Tempering Effect on Steel Hardness
The maximum hardness associated with as-quenched martensite decreases with increasing tempering temperature. Lower carbon steels have a lower hardness in the as-quenched condition and throughout tempering. Therefore, if maximum hardness is required, a high-carbon steel should be selected and tempering should be carried out in the 150 to 200C (300 to 400F) temperature range.
Tempering in this range produces a modest increase in toughness that is adequate in applications requiring high strength and fatigue resistance (medium-carbon steels) and in applications requiring the high hardness and associated good wear resistance that high-carbon martensite and light tempers provide, such as high-carbon steel bearings and gears where loading is primarily compressive
Two types of physical changes that cause residual stresses produced during cooling of heat treated parts are thermal contraction that occurs during cooling of a single phase or microstructure consisting of a mix of phases in the absence of a phase transformation, and transformation of austenite to the more open, higher specific volume crystal structures of ferrite, cementite, and martensite.
Volume expansion due to austenite transformation is the dominant factor in any heat treatment that involves cooling from the austenite phase field, while thermal contraction is the dominant factor in subcritical heat treatments. Residual stresses and distortion arise because cooling rate is a function of section size or position in a part.
Carbon and low-alloy steels slowly cooled from tempering at temperatures higher than 575C (1070F), or tempered for long times between the critical temperature range of 375 and 575C (710 and 1070F) suffer a loss in toughness, called temper embrittlement (TE).
This manifests itself in reduced notch-bar impact strength (increased impact transition temperature) compared with that resulting from relatively fast cooling rates. TE can occur due to the presence of impurities in steel (antimony, phosphorus, tin, and arsenic) on the order of 100 ppm (0.01 percent) or less.
Temper embrittlement is due to precipitation of compounds containing trace amounts of these elements. Large amounts of silicon and manganese also can be detrimental. TE is reversible; that is, de-embrittlement can be achieved by heating to about 575C for only a few minutes.