# Martensitic Transformation Mechanism In Situ Observation for the Simulated Coarse-Grained Heat-Affected Zone of DP1180 Steel

## TL;DR

This study observes how martensite forms in a specific steel zone during welding, revealing growth patterns that could improve steel material development and welding quality.

## Contribution

The study identifies seven martensitic lath growth modes in DP1180 steel's heat-affected zone using in situ observation techniques.

## Key findings

- Austenite grain growth occurs via continuous grain boundary annexation and migration during heating.
- Martensitic laths end growth due to collisions with grain boundaries or other laths, forming interlocking structures.
- Seven distinct growth modes of martensitic laths in the CGHAZ under laser welding conditions are identified.

## Abstract

The martensitic transformation mechanism in the heat-affected zone of DP1180 steel plays a decisive role in the strength of welded joints. In this work, the nucleation and growth kinetics of martensite laths in the coarse grain heat-affected zone (CGHAZ) are analyzed by a high-temperature laser scanning confocal microscope (LSCM). The grain distribution and stress distribution of the samples after in situ observation are analyzed by electron backscatter diffraction (EBSD). The results reveal that austenite grain growth is realized by continuous grain boundary annexation and grain boundary migration of small grains by large grains during the heating process. Seven growth modes of CGHAZ martensitic laths under laser welding conditions are proposed. Additionally, the end growth of martensitic laths is mostly attributed to the collision with grain boundaries or other laths to form a complex interlocking structure. The results of this study could provide important data support for the development of dual-phase steel materials and improvement of welding quality.