Low temperature nitriding of strain induced martensite and laser quenched austenite

The deformation behaviour and microstructure characteristics of 304L steel during bar extrusion at different strain after deep cryogenic treatment (DCT) and after low temperature plasma nitriding    were investigated. The relative volume fractions of transformed martensite at different stages of the deformation process were assessed using Ferritescope MP-30. It is found that at extrusion at the liquid nitrogen temperature the relative volume fraction of martensite increases with increasing strain. According to the enhance of magnetic and mechanical properties after low temperature nitriding in combination with isothermal treatment at 400°C, it is assumed that both, e-martensite and a’-martensite are present in deformation microstructure, indicating the simultaneous stress-induced-transformation and strain-induced-transformation of austenite. The effects of the laser surface treatment and local appearance of non-magnetic phase due to a’→ g  transformation after laser surface treatment are investigated. Analyses with Vickers hardness, light microscopy, SEM/EDS/EBSD of cross-sections of bars with laser marked, after low temperature plasma nitriding in combination with isothermal treatment were performed. EDS maps show high concentration of nitrogen in both alternating hard surface layers g_N and α’_N (expanded austenite and expanded martensite), but no significant higher concentration of chromium or iron. The results allow concluding that the interaction of deep-cryogenic treatment, plastic deformation, phase transitions, and low temperature pulse plasma nitriding in combination with isothermal treatment as well as laser surface quenching are promising area as they enable to explore new structures and find new applications. In addition, it provides a route to develop materials with exceptional combinations of magnetic properties, strength, ductility, as well as wear and corrosion resistance.