Perspectives on Quenching and Tempering 4340 Steel

Abstract

Steels are ubiquitous due to their affordability and the landscape of useful properties that can be generated for engineering applications. But to further expand the performance envelope, one must be able to understand and control microstructure development by alloying and processing. Here we use multiscale, advanced characterization to better understand the structural and chemical evolution of AISI 4340 steel after quenching and tempering (Q&T), including the role of quench rate and short-time, isothermal tempering below 573 K (300°C), with an emphasis on carbide formation. We compare the microstructure and/or property changes produced by conventional tempering to those produced by higher temperature, short-time ''rapid'' tempering. We underscore that no single characterization technique can fully capture the subtle microstructure changes like carbon redistribution, transition carbide and/or cementite formation, and retained austenite decomposition that occur during Q&T. Only the use of multiple techniques begins to unravel these complexities. After controlled fast or slow quenching, g transition carbides clearly exist in the microstructure, likely associated with autotempering of this high martensite start temperature (M s ) steel. Isothermal tempering below 598 K (325°C) results in the relief of carbon supersaturation in the martensite, primarily by the formation of g transition carbides that exhibit a range of carbon levels, seemingly without substitutional element partitioning between the carbide and matrix phases. Ha¨gg transition carbide is present between 300°C and 325°C. After conventional tempering at or above 598 K (325°C) for 2 h, cementite is predominant, but small amounts of cementite are also present in other conditions, even after quenching. Previous work has indicated that silicon (Si) and substitutional elements partition between the cementite, which initially forms under paraequilibrium conditions, and the matrix. Phosphorous (P) may also be preferentially located at cementite/matrix interfaces after high temperature tempering. Slower quench rates result in greater amounts of retained austenite compared to those after fast quenching, which we attribute to increased austenite stability resulting from ''autopartitioning''. Rapid, high temperature tempering is also found to diminish tempered martensite embrittlement (TME) believed to be associated with the extent of austenite decomposition, resulting in mechanical properties not attainable by conventional tempering, which may have important implications with respect to industrial heat treatment processes like induction tempering. Controlling the amount and stability of retained austenite is not only relevant to the properties of Q&T steels, but also next-generation advanced high strength steels (AHSS) with austenite/martensite mixtures.

Tempering influence on residual stresses and mechanical properties of AISI 4340 steel

“…Tempering at 300°C promoted low-temperature-tempered martensite formation, as shown in Figure 4(b), not causing significant changes concerning hardness, due to the presence of fine carbides of Fe2,4C, called ε-phase. This result is consistent with Clarke et al [1], who also verified carbides after conventional quenching and tempering at 300°C for 1 hour.…”

“…The absorbed energies of the specimens tempered at 300°C and 400°C were considerably low and below that found by Kwon et al [2] and Chi et al [3]. Temper embrittlement occurs, according to Clarke et al [1], with tempering in 200°C to 400°C probably due to the formation of intra-lath cementite from retained austenite. For this reason, this temperature range is critical, making it recommendable to use temperatures between 150°C and 200°C for low tempering treatment to obtain an adequate combination of strength and ductility.…”

“…The AISI 4340 steel is employed in quenched and tempered conditions, however, is susceptible to embrittlement phenomena when tempered between 300 and 400ºC. This problem has been investigated by several researchers, who have evaluated changes in microstructure and mechanical properties at different tempering temperatures [1][2][3].…”

Correlation of Heterogeneous Local Martensite Tetragonality and Carbon Distribution in High Carbon Steel

“…The average bulk carbon content of the APT tips, see Table 1 , shows no difference between the low and high tetragonality lift-out regions B–D, indicating that no significant autopartitioning was observed for medium carbon steels [ 43 , 44 ] at the cooling rates used for the 0.74 wt% C carbon steel [ 14 ] and subsequent RT ageing. Therefore, the decrease in average tetragonality during the cooling of high carbon steels [ 14 , 15 ] cannot be related to differences in the average carbon content on the size scale of an APT tip.…”

The Role of Retained Austenite in Tempered Martensite Embrittlement of 4340 and 300-M Steels Investigated through Rapid Tempering

“…The embrittlement associated with TME limits the strength-toughness properties achievable in medium carbon, high strength steels such as AISI 4340. However, rapid tempering on the scale of seconds has recently been shown to reduce TME in 4340 steel, leading to significantly improved strength-toughness combinations [2][3][4][5]. In addition to suppressing TME, rapid tempering is associated with a greater preservation of retained austenite upon tempering compared to more conventional treatments [2,3,6].…”