequently require grinding
to bring them to exact dimensions. This is usually done on emery-wheels
with an abundance of water, and as no temper colors are developed
indicating heat it is assumed that no harm can be done.
_Just here much valuable work is destroyed._ The tempered piece is put
on the wheel, in a “flood of water”; the work is rushed, and the piece
comes out literally covered with little surface-cracks running in every
direction, perfectly visible to the naked eye. Until the steel-worker
learns better he blames and condemns the steel.
This result is very common in the manufacture of shear-knives,
scissors, shear-blades, dies, etc.
Sometimes too a round bearing or expander-pin is hardened; examined
by means of a file it appears perfectly hard; it is then ground, not
quite heavily enough to produce surface-cracks, but still heavily,
and on a glazed wheel. It is found now that the surface is soft; only
a thousandth of an inch or so has been cut off, and the steel is
condemned at once because it will harden only skin deep. Let the file
be drawn heavily over the surface and it will be found that the soft
surface is only about a thousandth of an inch thick, and underneath the
steel is perfectly hard.
Now grind slightly on a sharp, clean wheel and re-harden; the surface
will be found to be perfectly hard. Ground heavily again on the glazed
wheel, it will become soft, as before. These operations can be repeated
with unvarying results until the whole piece is ground away.
These difficulties occur more with emery-wheels than with grindstones,
either because emery-wheels glaze more easily than grindstones,
or because, owing to their superior cutting powers under any
circumstances, they are more neglected than grindstones.
Experience shows that these bad results occur almost invariably on
glazed wheels. It is rare to find any bad work come off from a clean,
sharp wheel, unless the pressure has been so excessive as to show that
the operator is either foolish or stupid.
The remedy is simple: Keep the wheels _clean_ and _sharp_.
Many grinders who understand this matter will not run any wheel more
than one day without dressing, nor even a whole day if the work is
continuous and they have reason to apprehend danger.
A FEW WORDS IN REGARD TO PICKLING.
Pickling is the placing of steel in a bath of dilute acid to remove the
scale. It is a necessary operation in wire-making and for many other
purposes, and it may be hastened by having the acid hot.
Sulphuric acid is used generally; it is efficient and cheap. When thin
sheets are to be pickled, the acid should not be too hot, or it will
raise a rash all over the sheet in many cases. This indicates some
unsoundness in the steel, the presence probably of innumerable little
bubbles of occluded gases. This is possibly true, yet the same sheets
pickled properly and brought out smooth will polish perfectly, or if
cut up will make thousands of little tools that will show no evidence
of unsoundness.
Steel should never be left in the pickling-bath any longer than is
necessary to remove the scale; it seems unnecessary to warn readers
that the acid will continue to act on the steel, eat the steel after
the scale is removed. When taken from the pickle, the steel should be
washed in limewater and plenty of clean running water; but this does
not take out all of the acid. It should then be baked for several hours
at a heat of 400° to 450° F. to decompose the remaining acid. This
is just below a bluing heat, and it does not discolor or oxidize the
surface. It is known as the sizzling-heat, the heat that the expert
laundry-woman gets on her flat-iron which she tests with her moistened
finger.
Acid if not taken off completely will continue to act upon and rot the
steel; how far this will go on is not known exactly; for instance, it
is not known whether if a block six inches cube were pickled and merely
washed, the remaining acid would penetrate and rot the whole mass or
not. There must be some relation between the mass of the steel and the
power of a small amount of acid to penetrate.
The power of acid can be illustrated on the other extreme: A lot of
watch-spring steel is finished in long coils and .010 inch thick; when
last pickled, the baking was neglected; the steel is tough, it hardens
well, and when tempered it is springy and strong; by all of the tests
it is just right in every coil. It is shipped away and in three or four
weeks the spring-maker begins work on it. He reports at once that it
is rotten and worthless, it will not make a spring at all, and he is
angry. The steel is returned to the maker and he finds the report true:
the steel is rotten and worthless. Then by diligent inquiry he finds
that the last baking was omitted, and he pockets his loss, sending an
humble apology to the irate spring-maker.
Whether the residual acid can ruin a large piece of steel or not need
not be considered when the simple operation of baking will remove the
possibility of harm.
X.
IMPURITIES IN STEEL.
Any elements in steel which reduce its strength or durability in any
way may be classed as impurities.
A theoretical ideal of pure steel is a compound of iron and carbon; it
is an ideal that is never reached in practice, but it is one that is
aimed at by many manufacturers and consumers, because experience shows
that, especially in high steels, the more nearly it is attained the
more reliable and safe is the product.
All steel contains silicon, phosphorus, sulphur, oxygen, hydrogen,
and nitrogen, none of which add any useful property to the material.
It is admitted that, starting with very small quantities of silicon
or phosphorus in mild steel, small additions of either element will
increase the tensile strength of the steel perceptibly up to a given
amount, and that then the addition of more of either one will cause a
reduction of strength. The same increase of strength can be obtained
by the addition of a little carbon, producing a much more reliable
material. It is not known that even such slight apparent gain in
strength can be made by using oxygen, nitrogen, or hydrogen.
Manganese is present in all steel as a necessary ingredient, it gives
an increase in strength in the same way as phosphorus, and when
increased beyond a small limit it causes brittleness. Hadfield’s
manganese steel is a unique material, not to be considered in
connection with the ordinary steel of commerce.
Webster’s experiments are perhaps the most complete of any that show
the effects of small increases of silicon, phosphorus, sulphur, and
manganese, but as these are not completed they are not quoted here,
because Mr. Webster may reach additional and different results before
these pages are printed.
The chief bad qualities of steel that are caused by these impurities
are known as “red-shortness,” “cold-shortness,” and “hot-shortness.”
A steel is called red-short when it is brittle and friable at what is
known commonly as a low red heat--“cherry red,” “orange red.”
_Red-shortness_ is caused chiefly by sulphur or by oxygen; many other
elements may produce the same effects; it seems probable that nitrogen
may be one of these, but the real action of nitrogen is as yet obscure.
A red-short steel is difficult to work; it must be worked at a high
heat--from bright orange up to near the heat of granulation--or it will
crack. When hardened, it is almost certain to crack. When red-short
steel is worked with care into a sound condition, it may when cold
be reasonably strong, but hardly any engineer of experience would be
willing to trust it.
_Hot-short_ steel is that which cannot be worked at a high heat, say
above a medium to light orange, but which is generally malleable and
works soundly at medium orange down to dark orange, or almost black.
This is a characteristic of most of the so-called alloy steels, or
steels containing considerable quantities of tungsten, manganese, or
silicon. It is claimed that chrome steel may be worked at high heats
and that it is less easily injured in the fire than carbon steel.
This is not within the author’s experience. It is this property of
hot-shortness that makes the alloy steels so expensive; the ingots
cannot be heated hot enough nor worked heavily enough to close up
porosities, and therefore, there is a heavy loss from seams.
The range of heat at which they can be worked is so small that many
re-heatings are required, increasing greatly the cost of working.
As compared to good carbon steel they are liable to crack in hardening,
and when hardened they are friable, although they may be excessively
hard.
_Cold-short_ steel is steel which is weak and brittle when cold, either
hardened or unhardened. Of those which are always found in steel,
phosphorus is the one well-known element which produces cold-shortness.
