Abstract
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 20000 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 3000 s, and if all the ferrite, and pearlite can be obtained if the t 8/5 is more than 6000 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 20000 s. The cold cracking can be avoided only at the proper preheating temperature.