Local strain hardening behavior in a dissimilar metal welded joint with buttering layer of ultra-supercritical turbine rotor

Abstract

Stratified tensile tests were performed to quantitatively investigate strain hardening behavior along a dissimilar metal welded joint (DMWJ) with buttering layer of ultra-supercritical turbine rotor. The results show weld region has lower tensile strength but better strain hardening capacity than base metals (BMs). The buttering layer (BL) exhibits higher strain hardening capacity than weld metal (WM). Two strain hardening exponents were found based on the fitted stress-strain curves using Hollomon equation. The lower exponent is related to the ferrite matrix, whereas the larger exponent is related to the precipitation phase, including martensite, bainite and sorbite. The gradient distribution of strain hardening exponent in heat affected zones (HAZs) is related to the gradient change of tempered martensite and grain size. The strain hardening mismatch in interface is related to the drastic changes in microstructure and amount of carbides. The dislocation cell structure and sub grain were observed after tensile tests, and carbides were elastic during tensile deformation. Kocks–Mecking (K–M) type plots of strain hardening rate versus true stress presented two hardening stages (stages III and IV) in welded joint. More ferrite and finer tempered martensite are conductive to extending the strain hardening period. Local strain hardening parameters play an important role in the accurate evaluation of crack behavior in HAZ and interface.