It is clear that no one can have any use for cold-short steel.
Red-short or hot-short steel may be of some use when worked
successfully into a cold condition, but cold-short steel is to be
avoided in all cases where the steel is used ultimately cold.
If the theoretically perfect steel is a compound of iron and carbon,
it cannot be obtained in practice, and the only safeguard is to fix a
maximum above which other elements are not to be tolerated.
In tool-steel of ordinary standard excellence such maximum should be
.02 of one per cent; it may be worked to easily and economically,
except perhaps in silicon, which element is generally given off to some
extent by the crucible; it should be kept as low as possible, however,
say well under 10, one tenth of one per cent. Some people claim that a
little higher silicon makes steel sounder and better; but any expert
temperer will soon observe the difference between steels of .10 and
.01 silicon. For the highest and best grade of tool-steel the maximum
should be the least attainable. Every one hundredth of one per cent of
phosphorus, silicon, or sulphur will show itself in fine tool-steel
when it is hardened. It is assumed, of course, that such impurities as
copper, antimony, arsenic, etc., exist only as mere traces, or not at
all.
As oxygen must be at a minimum, no one has yet succeeded in making a
really fine tool-steel from the products of the Bessemer or of the
open-hearth process.
The removal of the last fractions of these impurities is difficult and
expensive; for instance, a steel melting iron of
Silicon .03 to .06
Phosphorus .03 “ .02
Sulphur .002 or less
may be bought for 2 cents a pound or less, whereas an iron of
Silicon <.02
Phosphorus <.01
Sulphur trace
can hardly be bought for less than 5 cents a pound.
This difference of three cents a pound is justifiable when the highest
grade of tool-steel is to be made; and it would be silly to require any
such material in any spring, machinery, or structural steel.
In addition to these impurities there are other difficulties to be
guarded against, chief among which is an uneven distribution of
elements.
In all steel there is some _segregation_; that is to say, as the
liquid metal freezes, the elements are to some extent squeezed out and
collected in that part of the ingot which congeals last. It is claimed
that in the Bessemer and Open-hearth processes any ferro-silicon added
to quiet a heat, or any ferro-manganese added to remove oxygen, are at
once absorbed and distributed through the mass, and so when any serious
irregularity is discovered it is charged to _segregation_.
A heat may produce billets of 75 carbon and 120 carbon, and again it is
called segregation.
As a rule, inertia has more to do with such differences than
segregation. One crucible of steel may produce an ingot containing
90 carbon and 130 carbon. Segregation has nothing to do with this: a
careless mixer has put a heavy lump of 140- or 150-carbon steel in
the bottom of the pot and covered it up with iron. The steel melted
first and settled in the bottom of the pot, the iron melted later and
settled on top of the steel, and they did not mix. The teeming was not
sufficient to cause a thorough mixing.
Segregation covers a multitude of sins.
Exactly how much is sin and how much is segregation will not be known
until analyses are made of the top, middle, and bottom of the bath,
and of the contents of the ladle, these to be compared to analyses
of the top, bottom, and middle of the ingots. There is certainly an
unavoidable amount of segregation, and as equally certain an amount of
curable irregularity due to inertia.
WILD HEATS.
After steel is melted, whether in a crucible, an open hearth, or a
Bessemer vessel, it boils with more or less violence. This boiling is
caused by ebullition of gases, and if steel be poured into moulds while
it is boiling the resulting ingot will be found to be honeycombed to an
extent that is governed by the degree of the boiling.
If a heat boils violently and persistently, it is said to be “wild,”
and if a wild heat be teemed the ingots will be honeycombed completely;
such ingots cannot be worked into thoroughly sound steel, and no melter
who has any regard for his work will teem a wild heat if he knows it.
To stop the boiling is called “dead-melting,” “killing” the steel, so
that it shall be quiet in the furnace and in the moulds.
A crucible-steel maker who knows his business can, and he will, always
dead-melt his steel. It only requires a few minutes of application
of a heat a little above melting temperature, and this can be
applied by a skilled melter without burning his crucible or cutting
down his furnace; this is indeed about all of the art there is in
crucible-melting, the remaining operations being easy and simple.
Dead-melting in the Bessemer vessel is not possible by increase of
time; wild heats are managed differently, probably by adding manganese
or silicon, or both, but exactly how is not within the author’s
experience.
Dead-melting in the open hearth would appear at first sight to be
always possible, but there are more difficulties in the way than in the
case of crucible-melting.
The heat may be wild when the right carbon is reached, and then the
melter must use a little ferro-silicon, or silico-spiegel, or highly
silicious pig, or aluminum, and he must use good judgment so as not to
have his steel overdosed with any of these. From half an ounce to an
ounce of aluminum to a ton of steel is usually sufficient, and although
any considerable content of aluminum is injurious to steel there is
little danger of its being added, because of its cost, and because a
little too much aluminum will cause the ingots to pipe from top to
bottom.
Silicon seems to be the most kindly element to use, and it is claimed
that a content of silicon as high as 20 is not injurious; some people
claim that it is beneficial. That it does help materially in the
production of sound steel there can be no doubt, and if such steel
meets all of the requirements of the engineer and of practice it would
seem to be wise not to place the upper limit for silicon so low as to
prevent its sufficient use in securing soundness. But the author cannot
concede that as much as 20 silicon is necessary. In crucible practice
high silicon is not necessary; in “melting-iron,” or iron to be melted,
it means so much dirt, indicating careless workmanship; but there will
always be a little silicon present which the steel has absorbed from
the walls of the crucible during the operation of melting. In high
tool-steel silicon should be at the lowest minimum that is attainable.
This discussion of wild heats may appear to be outside of the scope of
this work, and to belong exclusively to the art of manufacturing steel,
of which this book does not pretend to treat. This is true so far that
it is not recommended that the engineer shall meddle in any way with
the manufacturer in the management of his work; on the other hand, it
is vital to the engineer that he should know about it, because wild
steel may hammer or roll perfectly well, it may appear to be sound, but
the author cannot believe that it is ever sound and reliable.
Again, it has a scientific interest; that wildness is due to too much
gas, and probably to carbon-gas, may be shown by an illustration.
It has its parallel in the rising of the iron in a puddling-furnace at
the close of the boil, a phenomenon with which every one is familiar
who has watched a heat being boiled or puddled. That all of the iron
does not run out of the puddling-furnace at this stage is owing to the
fact that there is not heat enough in the puddling-furnace to keep the
iron liquid after it has been decarbonized.
During the running of a basic open-hearth furnace an apparently dead
heat was tapped; before the steel reached the ladle there was a sort
of explosion; the steel was blown all over the shop, the men had to
run for their lives, and not one tenth of the steel reached the ladle.
The manager was rated roundly for carelessness in not having dried his
spout, and the incident closed. A few days later another quiet heat was
tapped and it ran into the ladle; about the time the ladle was full the
steel rose rapidly, like a beaten egg or whipped cream, and ran out on
to the floor, cutting the sides of the ladle, the ladle-chains, and the
crane-beams as it flowed. The men ran, and there was no injury to the
person.
Again the manager was blamed, this time for having a damp ladle, and he
was notified of an impending dismissal if such a thing occurred again.
He protested that he knew the ladle and the stopper were red-hot, that
he had examined them personally and carefully, and knew he stated the
truth.
