Effects of Short-Time Tempering on Impact Toughness, Strength, and Phase Evolution of 4340 Steel Within the Tempered Mar
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SHORT-TIME tempering processes like induction or laser heating can decrease processing times, lower manufacturing costs, and impart location-specific heat treatments for bulk components. Increased understanding of short-time thermal processes is also applicable to heat-affected zone tempering associated with welding and joining.[1–3] Rapid tempering has been suggested[4–11] to improve toughness relative to conventional furnace tempering. For example, short-time tempering within the tempered martensite embrittlement (TME) regime was recently shown to improve impact toughness by up to 43 pct over conventional tempering conditions at a constant strength level.[4] Additionally,
V.K. EUSER, D.L. WILLIAMSON, K.D. CLARKE, K.O. FINDLEY, J.G. SPEER, and A.J. CLARKE are with the Colorado School of Mines, Golden, CO, USA. Contact e-mail: vjudge@ mymail.mines.edu Manuscript submitted December 10, 2018. Article published online May 29, 2019 3654—VOLUME 50A, AUGUST 2019
rapid tempering at higher temperatures (773 K to 973 K) has been reported to improve toughness by cementite refinement.[5–9] Tempering is utilized to achieve the desired combinations of strength, ductility, and toughness. However, high-strength steels tempered between 473 K and 673 K (200 C and 400 C) for 1 hour (3600 seconds) result in low toughness due to TME.[12–19] This embrittlement phenomenon is typically attributed to the decomposition of retained austenite into cementite (and ferrite) during the second stage of tempering.[13,17–20] In order to avoid TME, susceptible steels are often over-tempered at the expense of strength[21] or additional alloying is employed to increase the temperature at which austenite decomposition occurs.[19] Most short-time tempering studies[5–9] have examined the effects of rapid heating rates and short holding times within a relatively high-temperature regime (773 K to 973 K). These studies have primarily focused on the effect of rapid processing on dislocation density and cementite size distribution. It has been proposed that rapid tempering limits the opportunity for dislocation recovery compared to conventional tempering treatments. As a result, shorter tempering times promote a METALLURGICAL AND MATERIALS TRANSACTIONS A
finer distribution of cementite due to a greater number of nucleation sites associated with a higher dislocation density. An additional mechanism by which cementite is refined via rapid tempering was proposed by Furuhara et al.,[5] where rapid heating rates were associated with an increase in the temperature at which cementite precipitation occurs. Through steady-state nucleation rate calculations, it was shown that nucleation rate increases with temperature within certain thermal regimes. Therefore, Furuhara et al. associated rapid heating with increased nucleation rates and a resulting refinement of cementite. While few have experimentally related short-time tempered microstructures to corresponding toughness,[6] it is often assumed that the refinement of cementite improves toughness due to the understood effects
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