Tempering-induced Modulation of Hydrogen Embrittlement in Additvely Manufactured AISI 4340 Steel

Recent studies on additive manufacturing (AM) have indicated the necessity of understanding the hydrogen embrittlement (HE) of high-strength steels fabricated by AM due to the different microstructure obtained compared to their conventionally processed counterparts. This study investigated the influence of post-AM tempering (at 205 ◦C, 315 ◦C, and 425 ◦C) on the HE resistance of AM-fabricated AISI 4340 steel, a representative ultrahigh-strength medium-carbon low-alloy steel. The present results show that tempering effectively reduced the HE sensitivity of the steel. When tested in air, tempering at a low temperature of 205 ◦C slightly increased both the yield strength (YS) and ultimate tensile strength (UTS), accompanied by a reduction in elongation (EL). This behaviour is attributed to the precipitation of carbides. In contrast, higher tempering temperatures of 315 ◦C and 425 ◦C resulted in a progressive decrease in both YS and UTS, as anticipated. However, when tested in a hydrogen-rich environment, although the HE dramatically reduced the ductility, and YS could not even be determined for the samples tempered at 205 ◦C and 315 ◦C, the tempered samples retained higher UTS and EL compared to the as-AM-fabricated samples because of the increased HE resistance by tempering. Microstructural examination indicated that tempering at 205 ◦C and 315 ◦C retained the bainitic microstructure while promoting the formation of fine carbide precipitates, which softened the bainitic ferrite matrix, enhancing the hydrogen trapping capacity. Tempering at 425 ◦C promoted recovery of the AM-fabricated steel, reducing dislocation density, producing a lower subsurface hydrogen concentration, and higher hydrogen diffusivity, which led to an enhanced HE resistance. As a result, testing of the samples tempered at 425 ◦C in hydrogen resulted in a high YS (~1200 MPa), and only a ~5 % reduction in UTS and a 64 % reduction in EL compared with the untempered samples, of which the reductions were 31 % in UTS and 79 % in EL. Furthermore, this study underscores the critical role of the trap character in governing the HE behaviour, offering a pathway toward optimised heat treatment strategies for improved HE resistance of additively manufactured high-strength steels.