Technical Article

Analysis of Cracking in Austenitic Stainless Steel after Forging and Annealing

10Cr9Mo1VNb is an austenitic stainless steel with high oxidation resistance and high-temperature steam corrosion resistance, good impact toughness and persistent plasticity. It is used as a furnace tube, superheater, reheater steel tube, high-temperature header, steam pipeline, nuclear electric heat exchanger, etc. for subcritical and supercritical boilers with wall temperature ≤ 620 ℃ in high-temperature petroleum cracking units. When the steel is annealed after forging, it may experience cracking, sometimes even the entire length of the forging. Based on the summary of the causes of forging cracking, it is believed that due to the uneven furnace temperature of the annealing furnace, the insulation temperature of some steels is much lower than 680 ℃. Although the insulation time is long, the transformation from austenite to ferrite+carbide cannot occur during the insulation stage, and only the austenite state can be maintained. In the subsequent cooling process, although the speed is slow and the furnace temperature is low, the transformation from martensite still occurs. Therefore, there is a significant residual stress after cooling, and due to the slow cooling rate, thermal stress should be well released. In the residual stress, tissue stress dominates, with the core subjected to tangential compressive stress and the surface subjected to tangential tensile stress. When the residual stress exceeds the tensile strength, explosion occurs. Improvement measures: Increase the annealing temperature after forging and use an annealing furnace with a more uniform furnace temperature.