Abstract: Coarse grained samples were prepared by pre-stretching and solid solution treatment, and then isothermal aging was carried out on the samples. The microstructure and hardness changes of Super304H and TP347HFG steel pipes with abnormal austenite grain growth were simulated and studied, and the remaining service life was predicted. The results show that the average austenite grain size of the coarse-grained sample was grade 2 after pre-tensile deformation and solid solution treatment. During the isothermal aging process, the austenite grain size was basically unchanged, but the M₂₃C₆ phase particles continued to precipitate and became coarse, resulting in austenite grain boundary widening. The hardness of coarse-grained samples increased first and then decreased, then tended to be stable, and finally decreased rapidly and monotonously. Based on the hardness-P function method, a mathematical model for predicting the remaining service life of coarse-grained Super304H and TP347HFG steel tubes was established.