# Effect of Transformation Plasticity on the Residual Stress of Laser–MAG Hybrid Welding of 30MnCrNiMo High-Strength Steel

## TL;DR

This study shows how transformation plasticity affects residual stresses in high-strength steel welding, improving simulation accuracy.

## Contribution

A new finite element model incorporating transformation plasticity improves residual stress prediction in 30MnCrNiMo steel welding.

## Key findings

- Volume expansion during solid-state phase transformation shifts residual stress from tensile to compressive.
- Transformation plasticity reduces peak longitudinal tensile stress in the HAZ by 51 MPa.
- The model with transformation plasticity captures dual stress reversals with 8.8% average relative error.

## Abstract

In the current numerical simulation study of high-strength steel welding, ignoring the phase transformation plasticity effect in the coupling analysis led to a significant deviation between the simulated value of residual stress and the experimentally measured value. To investigate the influence mechanism of the Welding Residual Stresses (WRSs) of 30MnCrNiMo armor steel, the transformation plasticity (TP) coefficient (7.81 × 10−5 MPa−1) was measured via a Gleeble 3500, and a Finite Element Model (FEM) of thermal–metallurgical–mechanical coupling considering yield strength, volumetric strain and TP behavior in Solid-State Phase Transformation (SSPT) was developed. The results show that the volume expansion during the SSPT is the main factor for the shift in WRS from tensile to compressive. In contrast, the TP effect reduces the peak longitudinal tensile stress in the Heat-Affected Zone (HAZ) by 51 MPa. It also ultimately neutralizes the compressive component in this region. When the martensite fraction ranges from 0.12 to 0.45, transformation plastic strain becomes the dominant factor, leading to a characteristic evolution of longitudinal stress that initially decreases and subsequently increases. The FEM incorporating the TP effect successfully captures the dual reversals of residual stress in the HAZ. The average relative error between the simulated longitudinal stress and the experimental data obtained via X-ray diffraction (cosα method) is 8.8%. The TP coefficient database and the developed multi-field coupling model markedly enhance the predictive accuracy for WRS in 30MnCrNiMo steel, offering a robust theoretical foundation for the design of stress corrosion resistance and the service life assessment of welded joints in armored vehicles.