Microstructure of heat affected zone

The microstructure factor was studied of annealing reheat cracks with the objective of documenting the specific features of the zone susceptible to the formation of such cracks. Attention was paid to the occurrence of the carbide phase. The heat affected zone microstructure was studied in samples of overlaid forged test slabs of steels 15Kh2MFA and 15Kh2NMFA designed for pressure vessels of WWER-440 or WWER-1000 reactors. The microstructure was studied in four characteristic sites of the heat affected zone that also included extreme conditions, viz., a point without heat effect and a point on the limit of melting. The results of the tests are discussed in detail. They show among others that intergranular separations can occur, with respect to carbide precipitation, as a result of segragation of carbide particles and films on grain boundaries in sites where the carbide reaction did non finish, and of precipitation of fine vanadium carbonitride which reinforces the inside of the grain. (Z.M.). 6 figs., 10 refs