Thermal Simulation Research On Microstructure And Properties Of HAZ For 700 MPa Grade HSLA Steel

High strength low alloy steels(HSLA) have been widely used in engineering machinery, industries, energy etc, due to their excellent strength and toughness. However,with the improvement of strength, especially when the strength is more than 800 MPa, the welding of HSLA is complicated. The problems should be encountered in the joint include grain coarsening, mismatching of toughness and strength and poor performances of heat affected zone.In this paper, the microstructure and properties of the coarse-grained heat-affected zone(CGHAZ) was studied at different cooling time(t8/5) and the variation peak temperature(Tmax) by welding thermal simulation, electron microscopy analysis and mechanical property test. The SH-CCT curves of the yield strength 700 MPa grade HSLA steels were measured and the austenitic grain growing of CGHAZ were analyzed. The experimental results provide a theoretical basis and technical guidance for the choice of welding procedure and welding methods.By measuring the SH-CCT diagram of tested steel with the thermal dilation method and quantitative metallographic method, the influence of cooling rate on microstructure and hardness of CGHAZ was obtained to reveal the phase transition of HAZ at different cooling rates. It was found that:when the cooling rates are between 0.2 ℃/s and 1 ℃/s, the microstructure is mainly polygonal ferrite. When the cooling rates are between 1 ℃/s and 20 ℃/s, the microstructure is mainly bainite. When the cooling rates are more than 25 ℃ /s, the low-carbon martensite has been found. The Vickers hardness increase gradually with the increase of cooling rates.The austenite grain size of CGHAZ was increased with the increase of peak temperature and holding time, and increase more quickly with the peak temperature. When the holding time is less than 5s and the peak temperature is less than 1300 ℃, the austenite grain size of CGHAZ increased slowly and was relatively small However, the austenite grain will grow significantly when the peak temperature is greater than 1300 ℃ and holding time is more than 5 s.The effects of peak temperature and cooling rate on the microstructure and properties of heat affected zone were studied. The microstructure of HAZ is mainly small ferrite and a small amount of granular bainite when the peak temperature is less than 1150 ℃. However, when the peak temperature is higher than 1150 ℃, the microstructure of HAZ is mainly bainite, and the grains grow significantly with the increasing of peak temperature. When t8/5 is less than 15 s, the microstructure of HAZ is mainly lath bainite, a small amount of granular bainite and martensite. While t8/5 is more than 20 s, the microstructure of HAZ is mainly granular bainite and a little of ferrite.The impact energy increases at first and then decreases with the increasing of peak temperature. Especially, in CGHAZ, the impact energy decreases significantly with the increasing of peak temperature. However, the Vickers hardness increased with the increasing of the peak temperature.The impact energy increases at first and then decreases with the increasing of t8/5. When t8/5 is 15 s, the impact toughness of CGHAZ is the best. With the increasing of t8/5, the Vickers hardness of CGHAZ decreased gradually. So the cooling time should be controlled within 20 s in the actual welding process.