THE ALLOY STEELS
We have learned that steel, fundamentally, is an alloy of iron with
carbon, i.e., carbon is the characteristic element. We are now to note
what often seem to be exceptions to this rule. While in reality steel is
just this iron-carbon alloy, there are alloys known as steels to which
such strong characteristics are given by elements other than carbon,
that carbon seems not to be the defining constituent at all. Indeed, in
some of these, the carbon content may be small enough that, judging from
our experience with the carbon steel series, we would not expect any
such physical properties as some of these alloy steels show.
You remember that in olden days they distinguished between wrought iron
and steel by quenching the piece in water from a cherry-red heat. If the
piece was hardened and made brittle, by this treatment, it was thereby
proved to be “steel.” Also, it is generally known that by annealing a
piece of hardened steel, which usually means holding at a cherry-red
heat for a time and then cooling slowly, it is made soft.
Then what shall we say concerning a certain one of these new alloys,
Hadfield’s manganese steel, which is made very much less brittle and a
little softer by quenching, but which refuses absolutely to soften under
annealing treatment—in other words, is almost the opposite of what we
know as steel in these chief defining traits? The nickel steel which
contains 15 per cent of nickel, also, exhibits just these
characteristics, being softened by quenching but not by annealing.
Again, while iron, the carbon steels, and even the magnetic oxide of
iron which contains only about 72 per cent of the metal, are strongly
magnetic, manganese steel which has 85 per cent of iron is so
non-magnetic that it is sometimes used in place of brass or bronze where
an entirely non-magnetic material is required. The nickel steels with 24
per cent or more of nickel are also non-magnetic though both constituent
metals, alone, are strongly attracted by the magnet.
These are some of the things which make a logical classification of the
iron family so difficult. Though derived from the steels which we knew
and made from the same materials with the exception that a greater
amount of one constituent, manganese, is added, or perhaps, in other
cases, another element or two, the resulting alloys have markedly
different and often contradictory properties.
However, we must not be led astray. In all probability carbon is still
the necessary constituent, but much less of it is needed to produce
results when the other elements are present. There is no doubt, however,
that in “manganese steel” or in “nickel,” “chrome,” “tungsten,”
“silicon,” “vanadium,” “titanium,” and other alloy steels, the added
element or elements exert very strong modifying influences, and
sometimes obscure the influence of the carbon.
In the first place, we better at once dispose of certain of these steels
by terming the added element a “scavenger” only. Such usually are
“titanium” and “aluminum” steels. These are generally ordinary carbon
steels in which a very small amount of titanium or aluminum has been
used to rid the alloy of certain gaseous or other deleterious elements.
Upon analysis, steels so treated often show no trace of the element
which has been added to do the work, all of it having passed into the
slag, carrying with it the obnoxious substances, which, had they
remained would have injured the quality of the steel. Manganese and
silicon which were spoken of in the discussion of the Bessemer process
as deoxidizing the metal, also exert just this same influence, though
there is usually added of these enough that a certain percentage remains
in the finished steel. Vanadium and titanium have a particular affinity
for oxygen and nitrogen, and aluminum for oxygen. By chemically
combining with these gases in the metal, and through possible other
influence, they help to produce sound steel having very good physical
properties. Vanadium, however, is much more than a “scavenger” as will
be seen later on.
Manganese Steel
Manganese steel was discovered and highly developed by Robert Hadfield
of Sheffield, England, along about 1882. His 11 per cent to 14 per cent
manganese steel with about 1 per cent of carbon has such great hardness
that it cannot be drilled or cut with tools. In forgings and castings it
is used for milling machinery for ore treatment; manganese steel rails
inserted around sharp curves and for “frogs,” etc., under severe service
conditions outlast ordinary steel rails three or four times; it goes
into various rolls and crusher parts, steam and dredge shovels, grab
buckets, sand pumps, gears, pinions, etc., which have to resist heavy
wear. It is much used, too, as a material for burglar-proof safes. The
alloy is far too hard to drill and too tough and strong to be broken. It
is said that no manganese steel safe has ever been drilled or forcibly
entered.
In forming irregular shapes, manganese steel must be cast and finished
by grinding but for ordinary bars and rails it can be rolled. In the
“raw” condition it is quite brittle and extremely hard. Quenching from a
cherry-red heat greatly toughens it and makes it ductile. Though now it
can be dented by a hammer blow and marked with a file, it always is so
tough that it cannot be machined with any tool. Ordinary annealing
treatment has no softening effect on the alloy.
Silicon Steels
When alloyed in the steel in certain quantities, silicon gives desirable
properties. Steels with from one to two per cent of silicon in the
tempered condition are very tough. For this reason the leaves of
automobile springs are often made from it. Steels with from 3 to 5 per
cent of silicon are much used in electrical appliances because of their
improved magnetic properties.
