: The Working Of 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 a
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.