Enhanced Impact Toughness of Heat Affected Zone in Gas Shield Arc Weld Joint of Low‐C Medium‐Mn High Strength Steel by Post‐Weld Heat Treatment

Abstract

In the present study, two different post weld heat treatments (PWHT) are carried out on gas shield arc welded low-C medium-Mn high strength steel to explore the effect of PWHT on microstructure, microhardness, and low temperature toughness of heat affected zone (HAZ). The results show that when the weld joint is PWHT at 630 C for 30 min, the microhardness of HAZ is homogeneous and almost identical to base metal (BM). The microstructure of HAZ comprises of tempered martensite and strip-shaped reversed austenite. The impact energy of fusion line (FL), coarse-grained HAZ (CGHAZ), and fine grained HAZ (FGHAZ) at À40 C is enhanced by 29 J (from 42 to 71 J), 42 J (from 68 to 110 J), and 33 J (from 140 to 173 J), respectively, compared to the weld joint, which is PWHT at 200 C for 120 min. The formation of reversed austenite with certain stability in the HAZ is the main toughening mechanism when the weld joint is PWHT at 630 C. Reversed austenite enhances the impact energy significantly through the transformation-induced plasticity (TRIP) effect, which releases the stress concentration, improves the crack initiation energy, and effectively prevents the propagation of cleavage crack.

The relative contributions of TWIP and TRIP to strength in fine grained medium-Mn steels

“…This is enabled by a combination of either low Mn and high C (current alloy), or high Mn and low C. If the TWIP effect can be forgone during alloy design, it would be possible to enable low to medium Mn and low C (3-6 wt% Mn, 0.05-0.2 wt% C) compositions that exhibit the TRIP effect only. Such low Mn, low C compositions have been increasingly termed lean medium Mn steels [67], and are desirable in terms of lower segregation after casting [35], better weldability [68], processability, etc. However, it is worth noting that while the TWIP effect does little for TWIP + TRIP-type medium Mn steels, the tensile properties still tend to be better in terms of elongation than in pure TRIP-type medium Mn steels [19,26,27,69].…”

Recent developments and perspectives of advanced high-strength medium Mn steel: from material design to failure mechanisms

“…The high contents of C and Mn will degrade the weldability of MMS, particularly when using conventional fusion welding (FW) techniques such as inert gas welding [33,34], laser welding [35], and resistance spot welding [36,37], which will result in a brittle fusion zone that deteriorates the mechanical properties. The conventional FW process involves a high temperature at fusion zone that typically exceeds 1000 • C (i.e.…”

“…To enhance the mechanical properties of the welded MMS, a post welding heat treatment (e.g. annealing at 630 • C for 30 min) is usually performed [34,39]. Overall, in order to improve the weldability of MMS, the C and Mn contents should be controlled carefully, and the FSW can be used.…”

Influence of Tempering Temperature on the Microstructure and Mechanical Properties of a Cr–Ni–Mo‐Alloyed Steel for Rock Drill Applications

“…Temper brittleness in the experimental steel is substantiated by Figure b, in which the impact energy after tempering at 400 °C was the lowest. Although Mo coexisting with Cr and Mn is believed to restrain this temper brittleness, this effect is limited because the content of Mo was just 0.26% as listed in Table . Therefore, the temper brittleness tendency is substantial from 350 to 420 °C as depicted in Figure b for the experimental steel.…”