Study On Stability Of HAZ For High-strength Ship Plate Steel

Hulls serving in poor environment are built by welding the ship plate steel together, so it is important to research the stability of ship plate steel HAZ, which is helpful to improve the microstructure and properties of these steel furtherly. Studies on the stability of microstructure and properties of A36 steel are done by a series of tempering experiments on different conditions. Simulating the welding process and comparing with the actual welded joint help to study the microstructure and properties of the A36 steel HAZ, and further study of the effect of the post-weld heat treatment on the coarse grain zone is done. By comparing the thermal simulation results of A36, F40 and A32 coarse grain zone, the micro-alloying elements' influence on the HAZ is discussed. The main research results of this thesis are followed:(1) The A36 ship plate steel has good stability of structure and property during the tempering process. The microstructure had no significant changes, the Akv had slight reduction, and the hardness reached its maximum when tempering at about 600℃.(2) The cooling rate is expessed by the cooling time between 800℃and 500℃(t8/5) in the weld thermal cycle of micro-alloy steel. The microstructure of CGHAZ transforms form fine lath martensite into bainite gradually, along with the t8/5 increase from 10s to 100s. When the t8/5 is 40s, with the peak temperature rising, the microstructure of ICHAZ is fine pearlite+ferrite+a little original coarse pearlite which didn't austenitized; the microstructure of FGHAZ is fine ferrite and pearlite, the microstructure of CGHAZ is upper bainite and some granular bainite. The toughness of CGHAZ is the worst, and the hardness decreases with the cooling rate decreasing.(3) The the microstructure of CGHAZ transformed from upper bainite and a little granular bainite into stable lygonal ferrite, pearlite and some acicular ferrite after the samples with t8/5 being 40s were normalized. The microstructure of CGHAZ being 600℃tempered didn't have significant changes, still remaining to be bainite. Comparing with the untreated samples, the toughness was raised, and the hardness decreased slightly. The changes in toughness and hardness are closely related to the transformation of the structure.(4) The impact fractures of normalized samples have clear fibrous zone and shear lip, which are ductile fracture, and the radical zone is quasi-cleavage fracture and cleavage fracture with river pattern. While it is hard to see the fibrous zone or shear lip from the impact fracture of tempered samples, and most of the region is a kind of trans-granular brittle fracture with poor toughness, and accompanied by a bit of secondary cracks.(5) The A36 ship plate steel was welded by manual metal-arc welding with heat input about 19.6kJ/cm. Lath martensite is observed in the CGHAZ of welded joint, and the hardness of CGHAZ approaching the fusion line is highest. It is indicated that the microstructure and properties of HAZ thermal simulated are in accordance with the actual joint and the thermal simulation experiments can accurately reflect the changes of microstructure and properties in practical welding.(6) With the same cooling rate, there is less lath ferrite and more granular bainite and ferrite block in the CGHAZ of high Nb F40 steel than A36 steel and the microstructure is coarser than A36 too. Nb could restrain the ferrite formation, while promote the bainite formation. Besides excess Nb can reduce the amount of TiN particles, resulting in the decline in toughness and strength of CGHAZ.