A research team led by Prof. CHANG Keke from the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS) has solved a long-standing puzzle: why the growth rates of chromizing layers vary dramatically across different stainless steels under identical conditions. The sigma phase, which is typically harmful, actually plays an unexpected beneficial role in accelerating chromizing and opening a new pathway for enhancing surface protection.

These findings were published in Acta Materialia on March 12.

For over a century, chromizing has been a key technique for steel surface protection. It works by forming a chromium-rich surface layer on steel components that improves resistance to corrosion, heat, and wear. It is vital for demanding applications such as deep-sea engineering and next-generation nuclear power.

However, the same chromizing process often yields vastly different layer thicknesses across different steel grades. The underlying micro-mechanism behind this long-standing challenge has remained unclear, hindering precise control and optimization of the process.

Using 316 and 310S stainless steels as model materials, the researchers discovered that under identical chromizing conditions, the layer thickness on 316 rstainless steel was nearly four times that on 310S stainless steel.

Experimental characterization revealed that a continuous, coarse-grained sigma phase developed in the chromized layer of 316 stainless steel, whereas 310S showed no such phase. Conventionally regarded as a harmful and brittle intermetallic compound in stainless steels, the sigma phase plays an unexpected role here; its presence directly accelerates chromizing kinetics.

Thermodynamic calculations and AI-assisted, deep-potential molecular dynamics analyses revealed that molybdenum in 316 stainless steel promotes sigma phase formation at the chromizing temperature, while manganese in 310S stainless steel suppresses it. The coarse-grained sigma phase formed in 316 effectively inhibits the growth of surface chromium carbides, thus promoting chromium ingress.

A subsequent nitriding treatment completely transforms the sigma phase into protective CrxN, eliminating its potential embrittling effects and enhancing chromizing layer growth.

Leveraging these thermokinetic insights, the researchers proposed a novel strategy: intentionally inducing the sigma phase during chromizing through targeted elemental screening to accelerate layer growth.

This work introduces a paradigm-shifting concept of turning the conventionally "harmful" sigma phase into a promoter for superior surface engineering. This new approach offers a powerful way to improve the durability and longevity of critical mechanical components in harsh environments.

This study was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Zhejiang Province, the "Innovation Yongjiang 2035" Key R&D Program, and the CAS PIFI program.

Full-chain research strategy: revealing thermokinetics of sigma phase formation for precision chromizing control (Image by NIMTE)