Critical role of microstructure-driven hydrogen distribution in stress corrosion cracking behavior of medium Mn steels

Stress corrosion cracking (SCC) poses a tremendous threat to the service safety of high-strength steels. For duplex medium Mn steels, the SCC behavior becomes more complex due to the combined effects of strain-induced martensitic transformation, micro-galvanic effect, and the significant contrast in hydrogen solubility and diffusivity characteristics between ferrite and austenite. Here, we investigate two-type medium Mn steel specimens with markedly different phase characteristics: one containing 32.0% austenite and the other containing 57.5% austenite. Intriguingly, results indicate that the lower-austenite specimen exhibits only 9.1% SCC susceptibility, whereas the higher-austenite specimen, despite processing stronger corrosion resistance, demonstrates a severe SCC susceptibility of 67.1%. The inhomogeneous distribution of hydrogen in ferrite and austenite induces the distinct SCC behavior. Due to the fundamental difference in phase fractions and their percolation, all hydrogen resides in ferrite in the lower-austenite specimen, whereas in the higher-austenite specimen, the hydrogen is trapped in both ferrite and austenite. The dominant role of hydrogen in ferrite lies in its local plastic enhancement effect, and that in austenite consists in its decohesion effect, where the fresh martensite inherited with hydrogen is detrimental to hydrogen resistance. The SCC mechanism of medium Mn steel is primarily governed by hydrogen embrittlement. This work provides a vital insight for future microstructural design aimed at tailoring hydrogen diffusion and distribution to enhance SCC resistance of medium Mn steels.