Abstract
The residual stresses, microstructural heterogeneities, and mechanical properties were extensively examined in a 25 mm thick weld consisting of two regions which respectively undergo phase transformation at relatively low (100 °C) temperature (LTT) and high (670 °C) temperature (HTT) during cooling. Due to the LTT martensitic transformation, which features a transformation strain of ~4000 με, large compressive residual stress (−510 MPa) was generated as confirmed by neutron diffraction and contour method. Meanwhile, significant heterogeneities were observed in terms of the chemical composition, yield strength, and microhardness across the interface between the LTT and HTT regions. The dependency of martensitic transformation starting temperature (Ms) on chemical composition is empirically formulated, and it reveals that the dependency of Ms on the Ni and Cr compositions becomes stronger when the temperature is lower than 200 °C. The maximum change of residual stresses (σxmax) exponentially decreases as Ms decreases in the manner of σxmax = 97.5 exp(Ms/227)-649. The fracture behavior was found highly dependent upon the volume fraction of retained austenite influenced by Ms. While a ductile fracture mode was found in the cellular LTT region containing relatively higher amount of retained austenite (~10%), the interface shows transgranular brittle fracture features and sub-cracking due to the relatively small amount of retained austenite and the predominant martensite constituent.
Published Version
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