Study On Microstructures And Properties Of Welding HAZ Of ASTM4130 Steel

ASTM4130 steel is applied to high pressure pipeline of offshore platform because of its excellent mechanical properties. However, as a king of quenched and tempered medium carbon steel, welding heat affected zone is the area with weak performance. This thesis takes weld HAZ of ASTM4130 steel as the research object, simulates the real welding thermal cycle experienced by HAZ by means of welding thermal simulation testing, and research features of microstructure and properties of HAZ by using tools such as optical microscope, SEM, impact testing machine, Vickers and so on.The results showed that microstructure and properties of HAZ experiencing single welding cycle deteriorated without post-welding tempering. The fine grains zone was composed of granular bainite. The coarse grained zone was composed of cryptocrystalline martensite, acicular martensite and lath martensite. The impact energy of fine grains zone was only one sixth of that of base metal; the impact energy of coarse grained zone was only 10J, which was much lower than that of base metal. Hardness of coarse grained zone was very high and much higher than that of base metal because of hardening organization. With post-welding empering at 640℃for 1.5 hour, the microstructure with worsening performance of ASTM4130 steel HAZ experiencing single welding cycle was removed, toughness was improved, the hardness of coarse grained zone lowered down, which made toughness and hardness be equivalent to base metal.The microstructure of coarse grain zone without tempering after welding whose hardness is much higher than that of substrate is quench-hardening and impact energy is less than 14J when line energy is 12, 20 and 28KJ/cm; while quench-hardening microstructure is eliminated, toughness is improved and hardness is decreased with tempering at 640℃for 1.5 hour. Due to the effect of structure inheritance, the microstructure is coarse and toughness and harness are worsened when the peak temperature of second welding thermal cycle is higher than Ac1. The residual austenite generates quenched martensite during the process of tempering cooling owing to double quenching, which weakens the effect of tempering to improvement on toughness and hardness. The hardness is slightly higher and the toughness improves slightly.The results showed that With post-welding empering at 640℃for 1.5 hour, it was feasible to choose a bit high welding heat input and the microstructure and properties didn't deteriorate. In addition, to weaken the effect of double quenching on the microstructure and properties of ASTM4130 steel, multi-pass welding and temper bead should be adopted.