Welding Process Improvement of 347H Stainless Steel Boiler Tube for Mitigating Reheat Cracking

Abstract

Many failures at the welded joint of 347H tubes have recently occurred during the short-term operation of the boiler tubes. To mitigate reheat cracking or strain induced precipitation hardening, the finite element analysis and experiment for optimization of welding procedure were performed. Preheating, high inter-pass temperature, low and high heat input conditions were compared with existing welding procedure. According to mechanical analysis, high inter-pass temperature and high heat input condition showed the lower plastic strain and tensile residual stress at the inner area of heat-affected zone. At high temperature tensile test, the high inter-pass temperature condition had lower yield strength and higher elongation, while the high heat input condition showed higher yield strength and smaller elongation. In the specimen of high heat input condition, the dissolution of most precipitates and coarse grain were observed in the heat-affected zone. Relatively small amount of dissolution and smaller grain got shown in high inter-pass temperature condition. Therefore, high inter-pass temperature condition can be an alternative. Through X-ray diffraction analysis, it was confirmed that nitrogen was added to Super304H and carbon was substituted for the existing crystal structure of Nb(C,N). This will mitigate reheat cracking by increasing the solvus temperature of Nb(C,N) and reduce the grain coarsening. Key words: Strain induced precipitation hardening, Carbide dissolution, Plastic strain, Heat input, Inter-pass temperature