Molybdenum Steels
To a certain extent steels containing 3 or 4 per cent of the metal
molybdenum, and 1 or 1½ per cent of carbon are used in the construction
of permanent magnets. It is said that molybdenum is used in some modern
guns, which longer resist the corrosive effect of the powder-gases
because of it. A certain amount goes into the high-speed steels where it
replaces part or all of the tungsten. Here, however, it has been a
disappointment and the amount so used seems to be decreasing rather than
increasing.
Tungsten Steels
Steels with about ½ per cent of each of carbon and tungsten are
occasionally used for manufacture of springs, and with greater amounts,
e.g., ¾ per cent of carbon and 5 or 7 per cent of tungsten, for
permanent magnets for which they are claimed to be the best material
known. The use of tungsten in the tool steels (other than the high-speed
steels) is considerable.
Nickel Steel
Nickel steel is much used on account of its high strength. The most
usual alloy, perhaps, is the one which contains about 3½ per cent of
nickel. This is in addition to the carbon which may vary between .15 per
cent and ½ per cent. This 3½ per cent of nickel adds several thousand
pounds per square inch in strength to the steel, and when tempered, both
the strength and toughness are greatly improved.
Nickel steels of these compositions can readily be forged and rolled.
They are used for drop forgings, machine parts, engine and automobile
parts, in seamless tubes and for bridge members of great span.
Nickel will not rust so it does not surprise one that with 22 per cent
or more of nickel the steel is almost immune from ordinary corrosion.
Steels containing from 25 to 46 per cent nickel are variously used for
resistance wire, for valve stems, valves for motors, etc. The 36 per
cent nickel steel is the alloy, “Invar,” which has such slight expansion
and contraction with heat and cold that it is used for clock pendulums,
watch parts and for parts of measuring instruments.
Forty-six per cent nickel steel is called “Platinite.” It has
practically the same rates of expansion and contraction with heat and
cold as glass and for this reason it finds extensive use in incandescent
electric lamps. Wires of the alloy are fused into the glass bases and
connect with the filaments in place of the expensive platinum which
formerly was used.
As remarked above, the 13 per cent to 15 per cent nickel-iron alloys
soften with quenching but not with annealing. The 15 per cent nickel
steel has the highest strength of the nickel-iron-carbon series. Though
nickel and iron are each strongly magnetic, alloys of the two which
contain 24 per cent or more of nickel are not magnetic.
Chrome and Nickel-Chrome Steels
While “simple” chrome steel is pretty well known as a material for
products which require great hardness, such as balls, roller bearings,
files, rolls, five-ply safes, stamp shoes, projectiles, etc., and
heat-treated chrome-vanadium steels are now extensively used in forged
frames and shafts of automobile and other machines, a combination of
nickel and chromium gives steels which have been great favorites. With 2
to 3½ per cent of nickel, not over 3 per cent of chromium and ½ per cent
of carbon, these steels, when expertly heat-treated, can give “elastic
limits” anywhere between 40,000 and 250,000 pounds per square inch, with
good freedom from brittleness. They are very largely used for automobile
gears, axles, and other parts, for armor plate, for projectiles and for
many other purposes.
These alloys are also used for castings.
Chrome-Vanadium Steels
Within a comparatively short time the chrome-vanadium steels have come
to be very largely used, often in place of the chrome and nickel-chrome
steels. As vanadium is a “deoxidizer,” whereas nickel is not, the
chrome-vanadium steels show fewer imperfections than the nickel-chrome
steels and they also roll, forge and machine better.
They are used for automobile frames, shafts, for miscellaneous forged
and rolled articles and for heat-treated armor plates. Of this
comparatively new material about 90,000 tons were made during 1913,
according to a recently issued bulletin of the Department of the
Interior.
It is impossible, of course, to even begin to impart any adequate
conception of the qualities and great importance of the alloy steels for
purposes of construction. As has been shown they are special steels for
special purposes and their application is wide. Incorporation of the new
element in the alloy imparts peculiar and valuable properties: for
example, 12 per cent of manganese, great hardness and toughness; 23 per
cent of nickel, non-corrosive properties and great strength; chromium,
nickel with chromium or chromium with vanadium, strength and high
elastic limit (resistance to distortion) as well as great hardening
power when desired, this, of course, the usual hardening through
quenching from a cherry-red heat.
Very often instead of the single defining element, a combination of two,
three or even four of them is used. Such, of course, are rather
complicated steels having combinations of properties as might naturally
be expected, though very often these resulting properties are not those
which are expected. In fact no one can tell in advance what properties
any new combination of metals in an alloy will produce and often new
proportions of the same constituent metals give entirely different and
unique results.
The only certain method of ascertaining what characteristics and
properties a new alloy will have is to develop it and in that way find
out.
Description of a special and extremely important class of these alloys,
the “high-speed steels,” will serve to show how laborious, slow and
expensive a process development of new alloys may be and what
unlooked-for results are sometimes obtained.