Correlation Between Microstructures and Mechanical Properties during PWHT Process of TMCP Steel Welded HAZ

Abstract

Correlation between Heat-Affected Zone (HAZ) microstructures and the mechanical properties during Post-Weld Heat Treatment (PWHT) on two types of Thermo-Mechanical Control Process steels (TMCP steels), especially the relation between the stress relaxation behavior and high temperature deformability was investigated. Simulated-weld heat treatment was carried out using a weld-heat cycle simulator with a maximum temperature of 1623 K. PWHT was done at a heating rate of 55.6 K/ks. The mechanical properties during the PWHT process were evaluated by means of the constant-strain rate test on heating and the constant-load test on heating. The influence of PWHT on the reducing tendency of ductility was discussed from the viewpoint of precipitation behavior of inter-or transgranular carbides and the deformability of the matrix on each HAZ microstructure. The results are summarized as follows :
(1) The stress relaxation behavior of the specimens of bainitic structure on both AcC type (Controlled rolled+Accelerated Cooled) and Non AcC type (only Controlled Rolled) TMCP steels were delayed clearly above 600 K. The stress relaxation behavior of the Non AcC type TMCP steel was further delayed to the higher temperature range during PWHT than that of the AcC type TMCP steel.
(2) High temperature deformability of the specimens of bainitic structure on both TMCP steels had a remarkable reducing tendency of ductility in the range of 850-900 K. The Non AcC type TMCP steel had a remarkable reducing tendency of ductility than that of the AcC type TMCP steel.
(3) The reducing tendency of ductility was caused by the difference in precipitation behavior of inter-or transgranular carbides in each HAZ microstructure during PWHT process. The tendency of the Non AcC type TMCP steel which contained more alloying elements occurred more strongly because of precipitation of alloying carbides.
(4) When the high temperature ductility or embrittlement of HAZ microstructure of the TMCP steels was evaluated, it was very important to measure the deformability of the material during the PWHT process. An evaluating method of ductility of TMCP steels was proposed using the relation between deformability of material and the displacement of each microstructure during PWHT process.