Influence of Fine-Grain Bands in Forged 690 Alloy Bars on Microstructure of Final Tubes

Heat treatment experiments were conducted on forged 690 alloy bars for heat-exchange tubes of steam generator in nuclear power plant according to hot extrusion heating process. Annealed intermediate tubes and thermal-treatment final tubes were prepared using product process for heat-exchange tubes. The microstructure of the samples at different states were analyzed. The results show that the non-homogeneous precipitation of carbides was the main reason for abnormal fine-grain bands existing in the microstructure of forged bars. The abnormal fine-grain bands in forged bars would have adverse effects on the microstructure uniformity of 690 alloy tubes in succeeding processions and result in non-homogeneous precipitation of carbides in the thermal-treatment final tubes.