Abstract: In order to explore whether the cooling rate had adverse effects on the thermal stress of the 304LN steel when it was cryogenic treated, 304LN specimens with welded joints was used to calibrate the base metal zone, weld zone and heat affected zone, respectively with the cooling treatment of 2 ℃/min, 5 ℃/min and direct liquid nitrogen immersion, and the rationality of the test results was verified by the finite element theory. The results show that with the decrease of temperatures, the perceived temperature of the upper and lower surfaces of the steel plate is different, and the temperature at the edge of the specimen is lower than that in the center, but the temperature difference is small. In the process of continuous cooling, the temperature stress of the specimen increases with the increase of cooling rates. According to the thermal stress-temperature curves, it is found that they have a nonlinear relationship. The precipitates are more uniform and stable at cooling rate of 5 ℃/min. Therefore, it can be concluded that there is little difference between the first two kinds of rates of cooling in general cryogenic processing, while 5 ℃/min is recommended based on the results of the practical mechanical properties testing and related processes.

Key words: 304LN steel welded parts, cryogenic treatment, finite element modelling, microstructure