# Investigation on the Microstructure and Mechanical Properties of X70 Pipeline Steel Fabricated by Laser-Directed Energy Deposition

## TL;DR

This study examines how laser repair affects the structure and strength of X70 pipeline steel, finding that it improves ductility but reduces strength.

## Contribution

The paper reveals a thermal history-dependent strength-ductility trade-off in laser-repaired X70 steel, offering insights for optimizing repair processes.

## Key findings

- L-DED repaired X70 steel shows increased ductility with 42.9% elongation, 58% higher than base material.
- Yield and tensile strengths of L-DED X70 are lower than base material and API 5L standards.
- Grain coarsening and thermal accumulation influence microstructure and mechanical properties along the building direction.

## Abstract

The laser-directed energy deposition (L-DED) technique, with its excellent environmental adaptability and superior repair capability, shows great potential for the repair of damaged X70 pipeline steel. In this work, the microstructure and mechanical properties of L-DED repaired X70 steel were systematically investigated. The deposited material exhibited inhomogeneity along the building direction. From the bottom to the top, the grains gradually coarsened, and the proportion of polygonal ferrite increased. This was mainly attributed to increasing thermal accumulation with deposition height, which reduced the cooling rate and promoted solid-state transformations at higher temperatures. Meanwhile, the heat accumulation and intrinsic heat treatment reduced the dislocation density and promoted Fe3C precipitation within grains and along boundaries. Microhardness was highest in the bottom region and decreased along the building direction due to the gradual coarsening of microstructure and decreasing in dislocation density. The L-DED X70 showed lower yield strength (435 MPa) and ultimate tensile strength (513 MPa) compared to the base material and API 5L requirements. The elongation of the L-DED X70 was 42.9%, which was 58% higher than that of the base material, indicating excellent ductility. These results revealed a thermal history-dependent strength–ductility trade-off in the L-DED repaired X70 steel. Therefore, more efforts are needed to control the L-DED thermal process, tailor the microstructure, enhance strength, and meet the service requirements of harsh environments.