29. MAKING STEELS OF VERY HIGH STRENGTHS
by Arthur Kenneford
Design engineers are forever demanding materials giving more strength for less weight, and a reduction in size and weight of components. Considerable attention has been paid over the past few years, particularly in the United States, to steels heat-treated to ultimate tensile strengths of around 120 to 130 tons per square inch - that is to say, about three times as strong as ordinary high tensile steels. The new steels are now regularly used in aircraft landing-gear components, where weight reduction is of great importance. In the United States these high tensile strengths have been achieved mainly by using low tempering temperatures during the heat treatment of standard commercial alloy steels. There have been some minor modifications in composition of the standard American steels; the beneficial effects of silicon in increasing impact strength and ductility, at high tensile strengths, and its effects on resistance to softening on tempering seem to have been recognised.
It should perhaps be explained that steel, which basically consists of an alloy of carbon and iron, exists in its unhardened state as a mixture of iron carbide in iron. On heating to a temperature known as the critical temperature the iron undergoes a crystallographic change and becomes gamma-iron, in which iron carbide is soluble. This change is accompanied by a contraction on heating through the critical range and, vice versa, by an expansion on cooling. To harden a steel, rapid cooling (quenching) must normally take place from a temperature above the critical temperature so that the iron carbide is retained in solution. Considerable internal stresses can be generated in a piece of steel so treated. The magnitude depends in part on the volume changes occurring during heat treatment, caused by the crystallographic changes as distinct from pure thermal .expansion and contraction. The smaller these volume changes are, the lower the internal stresses, and the less likely is the steel to crack when hardened. Fundamental work some years ago on the effect of various elements used in steel showed that the volume changes through the critical range of temperatures could be considerably influenced by two elements, chromium and silicon. Chromium increased this volume change, whilst silicon decreased it, and both elements raised the temperature at which the transformation occurred. The use of silicon as an alloying element, therefore, was thought likely to be beneficial in increasing the resistance of a steel to cracking during quenching and, incidentally, its .resistance to "craze-cracking" by repeated heating and cooling.
A hardened steel may be brittle, and reheating at some temperature below the critical range (tempering) is usually employed to improve ductility. This re-heating also softens the steel by allowing iron carbide to precipitate from the supersaturated solution formed during quenching.
It was discovered, however, that this rate of softening with .increasing tempering temperature could be profoundly affected by certain alloy additions, particularly silicon, copper, molybdenum and vanadium. The effect of these alloy additions was to retard the softening which occurred on tempering after hardening. Silicon delayed the tempering of a quenched steel by raising the temperature at which martensite (the hard microstructure) decomposed, while molybdenum and vanadium, which had no effect on the breakdown of martensite, retarded softening by the precipitation of complex alloy carbides. Copper delayed martensite breakdown and also gave "secondary hardening" by precipitation.
As these alloying elements operated in a manner different from each other, and as each operated over a different range of tempering temperatures, it was possible to combine their effects to produce a steel which softened very little after tempering even at temperatures up to 600°C.
Improved resistance to softening on tempering is of potential value for several reasons, the most important of which are:
(from New Scientist, 3rd August, 1961)