Prof. CHANG Keke’s team at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS) has solved a long-standing puzzle: why chromizing layer growth rates vary dramatically across different stainless steels under identical conditions. The typically harmful sigma phase actually plays an unexpected beneficial role in accelerating chromizing, opening a new pathway for enhancing surface protection.
The findings were published in Acta Materialia.
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 like 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, researchers at NIMTE discovered that under identical chromizing conditions, the layer thickness on 316 reached nearly four times that on 310S.
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 takes on an unexpected role here: its presence directly accelerates chromizing kinetics.
Thermodynamic calculations and AI-assisted deep potential molecular dynamics analyses revealed the mechanism: molybdenum in 316 promotes the sigma phase formation at the chromizing temperature, while manganese in 310S suppresses its formation. The coarse-grained sigma phase formed in 316 effectively inhibits the growth of surface chromium carbides, thus boosting chromium ingress.
Crucially, a subsequent nitriding treatment completely transforms the sigma phase into protective CrxN, eliminating its potential embrittling effects while enhancing the growth of chromizing layers.
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: turning the conventionally "harmful" sigma phase into a promoter for superior surface engineering. This new approach offers a powerful pathway to enhance the durability and longevity of critical mechanical components in harsh environments.
The study was supported by the National Natural Science Foundation of China (Nos. U23A6016 and 52471104), the Natural Science Foundation of Zhejiang Province (No. LD24E010002), the "Innovation Yongjiang 2035" Key R&D Program (Nos. 2024Z095 and 2024Z138), and the CAS PIFI program.
Full-chain research strategy: revealing thermokinetics of sigma phase formation for precision chromizing control (Image by NIMTE)
