Effects of carbon and nitrogen on austenite stability and deformation behavior of 15Cr 15Mn-4Ni based austenitic stainless steels

Abstract

Austenitic stainless steels (ASSs) which are known to have excellent corrosion resistance, toughness, and formability have been widely used. In general, Ni is included to stabilize the austenite. However, since Ni is an expensive element, the price of austenitic stainless steels is governed by the Ni content. In recent years, Mn and N have been incorporated to replace Ni as austenite stabilizers due to their cheaper price. Apart from Ni, Mn and N, C is also a well-known austenite stabilizer and can be considered as another candidate to replace Ni in ASSs. However, the addition of C and N to ASSs largely influences the properties and the effects can be vastly different between C and N. In order to understand the roles of C and N in ASSs, the austenite stability and deformation behavior are studied in the 15Cr-15Mn-4Ni based ASSs containing different concentrations of C and N. The results from thermodynamic calculation and the stacking fault energy (SFE) estimated from XRD spectra show that the thermal and mechanical stabilities of austenite were enhanced with increasing C and N contents. The tensile test data at room temperature displays that both C and N linearly increased the yield strength and N is a more effective element. The strain hardening of C-containing alloys is related to the fraction of α´-martensite formed during deformation which was changed with increasing C content. On the other hand, increasing N content does not influence strain hardening. The microstructure observation results suggest that the difference between the strain hardening of C and N containing alloys is attributed to the occurrence of planar slip in N-containing alloys at low strain, while the inability to recover of SRO during deformation of N-containing alloys and the larger increase of SFE by N addition than C govern the difference at high strain. Further details on austenite stability and deformation behavior at various deformation temperatures and strain rates are also provided in this study. The mechanical stability of austenite is shown to increase with increasing temperature. Moreover, the increase of yield strength with decreasing temperature in N-containing alloys is two times of the C-containing one which is attributed to the existence of SRO, higher binding energy of N with dislocation than C and, the splitting of dislocation core by N atoms. The transformation of α´-martensite during was also observed to mainly connect to the ∆G γ → α´ value not the SFE. With increasing strain rate, the mechanical properties were deteriorated and the tensile behavior was split into two for interstitial containing alloys. The reason is attributed to the occurrence of adiabatic heating which caused the increase of SFE resulting in the enhancement of mechanical stability of austenite. Interestingly, when deforming the alloys containing similar content of C or N with initial strain rate of 10-5 s-1, the alloy containing N showed lower mechanical stability than the one containing C. The estimated SFEs imply that the behavior is attributed to the higher sensitivity of SFE with increasing temperature in N-containing alloys. The results from present work suggest that the addition of C and N in 15Cr 15Mn-4Ni based ASS affects the stability of austenite and deformation behavior differently. The reasons are mainly attributed to the existence of SRO in N-containing alloys and their effects on SFE of austenite. Furthermore, the additional experiment results show that the effects of C and N on the segregation behavior, grain growth behavior, Hall-Petch relationship, phase hardness and, internal friction were considerably different. Therefore, this study provides a basic understanding on the effects of C and N on deformation behavior and austenite stability in ASSs however, further studies on other properties are needed in order to clarify the overall effects of both interstitial elements.