Thermal stress evolution and mechanical/physical properties of 304LN weldments subjected to deep cryogenic treatment

In this work, 304LN weldments were selected as the experimental material for the new generation of piping steel. The 304LN stainless steel plates were welded by means of Tungsten Inert Gas (TIG). One set of samples was untreated and the rest of samples were treated by different deep cryogenic treatments (DCTs). The DCT process can change the unstable austenite to martensite, thus the workpiece could gain stable performance. To gain the optimal DCT curve, the effect of processes on the metallographic change and physical/mechanical properties of 304LN welded joints were analyzed. Also, the distribution and evolution of thermal stress in cryogenic treatment of 304LN weldments were studied by means of strain test and simulation. From these results it can be inferred that the faster DCT cooling rate will generate much stronger thermal stress, but it was much less than yield strength of the steel. Due to the existence of thermal stress, supersaturated martensite was further broken down into fine and dispersed carbides and nitrides, which made the strength and toughness increased. At the same time treated with 5 °C min⁻¹, the tensile strength and impact toughness of the welded joints increased 4.77% and 23% respectively, while these indexes increased 1.24% and 16% with 2 °C min⁻¹. Hence, considering two processes comprehensively, it is better to treat the 304LN weldments with 5 °C min⁻¹. This process can be used as a strengthening and toughening method for 304LN welded joints. (paper)