Strengthening mechanism of 16Cr-5Ni metastable austenitic stainless steel: martensitic transformation and static strain aging

Abstract

The expansion of stainless steel application in electronic devices raised additional requirements for the materials to be satisfied. Not only the surface quality and corrosion resistance, enhanced strength and elasticity are needed for the stainless steels to be used as various components in the devices. Previous researches have accumulated comprehension in martensitic transformation kinetics and evolution of mechanical properties of the steel during the transformation. However, complex microstructure of cold rolled metastable austenitic stainless steel with mixture of retained austenite and martensite is still very difficult to analyze with existing methodology and extend it to understand mechanical properties. Further understanding of the strengthening mechanism of martensite associated with dislocation density and microstructure change is needed. In addition, when additional heat treatment process is introduced, change of microstructure such as formation of precipitates, movement of interstitials must be carefully analyzed with proper techniques. In this study, comprehensive strengthening mechanism of 16Cr-5Ni based metastable austenitic stainless steel was investigated by analyzing the deformation-induced martensitic transformation and static strain aging behavior.

A constitutive equation was derived by analyzing the YS change as a function of composition and reduction ratio in cold rolled metastable austenitic stainless steels. In addition to the as rolled state mechanical properties, aging phenomenon and resulting change of tensile properties were investigated. The strain aging behaviour in steels is defined as the time and temperature dependent diffusion of interstitial C or N to dislocation sites, contributing to the local pinning of dislocation which results in an increase of strength and hardness. By categorizing the factors affecting the aging phenomenon into external condition, i.e. aging temperature and time, and internal condition, i.e. alloying composition and martensite fraction, the static strain aging behaviour was analysed.

In order to investigate the mechanism of the static strain aging behavior of cold-rolled 16Cr-5Ni metastable stainless steel was investigated when the steel is aged at 400 °C for 30 min. In uniaxial tensile tests of 20% cold rolled specimens, increase of yield strength to approximately 300 MPa and decrease of strain hardening rate were observed in the aged specimen as compared to the as-rolled specimen. In the interrupted tensile tests, delayed martensitic transformation was observed in the aged specimen. Volume shrinkage during the 30 min holding period at 400 °C was observed in dilatometry analysis indicating C partitioning from α’ martensite to austenite. The clustering of C in α’ martensite phases was confirmed by impulse internal friction technique with observation of Cottrell atmosphere formation. 3D atom probe tomography analysis revealed the partitioning of C atoms in austenite phase and clustering of C atoms in α’ martensite phase. The redistribution of C atoms during the aging treatment resulted in the remarkable increase of yield strength.