Research On Weldability Of The Under-matching Weld Joint Of Super-high Strength Steel Welded By High-chromium-nickel Austenitic Welding Consumables

When super-high strength steel whose yield strength is greater than 700 MPa and thickness is greater than 50 mm is welded at high restraint conditions, even using conventional measures such as preheating, postheating and so on is still unable to avoid cold cracking. In order to control cold cracking of super-high strength steel thick plate high restraint structure, under-matching weld joint is considered to use. Therefore the high-chromium-nickel austenitic stainless welding wire whose strength is smaller than base metal is developed in the project. The weldability of newly developed austenitic stainless welding wire is studied from residual stress, microstructures, properties and cold crack test. The results provide a theoretical basis and data support for newly developed austenitic stainless welding wire applications.The stress and distortion under two cases including equal-matching case and under-matching case of three finite element models which were established using MARC software were researched. The results show that the equivalent stress and longitudinal stress in under-matching weld joint are significantly smaller than those in equal-matching weld joint, but residual deformation in under-matching weld joint is slightly bigger than that in equal-matching weld joint. The effect of different preheating temperatures on residual stress of weldment in overlaying model has been studied using MARC software. With the increase of preheating temperature, welding residual stress gradually decreases. Preheating temperature has a significant effect on residual stress of both weld and HAZ and has small effect on residual stress of base metal away from weld in equal-matching weld joint. In under-matching weld joint, preheating temperature only has a significant effect on residual stress of HAZ. Under-matching could reduce preheating temperature even without preheating.Macro morphology of welding joint welded by the high-chromium-nickel austenitic stainless welding wire under different shielding gases was studied by using SZX12 stereo microscope. The results show that to acquire good quality welding joint, shielding gas composition is chosen as 40%Ar+58%He+2%CO2. The microstructure of the good quality welding joint has been analyzed by VHX-1000 E optical microscope. The microstructure of weld is single phase austenite, the microstructure of fusion zone consist of “the bright band” which microstructure is single phase austenite and “follow-up melted stagnant layer” which microstructure is austenite+martensite, CGHAZ’s microstructure is coarse lath martensite, the microstructure of GRHAZ is fine martensite, the microstructure of incomplete recrystallization zone is martensite+tempered sorbite. The analysis results of micro-hardness show that the maximum hardness appears in CGHAZ, the hardness of weld is smaller than other zones.The results of window-like restraint weld cracking test show that weld joint which is acquired by using austenitic welding consumables has excellent resistance to cold cracking. Based on the research results that residual stress decreases and the microstructure of weld is single phase austenite which results in small hardness value and small tendency of hardening in under-matching weld joint. So tendency to produce cold cracks decreases by using austenitic welding consumables to weld super-high strength low alloy steels.