COMPOSITION AND PROPERTIES OF STEEL
It is a remarkable fact that one can look through a dozen text
books on metallurgy and not find a definition of the word "steel."
Some of them describe the properties of many other irons and then
allow you to guess that everything else is steel. If it was difficult
a hundred years ago to give a good definition of the term when the
metal was made by only one or two processes, it is doubly difficult
now, since the introduction of so many new operations and furnaces.
We are in better shape to know what steel is than our forefathers.
They went through certain operations and they got a soft malleable,
weldable metal which would not harden; this they called iron. Certain
other operations gave them something which looked very much like
iron, but which would harden after quenching from a red heat. This
was steel. Not knowing the essential difference between the two,
they must distinguish by the process of manufacture. To-day we
can make either variety by several methods, and can convert either
into the other at will, back and forth as often as we wish; so
we are able to distinguish between the two more logically.
We know that iron is a chemical element--the chemists write it
Fe for short, after the Latin word "ferrum," meaning iron--it is
one of those substances which cannot be separated into anything
else but itself. It can be made to join with other elements; for
instance, it joins with the oxygen in the air and forms scale or
rust, substances known to the chemist as iron oxide. But the same
metal iron can be recovered from that rust by abstracting the oxygen;
having recovered the iron nothing else can be extracted but iron;
_iron is elemental_.
We can get relatively pure iron from various minerals and artificial
substances, and when we get it we always have a magnetic metal,
almost infusible, ductile, fairly strong, tough, something which
can be hardened slightly by hammering but which cannot be hardened
by quenching. It has certain chemical properties, which need not be
described, which allow a skilled chemist to distinguish it without
difficulty and unerringly from the other known elements--nearly
100 of them.
Carbon is another chemical element, written C for short, which is
widely distributed through nature. Carbon also readily combines
with oxygen and other chemical elements, so that it is rarely found
pure; its most familiar form is soot, although the rarer graphite and
most rare diamond are also forms of quite pure carbon. It can also
be readily separated from its multitude of compounds (vegetation,
coal, limestone, petroleum) by the chemist.
With the rise of knowledge of scientific chemistry, it was quickly
found that the essential difference between iron and steel was that
the latter was _iron plus carbon_. Consequently it is an alloy,
and the definition which modern metallurgists accept is this:
"Steel is an iron-carbon alloy containing less than about 2 per
cent carbon."
Of course there are other elements contained in commercial steel,
and these elements are especially important in modern "alloy steels,"
but carbon is the element which changes a soft metal into one which
may be hardened, and strengthened by quenching. In fact, carbon,
of itself, without heat treatment, strengthens iron at the expense
of ductility (as noted by the percentage elongation an 8-in. bar
will stretch before breaking). This is shown by the following table:
--------------------------------------------------------------------------
| | |Elastic |Ultimate|Percentage.
Class by use. | Class by | Per cent | limit |strength|elongation
| hardness. | carbon. |lb. per |lb. per |in 8 inches.
| | |sq. in. |sq. in. |
------------------|-----------|------------|--------|--------|------------
Boiler rivet steel|Dead soft |0.08 to 0.15| 25,000 | 50,000 | 30
Struc. rivet steel|Soft |0.15 to 0.22| 30,000 | 55,000 | 30
Boiler plate steel|Soft |0.08 to 0.10| 30,000 | 60,000 | 25
Structural steel |Medium |0.18 to 0.30| 35,000 | 65,000 | 25
Machinery steel |Hard |0.35 to 0.60| 40,000 | 75,000 | 20
Rail steel |Hard |0.35 to 0.55| 40,000 | 75,000 | 15
Spring steel |High carbon|1.00 to 1.50| 60,000 |125,000 | 10
Tool steel |High carbon|0.90 to 1.50| 80,000 |150,000 | 5
--------------------------------------------------------------------------
Just why a soft material like carbon (graphite), when added to
another soft material like iron, should make the iron harder, has
been quite a mystery, and one which has caused a tremendous amount
of study. The mutual interactions of these two elements in various
proportions and at various temperatures will be discussed at greater
length later, especially in Chap. VIII, p. 105. But we may anticipate
by saying that some of the iron unites with all the carbon to form a
new substance, very hard, a carbide which has been called "cementite."
