Category: ALLOYS AND THEIR EFFECT UPON STEEL
Tungsten, as an alloy in steel, has been known and used for a long
time. The celebrated and ancient damascus steel being a form of
tungsten-alloy steel. Tungsten and its effects, however, did not
become generally realized until Robert Mushet experimented and
developed his famous mushet steel and the many improvement made
since that date go to prove how little Mushet himself understood
the peculiar effects of tungsten as an alloy.
Tungsten acts on steel in a similar manner to carbon, that is,
it increases its hardness, but is much less effective than carbon
in this respect. If the percentage of tungsten and manganese is
high, the steel will be hard after cooling in the air. This is
impossible in a carbon steel. It was this combination that Mushet
used in his well-known air-hardening steel.
The principal use of tungsten is in high-speed tool steel, but
here a high percentage of manganese is distinctly detrimental,
making the steel liable to fire crack, very brittle and weak in
the body, less easily forged and annealed. Manganese should be
kept low and a high percentage of chromium used instead.
Tools of tungsten-chromium steels, when hardened, retain their
hardness, even when heated to a dark cherry red by the friction of
the cutting or the heat arising from the chips. This characteristic
led to the term red-hardness, and it is this property that has
made possible the use of very high cutting speeds in tools made
of the tungsten-chromium alloy, that is, high-speed steel.
Tungsten steels containing up to 6 per cent do not have the property
of red hardness any more than does carbon tool steel, providing
the manganese or chromium is low.
When chromium is alloyed with tungsten, a very definite red-hardness
is noticed with a great increase of cutting efficiency. The maximum
red-hardness seems to be had with steels containing 18 per cent
tungsten, 5.5 per cent chromium and 0.70 per cent carbon.
Very little is known of the actual function of tungsten, although
a vast amount of experimental work has been done. It is possible
that when the effect of tungsten with iron-carbon alloys is better
known, a greater improvement can be expected from these steels.
Tungsten has been tried and is still used by some steel manufacturers
for making punches, chisels, and other impact tools. It has also
been used for springs, and has given very good results, although
other less expensive alloys give equally good results, and are
in some instances, better.
Tungsten is largely used in permanent magnets. In this, its action
is not well understood. In fact, the reason why steel becomes a
permanent magnet is not at all understood. Theories have been evolved,
but all are open to serious questioning. The principal effect of
tungsten, as conceded by leading authorities, is that it distinctly
retards separation of the iron-carbon solution, removing the lowest
recalescent point down to atmospheric temperature.
A peculiar property of tungsten steels is that if a heating temperature
of 1,750 deg.F. is not exceeded, the cooling curves indicate but one
critical point at about 1,350 deg.F. But when the heating temperature
is raised above 1,850 deg.F., this critical point is nearly if not
quite suppressed, while a lower critical point appears and grows
enormously in intensity at a temperature between 660 and 750 deg.F.
The change in the critical ranges, which is produced by heating
tungsten steels to over 1,850 deg.F., is the real cause of the red-hard
properties of these alloys. Its real nature is not understood,
and there is no direct evidence to show what actually happens at
these high temperatures.
It may readily be understood that an alloy containing four essential
elements, namely: iron, carbon, tungsten and chromium, is one whose
study presents problems of extreme complexity. It is possible that
complex carbides may be formed, as in chromium steels, and that
compounds between iron and tungsten exist. Behavior of these
combinations on heating and cooling must be better known before
we are able to explain many peculiarities of tungsten steels.