Prediction of impact toughness based on the microstructural characteristics of steel weld metals is complicated due to the innumerous parameters involved. The common practice that associates this property with the microstructure of the last bead of multipass weldments has been proven to be unsatisfactory, because the amount of acicular ferrite, the most desirable constituent, may not always be the main contributor to toughness. Parameters such as the recrystallized fraction, the presence of micro-phases and inclusions may also have a relevant role. Thus, to consider the influence of all these parameters, the method proposed by the International Institute of Welding (IIW) is not sufficiently comprehensive and so complementary techniques are necessary. This situation is more relevant for high strength steel weld metals, where very refined microstructures may not be adequately resolved, resulting in misidentification of the microstructure. The use of scanning electron microscopy as an auxiliary technique to optical microscopy has been successfully used for many decades to study C–Mn and low alloy weld metals, mainly when refined microstructures are to be assessed. Recently, in addition to the previously mentioned techniques, Electron Back Scattering Diffraction (EBSD) has also been used to enable a more effective analytical procedure. This technique, which provides valuable information about grain boundaries, is advantageous for refined microstructures to confirm constituents such as acicular ferrite, bainite, and martensite. This work proposes a contribution to the microstructural characterization of C–Mn and high strength steel weld metals based on the analysis carried out by optical microscopy, scanning electron microscopy and EBSD techniques, involving the influence of recrystallization in multipass welds, microstructural constituents, microphases, and inclusions. The microstructure/toughness relationship analysis of some experimental results obtained over the last decades for weld metals with ultimate tensile strength varying from 400 to 1000 MPa was done using the methodology proposed in this work to check its effectiveness and to explain the impact toughness behavior.

Microstructure characterization and its relationship with impact toughness of C–Mn and high strength low alloy steel weld metals – a review

The current study characterizes the degradation of cross-tension properties in medium-Mn transformation-induced plasticity (TRIP) steel weldments by comparison of fracture behaviors in both similar and dissimilar weldments with DP980 dual phase steel. The experimental results reveal that the cross-tension strengths of medium-Mn TRIP steel weldments is inferior to those of conventional TRIP steel having lower carbon equivalent. Although the failure modes of similar and dissimilar weldments were different from each other, the brittleness of the martensite was a common key factor governing crack propagation. In similar weldment, the decreased tensile toughness in the coarse grain heat-affected zone due to the insufficient self-tempering of martensite and the decreased retained austenite fraction. Meanwhile, understanding the changes in the carbon equivalent caused by dilution in the dissimilar welds and segregation in the fusion zone is necessary to characterize medium-Mn TRIP steel spot welds.

A comparison of cross-tension properties and fracture behavior between similar and dissimilar resistance spot-weldments in medium-Mn TRIP steel

Correlation between the microstructure and hydrogen embrittlement resistance in a precipitation-hardened martensitic stainless steel

Effect of shot peening coverage on hydrogen embrittlement of a ferrite-pearlite steel

Effect of vanadium content on hydrogen embrittlement of 1400 MPa grade high strength bolt steels

Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments

We elucidate here the stability of reversed austenite (RA) and its effect on mechanical properties in 005C–50Mn steel With increased annealing temperature from 903 to 943 K, the volume f

Effect of austenite stability on toughness, ductility, and work-hardening of medium-Mn steel

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

Fracture toughness was studied in terms of crack-tip opening displacement (CTOD) in low-C medium-Mn high-strength steel at both room temperature and − 40 °C, and excellent fracture toughness was obtained. The critical CTOD values (δ) and crack extension (∆a) followed the relationship: δ = 0.01343 + 0.62315∆a0.47531 and δ = 0.07391 + 0.48466∆a0.60103 at room temperature and − 40 °C, respectively. With the decrease in test temperature from room temperature to − 40 °C, the corresponding δ when ∆a = 0.2 mm (δ0.2) was reduced from 0.30341 to 0.25813 mm, and intersecting point in the crack extension resistance curve with a 0.2-mm passivation line (δQ0.2BL) was reduced from 0.42132 to 0.33941 mm. The large fraction of high misorientation boundaries between tempered martensite effectively hindered the crack propagation and increased the fracture toughness. Furthermore, the submicron-scale complex laminated microstructure of tempered martensite and reversed austenite refined the effective fracture grain size, which inhibited crack propagation and led to high fracture toughness. Also, the excellent fracture toughness is attributed to strain-induced martensite transformation of reversed austenite in the small plastic deformation zone ahead of the crack tip, which absorbed the strain energy, relaxed the local stress concentration, suppressed the crack propagation, and enhanced the fracture toughness.

Fracture toughness behavior of low-C medium-Mn high-strength steel with submicron-scale laminated microstructure of tempered martensite and reversed austenite

The structure–property relationship in heat-affected zone (HAZ) of a low-carbon steel bearing V–N subjected to gas-shielded arc welding was explored. The microstructural characteristics of base metal (BM), coarse-grained HAZ (CGHAZ), fine-grained HAZ, and intercritical HAZ were significantly different. The effect of grain-refinement strengthening and transformation hardening on HAZ contributed to equivalent hardness of 260.8–278.5 HV in comparison with BM hardness of 272.0 HV. Moreover, excellent impact toughness at − 20 °C was obtained because of high resistance to crack propagation by high-misorientation boundaries, leading to impact fracture consisting of dimples. In CGHAZ, free N was partly fixed by V(C, N) precipitates, such that the deterioration effect of N on toughness was considered to be nearly eliminated. In comparison with CGHAZ, weld metal contained higher fraction of acicular ferrite with fine plates, while the impact toughness was inferior because of the detrimental influence of coarse inclusions from the welding wire. The nanoscale V(C, N) precipitates in CGHAZ had weak effect on toughness because of small size.

Qi Xiangyu

Papers

Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments

Effect of austenite stability on toughness, ductility, and work-hardening of medium-Mn steel

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

Fracture toughness behavior of low-C medium-Mn high-strength steel with submicron-scale laminated microstructure of tempered martensite and reversed austenite

Structure–property relationships in heat-affected zone of gas-shielded arc-welded V–N microalloyed steel