The Research Of Rapid Cooling Process Effect Mechanism On Nickel-base Alloys Welding Hot Cracking Susceptibility

Nickel-base alloys have the favorable high temperature mechanical properties and corrosion resistance,therefore they are widely used in fields like petrochemical industry,aerospace and thermal power generation.However,the various alloy elements in Nickel-base alloys and the low thermal conductivity of austenitic matrix make the welding process generate coarse grains and severe segregation easily,which could result in the increased hot cracking susceptibility.The previous investigates indicated that grain refinement and segregation decrease could be effective to reduce the welding joint hot cracking susceptibility of Nickel-base alloys,and this paper is attended to achieve this objective by the usage of additive heat sink rather than introducing into other impurity elements.Inconel 601 H was used as the experimental material and liquid nitrogen was used as the cooling agent in this paper.It could be found in the study of liquid nitrogen cooling effect and its different cooling positions relative to molten pool that: The cooling method through adding the copper pipe below coupons reduced the arc interference by liquid nitrogen and improved the cooling effect;Compared to the heat sink located on front of the molten pool,the welding joints microstructures variation was more obvious when the heat sink located on one side and rear side;When the cooling distance reached too close relative to molten pool,the liquid nitrogen spraying impact made the welding seam shape become undesirable,and in this condition,the reduced liquid nitrogen cooling effect also failed to get its microstructure to the best.When the heat sink located on one side 10 mm relative to molten pool and the welding heat input reached 8.4kJ/cm,the welding joint microstructure had the largest change after liquid nitrogen cooling,thus the following experiments took this heat input as the welding parameter.Compared to conventional GTAW,the microstructure under liquid nitrogen cooling process had changes as follows: The rapid cooling changed the Inconel 601 H molten pool morphology and its welding seam width,and the size of HAZ was also be affected;More equiaxed grains formed in this condition and the welding seam microstructure became more homogeneous,and the epitaxial growth phenomenon began to disappear when the cooling intensity reached to some extent;The change of grain morphology and the increase of cooling rate reduced the alloy elements segregation degree,and significantly affected the ?? phase distribution,chemical composition and its size,therefore the welding joint hardness distribution was ultimately changed;Under all cooling processes,when the heat sink located on one side 10 mm relative to molten pool,its welding seam width and HAZ size reached to the smallest and the microstructure in welding seam mostly consisted of fine equiaxed grains,and the element segregation and ?? phase distribution were also became the most homogeneous.All these results could improve the alloy grain strength and reduce the liquid films incidence along grain boundaries,and ultimately attained the objective of preventing Nickel-base alloys welding hot cracks initiating.