Research on the micro zone strength and strain hardening behavior in the 30Cr2Ni4MoV rotor welded joint

In order to investigate the micro zone tensile properties of a 30Cr2Ni4MoV rotor welded joint used for an ultra-supercritical steam turbine, a thin plate tensile test method is put forward by taking the micro zone size into account. The microstructures of welded joint are observed by OM, SEM and TEM, and a strength prediction model based on the microhardness and the strain hardening exponent is established. The researches show that the weld metal (WM) has lower strength than the base metal (BM), but similar plasticity and strain hardening ability, that the heat affected zone (HAZ) has a sharp distribution of tensile strength due to the great changes in grain size and microstructure. Based on the hardening rate analysis with Kocks-Mecking (K-M) model, the coarse-grained HAZ (CGHAZ) only experienced two strain hardening stages Ⅲ and Ⅳ, and other micro zones of the welded joint exhibited three strain hardening stages Ⅲ, IV and Ⅴ. The coarse tempered martensite in CGHAZ has poor coordination deformation ability, resulting in its short strain hardening stage IV. More tempered bainite and finer tempered martensite are beneficial to extending the strain hardening period. By observing the microstructures after tensile tests, it is found that the dislocation cells formed more uniformly in the tempered bainite than in the tempered martensite, and only the intragranular carbides have a beneficial effect on the strain hardening behavior. An equation for predicting the micro zone strength is established, and the micro zone strength distribution along the welded joint is successfully predicted.