Study of hot cracking behaviour of nitrogen-enhanced austenitic stainless steels using Varestraint and hot ductility tests

Hot cracking is a major problem in the welding of austenitic stainless steels (SS), particularly the fully austenitic grades. A group of alloys of high nitrogen stainless steel is being developed for structural components of the Indian Fast Reactor programme. Studying the hot cracking behaviour of this nitrogen-enhanced austenitic stainless steel is an important consideration during welding, as this material solidifies without any residual delta-ferrite in the primary austenitic mode. Nitrogen has potent effects on the solidification microstructure; hence, it is expected to have a strong influence on the hot cracking behaviour. Both Varestraint and hot ductility tests were used to evaluate its solidification and liquation cracking susceptibility. Different heats of this material were investigated, which included fully austenitic (high nitrogen stainless steels) containing 0.07-0.22 wt. (%) nitrogen. Varestraint tests were carried out on these alloys using specimens of 3 mm thickness at four strain levels between 0.5 and 4.0 %. The Brittleness Temperature Range (BTR) was also evaluated from these tests. The Varestraint test results showed that the solidification cracking susceptibility is higher for 0.22 wt. (%) steel and the liquation (HAZ) crack significantly increases with increasing nitrogen content. Hot ductility tests were conducted on these alloys using a thermomechanical simulator and the Nil Strength Temperature (NST), Nil Ductility Temperature (NDT) and Ductility Recovery Temperature (DRT) were determined. The hot ductility test results showed that the nil ductility range (NDR), the difference between NST and DRT of the nitrogen-enhanced steel containing 0.22 % N, is higher (50 °C) than that of the alloys containing 0.07 % N (40 °C) and 0.14 % N (30 °C), indicating high susceptibility of the 0.22 % N alloy to liquation cracking. This paper presents and discusses the role of high nitrogen content on hot cracking susceptibility of this class of steels using Varestraint and hot ductility tests.