There were several reasons for looking into the matter farther: first,
the man in charge was known to be truthful and careful, so that there
was no reason for doubting his word; second, if the vessel and rod were
red-hot, there could be no aqueous moisture there; and, finally, such
an ebullition from dampness was contrary to experience, as a small
quantity of water _under_ a mass of molten iron, or slag, results
almost invariably in a violent explosion, like that of gunpowder or
dynamite.
Upon inquiry it was found that prior to both ebullitions there had been
a large hole in the furnace-bottom, requiring about a peck of material
to fill it in each case. Magnesite was used; the magnesite was bought
raw, and burned in the place. It is well known that it takes a long
time and high heat to drive carbonic acid out of magnesite, and it was
surmised that insufficient roasting might have caused the trouble.
Samples of burned and of raw magnesite were sent to the laboratory, and
the burned was found to contain about as much carbonic acid as the raw
magnesite. Then the case seemed clear: This heavily charged magnesite
was packed into the hole; the heat was charged and melted. The
magnesite held the carbonic acid until near the close of the operation;
then the intense heat of the steel forced the release of the gas, which
was at once absorbed by the steel. Owing to the superincumbent weight
of the steel the gas was absorbed quietly, and when the weight was
removed the gas escaped, exactly as it does at the close of puddling or
in the frothing of yeast.
Whether the carbonic acid remained such, or whether it took up an
equivalent of carbon and became carbonic oxide, and then again took
up oxygen from the bath, and so kept on increasing in volume, is not
known.
The facts seem clear, and the collateral proof is that thorough burning
of the magnesite, and of any dolomite that was used, prevented a
recurrence of any such accidents.
Such ebullitions have occurred and caused the burning to death of
pitmen, and the statement of the above case may be of use to melters in
the future who have not met such an experience.
OXYGEN AND NITROGEN.
Oxygen and nitrogen are present in all steel and both are injurious,
probably the most so of all impurities.
The oxides of iron are too well known to need discussion or
description; they are the iron ores mixed with gangue. They are
brittle, friable, hard, and weak, like sandstones. Mixed in steel they
can be nothing but weakeners, elements of disintegration. Let any
one take a handful of scale--or rust--oxide of iron, in his fingers
and crumble it, and it will be difficult for him to imagine how such
material could be anything but harmful when incorporated in steel.
Langley has shown, and other scientists have confirmed him, that
oxygen may exist in iron in solution, and not as oxide; the discovery
was attended with the assertion that such dissolved oxygen produced
excessive red-shortness. The proof that red-shortness was caused in
this way was completed by the removal of the oxygen from some extremely
red-short steel; the red-shortness disappeared with the oxygen and the
steel worked perfectly.
When steel is melted very low in carbon, by any process, it is certain
to be red-short and rotten unless the greatest care be used to prevent
the introduction of oxygen. Crucible-steel of 15 carbon or less will as
a rule be red-short and cold-short; it will not weld, and is generally
thoroughly worthless. The same material melted to contain 18 to 25
carbon will be tough and waxlike, hot or cold. It will weld easily into
tubes, and may be stamped cold into almost any desired shape.
Bessemer or open-hearth steel of less than 8 carbon is almost certain
to be equally worthless, whereas the same material blown or melted not
below 10 or 12 carbon, and re-carbonized not above 20, will be tough
and good at any heat under granulation, and equally good and tough when
cold.
As to Bessemer steel, the author cannot say whether it would be
possible to stop the blow between 10 and 15 carbon or not, but it
seems certain that if there be no overblowing red-shortness and
cold-shortness may be avoided by carbonizing back to about 15 by the
use of manganese or silicon, or both together.
In the open hearth it is always possible to stop the melt at 10 carbon,
and to deoxidize the heat so as to avoid shortness, and not to go above
20 carbon. Such steel will be sound and tough; it will weld and stamp
perfectly, and will be satisfactory for all reasonable requirements.
The reason of this seems to be simple and plain: In melting or blowing
out the last fractions of carbon below 10 to 15 the same quantity of
air per second or minute must be used as when burning out the higher
quantities, and now there is so little carbon to be attacked that the
oxygen necessarily attacks the iron in greater and greater force as the
carbon decreases.
This leaves an excess of oxygen in the steel which cannot be removed by
the ordinary quantities of silicon, or manganese, or aluminum.
If more manganese or silicon be used, the red-shortness and weakness
can be cured largely; but then the carbon is raised considerably, and
thus the steel is brought up to where it would have been without this
excessive decarbonizing, with the difference that it is not quite so
strong.
What good is there, then, in extremely low melting?
It must be admitted that there are tough, good-working steels in the
market of carbon < 5, manganese < 20. They are made in small furnaces,
worked with great care; the product is expensive, and, unless it is
wanted to be welded in place of common wrought iron, it is in no case
as good as well-made steel of 12 to 20 carbon; even for welding the
latter is superior if the worker will only be satisfied to work at a
lemon instead of a scintillating heat.
These special cases do not militate against the general fact that
extremely low steel is usually red-short and weak.
The above is written for the consideration of those engineers who
think they are going safe when they prescribe low tensile strength and
excessive ductility. If these requirements meant the reception of pure,
or nearly pure, iron, indicated by the low tenacity and high stretch,
then they would be wise; but if they result, as they almost certainly
do, in initially good material rotted by overdoses of oxygen the wisdom
may not be so apparent.
NITROGEN.
The real influence of nitrogen is not known to the author. Percy shows
that nitrogenized iron is hard, exceedingly friable, and causes a
brilliant, brassy lustre. He also says nitrogen is driven out at a
yellow heat; doubtless this is true of the excess of nitrogen, but it
has been shown in Chapter II that melting in a crucible will not drive
the nitrogen out of Bessemer steel.
When crucible-steel not made from Bessemer scrap and Bessemer steel of
equal analysis are compared in the tempered condition, there is almost
invariably a yellowish tinge over the fresh Bessemer fracture which
distinguishes it from the crucible-steel. The Bessemer steel is also
the weaker. These differences are believed to be due to nitrogen.
Langley maintains his belief that oxygen is still the chief
mischief-maker; the author believes nitrogen to be the more potent of
the two; there is no known way to remove the nitrogen, and there the
question stands.
ELEMENTS OF DISINTEGRATION.
It has been stated time and again that these impurities are elements of
disintegration, and that it would be wise in every case to restrict the
quantities allowable within reasonable limits, giving the steel-maker
sufficient leeway to enable him to work efficiently and economically,
and at the same time to keep the quantities of these impurities as low
as possible.
On the other hand, able, successful, and conservative engineers have
claimed that if the steel-maker meets their physical requirements as
shown by prescribed tests they, the engineers, should be satisfied;
that they should not interfere with chemical composition, as they had
no fear of subsequent disintegrations.
This argument was answered by the statement that skilled steel-workers
could manipulate poor steel so as to bring it up to the requirements;
that the well-trained workers in the bridge-shops would not abuse the
steel; that the inherent deficiencies would not be developed; the work
would go out apparently satisfactory; and that it might remain so for
a long time, in the absence of unusual shocks or strains, but that in
an emergency such material might fail because of deterioration where
a purer material would have held on. In the absence of proofs such
statements have been met with a smile of incredulity.
