Analysis on continuous cooling transformation curves of simulated heat affected zone for SA508-3 steel in nuclear power

The SA508-3 steel used in nuclear power was used to determine continuous cooling transformation(SH-CCT) diagram of simulated heat affected zone(HAZ) by thermal expansion method on Gleeble 1500D thermal simulation testing machine,and the microstructure characteristics of HAZ at t_8/5 from 3.75 s to 20 000 s were investigated.It was found that the microstructures were changed much from the base metal because of the influence of the cooling time(t_8/5).When the t_8/5 is less than 15 s,the phase transformation to obtain all martensite happens,the cooling time range to get all the bainite is from 60 s to 3 000 s,and if all the ferrite,and pearlite can be obtained if the t_8/5 is more than 6 000 s.The hardness of HAZ is higher than 350 HV and there are harden quenching tendency and the crack sensitivity when the t_8/5 is less than 100 s,softening phenomena easily happens and the hardness of HAZ is lower than that of the base metal when the t_8/5 is more than 20 000 s.The cold cracking can be avoided only at the proper preheating temperature.