The compound always contains iron and carbon in the proportions
of three atoms of iron to one atom of carbon; chemists note this
fact in shorthand by the symbol Fe3C (a definite chemical compound
of three atoms of iron to one of carbon). Many of the properties
of steel, as they vary with carbon content, can be linked up with
the increasing amount of this hard carbide cementite, distributed
in very fine particles through the softer iron.
SULPHUR is another element (symbol S) which is always found in
steel in small quantities. Some sulphur is contained in the ore
from which the iron is smelted; more sulphur is introduced by the
coke and fuel used. Sulphur is very difficult to get rid of in
steel making; in fact the resulting metal usually contains a little
more than the raw materials used. Only the electric furnace is
able to produce the necessary heat and slags required to eliminate
sulphur, and as a matter of fact the sulphur does not go until
several other impurities have been eliminated. Consequently, an
electric steel with extremely low sulphur (0.02 per cent) is by
that same token a well-made metal.
Sulphur is of most trouble to rolling and forging operations when
conducted at a red heat. It makes steel tender and brittle at that
temperature--a condition known to the workmen as "red-short." It
seems to have little or no effect upon the physical properties
of cold steel--at least as revealed by the ordinary testing
machines--consequently many specifications do not set any limit
on sulphur, resting on the idea that if sulphur is low enough not
to cause trouble to the manufacturer during rolling, it will not
cause the user any trouble.
Tool steel and other fine steels should be very low in sulphur,
preferably not higher than 0.03 per cent. Higher sulphur steels
(0.06 per cent, and even up to 0.10 per cent) have given very good
service for machine parts, but in general a high sulphur steel
is a suspicious steel. Screw stock is purposely made with up to
0.12 per cent sulphur and a like amount of phosphorus so it will
cut freely.
Manganese counteracts the detrimental effect of sulphur when present
in the steel to an amount at least five times the sulphur content.
PHOSPHORUS is an element (symbol P) which enters the metal from
the ore. It remains in the steel when made by the so-called acid
process, but it can be easily eliminated down to 0.06 per cent
in the basic process. In fact the discovery of the basic process
was necessary before the huge iron deposits of Belgium and the
Franco-German border could be used. These ores contain several
per cent phosphorus, and made a very brittle steel ("cold short")
until basic furnaces were used. Basic furnaces allow the formation
of a slag high in lime, which takes practically all the phosphorus
out of the metal. Not only is the resulting metal usable, but the
slag makes a very excellent fertilizer, and is in good demand.
SILICON is a very widespread element (symbol Si), being an essential
constituent of nearly all the rocks of the earth. It is similar to
carbon in many of its chemical properties; for instance it burns
very readily in oxygen, and consequently native silicon is unknown--it
is always found in combination with one or more other elements.
When it bums, each atom of silicon unites with two atoms of oxygen
to form a compound known to chemists as silica (SiO2), and to the
small boy as "sand" and "agate."
Iron ore (an oxide of iron) contains more or less sand and dirt
mixed in it when it is mined, and not only the iron oxide but also
some of the silicon oxide is robbed of its oxygen by the smelting
process. Pig iron--the product of the blast furnace--therefore
contains from 1 to 3 per cent of silicon, and some silicon remains
in the metal after it has been purified and converted into steel.
However, silicon, as noted above, burns very readily in oxygen,
and this property is of good use in steel making. At the end of
the steel-making process the metal contains more or less oxygen,
which must be removed. This is sometimes done (especially in the
so-called acid process) by adding a small amount of silicon to
the hot metal just before it leaves the furnace, and stirring it
in. It thereupon abstracts oxygen from the metal wherever it finds
it, changing to silica (SiO2) which rises and floats on the surface
of the cleaned metal. Most of the silicon remaining in the metal
is an excess over that which is required to remove the dangerous
oxygen, and the final analysis of many steels show enough silicon
(from 0.20 to 0.40) to make sure that this step in the manufacture
has been properly done.
