# Evaluation of the Relationship Between Fracture Toughness and Hydrogen-Induced Damage in X70 Line Pipe Steel for Low-Temperature Service

## TL;DR

This study examines how different rolling treatments affect the toughness and hydrogen resistance of X70 steel used in low-temperature pipelines.

## Contribution

The study identifies specific microstructural features that enhance toughness and hydrogen damage resistance in X70 line pipe steel.

## Key findings

- Medium R/F reduction steel showed better DWTT performance and hydrogen resistance due to stronger TBr and TC components.
- Higher fractions of blocky austenite/martensite and RC texture negatively impact toughness and hydrogen resistance.
- Gamma (γ)-fiber and {332}<113> textures improve both toughness and hydrogen damage resistance.

## Abstract

In this study, X70 line pipe steels were subjected to different hot rolling treatments under three conditions with varying roughing (R) and finishing (F) reductions while maintaining the same total reduction to investigate the effect on drop weight tear test (DWTT) toughness and hydrogen-induced damage as assessed through electrochemical charging. Scanning Electron Microscope (SEM) images were used to analyze microstructure phases and their volume fractions, while Electron Backscatter Diffraction (EBSD) provided quantitative microscopy, and X-ray analysis examined crystallographic texture. Although all steels exhibited similar microstructure phases, the effective grain size and morphology varied slightly across the thickness. As these variations were minor, the focus shifted to other microstructural features such as textural characteristics. Overall, the steel with the medium R/F reduction demonstrated improved DWTT performance and greater hydrogen cracking and blistering resistance. This was attributed to stronger Transformed Brass (TBr) and Transformed Copper (TC) components, weaker Rotated-Cube (RC) texture, and lower Kernel Average Misorientation (KAM) values. Across the three steels in this work, this study demonstrates that increased fraction of blocky austenite/martensite as secondary phases, high geometrically necessary dislocation (GND) density, and RC texture negatively affect both DWTT and hydrogen damage resistance, whereas gamma (γ)-fiber and {332}<113> textures have positive effects. Improving these metallurgical factors can therefore boost toughness and reduce hydrogen-induced damage in line-pipe steels.