Fortunately some proofs are now at hand, and as the method of getting
them has been obtained, more will follow from time to time.
In _Engineering_, Jan. 17, 1896, Mr. Thomas Andrews, F.R.S.,
M.Inst.C.E., gives the following cases:
A fracture of a rail into many pieces, causing a serious accident.
A broken propeller-shaft which nearly caused a disastrous accident.
Analysis of the rail:
Carbon 0.440
Silicon 0.040
Manganese 0.800
Sulphur 0.100
Phosphorus 0.064
It is clear that the sulphur is excessive, and that it was neutralized
so as to make the steel workable by an excess of manganese.
Of the propeller-shaft Mr. Andrews says chemical analysis of outside
and central portions of the shaft showed serious segregation.
“The percentage of combined carbon was nearly 50 per cent greater in
the inside of the shaft than on the outside; the manganese was also
in excess in the inside of the shaft; the phosphorus and sulphur had
also segregated in the interior of the shaft to nearly three times the
percentage of these elements found near the outside of the shaft.”
Unfortunately Mr. Andrews does not give the analysis of the shaft.
A number of micro-sections of the rail and of the shaft were made and
examined.
“Numerous micro-sulphur flaws were found, varying in size from 0.015
inch downward, interspersed or segregated in the intercrystalline
junctions of the ultimate crystals of the steel, and being located in
such a manner as to prevent metallic cohesion between the facets of the
crystals, thus inducing lines of internal weakness liable to be acted
upon by the stress and strain of actual wear.”
The dimensions of these flaws in the rail varied from .0150 × .0012 to
.0010 × .0004 parts of an inch.
In the shaft from .0160 × .0030 to .0020 × .0016 parts of an inch.
In the rail he found as many as 14 flaws in an area of only 0.00018
square inch, equal to nearly 60,000 flaws per square inch.
In the shaft he found as many as 34 flaws in an area of only 0.00018
square inch, equal to nearly 190,000 per square inch.
In speaking of the shaft he says: “In addition to blow-holes,
air-cavities, etc., the interior of the shaft was literally honeycombed
with micro-sulphide of iron flaws, which were meshed about and
around the primary crystals of the metal in every direction.” “The
deleterious effects of an excess of manganese in interfering with the
normal crystallization of the normal carbide of iron areas were also
perceptible.”
As the number of micro-sulphur flaws in the shaft were about three
times as many as in the rail, we may assume that the shaft contained at
least as large a percentage of sulphur as the rail, and, owing to the
general honeycombed structure, it would not be a far guess to assume
that the steel was teemed wild.
“The deleterious effect of these treacherous sulphur areas and other
microscopic flaws, with their prolonged ramifications spreading along
the intercrystalline spaces of the ultimate crystals of the metal and
destroying metallic cohesion, will be easily understood.”
“Constant vibration gradually loosens the metallic adherence of the
crystals, especially in areas where these micro-flaws exist. Cankering
by internal corrosion and disintegration is induced whenever the
terminations of any of the sulphide areas or other flaws in any way
become exposed at the surface of the metal, either to the action of
sea-water, or atmospheric or other oxidizing influences. In many other
ways, also, it will be seen how deleterious is their presence.”
“Internal micro-flaws of various character are nevertheless almost
invariably present in masses of steel, and constitute sources of
initial weakness which not unfrequently produce those mysterious and
sudden fractures of steel axles, rails, tires, and shafts productive
of such calamitous results. A fracture once commencing at one of these
micro-flaws (started probably by some sudden shock or vibration, or
owing to the deterioration caused by fatigue in the metal) runs
straight through a steel forging on the line of least resistance, in a
similar manner to the fracture of glass or ice.”
It is understood that similar investigations are being carried out
on an extensive scale by Prof. Arnold; in the meantime the above
cases should satisfy any one that these impurities are elements of
disintegration, and that the less there are of them in any steel the
better for the steel.
It seems clear that if 10 sulphur will cause 60,000 flaws per square
inch, 01 sulphur ought not to cause more than one tenth of that number;
or, if an equal number, then they could only be one tenth of the size.
The segregation found in the shaft is so excessive that it would seem
probable that there was a good deal of sin there also; but, even if it
were unavoidable segregation, the harm would have been just so much the
less if there had been less of total impurities present to segregate.
ARSENIC.
Arsenic is known to be very harmful in tool-steel, and it is proper to
assume that it can do no good in structural steel. In any case where
the properties of steel do not come up to the standard to be expected
from the regular analysis examination should be made for arsenic,
antimony, copper, etc. These are not as universal constituents of steel
as silicon, phosphorus, sulphur, and manganese, but they are present
frequently, and in any appreciable amount they are bad.
XI.
THEORIES OF HARDENING.
The hardening of steel is such a marked phenomenon, and one of so great
importance, that it has always attracted a great deal of attention, and
many theories have been put forward in explanation.
Before chemistry was brought to bear upon the subject the proposed
theories were based upon assumption, and as there were no proofs one
had as much right to consideration as another, and none seemed to be
altogether satisfactory.
Since science has taken up the question the theories are about as
numerous as the investigators, and while no one can claim as yet to
have settled the matter definitely, each one has an apparent basis of
reason deduced from observed facts.
Among early observations it was noted that when unhardened steel and
hardened steel were dissolved in acid a much larger amount of carbon
was found in the solution of the unhardened than in that of the
hardened steel. This led, first, to the distinction of combined carbon
and graphitic carbon, a distinction that has been maintained through
subsequent investigations. It seems to be well established now that
there is a definite carbide of iron, Fe₃C, and some observers believe
it to be the hard substance in hardened steel.
Following this came the announcement that these conditions, _combined_
and _graphitic_ carbon, represented two different forms of carbon, and
they were designated as _cement_ carbon and _hardening_ carbon; also
as _non-hardening_ and _hardening_ carbon. Later investigation having
established the existence of the carbide Fe₃C, this was claimed to be
the hard body, but this has not met universal acceptance.
Another investigator, studying by means of the pyrometer and observing
heat phenomena, concludes that hardening is due to an allotropic
condition of the iron itself; that when iron is heated above the
recalescent-point, and presumably below granulation, it becomes in
itself excessively hard; that sudden cooling prevents its changing
from this form, and so, when there is carbon present, the result of
quenching is great hardness.
When steel is allowed to cool slowly to below recalescence, the iron
assumes another form, and one which cannot be hardened by quenching;
this latter is known as α iron, and the hardening kind as β iron. A
later investigator finds it necessary to have a third allotropic form
to meet some of the phenomena, which he designates by another Greek
letter.
Another investigator establishes independently the saturation-point,
which was pointed out and published twenty years ago, viz., somewhere
about 90 to 100 carbon; he fixes the saturation-point at 89 carbon
and gives the formula Fe₂₄C. He assumes that this is an exceedingly
unstable carbide, that it is formed between recalescence and
granulation, and can only be fixed by quenching, and that when steel is
quenched the fixing of this carbide is the cause of hardness.
A still later investigation establishes this saturation-point at about
100 carbon by observing that in hardened steel of 135 carbon there is
a combination of 100 carbon which is the excessively hard part of the
steel, and a portion containing the remaining 35 parts of carbon that
is not quite so hard, and he suggests a fourth allotropic form to cover
this part.
