Preparation and characterization of ultra-refined expanded martensite α′N

Abstract Theoretical and experimental methods are utilized to design and prepare the expanded martensite (α′ N ) on the steel 38CrMoAl by plasma nitriding. Lehrer diagram is firstly calculated by thermodynamic method, based on which the plasma nitriding experiments are conducted to prepare single-phase α′ N layer. Microstructure and mechanical properties for α′ N layer are further characterized by XRD, TEM and nanoindentation methods. Results show that an expected single-phase α′ N layer is obtained by nitriding at 590 °C in atmosphere with the nitrogen-hydrogen flow ratio of 0.05:0.4. Moreover, single-phase α′ N layer is also detected beneath the outermost compound layer about 20 μm thick in 460 °C nitrided layer. The maximal peak offset of 0.5° and broadening for α′ N (110) indicate that the supersaturated N dissolved in refined grains. Ultra-refined grains about ~ 10 nm of α′ N observed by TEM, elucidate the nano-crystalline layer can be obtained by one-step low-temperature plasma nitriding at 460 °C. However, no evident refinement for α′ N is observed in the 590 °C nitrided layer. More importantly, compare with the γ′-Fe 4 N and e-Fe 2 − 3 N, significantly improved H and the ratios of H/E and H 3 /E 2 of α′ N layer demonstrate its superior hardness, strength and toughness.