MANGANESE is a metal much like iron. Its chemical symbol is Mn. It
is somewhat more active than iron in many chemical changes--notably
it has what is apparently a stronger attraction for oxygen and
sulphur than has iron. Therefore the metal is used (especially in
the so-called basic process) to free the molten steel of oxygen,
acting in a manner similar to silicon, as explained above. The
compound of manganese and oxygen is readily eliminated from the
metal. Sufficient excess of elemental manganese should remain so
that the purchaser may be sure that the iron has been properly
"deoxidized," and to render harmless the traces of sulphur present.
No damage is done by the presence of a little manganese in steel,
quite the reverse. Consequently it is common to find steels containing
from 0.3 to 1.5 per cent.
ALLOYING ELEMENTS.--Commercial steels of even the simplest types
are therefore primarily alloys of iron and carbon. Impurities and
their "remedies" are always present: sulphur, phosphorus, silicon
and manganese--to say nothing of oxygen, nitrogen and carbon oxide
gases, about which we know very little. It has been found that other
metals, if added to well-made steel, produce definite improvements
in certain directions, and these "alloy steels" have found much
use in the last ten years. Alloy steels, in addition to the
above-mentioned elements, may commonly contain one or more of the
following, in varying amounts: Nickel (Ni), Chromium (Cr), Vanadium
(Va), Tungsten (W), Molybdenum (Mo). These steels will be discussed
at more length in Chapters III and IV.
PROPERTIES OF STEEL
Steels are known by certain tests. Early tests were more or less
crude, and depended upon the ability of the workman to judge the
"grain" exhibited by a freshly broken piece of steel. The cold-bend
test was also very useful--a small bar was bent flat upon itself,
and the stretched fibers examined for any sign of break. Harder
stiff steels were supported at the ends and the amount of central
load they would support before fracture, or the amount of permanent
set they would acquire at a given load noted. Files were also used
to test the hardness of very hard steel.
These tests are still used to a considerable extent, especially in
works where the progress of an operation can be kept under close
watch in this way, the product being periodically examined by more
precise methods. The chief furnace-man, or "melter," in a steel
plant, judges the course of the refining process by casting small
test ingots from time to time, breaking them and examining the
fracture. Cutlery manufacturers use the bend test to judge the
temper of blades. File testing of case-hardened parts is very common.
However there is need of standardized methods which depend less
upon the individual skill of the operator, and which will yield
results comparable to others made by different men at different
places and on different steels. Hence has grown up the art of testing
materials.
TENSILE PROPERTIES
Strength of a metal is usually expressed in the number of pounds
a 1-in. bar will support just before breaking, a term called the
"ultimate strength." It has been found that the shape of the test
bar and its method of loading has some effect upon the results,
so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in.
in diameter for a central length of 2-3/8 in., ending the turn
with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so
the load it bears at rupture multiplied by 5 will represent the
"ultimate strength" in pounds per square inch.
Such a test bar is stretched apart in a machine like that shown
in Fig. 9. The upper end of the bar is held in wedged jaws by the
top cross-head, and the lower end grasped by the movable head.
The latter is moved up and down by three long screws, driven at
the same speed, which pass through threads cut in the corners of
the cross-head. When the test piece is fixed in position the motor
which drives the machine is given a few turns, which by proper
gearing pulls the cross-head down with a certain pull. This pull
is transmitted to the upper cross-head by the test bar, and can
be weighed on the scale arm, acting through a system of links and
levers.
Thus the load may be increased as rapidly as desirable, always
kept balanced by the weighing mechanism, and the load at fracture
may be read directly from the scale beam.
This same test piece may give other information. If light punch
marks are made, 2 in. apart, before the test is begun, the broken
ends may be clamped together, and the distance between punch marks
measured. If it now measures 3 in. the stretch has been 1 in. in 2,
or 50 per cent. This figure is known as the elongation at fracture,
or briefly, the "elongation," and is generally taken to be a measure
of ductility.