It is also suggested that steel should be considered and treated as an
igneous rock; judging from the appearance of magnified micro-sections,
this suggestion appears to be a happy one for the purpose of making
comparisons.
The above theories of hardening, and others, are not to be regarded as
antagonistic or contradictory, doubtless there are germs of truth in
every one of them, or each one may be merely the individual’s way of
suggesting an explanation of the same observed phenomena, so that when
a final conclusion is reached each may be found to have been travelling
in the same direction by a different path. It is certain that able,
patient, painstaking, men are working faithfully to produce a solution
of the problem, and even if their ideas, as briefly given above, do
seem to be contradictory it would only evince deeper ignorance and a
stupid mind in any who should attempt to ridicule or unduly criticise
honest work before it is completed. While these investigations are
going on, and before any definite conclusion is reached, is there any
well-established safe ground for the steel-worker and the engineer
to stand upon? There certainly is a good working hypothesis for all
to use, and one which it is believed will always be the right one to
follow no matter what the final explanation of the remarkable phenomena
of hardening, tempering, and annealing may prove to be.
After many years of careful experimenting and study Prof. J. W. Langley
came to the conclusion that no matter what the final result might be as
to carbides, allotropic conditions, etc., that if steel were considered
as iron containing carbon in solution, whether it were a chemical
combination or a mere solution, and that cold steel be regarded as a
congealed liquid in a state of tension, then all known phenomena could
be accounted for, and all known conditions could be produced with
certainty by well-known applications of heat and force.
When carbon is in the so-called combined condition, then the solution
maybe compared to pure sea-water; when the carbon is partly combined
and partly graphitic, the solution may be compared to muddy sea-water,
the mud representing the graphitic carbon.
When the carbon is practically all graphitic, as in over-annealed
steel, then the solution may be compared to thoroughly muddy fresh
water.
This hypothesis of solution agrees well with the saturation noted; then
about 100 carbon is all that iron will dissolve without extraneous
force; and higher carbon must be forced into solution by the work of
hammers, presses, or rolls.
This gives reason to the experienced tool-maker’s well-known preference
for well-hammered steel.
The hypothesis of tension, probably molecular, covers all of the
phenomena of excessive hardness due to high heat, which means high
molecular motion checked violently by sudden quenching. It accounts
for the progressive softening due to every added degree of heat, and
it accounts for rupture, cracking, due to excessive heat or to any
unevenness of heat.
Without this hypothesis of tension it is difficult to understand why
quenching should rupture a piece of steel, no matter what the degree of
heat, or how uneven it might be.
Without it, too, it is hard to see how successive additions of heat can
cause gradual changes from β to α iron, or from an unstable carbide to
an imperfect solution. It would seem that the allotropic changes, or
the decompositions of carbides, must be more marked than the gradual
changes from hard to soft which we know to take place by slow and
gentle accretions of heat.
There is no property of steel known to the author which is not
covered by Langley’s hypothesis, and therefore it is put forward
with confidence for engineers and steel-users to work by until the
scientists shall have completed their investigations, and after that it
is believed that it will be a safe working hypothesis, because science
does not change facts, it only collates them and reveals the laws of
action.
Under this hypothesis of Langley’s we may define _hardness_ as
_tension_, _softness_ as absence of tension.
This is not stated as established fact; it is given as a simple
definition to cover the known phenomena until the final solution of the
problem shall lead to a better explanation.
Regarding steel as a solution of carbon in iron, one important fact may
be set down as established thoroughly: that is, that the more perfect
the solution under all circumstances the better the steel.
Continued application of heat in any part of the plastic condition
allows carbon to separate out of solution into a condition of mere
mixture; it converts the clear sea-water into muddy water; this is the
reason why so much emphasis has been given in previous chapters to the
harmfulness of long-continued heating.
In every case, when steel is hot enough for the purpose desired, it
should be removed at once from the fire.
XII.
INSPECTION.
Careful and systematic inspection is of the utmost importance from the
first operation of melting to the last act of the finisher.
Assuming that every operator is honest and conscientious in the
performance of his work, the personal equation must be considered,
as well as the exigencies of the many operations. The steel-maker
must inspect his ingots to see that they are melted well and teemed
properly, that they are sound and clean, and to determine their proper
temper.
When work is finished, he must inspect it to see that it has been
worked at proper, even heats, that it is correct in dimensions, and
that all pipes and seams have been cut out. After all this has been
done faithfully it were well that his work were done when it were well
done. Such is not his happy lot; every successive manipulator may ruin
the steel by carelessness or ignorance, and it is a gala day for a
steel-maker when he does not receive some sample of stupid ignorance or
gross carelessness, with an intimation that it would be well for him
to learn how to make steel before he presumed to offend by sending out
such worthless material. And sometimes, though not so often if he knows
his business, he finds a complaint well founded; then he must regulate
his own household and make his peace with his angry customer as best he
can.
The engineer must inspect his steel to see that it is sound, and clean,
and finished properly, as he has a right to expect that it should be.
It is not intended here to lay down rules for shop and field
inspection,--that is an art in itself outside of the function or the
experience of a steel-maker,--but some hints may be given as to the
examination of steel as it comes from the mill, and it has been the aim
in previous chapters to give such information as may enable an engineer
to form a good judgment as to matters which are not likely to come to
his knowledge in the course of ordinary practice.
Steel should be sound; it should be examined before it is oiled or
painted. All pipe should be cut off; a pipe of any considerable size
will show in the end of a sheared bar, and a careful observer will soon
learn to detect it. If there is reason to suspect a pipe, file the
place and the pipe will be revealed if it is there. Do not chip at it,
for a chisel will often smooth a line which a file will bring out. In
tool-steel there should not only be no pipe, there should be no star
left in the bar. A “star” is a bright spot which shows the last of the
pipe, not quite cut away; the steel is not solid in the star and it
will not make a good cutting-edge; it may even cause a sledge to split.
SEAMS.
In tool-steel there should be no seams at all. Some makers declare that
in high steel, seams are evidences of good quality; such a statement is
the veriest fraud; it is hard to get any high steel free from seams,
and therefore if the maker can get the user to believe that a seam is a
good thing he can enhance his profit; that is, he can enhance it for a
time until his fraud is understood.
Some seams are hard to see; when there is reason to suspect one, a
little filing across the line will show it in a distinct black line if
it is there. A file is an indispensable tool for an inspector, better
than a chisel or a grindstone.
In machinery and structural steel a few small seams may be
unobjectionable; too close inspection may lead to unnecessary cost
without a compensating gain; still every engineer should reserve the
right to determine what seams are allowable and what are not, for his
own safety.
_Laps_ should not be tolerated in any work.
_Torn cracks_ on edges or surface indicate burned steel or red-short
steel; they should not be allowed.
The grain of steel should be practically uniform, not too coarse, not
with brilliant lustre, nor with a dark india-ink tint. With an even
fine grain, a bright lustre may indicate a mild steel not worked badly.
Inspectors must learn by practice what is tolerable and what is not, as
it is impossible to lay down hard and fast rules; it is safe, however,
to say that a fairly fine grain of even texture, not much lustre, and
no india-ink shade, is indicative of good heating and proper working.
With these few general hints the subject must be left, for, like
tempering, inspecting is an art in itself, and it cannot be taught in a
book.
