Abstract：Martensitic transformation can significantly reduce the macroscopic residual stress in welds. Existing studies have generally attributed this effect to transformation-induced volumetric expansion. However, residual stress origin and evolution due to martensitic transformation at the grain scale remain unclear. To address this issue, this study investigates the effect of martensitic transformation on residual stress for P91 steel during laser welding by combining microscale transformation volumetric strain with EBSD-based microstructure reconstruction and crystal plasticity finite element simulation. The results show that the proposed model can effectively capture the microscale evolution characteristics of residual stress in the weld zone. The transformation volumetric strain is introduced as eigenstrain via the deformation gradient. At the macroscopic scale, it compensates for cooling shrinkage deformation. At the microscopic scale, transformation mismatch among grains induces local stress accumulation and release. This leads to a strongly heterogeneous residual stress field. Dislocation density analysis reveals a threshold effect of cooling rate on transformation volumetric strain. The effect is significant below 10 °C/s but insignificant above this value. This work clarifies the mechanism of martensitic transformation-induced micro-stress at the grain scale and reveals the role of cooling rate in regulating phase transformation behavior. These findings provide a theoretical basis for understanding residual stress formation in the weld zone of P91 steel.
2026, Vol. 47 