When steel shows any elongation, it also contracts in area at the
same time. Often this contraction is sharply localized at the fracture;
the piece is said to "neck." A figure for contraction in area is
also of much interest as an indication of toughness; the diameter
at fracture is measured, a corresponding area taken out from a
table of circles, subtracted from the original area (0.200 sq.
in.) and the difference divided by 0.2 to get the percentage
contraction.
[Illustration: FIG. 9.--Olsen testing machine.]
Quite often it is desired to discover the elastic limit of the
steel, in fact this is of more use to the designer than the ultimate
strength. The elastic limit is usually very close to the load where
the metal takes on a permanent set. That is to say, if a delicate
caliper ("extensometer," so called) be fixed to the side of the
test specimen, it would show the piece to be somewhat longer under
load than when free. Furthermore, if the load had not yet reached the
yield point, and were released at any time, the piece would return
to its original length. However, if the load had been excessive, and
then relieved, the extensometer would no longer read exactly 2.0
in., but something more.
Soft steels "give" very quickly at the yield point. In fact, if
the testing machine is running slowly, it takes some time for the
lower head to catch up with the stretching steel. Consequently at
the yield point, the top head is suddenly but only temporarily
relieved of load, and the scale beam drops. In commercial practice,
the yield point is therefore determined by the "drop of the beam."
For more precise work the calipers are read at intervals of 500 or
1,000 lb. load, and a curve plotted from these results, a curve
which runs straight up to the elastic limit, but there bends off.
A tensile test therefore gives four properties of great usefulness:
The yield point, the ultimate strength, the elongation and the
contraction. Compression tests are seldom made, since the action
of metal in compression and in tension is closely allied, and the
designer is usually satisfied with the latter.
IMPACT TESTS
Impact tests are of considerable importance as an indication of
how a metal will perform under shock. Some engineers think that
the tensile test, which is one made under slow loading, should
therefore be supplemented by another showing what will happen if
the load is applied almost instantaneously. This test, however, has
not been standardized, and depends to a considerable extent upon
the type of machine, but more especially the size of the specimen
and the way it is "nicked." The machine is generally a swinging
heavy pendulum. It falls a certain height, strikes the sample at
the lowest point, and swings on past. The difference between the
downward and upward swing is a measure of the energy it took to
break the test piece.
FATIGUE TESTS
It has been known for fifty years that a beam or rod would fail
at a relatively low stress if only repeated often enough. It has
been found, however, that each material possesses a limiting stress,
or endurance limit, within which it is safe, no matter how often
the loading occurs. That limiting stress for all steels so far
investigated causes fracture below 10 million reversals. In other
words, a steel which will not break before 10,000,000 reversals
can confidently be expected to endure 100,000,000, and doubtless
into the billions.
About the only way to test one piece such a large number of times
is to fashion it into a beam, load it, and then turn the beam in
its supports. Thus the stress in the outer fibers of the bar varies
from a maximum stretch through zero to a maximum compression, and
back again. A simple machine of this sort is shown in Fig. 10,
where _B_ and _E_ are bearings, _A_ the test piece, turned slightly
down in the center, _C_ and _D_ ball bearings supporting a load
_W_. _K_ is a pulley for driving the machine and _N_ is a counter.
[Illustration: FIG. 10.--Sketch of rotating beam machine for measuring
endurance of metal.]
HARDNESS TESTING
The word "hardness" is used to express various properties of metals,
and is measured in as many different ways.
"Scratch hardness" is used by the geologist, who has constructed
"Moh's scale" as follows:
Talc has a hardness of 1
Rock Salt has a hardness of 2
Calcite has a hardness of 3
Fluorite has a hardness of 4
Apatite has a hardness of 5
Feldspar has a hardness of 6
Quartz has a hardness of 7
Topaz has a hardness of 8
Corundum has a hardness of 9
Diamond has a hardness of 10
A mineral will scratch all those above it in the series, and will
be scratched by those below. A weighted diamond cone drawn slowly
over a surface will leave a path the width of which (measured by
a microscope) varies inversely as the scratch hardness.
