In this installment we talk about the variables that affect the transition temperature and look at the future of impact testing.
The transition temperature in steel can be affected by a number of factors, the most interesting of which is the microstructure (Fig. 3). If we compare a quenched-and-tempered microstructure with that of a normalized-and-tempered microstructure, we can see the effect of these two different heat treatments on the low-temperature impact strength of a 4340 medium-carbon alloy steel with high hardenability. While both heat treatments produced the identical hardness, the microstructure of the quenched-and-tempered material consisted of spheroidized carbides while the normalized structure consists of pearlite. As one quickly observes, the transition temperature is approximately 150ºC (300ºF) lower for the quenched-and-tempered microstructure.