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

Abstract

Tempered martensite embrittlement (TME) is investigated in two medium carbon, high strength steels, 4340 (low silicon) and 300-M (high silicon), via rapid (1, 10, or 100 s) and conventional (3600 s) tempering. Rapid tempering of 4340 diminishes the depth of the TME toughness trough, where improvements in impact toughness correspond to the suppression of retained austenite decomposition. In 300-M, retained austenite decomposition is suppressed to an even greater extent by rapid tempering. While toughness improves overall after rapid tempering, TME severity remains consistent in 300-M across the tempering conditions examined. Through interrupted tensile tests, it was found that the 300-M conditions that exhibit TME are associated with mechanically unstable retained austenite. Unstable retained austenite is shown to mechanically transform early in the deformation process, presumably resulting in fresh martensite adjacent to interlath cementite that ultimately contributes to TME. The present results emphasize the role of both the thermal decomposition and mechanical transformation of retained austenite in the manifestation of TME.

Volume fraction of retained austenite in 1.2842 tool steel as a function of tempering temperature

“…RA volume dropped suddenly above the tempering temperature of 200ºC due to the thermal destabilization of this phase [21]. As stated by Dong et al [9], Kokosza and Pacyna [22] and Talebi et al [23], after the tempering temperature of 200°C, untransforfed austenite within the structure could be decomposed by a diffusion mechanism and causes precipitation of more carbides. This causes reduction in the austenite stability and results the transformation of austenite to a hard martensitic structure during the cooling process.…”

Quench and Tempered Embrittlement of Ultra-High-Strength Steels with Transition Carbides

“…The coarse plates of η-Fe 2 C carbide facilitate crack propagation along boundaries of packets and PAGs but no transition from transgranular fracture to intergranular one occurs. It is worth noting that authors [14,23] assumed that interlath cementite plays the role of the embrittling agent in Si-enriched 300M steel under low-temperature tempering. However, this presumption was not supported by structural characterization [14,23].…”

“…Almost no boundary carbides precipitate in these steels up to ~450 • C [20,29]. No cementite appears due to the decomposition of RA occurring at T ≤ 400 • C [20,29], that is, in contrast with steels with Si ≤ 0.5 wt.% [3,6,7,10,14,19,31]. In addition, the volume fraction of RA after water quenching in these steels is negligible (~1%).…”

“…Sufficient ductility and fracture toughness is attained in these steels by vacuum arc remelting, the use of modified heat treatment routes, and/or alloying the steels with ≥1.5 wt.%Si [1,[7][8][9][10][11]. Additions of Si ≥ 1.5 wt.% reduce significantly the precipitation of cementite which enhances the strength and toughness of UHSS [1,4,10,[12][13][14][15][16]. The microstructure of Si-enriched UHSS consists of lath martensite with transition carbides located in the matrix and film-like retained austenite (RA) located at lath and block boundaries [1,14,17].…”

“…Additions of Si ≥ 1.5 wt.% reduce significantly the precipitation of cementite which enhances the strength and toughness of UHSS [1,4,10,[12][13][14][15][16]. The microstructure of Si-enriched UHSS consists of lath martensite with transition carbides located in the matrix and film-like retained austenite (RA) located at lath and block boundaries [1,14,17].…”