Effect of rapid tempering on microstructural evolution and toughness within the tempered martensite embrittlement regime of 4340 and 300-M, The

Abstract

Conventional tempering of steel, utilizing times on the scale of minutes to hours, has been widely studied within the literature, and provides background understanding of time and temperature effects on microstructural and mechanical response. However, the tempering response associated with rapid tempering (on the scale of seconds) has not been widely studied and is not as well understood. Rapid tempering, through processes such as induction heating, is attractive due to the opportunity for decreased processing times, increased energy efficiency, and site specific heat treatments. Thus, the goal of the present work is to gain understanding of the microstructural evolution and toughness behavior associated with rapid tempering of 4340 and 300-M steel. Rapid tempering is specifically investigated within a tempering regime associated with the phenomenon known as tempered martensite embrittlement (TME), as significant improvements in strength-toughness properties might be helpful via rapid tempering within this regime. The rapid tempering response of two medium carbon high strength steels, 4340 (low silicon) and 300-M (high silicon), were investigated using short (1, 10, and 100 s) and conventional (3600 s) tempering times. All properties were evaluated at a constant hardness in an effort to isolate the effect of rapid tempering from the overall degree of tempering. Mossbauer effect spectroscopy (MES) was used to assess microstructural evolution associated with retained austenite, cementite, and transition carbides as a function of tempering. Retained austenite carbon partitioning was also monitored via MES. Cementite size and dislocation recovery were evaluated as a function of tempering using electron microscopy and x‑ray diffraction, respectively. The effect of rapid tempering on mechanical properties was analyzed through tensile and Charpy impact testing. When tempered to an equivalent hardness, rapid tempering systematically retained greater amounts of austenite compared to conventional conditions. All other evaluated microstructural changes including cementite content and size, transition carbide precipitation and dissolution, and dislocation recovery remained consistent between rapid and conventional tempering conditions at a given hardness. Rapid tempering generally improved strength-toughness properties across hardness conditions in both 4340 and 300-M, and resulted in different behavior in the two steels with respect to TME depending on retained austenite stability. Suppressing retained austenite decomposition via rapid tempering resulted in reduced embrittlement (TME) when the remaining retained austenite was chemically/mechanically stable. In contrast, no change in embrittlement was observed in conditions associated with high levels of retained austenite preservation upon tempering, but in which the remaining austenite was relatively unstable. Thus, the suppression of TME is possible via rapid tempering, where the prevention of both retained austenite decomposition and retained austenite carbon depletion are important. In addition, transition carbide formation was found to be suppressed with the addition of silicon (4340 300-M), a new observed phenomenon that results in higher levels of carbon in solution at low tempering temperatures.