Effect of Nitrogen Content on Grain Refinement and Mechanical Properties of a Reversion-Treated Ni-Free 18Cr-12Mn Austenitic Stainless Steel

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 45; no. 13; pp. 6317 - 6328
• Main Authors: Behjati, P., Kermanpur, A., Najafizadeh, A., Samaei Baghbadorani, H., Karjalainen, L. P., Jung, J. -G., Lee, Y. -K.
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.12.2014; Springer; Springer Nature B.V
• Subjects: Annealing; Applied sciences; Austenitic stainless steels; Characterization and Evaluation of Materials; Chemistry and Materials Science; Cold; Cold rolling; Ductility; Engineering; Exact sciences and technology; Grain size; Materials Science; Mechanical properties; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metallic Materials; Metals. Metallurgy; Microstructure; Nanotechnology; Nitrogen; Reversion; Scanning electron microscopy; Stainless steel; Strain hardening; Structural Materials; Surfaces and Interfaces; Tensile strength; Thin Films; Yield strength; Yield stress; Iran; Austenitic Stainless Steel; Cold Rolled; Martensite; Size Refinement; Austenite; Grain size; Austenitic stainless steel; Austenitic steel; Stainless steel; Mechanical properties; Composition effect; Nitrogen; Microstructure; Grain refinement
• DOI: 10.1007/s11661-014-2595-4
• ISSN: 1073-5623

Martensite reversion treatment was utilized to obtain ultrafine grain size in Fe-18Cr-12Mn-N stainless steels containing 0 to 0.44 wt pct N. This was achieved by cold rolling to 80 pct reduction followed by reversion annealing at temperatures between 973 K and 1173 K (700 °C and 900 °C) for 1 to 10 4  seconds. The microstructural evolution was characterized using both transmission and scanning electron microscopes, and mechanical properties were evaluated using hardness and tensile tests. The steel without nitrogen had a duplex ferritic-austenitic structure and the grain size refinement remained inefficient. The finest austenitic microstructure was achieved in the steels with 0.25 and 0.36 wt pct N following annealing at 1173 K (900 °C) for 100 seconds, resulting in average grain sizes of about 0.240 ± 0.117 and 0.217 ± 0.73  µ m, respectively. Nano-size Cr 2 N precipitates observed in the microstructure were responsible for retarding the grain growth. The reversion mechanism was found to be diffusion controlled in the N-free steel and shear controlled in the N-containing steels. Due to a low fraction of strain-induced martensite in cold rolled condition, the 0.44 wt pct N steel displayed relatively non-uniform, micron-scale grain structure after the same reversion treatment, but it still exhibited superior mechanical properties with a yield strength of 1324 MPa, tensile strength of 1467 MPa, and total elongation of 17 pct. While the high yield strength can be attributed to strengthening by nitrogen alloying, dislocation hardening, and slight grain refinement, the moderate strain-induced martensitic transformation taking place during tensile straining was responsible for enhancement in tensile strength and elongation.