Thermo-Mechanical Simulation of Reheat Cracking in Welding CGHAZ of Vanadium-Modified 2.25Cr1Mo Steel

Abstract

The reheat cracking, also known as stress relief cracking, has occurred many times in the welding coarse grained heat affected zones (CGHAZ) of Vanadium-modified (V-mod) 2.25Cr1Mo steel, but seldom researches on this problem have been conducted until now. In this paper, reheat cracking in welding CGHAZ of V-mod 2.25Cr1Mo steel was studied by the thermo-mechanical simulation methods. A screening test was carried out for simulating similar material to the real CGHAZ. The high temperature ductility of simulated CGHAZ was measured in the range of 600~705°C for determining the most sensitive temperature to reheat cracking. And then, the reheat cracking phenomena at the sensitive temperature was reproduced by Isothermal stress relaxation test and the micro-morphology of cracks was observed and analyzed. The results show that the thermal simulation parameters for acquiring the most similar materials to real CGHAZ are heat input of 35kJ/cm, heating rate of 1000°C/s and holding time at 1320°C of 1s. The reduction of area (RoA) of the simulated CGHAZ specimen first decreases and then increases with increasing temperature. It exhibits a minimum value at 675°C, corresponding to the most sensitive temperature to reheat cracking. In the stress relaxation test, the sample failure typically occurs with very low stress relaxation proportion (less than 15%) and extremely poor ductility (less than 5% in RoA). Both microvoid coalescence along the PAGB and intergranular wedge type cracks were observed in the fractured sample, proposing a mixed cracking mode of W-type and R-type in stress relaxation.