Stainless steel, Co-Cr and Ni-Cr alloys have played an important role in many industrial sectors to combat environmental degradation. However, low hardness and poor wear properties have impeded their tribological and tribochemical applications. Conventional thermochemical treatments can be used to significantly harden these passive alloys but at the expense of their corrosion resistance due to precipitation induced depletion of Cr in the matrix. Research in 1980s led to the discovery of a new expanded austenite phase, i.e. so called S-phase with combined improvement in wear and corrosion resistance. Recent research has revealed that S-phase can be formed not only in stainless steels but also in Co-Cr alloys and Ni-Cr alloys. It is the purpose of this paper to critically review the S-phase surface engineering of stainless steels, Co-Cr alloys and Ni-Cr alloys. Particular attention will be paid to the structure, formation conditions, supersaturation, hardening mechanisms and metastability of...

S-phase surface engineering of Fe-Cr, Co-Cr and Ni-Cr alloys

An improvement of the surface properties of austenitic stainless steels and nickel-based alloys is obtained by low temperature plasma assisted nitriding (PAN), plasma assisted carburizing (PAC) and plasma assisted nitrocarburizing (PANC). At low temperature, PAN treatment produces a peculiar phase usually called expanded austenite, S phase, m phase or γ N phase. Replacing nitrogen by hydrocarbon for conducting a low temperature PAC gives a carburized layer containing the carbon expanded austenite phase (γc). These two phases can simultaneously be obtained by combining hydrocarbon and nitrogen in the gas phase through a low temperature PANC. Several fundamental aspects concerning the expanded austenite phases are presented in this paper.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ppap.200930003

Fundamental and Innovations in Plasma Assisted Diffusion of Nitrogen and Carbon in Austenitic Stainless Steels and Related Alloys

Austenitic stainless steels exhibit excellent corrosion resistance but relatively poor wear resistance. Previous investigations have demonstrated that surface treatments consisting of plasma carburizing and plasma nitriding were able to successfully increase the wear resistance of austenitic stainless steels. In this work, the effect of a prior shot peening (SP) process on wear and corrosion resistance of sequentially plasma carburized and plasma nitrided AISI 316L austenitic stainless steel was investigated. Triode plasma carburizing (TPC) and triode plasma nitriding (TPN) were sequentially carried out at two temperatures: 400 °C and 475 °C. SP processing prior to sequential plasma treatments led to a significant increase in the near-surface hardness. The sequential plasma treatment at 475 °C in combination with a SP pre-treatment promoted a further increase in thicknesses of carburized and nitrided layers, leading to a greater hardening depth. The best wear resistance exhibited by austenitic AISI 316L samples subjected to SP and sequential plasma processing at higher temperature could be attributed to their high surface hardness and greater treatment depth. However, this treatment at higher temperature yielded CrN precipitates that impaired the corrosion resistance in aerated 0.5 M H2SO4 aqueous solution. Electrochemical tests also revealed that the use of shoot peening prior to sequential plasma treatments at 475 °C could partially counteract the deleterious reduction in corrosion resistance promoted by chromium nitride precipitation at this higher processing temperature. Although the precipitation of chromium nitrides at higher plasma processing temperatures promoted a reduction in corrosion resistance in such acidic environment, results indicate that austenitic stainless steels modified by shot peening (SP) followed by sequential plasma treatments at high processing temperatures could be potentially used in applications where high wear resistance and moderate corrosion resistance in specific environments are required.

Effect of shot peening and treatment temperature on wear and corrosion resistance of sequentially plasma treated AISI 316L steel

Pulsed plasma nitriding (PPN) treatment is one of the new processes to improve the surface hardness and tribology behavior of austenitic stainless steels. Through low temperature treatment (

Microstructural characterization of pulsed plasma nitrided 316L stainless steel

Low temperature carburising (LTC) was applied to AISI316L austenitic stainless steel and its effect on microstructure and fatigue behaviour was investigated. LTC treatment enhances surface hardness and wear resistance of the steel without reducing its corrosion resistance. Surface hardness up to 1150 Vickers was achieved in the carburised layer, thanks to the formation of the so-called “S-phase”, a carbon-supersaturated austenite phase. The XRD evaluation of treated material verified expanded austenite with no evidence of carbide precipitation. Rotating bending fatigue tests showed that the low temperature carburising treatment enhances the fatigue strength of the 316L steel by 40% with respect to the untreated material due to the high residual stresses present in the treated layer. A major temperature increase was found testing the LTC specimens, with a peak value at the end of the test up to 600 °C. By air cooling the LTC specimens during the tests, a further increase of fatigue strength up to 70% was achieved with respect to the untreated material. Fatigue cracks in the surface-treated specimens always nucleated near the boundary between the carburised case and the core.

Fatigue behaviour of low temperature carburised AISI 316L austenitic stainless steel

Plasma nitriding of an AISI 316L austenitic stainless steel at low (400 °C) and high temperatures (550 °C) was performed under different nitriding gas mixtures. Nitrided surfaces were characterized by XRD using the Rietveld method. Expanded austenite “γN” with a special triclinic (t) crystalline structure was formed during the low-temperature nitriding treatment. Minor volume fractions of Fe3N, Fe4N and Cr2N nitrides were also found. The expanded austenite phase showed a distortion e of the lattice angles due to a very high nitrogen content dissolved in austenite, supersaturating the solid solution and leading to a 10% lattice distortion and to high compressive residual stresses at the surface. After nitriding the specimens at 550 °C the case was composed primarily by a high volume fraction of Fe4N, Cr2N and CrN nitrides, leading to a low distortion of the parent austenitic phase, maintaining the original cubic lattice.

/pdf/on-the-formation-of-expanded-austenite-during-plasma-5dz04mzpd9.pdf

On the formation of expanded austenite during plasma nitriding of an AISI 316L austenitic stainless steel

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On the formation of expanded austenite during plasma nitriding of an AISI 316L austenitic stainless steel