Assessment of Deep Cryogenic Heat-Treatment Impact on the Microstructure and Surface Chemistry of Austenitic Stainless Steel

This systematic study deals with the influence of deep cryogenic treatment (DCT) on microstructure and surface properties of austenitic stainless steel AISI 304 L on different length scales and in the surface region. The study incorporates different analysis techniques, such as light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ions mass spectrometry (ToF-SIMS). DCT modifies the microstructure of treated samples through promoted precipitation of Cr7C3 carbides, induced twinning and alpha-martensite formation. Additionally, XPS/AR-XPS and ToF-SIMS results also provide evidence of modified oxidation dynamics of DCT samples compared to conventionally heat-treated samples with increase of the Fe-oxide fraction and lower Cr-oxide fraction in the surface oxide layer. An evaluation of oxidation states and ions distribution within the surface layer of deep cryogenically heat-treated stainless steel AISI 304 L is conducted with XPS/ToF-SIMS. These results are correlated with the microstructural changes and nitrogen diffusivity induced by DCT, which are associated with modified oxidation behaviour of AISI 304 L. These results provide further understanding of DCT dynamic on the overall microstructure and the corresponding surface behaviour.