The Evaluation Of Ductility Dip Cracking Susceptibility In Austenitic Stainless Steel During The Process Of Hot Working

Hot ductility in austenitic stainless steel (ASS) is low and prone to suffer from cracking during hot deformation. In this paper, the as-cast microstructure of304ASS slab and weld joint is studied. Presents a kind of variable cross-section samples to evaluate the susceptibility of cracking. The size of samples is determined by the finite element model(FEM). The tensile test at elevated temperatures was performed by using the Thermorestor-W simulator, studying the cracking on the fractured samples and evaluate the susceptibility of cracking. From the angle of mechanics, the mechanism of crack formation is analyzed. And the reasons of difference between the two is discussed according to the crack nucleation position.Finite element model of high temperature tensile test is established by ABAQUS software. The size of deformation zone is determined according to the gradient distribution of strain, with the length of15mm, width of11mm, thickness changing from10mm to4mm, and the minimum thickness at the1/3length of deformation zone. With the sample, the threshold strain of producing cracks can be calculated. The slab samples is taken from casting billet, and the weld samples from the weld area of the hot rolled plate by TIG welding. The experiment is conducted in the temperature range of950and1200℃, with the interval of50℃.As for the two kinds of samples, when the deformation reaches to a certain amount, samples will crack on the surface at all temperatures. They both have the same most sensitive temperature range, but the threshold strain of the slab is smaller than the welding joint. From the angle of mechanics, the cracking is related to the flow stress. If the flow stress is bigger than the threshold stress, the cracks will occur. Therefore, the cracking in the slab and weld joint is belong to the same type in the process of hot deformation.Observing the microstructure of the fractured samples, the position of cracking nucleation of slab and weld joint is different. As for slab, at950℃, the cracking nucleation is at dendritic branches of ferrite; at1050℃, it is at Ferrite dendrite and austenite, each accounting for about50%according to the statistics; at1200℃, it is mostly likely to occur at austenite. While for weld joint, at950℃, the cracking nucleation is at austenite grain boundary; at1000℃, it is at Ferrite dendrite and austenite; at1150℃, it is mostly likely to occur at ferrite.Between the slab and weld joint, The differences of the threshold strain and the cracking nucleation are due to the difference of the cooling speed. In the weld, the dendritic ferrite is distributed in austenitic grains at high cooling speed; while in the slab, the ferrite is distributed in austenitic grains and grain boundaries.