Abstract：To provide theoretical and experimental basis for the proper selection of welding parameters in engineering application，the effect of cooling time from 800 to 500 ℃(t8/5) on the microstructure transformation and properties variation of the coarse grained heat affected zone (CGHAZ) of the 1100, 1400 MPa grade steel was investigated by using the Formastor-FⅡ, optical microscope(OM) and Vickers hardness tester. The results indicate that when the t8/5 of LG1100QT steel was less than 30 s or the t8/5 of LG1400QT steel was less than 60 s, the heat-affected zone was transformed from lath martensite/bainite → lath bainite + granular bainite → granular bainite → granular bainite + ferrite + pearlite, and the size of the grain and microstructure gradually increased. The microstructure transformation of LG1100QT steel was earlier than that of LG1400QT steel, and at the same cooling rate, the microstructure transformation such as bainite and pearlite first appeared in the heat affected zone of LG1100QT steel, which was related to the content of alloying elements in the steel. With the increase of t8/5 and transformation of its microstructure from lath martensite to bainite and ferrite/pearlite, the hardness of the heat affected zone decreased. When the t8/5 of LG1100QT steel exceeded 75s and the t8/5 of LG1400QT steel exceeds 30s, the hardness of the heat-affected zone was lower than that of the test steel. The carbon equivalent and cold cracking susceptibity coefficient of LG1100QT steel is 0.52% and 0.27%, the carbon equivalent and cold cracking susceptibity coefficient of LG1400QT steel is 0.66% and 0.37%, respectively, indicating that two test steels all had strong harden quenching tendency and cold cracking susceptibility. In order to avoid welding cold cracks and other defects and obtain joints with properties equivalent to the base metal, the t8/5 time of LG1100QT steel and LG1400QT steel should be controlled within 75s and 30s respectively, and preheating and post-weld heat treatment should be taken during welding.
2023, Vol. 30 