Evaluation of HAZ Fracture Toughness for 800MPa Grade HSLA Used for Construction Machinery
As construction machinery is large scaling, application of 800MPa grade high strength low alloy (HSLA) steels is essential. In order to obtain good toughness of welded joints, rational control of welding heat input is needed to avoid brittleness of welded joints.The effects of welding cooling time and heat temperature on fracture toughness and microstructure feature for an 800MPa grade HSLA steel and heat affected zone(HAZ) were investigate using thermal simulation specimens.It is found that the fracture toughness of the simulated HAZ is the best when the cooling time of t8/5 is 18s. With gradually increase in t8/5, austenite grain size grows, fraction of martensite volume decreases, quantity of grain bainite increases, and fracture toughness decreases. If t8/5 is larger than 45s, brittle instability occurs for the LT specimens. Among the toughness distribution of various zones of HAZ, the fracture toughness of overheat zone is the best, that of dual phase zone is the lowest. In addition the toughness of coarse grain zone is better than that of fine grain zone. The main factor affecting toughness of HAZ is the configuration and distribution of bainite. The results will be valuable for determination of welding procedure and fracture evaluation of large scale construction machinery.