"Cutting hardness" is measured by a standardized drilling machine,
and has a limited application in machine-shop practice.
"Rebounding hardness" is commonly measured by the Shore scleroscope,
illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter,
3/4 in. in length, and weighing about 1/12 oz. is dropped a distance
of 10 in. upon the test piece. The height of rebound in arbitrary
units represents the hardness numeral.
[Illustration: FIG. 11.--Shore scleroscope.]
Should the hammer have a hard flat surface and drop on steel so hard
that no impression were made, it would rebound about 90 per cent
of the fall. The point, however, consists of a slightly spherical,
blunt diamond nose 0.02 in. in diameter, which will indent the steel
to a certain extent. The work required to make the indentation
is taken from the energy of the falling body; the rebound will
absorb the balance, and the hammer will now rise from the same
steel a distance equal to about 75 per cent of the fall. A permanent
impression is left upon the test piece because the impact will
develop a force of several hundred thousand pounds per square inch
under the tiny diamond-pointed hammer head, stressing the test
piece at this point of contact much beyond its ultimate strength.
The rebound is thus dependent upon the indentation hardness, for
the reason that the less the indentation, the more energy will
reappear in the rebound; also, the less the indentation, the harder
the material. Consequently, the harder the material, the more the
rebound.
"Indentation hardness" is a measure of a material's resistance
to penetration and deformation. The standard testing machine is
the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter,
is forced into the test piece with a pressure of 3,000 kg. (3-1/3
tons). The resulting indentation is then measured.
[Illustration: FIG. 12.--Hydraulic testing machine. (Brinell
principle.)]
While under load, the steel ball in a Brinell machine naturally
flattens somewhat. The indentation left behind in the test piece is
a duplicate of the surface which made it, and is usually regarded
as being the segment of a sphere of somewhat larger radius than
the ball. The radius of curvature of this spherical indentation
will vary slightly with the load and the depth of indentation.
The Brinell hardness numeral is the quotient found by dividing the
test pressure in kilograms by the spherical area of the indentation.
The denominator, as before, will vary according to the size of the
sphere, the hardness of the sphere and the load. These items have
been standardized, and the following table has been constructed
so that if the diameter of the identation produced by a load of
3,000 kg. be measured the hardness numeral is found directly.
TABLE FOR BRINELL BALL TEST
------------------------------------------------------------------------
Diameter of Ball | Hardness Number | Diameter of Ball | Hardness Number
Impression, mm. | for a Load of | Impression, mm. | for a Load of
| 3,000 kg. | | 3,000 kg.
-----------------|-----------------|------------------|-----------------
2.0 | 946 | 4.5 | 179
2.1 | 857 | 4.6 | 170
2.2 | 782 | 4 7 | 163
2.3 | 713 | 4.8 | 156
2.4 | 652 | 4.9 | 149
2.5 | 600 | 5.0 | 143
| | |
2.6 | 555 | 5.1 | 137
2.7 | 512 | 5.2 | 131
2.8 | 477 | 5.3 | 126
2.9 | 444 | 5.4 | 121
3.0 | 418 | 5.5 | 116
| | |
3.1 | 387 | 5.6 | 112
3.2 | 364 | 5.7 | 107
3.3 | 340 | 5.8 | 103
3.4 | 321 | 5.9 | 99
3.5 | 302 | 6.0 | 95
| | |
3.6 | 286 | 6.1 | 92
3.7 | 269 | 6.2 | 89
3.8 | 255 | 6.3 | 86
3.9 | 241 | 6.4 | 83
4.0 | 228 | 6.5 | 80
| | |
4.1 | 217 | 6.6 | 77
4.2 | 207 | 6.7 | 74
4.3 | 196 | 6.8 | 71.5
4.4 | 187 | 6.9 | 69
------------------------------------------------------------------------