An expert inspector will see seams and pipes with his naked eye that a
novice could not detect with an ordinary magnifying-glass.
It may do no harm to the inspector to suggest to him that amiability
and good sense are the best ingredients to mix with sound judgment.
If he will cultivate these, and learn to distinguish between a mere
blemish and a real defect, he will find his work made easy and
pleasant; and he will be far less likely to have bad work thrust at him
than he will if he makes it apparent that he regards himself as the
only honest man.
XIII.
SPECIFICATIONS.
Specifications should cover three principal points:
_Physical properties_: Elastic limit; ultimate tensile strength;
elongation; reduction of area.
_Chemical constituents_: Limiting silicon, phosphorus, sulphur,
manganese, and copper; all other elements to be absent or mere traces
in quantity, except carbon.
_Finish and general condition_: Fixing limit of variation in size from
a given standard; conditions as to pipes, seams, laps, uniformity of
grain, and other defects; no red-shortness.
PHYSICAL PROPERTIES.
It has been shown in Chap. V that tensile strength may be had from
46,800 lbs. per square inch to 248,700 lbs. per square inch.
There are published in many transactions and technical periodicals
thousands of tests giving elastic and ultimate strength, ductility,
etc., so that every engineer can find easily what has been done to
guide him as to what he can get.
In almost every case the engineer must be the judge as to the
requirements in each; therefore it would be useless to attempt to lay
down any fixed rules or limits.
Many engineers adhere to low tenacity and high ductility in the belief
that they are securing that material which will be safest against
sudden shocks and violent accidental strains.
Theoretically this appears to be correct, but if the statements made
in the preceding chapters are credible it is plain that the limit to
such safety can be passed, and that in insisting upon too low tenacity
and high ductility the engineer may be getting simply a rotten,
microscopically unsound material, through no fault of the manufacturer,
who has been compelled to overmelt or overblow his steel to meet the
requirements, and so reducing the quality of otherwise good material at
no saving in cost to himself, and at a considerable cost in quality to
the consumer.
Any manufacturer would rather check his melt between 10 and 15 carbon,
or stop his blow so as to be sure not to overblow, if he were asked to
do so, because it would save him time and expense, and it would yield
sounder, better, and easier working steel.
It may not be wise yet for an engineer to fix limits as to blowing
or melting, for the reason that neither he nor his assistants would
know how to insure compliance, and in attempting to do it they might
interfere too far with manufacturing operations and so involve
themselves in responsibilities which they ought not to assume.
On the other hand, if they will let the carbon and tensile strength run
up a little and reduce ductility slightly, it is safe to say that any
manufacturer will be glad of the chance to help them to get the best
results, which involve no extra cost.
Boiler-steel and rivet-steel usually suffer the most in this respect.
A boiler should be tough, yet it is the belief of the author that
boilers made of the 46,800-lb. steel of which the analysis is given in
Chap. V would not last half as long as boilers made of 65,000-lb. to
70,000-lb. steel when the increased strength was gained by added carbon
and no overmelting was allowed.
In the same table the “Crucible-sheet” column gives a mean of 24
tests, and a mean analysis, of boiler-steel which has been in use in
12 boilers for nearly 16 years. The boilers are in perfectly good
condition; they have been subjected to severe and very irregular usage,
and they have been in every way satisfactory. Only one test-piece
of the 24 was mild enough to stand the ordinary bending test after
quenching.
That 46,800-lb. steel is remarkably pure chemically; it is unusually
red-short. It would appear to some to be an ideal rivet-steel; it would
stand a very high heat, it would head well and finish beautifully under
a button-set. There is every probability that the majority of rivets
driven of that steel would be cracked on the under side of the head,
where the cracks would never be discovered until in service the heads
flew off.
Rails are usually made of 40 to 45 carbon, tires from .65 up to 80
carbon, crank-pins as high as 70 carbon, with 85,000 lbs. to 95,000
lbs. tensile strength and 12% to 15% elongation.
It is difficult to see how a bridge or a boiler is to be subjected to
any such violent usage as these receive daily; and while it is not
advised that even 40 carbon should be used in boilers or bridges,
although it would be perfectly safe, it does seem to be unreasonable to
run to the other extreme to the injury of the material.
For steel for springs, and for all sorts of tools that are to be
tempered, there is no need of a specification of physical properties as
they are indicated by testing-machines.
The requirement that they shall harden safely and do good work
afterwards involves necessarily, high steel of suitable quality.
CHEMICAL CONSTITUENTS.
No engineer should, unless he be an expert steel-maker, attempt
to specify an exact chemical formula and a corresponding physical
requirement; in doing so he would probably make two requirements which
could not be obtained in one piece of steel, and so subject himself to
a back down or to ridicule, or both.
On the other hand, he may properly, and he should fix, a limit beyond
which the hurtful elements would not be tolerated. Notwithstanding
satisfactory machine tests, successful shop-work, and a liberal
margin of safety, no steel can be relied upon that is overloaded with
phosphorus, sulphur, manganese, oxygen, antimony, arsenic, or nitrogen.
In regard to silicon, it is common to have as much as 20 to 25 points
in tires, with 55 to 80 carbon; such tires are made by the best
manufacturers, and they endure well. But it is certain that good, sound
steel can be made for any purpose with silicon not exceeding 10.
Structural steel can be made cheaply within the following limits:
Silicon < .10
Phosphorus < .05
Sulphur < .02
Manganese < .50 or even < .30
Copper < .03
Carbon to meet the physical requirements.
Steel made within these limits and not overblown or overmelted must be
better in every way than steel of
Silicon > .20
Phosphorus > .08
Sulphur > .05
Manganese > .60
Carbon to meet the same requirements
A steel of the latter composition, or with no fixed limits, may be made
cheaper than the first by a dollar or two a ton; but for any large
lot it is believed that the first specification would be bid to at
as low a price as if there were no specification; competition among
manufacturers would fix that. At any rate there is no reason why an
engineer should refuse to demand fairly pure material when he can do so
at little or no extra cost.
Arsenic, antimony, or any other elements should be absent, or < .005.
FINISH AND GENERAL CONDITIONS.
As there can be no such thing as exact work done, there must be some
tolerance as to variation in size. In standard sections, sheets, and
plates this is usually covered by a percentage of weight; in forgings
or any pieces that are to be machined the consumer should allow enough
to insure a clean, sound surface. But it would be unwise to lay down
any rule here, because conditions vary; a rolled round bar may finish
nicely by a cut of from ¹/₃₂ to ¹/₁₆ of an inch, and so also a neatly
dropped forging; an ordinary hammered forging might require a cut of ¼
or ⅜ of an inch; such a forging might be made closer to size at a cost
for extra time at the hammer far exceeding the saving of cost in the
lathe. These are cases where common-sense and good judgment must govern.
Pipes should not be tolerated if they can be discovered; because a pipe
appears small in the end of a bar it is no evidence that it is not
larger farther in.
Seams should not be allowed in any steel that is to be hardened; they
should be a minimum in any steel, as they are of no possible use; small
seams when not too numerous may do no harm in structural or machinery
steel, and consumers should be reasonable in regard to them, or else
they may have too high prices put upon their work, or too high heat
used in efforts to close the last few harmless seams.
