# Diffusion and Critical Hydrogen Content of Carbon Steels with Different Strengths and Microstructures

## TL;DR

This study examines how different carbon steels resist hydrogen embrittlement based on their strength and microstructure.

## Contribution

The study identifies the critical hydrogen content for various carbon steels and highlights the superior resistance of bainitic steels.

## Key findings

- Bainitic steel S1100M has higher resistance to hydrogen embrittlement than lower-strength martensitic steel S960M.
- Higher-strength steels generally have lower resistance to hydrogen embrittlement.
- Critical hydrogen content varies significantly among the tested steels.

## Abstract

Four thermomechanically rolled carbon steels with different strength levels and microstructures, namely S355M, X65M, S960M, and S1100M, were selected, and their critical hydrogen contents Hcrit were determined. Hcrit is the hydrogen content where brittle fracture of the steel can occur. The Hcrit values for the four carbon steels are 2.03 (S355M), 0.91 (X65M), 0.32 (S960M), and 0.53 ppm (S1100M). Bainitic carbon steel S1100M outperforms lower-strength tempered martensitic steel S960M in terms of resistance to hydrogen embrittlement. All other steels follow the trend that a higher strength of steel results in a lower resistance to hydrogen embrittlement. The root cause for the beneficial behavior of bainitic steels is discussed.