Characterization Of Weld Metal Mechanical Properties Of The Scale Rotor

As large-scale of generating set,steam turbine low pressure rotor not only runs under the harsh environment with high temperature and high stress for a long time,will also withstand dynamic thermal stress caused by the behavior of start-stop and peak shaving.Therefore,the steam turbine rotor must be equipped with great mechanical properties.Using welding method to manufacture large steam turbine low pressure rotor has many advantages and has become an important direction of power equipment manufacturing industry.But,due to the complexity of the welding heat process,the composition,microstructure and mechanical properties of welding joint are heterogeneous,thus the mechanical properties of welding rotor often become the weak part,which affects the reliability and security of welding rotor.In order to improve the toughness of rotor welded joint and reduce residual stress,high temperature tempering is necessary after welding.In the actual production process,it is difficult to improve the cooling rate after tempering,which can easily cause the temper brittleness and reduce the toughness.This paper focuses on the toughness problem of the weld metal of the CrMo V scale rotor,which mainly includes the following two aspects:On the one hand,through changing the cooling rate after tempering,the relationship between tempering cooling rate and impact toughness of weld metal at room temperature were investigated,and the results indicated that the slower tempering cooling rate is,the poorer impact toughness of weld metal is.A large number of the intergranular fracture dominated the fracture surface of the specimens with lower impact toughness.By means of TEM and AES analysis,it is believed that the double segregation between impurity element S and alloy elements Mn、Cr、C at the cementite distributed along prior austenite grain boundary of reheated zone with 5℃/h cooling rate specimen causes the high density dislocation and stress concentration of the interface between cementite and ferrite matrix,reducing the interface bonding strength and causes the intergranular cracking.Based on the research of the temper embrittlement mechanism,combining with the existing literatures,a de-embrittling heat treatment was used to recover impact toughness of weld metal at room temperature,and there is no segregation of impurity element S observed on the intergranular section,and further verify the mechanism that segregation of impurity element S causes temper embrittlement.On the other hand,the chemical composition of the weld metal was changed to study the effects of Ti content on impact toughness.and the results indicated that the trace Ti element was found to have a significant effect on improving the impact toughness of Cr MoV heat-resistant steel weld metal.By the analysis of the microstructure of weld metal,the composition of the non-metallic oxide inclusions in weld metal was changed by joining trace Ti elements,the Mn₂TiO₄ formed at the inclusion surface can contribute to the nucleation of acicular ferrite,thereby increasing the proportion of acicular ferrite in weld metal,prohibiting crack propagation,improving the ability of weld crack propagation resistance,and enhance toughness ultimately.At the same time,Ti elements can refine the grain size and produce fine grain strengthening effect on the matrix structure to improve the toughness.This research results of this paper have important engineering significance on how to improve impact toughness of CrMoV heat resistant steel low pressure rotor welding joint at room temperature.At the same time,the research also has a scientific value on the mechanism of the temper embrittlement and the nucleation of acicular ferrite.