Burns, rough, ragged holes in the faces or on the corners, are
inexcusable and should be rejected; the steel has been abused, or it is
red-short; in either case the ragged breaks are good starting-points
for final rupture.
Laps should not be permitted; they are evidences of carelessness; there
can be no excuse for them.
Fins are sometimes unavoidable in a difficult shape; for instance, if a
trapezoid is wanted, it may be rolled in this form:
[Illustration]
or in this:
[Illustration]
The consumer must decide which; if he wants sharp angles he must accept
the fin and cut it off, or have it cut off by the manufacturer.
Rivet-steel should be tested rigidly for red-shortness, because
red-short steel may crack under the head as the steel cools.
Emphasis is laid upon this because engineers will insist upon excessive
ductility in rivet-steel, not realizing that they may be requiring the
manufacturer to overdose his steel with oxygen to its serious injury.
No sharp re-entrant angles should be allowed under any circumstances
where there is a possibility of vibrations running through the mass.
All re-entrant angles should be filleted neatly.
No deep tool-marks should be allowed; a fine line scored around a piece
by a lathe-tool, or a sharp line cut in a surface by a planing-tool
will fix a line of fracture as neatly as a diamond-scratch will do it
on a piece of glass.
Indentations by hammers or sledges should be avoided; they may not be
as dangerous as lathe-cuts, but they can do no good, and therefore they
are of no use.
XIV.
HUMBUGS.
Steel is of such universal use and interest in all of the arts that
it attracts the attention of would-be inventors perhaps more than any
other one material.
Half-informed, or wholly uninformed, men get a smattering of knowledge
of some one or more of the well-known properties of steel, make an
experiment which produces a result that is new and startling to them,
and at once imagine that they have made a discovery; this they proceed
to patent and then offer it to the world with a great flourish of
trumpets.
Many steel-workers, even men of skill, who know something of the
difficulties that follow irregular work, or who are not quite fully
informed as to the properties of steel, seize upon these discoveries
in the hope that they have found a royal road to success where all old
pitfalls are removed and their path is made easy.
Not wishing to discourage pioneers in legitimate efforts to improve, it
is the object of this chapter to warn them against being too ready to
spend their money because of flaming circulars or glib tongues. It is
the duty and the interest of a steel-maker to examine and test every
apparently new suggestion, for the reason that there is still room for
improvement, and he should let no opportunity for a betterment slip
past him.
As a rule the steel-maker does test every claim that is laid before
him, unless it be a repetition of some old plan long since tried and
found worthless. This is the bane of the steel-maker’s life, and yet he
must keep at this work so that he may know for himself whether anything
of value has been discovered, and also that he may advise his clientage
properly.
Inventions relating to the manufacture of steel have no interest for
steel-users except as lively manufacturers may adopt the mistaken
plan of flourishing trumpets to attract trade, not always giving a
corresponding benefit to the consumer.
Examples of this sort of thing may be illustrated by so-called
phosphorus steel, silicon steel, and aluminum steel; also the
case mentioned before of parties recommending seams as evidences
of excellence in high steel. Such efforts are sometimes costly to
consumers until active competitive manufacturers expose the humbug.
Among the most absurd of such claims are those where a nostrum is used
to convert ordinary Bessemer or open-hearth steel into the finest of
tool-steel, equal to the best crucible-steel; for example, a patent
to convert mild Bessemer steel into the finest tool-steel by merely
carbonizing it by the old cementation process; this takes no account
of the silicon, manganese, oxygen, and nitrogen in the mild Bessemer,
makes no provision for their removal, and involves a costly method of
putting carbon into poor stock in face of the fact that a Bessemer
steel maker can put the same amount of carbon there at practically no
cost, and so produce a better material.
Among the humbugs that do not involve the manufacturer, the pet one is
a nostrum for restoring burnt steel; these have been evolved by the
dozen, in face of the fact that burned steel cannot be restored except
by smelting, and that overheated steel, coarse-grained steel, can be
restored by merely heating it to the right temperature, a process which
has been explained fully in Chapter VI.
Another pet is some greasy compound for toughening high steel so as
to make it do more work. This is done by heating the steel to about
recalescence and plunging it into the grease, perhaps once, or possibly
two or three times; then working it into a tool and proceeding in the
ordinary way. This will make a good tool; it is the partial annealing
plan explained in a previous chapter. Now take a similar piece of
steel, heat it the same way, lay it down in a warm, dry place alongside
the forge-fire, and let it cool; then heat it and work it into a tool
and it will beat the greased tool.
When all of these operations of restoring, partial annealing,
annealing, etc., depend merely upon temperature and rate of cooling,
why spend money for nostrums that add no possible benefit?
There is room for improvement in steel, great room for great
improvements; they will come in time as science and knowledge advance,
and great benefits to the consumers will come with them.
This chapter is not written to place difficulties in the way of
legitimate improvement, but to warn unsuspecting people against
quackery. Some of the humbugs are honest productions of well-meaning
ignorance, and some that come from designing manufacturers are not
entitled to such charitable designation. A knowledge of the simplest
properties of steel will enable a thoughtful man to judge as to whether
a proposed improvement is likely to be of any value or not, and the
warnings given are intended as a protection to the unsuspecting and
credulous.
XV.
CONCLUSIONS.
After perusal of the preceding chapters the reader may form a hasty
conclusion that if steel be so sensitive as it is stated to be its use
may be difficult and precarious, and that it must be handled in fear
and trembling, lest the result should be a dangerous structure, and the
builder must be in doubt as to its safety.
The conveyance of any such impressions is not intended at all; emphasis
has been laid upon practices that are hurtful in order that every
steel-user may know what to avoid, solely that he may then be sure that
he has the best, the most reliable, and most useful material that is
known to man.
WHAT TO AVOID.
He should avoid uneven heat, excessive heat, or too low heat. The range
between orange red and the heat that will granulate is so great that no
one who is not a bungler or indifferent need ever get outside of it.
The uniformity of temperature that is insisted upon is so easily seen
that any person who is not color-blind should have no trouble in
securing it by the simplest manipulations of the furnace.
Practical uniformity of the work put on a piece is readily secured by
any mechanic of ordinary skill.
Red-short, cold-short, or honeycombed steel are easily detected, and,
under reasonable specifications, the steel-makers can as easily avoid
them.
Steel a little higher than most engineers favor in their specifications
is certainly as safe as, and likely to be sounder than, extremely
ductile steel.
Wild steel, resulting almost certainly in micro-honeycombs, if not
worse, can only be avoided by the co-operation of the manufacturer, and
engineers should impress this point with energy.
Such micro-unsoundness as is shown in Mr. Andrews’s report upon
a broken rail and propeller-shaft can be reduced to a minimum by
insisting upon reasonably pure steel.
If sulphur, phosphorus, silicon, and oxygen are kept at a reasonable
minimum, sulphides, phosphides, silicides or silicates, and oxides must
be at a corresponding minimum.
That there is much room for improvement in the manufacture of steel is
evident, and when means of getting rid of oxygen, nitrogen, and all
other undesirable elements have been found the steel of the future will
be very different in kindliness of working and in endurance of strains
than that with which we are familiar.
It is believed, however, that no matter how perfect the manufacture may
become, nor what the final theories of hardening, etc., may be, the
properties stated in these pages will remain the same as long as steel
continues to be essentially a union of iron and carbon.
