Study On The Susceptibility To Reheat Cracking In The CGHAZ Of T23 Steel

T23 steel is a new ferritic heat resistant steel. Due to its good weldability and high creep strength, T23 steel is widely used in ultra-supercritical bolier water wall, superheater et al. However, in manufacturer and power station, cracks were found in the welded joint of water wall made with T23 steel. These cracks led to the bursting and leaking of the water walls, which had a serious effect on the safety of the generator unit. Therefore, it is significant to further study the susceptibility to RC of T23 welded joints.Y-slit groove of actual welded joint tests and thermal simulation CGHAZ STF tests performed on Gleeble-3500 thermo-mechanical simulator were used to evaluate the RC susceptibility of domestic T23 steel. Transmission electron microscopy(TEM), electronic backscattered diffraction(EBSD) combining with Jmat Pro software were used to characterize and analyze the precipitates at grain boundaries, misorientation of cracked grain boundaries, and strain accommodation process around the grain boundaries. It was concluded that the RC susceptibility at different temperatures depended on the relative strength of grain boundary and grain interior. The strength of grain boundary depended on the misorientation of grain boundary and precipitates at grain boundary, while the secondary hardening and dislocation density determined the strength of grain interior. In addition, the relation between reheat cracking and misorientation of grain boundary provides a basis for reducing the susceptibility to RC from the view of grain boundary engineering.In order to clarify the factors affected the strength of grain interior and grain boundary, the experiments were designed. The strength of grain interior was adjusted by heat treatment at 750℃, the RC susceptibility was assessed by STF tests. SEM, TEM combining with SAXS were used to study the types of carbides, dislocation density, the change of locations of voids and micro-cracks. The results showed that within a certain range of time(1min-1.5h), the dislocation density decreased greatly and effectively softened the grain interior. It led to the plastic deformation happened mainly in grain rather than at grain boundary, which reduced the susceptibility to reheat cracking. Beyond the time, the recovery speed of the dislocation density declined and lots of MC carbides formed in grain, thus the strength of the grain interior was slightly increased. The plastic deformation might be accommodated by the cracking of grain boundary. However, coherent/semi-coherent smaller M23C6 carbides precipitated at grain boundary. The weakening effect of this type of carbides was less than other types. Therefore, the harmful effect of increased strength in grain was counteracted. Due to the increased strength of grain interior and grain boundary, plastic deformation was accommodated only by the cracking or shearing of low strength blocks. Therefore, the susceptibility of reheat cracking was still low. The above results provided an instruction for heat treatment process, which ensured the safety after the welded joint was put into operation.An intercritical thermal cycle process was designed to change the grain boundary character. Combined with TEM and EBSD, the crystallographic characteristics of recrystallized grains, distribution of grain boundary misorientation, strain concentration were studied after intercritical thermal cycle. It was found that recrystallized grains reduced the proportion of high-angle grain boundary and increased the curvature of grain boundary, thus the crack was prevented from straight propagating along the prior austenite grain boundary. Additionally, the M/A(martensite-austenite) constituents consumed carbon element, which deprived C from carbides and precipitates were hard to coarsen at grain boundary. Meanwhile, the strain concentration located at M/A constituents due to its formation other than at prior austenite grain boundary. These above factors increased the strength of grain boundary. The grain boundaries were strengthened effectively and the susceptibility was reduced.Based on the cracking mechanisms as well as analyzing and verifying influence factors which affected the strength of grain interior and grain boundary, the cracking model in the CGHAZ of T23 steel was constructed. This model provides a basis for the heat treatment of welded joint, also gives a new method and angle for reheat cracking research of other materials.