Study On Fracture Properties Of Q390 Low Alloy High Strength Steel Welded Joint

Low-alloy high-strength steel has good toughness and weldability,and its welding structure has been widely used in engineering machinery,energy,construction,pressure vessels,vehicle vessels and other fields.But at the same time,the fracture accidents of the welded structure have often occurred.In recent years,domestic and foreign experts are studying on the welding cracks,toughness mismatch,fusion zone and heat-affected zone performance changes,and other issues of low-alloy high-strength steel welded joints.The result showed that,in the design of welding structure,only relying on the traditional strength design principles is not enough to ensure the safety of the welding structure.Therefore,in order to ensure the safety of welding structure,welding structure design should take into account the welding strength and toughness.In this paper,fracture properties of Q390 steel welded joint was studied.Q390 steel was welded by CO2 gas shielded welding in accordance with the principle of equal strength matching welding,meeting the actual field welding process in field work;.The cold crack sensitivity of hot-rolled Q390 steel was analyzed by measuring the hardness of welded joints.The tensile and impact tests on the welded joint were carried out to analyze the influence of temperature on the fracture properties of the welded zone and the heat affected zone.The microstructure of the welded joint was observed by scanning electron microscopy.According to the structural integrity criterion,the existing relationship between the impact energy and fracture toughness of the ductile-brittle transition zone is corrected by the lower boundary of the main curve,and the relation between impact energy and fracture toughness applicable to the weld zone of Q390 steel which was welded in accordance with the principle of equal strength matching welding is obtained.The process of impact test was simulated and the fracture toughness of welded joint was predicted.The main class curve model of impact energy of welded joints was established.The results show that:(1)When Q390 low-alloy high-strength steel is made into a welded joints,the base material is little affected by the heat cycle,it will make the heat-affected zone after welded cools quickly,its surface hardening,acicular ferrite increasing,impact fracture performance better than the weld and base metal zone.And weld grain increase,its microstructure is sensitive to temperature changes,low temperature performance deterioration,the weak area of welded joints,reducing the welded joints' service capacities.(2)The fracture toughness distribution of weld zone accords with the relationship between fracture toughness and impact energy absorbed by the existing empirical formula.In the ductile-to-brittle transition range of weld,the main curve method was used to establish the relationship among the cumulative failure probability,temperature and impact energy of equal intensity matching welded joints of Q390 steel,which provides a reference for evaluating the cumulative failure probability by impact test.(3)The resistance to damage of cracked welded joints increases as the increase of the tensile strength of the material and decreases with the increase of the elastic modulus.