Some other alloy or compound may displace carbon steel, and present an
entirely new set of properties, but there is nothing of the kind in
sight now, and engineers need have no fear of having a new art to learn
very soon.
To one who has spent an ordinary business lifetime in making steel,
studying it, and working with it it becomes a subject of absorbing
interest, if not of love; and steel when handled reasonably is so true
that “true as steel” ceases to be a metaphor, it is then a fact which
fills him with the most entire confidence.
Once more, steel highly charged with sulphur, phosphorus, arsenic,
oxygen, and nitrogen is certainly highly charged with so many elements
of disintegration; it takes more serious harm from ordinary deviations
from good practice, such little irregularities as occur inevitably in
daily working, than steel does which is more free from these elements.
Reasonably pure, sound, reliable steel can be had at moderate cost, and
all consumers should insist upon having it.
Regular, uniform, reliable working can be had where it is required, and
there should be no excuse for irregular grain, overheated work, uneven
work, or any other bungling. Where skill is required and reasonable
discipline is enforced, good work will not cost any more than bad work.
Many people still hold to the idea that there are many mysteries
connected with steel, and that many unaccountable breaks occur which
make it an unreliable material. It is hoped that what has been set down
in these pages will go far to dissipate these supposed mysteries, and
to give confidence to steel-users.
Many breaks are unaccounted for, but it is not within the author’s
experience that any fracture ever occurred that could not have been
explained if it had been examined thoroughly in the light of what we
know now. There is much to be learned, but there are no mysteries.
GLOSSARY.
There are many shop terms used in this book which may not be familiar
to all steel-users.
They are in common use in steel-manufactories, and definitions of them
will enable a steel-user to understand more clearly the common talk he
will hear in the shops.
=Blow-holes.=--Blow-holes are the small cavities, usually spherical,
which are formed in ingots as the steel congeals by bubbles of gas
which cannot escape through the already frozen surface.
=Burned.=--Burned steel is steel that is reduced to oxide in part by
excessive heating.
=Check.=--A check is a small rupture caused by water; it may run in any
direction; it is usually not visible until steel is ruptured.
=Chemical Numeration.=--Chemical quantities are almost universally
expressed in hundredths of one per cent, as explained in the body
of the work. It is a very convenient numeration; any steel-worker,
melter, hammerman, etc., will talk of 20, or 50, or 130 carbon;
or 8 phosphorus; or 10, 15, or 25 silicon, etc.; and will talk
intelligently, although he may not know the exact mathematical value of
these points.
=Dead-melting; synonym, killing.=--Dead-melting--killing--means melting
steel in the crucible or open hearth until it ceases to boil or evolve
gases; it is then dead, it lies quiet in the furnace, and killed
properly it will set in the moulds without rising or boiling.
=Dry.=--Steel is called _dry_ when its fracture is sandy-looking,
without lustre or sheen, and without a proper blue cast. There is more
of a shade of yellowish sandstone. It is an evidence of impurity and
weakness.
=Fiery.=--Fiery steel has a brilliant lustre; it is an evidence of high
heat.
If the grain be fairly fine and of bluish cast, it is not necessarily
bad in mild steel; in high steel or in tool-steel it should not be
tolerated.
If the grain be large and of brassy cast, it is sure evidence of bad
condition; the grain should be _restored_ before the steel is used.
In hardened steel it is always bad, except in dies to be used under the
impact of drop-hammers; in this case steel must be so hard as to be
slightly fiery.
=Grade.=--Grade applies to quality, as crucible, Bessemer, or
open-hearth grade. Or in the crucible, common, spring, machinery,
tool, special tool, etc., etc. It does not indicate temper or relative
hardness.
=Honeycombed.=--Unsound from many blow-holes. Usually applied to
ingots. It is a bad condition.
=Lap.=--A lap is caused by careless hammering, or by badly proportioned
grooves in rolls, or by careless rolling. A portion of the steel is
folded over on itself, the walls are oxidized and cannot unite. A lap
generally runs clear along a bar, practically parallel with its axis;
it may be seen by a novice. Lapped steel should be rejected always.
=Overblown.=--Steel that has been blown in a Bessemer converter after
the carbon is all burned; then there is nothing but steel to burn, and
the result is bad.
=Overheated.=--Steel that has been heated too hot, and not quite
burned; its fiery fracture exposes it. The grain of overheated steel
may be restored, but restored steel is never as reliable as steel that
has not been overheated. Overheating is a disintegrating operation.
=Overmelted.=--Steel that has been kept too long in fusion. The finest
material may be ruined in a crucible by being kept in the furnace any
considerable time after it has been killed. Open-hearth steel may be
injured seriously in the same way. Prompt teeming after killing should
be the rule.
=Pipe.=--A pipe is the cavity formed in an ingot when it cools; the
walls chill first and nearly to the full size of the mould, then the
shrinking mass separates in the middle, forming a pipe. A pipe should
be at the top of the ingot; it may occur anywhere by bad teeming.
=Point.=--One hundredth of one per cent of any element. You have say 10
points of carbon, or 10 carbon; you want it raised a few points to 15
or 18 carbon.
=Recalescence.=--When a piece of steel is heated above medium orange
color and cools slowly, at about medium orange--1100° to 1200° F.--the
change of color ceases, then the color rises sometimes to bright
orange, and afterwards the cooling goes on; this phenomenon is called
recalescence. This is not yet a common shop term.
=Restoring.=--When a piece of overheated steel is re-heated to
recalescence, kept there a few minutes, and then cooled slowly, its
grain becomes fine and its fiery lustre disappears; this is called
_restoring_. No nostrums are necessary.
=Sappy.=--Well-worked, good steel has a bluish cast, a fine grain, and
a silky sheen. It is sappy; it is as good as it can be made.
=Seam.=--A seam is a longer or shorter defect, caused by a blow-hole
which working has brought out to the surface and not eliminated.
It usually, or always, runs in the direction of working. Seams are
distinguished from laps by not being continuous; they are usually only
an inch or two in length.
=Short (Cold, Red, Hot).=--
_Cold-short_ steel is weak and brittle when cold.
_Red-short_ steel is brittle at dark orange or medium orange
heat or at the common cherry red heat. It may forge well at
a lemon heat, and be reasonably tough when cold.
_Hot-short_ steel is brittle and friable above a medium orange
color; it may forge well from medium orange down to black heat.
=Star.=--A brilliant spot in mid-section showing that the pipe is not
all cut away. It should be removed from tool-steel especially, as it
may have considerable depth. It is of no use in any steel.
=Temper.=--Used by the steel-maker it means the quantity of carbon
present. It is low temper, medium, or high; or number so and so by his
shop numbers.
Used by the steel-user or the temperer it means the color to which
hardened steel is drawn: straw, brown, pigeon-wing, blue, etc., etc.
Or, it is the steel-maker’s measure of initial hardness; and it is the
steel-user’s measure of final hardness.
=Water-crack.=--A crack caused in hardening; it may run in any
direction governed by lines of stress in the mass. It is distinguished
from a _check_ by being larger, and usually plainly visible.
=Wild Steel.=--Steel in fusion that boils violently, and acts in the
moulds like lively soda-water or beer does when poured into a glass.