Physical Simulation Of Microstructure And Property Of HAZ In 2.25Cr-1Mo-0.25V Steel

The 2.25Cr-1Mo-0.25V steel had been widely using in manufacturing hot wall hydrogen reactor for its higher strength and stronger resistance to the high temperature creeping, and hydrogen corrosion and tempered brittleness. However, the fundamental research works on applying are still behind practical manufacture, furthermore, the further optimization of welding process were firmly retarded.In this thesis, one and twice weld thermal cycle experiments were carried out by using of Gleeble-3500 thermal simulator. The microstructures, properties and solution and precipitating behaviors of second phase particles were investigated by using methods of optical microscope, SEM+EDS, TEM and cryogenic impact test.The results show that after gone through one and twice weld thermal cycles the grain size and microstructure become coarse, which is main reason resulting in sever brittlemnt tendency. In addition, the strip carbides may be other reasons. Small grains were found in both sides of initial austenite grain boundary when the peak temperature is 1000℃and 900℃. The former one result from the austenite spontaneous recrystallization and the later one result from grain boundary effect of spherical austenite. The cryogenic impact values can be obviously increased by using of post weld heat treatment (PWHT), as well as this property can be further increased with the tempering time increasing. The fracture surfaces exhibit quasi-cleavage character before PWHT and ductile fracture after PWHT, repectively. But to some tempering samples, if the particles joint to each other among grain boundary, the impact value and the fracture style didn't change.The dimension of the second phase particles after gone through weld thermal cycles is about 100nm. With the increase of the welding pass and the decrease of the peak temperature of second welding thermal cycles, the numberof the precipitates increases. In the center of the square carbonitride is rich in titanium and the edge is rich in niobium and vanadium. The strip particles between the laths are rich in niobium. The particle dimensional range is about 200-300nm and its shape became regular after gone through post welding tempering. The number of precipitate increase with the tempering time increasing and the precipitates distributing among the grain boundary is rich in chromium, vanadium, iron and manganese.