Transformation Rate Effect
When alloying additions are made to plain-carbon steels, the effect is to shift the time-temperature-transformation (TTT) curves to the right. What this means is that the critical cooling rate (i.e. the cooling rate required to promote a martensitic transformation) is reduced.
For example, in order to obtain a completely martensitic transformation in the case of plain-carbon (0.8%) steel, we must cool it from austenitizing temperature, that is, from above 723°C (1333°F) to room temperature in approximately 1 second. And as such a brine, caustic or water quench may be required (Fig. 1). The quench rate needed is so severe that often distortion and/or cracking may occur.
In contrast, by the addition of suitable alloying elements (e.g., nickel and/or chromium), the critical cooling rate can be changed such that oil-quenching technology may be employed while still producing a martensitic microstructure. Further increases in the amounts of alloying elements will reduce the rate of transformation still further, allowing hardening by air quenching and dramatically reducing the tendency toward distortion or cracking.
A slight negative is that alloying elements, except cobalt, also reduce the martensite start (Ms) and martensite finish (Mf) temperatures to the extent that for many alloy steels the Mf temperature is well below room temperature, resulting in the formation of retained austenite in the as-quenched microstructure.
Alloy elements such as copper, aluminum, silicon and chromium can have a dramatic effect on improving the corrosion resistance of steels. These elements tend to form thin, dense and highly adherent oxide films that protect the surface of the steel from attack.
One of the main reasons for alloying is to affect improvements in the mechanical properties of steel. In general, hardness is increased by elements that stabilize carbides; strength is increased by all alloying elements that dissolve in ferrite; and toughness is improved by elements that refine the grain size.