Self-organized Formation of Multilayer Structure in a High Nitrogen Stainless Steel during Solution Treatment
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.42
Self-organized Formation of Multilayer Structure in a High Nitrogen Stainless Steel during Solution Treatment Rui Zhou1, Xuan Wang1, Cheng Liu1, and Derek O. Northwood2 1
College of Mechanical Engineering, Yangzhou University, Yangzhou, Jiangsu, P R China
2
Mechanical, Auto and Materials Engineering, University of Windsor, Windsor, Ontario, Canada
ABSTRACT Compared with traditional stainless steels, high nitrogen stainless steels (HNSS), have been widely used due to their high strength, toughness along with excellent corrosion resistance and low cost, formed by partial replacement of Ni (austenite-forming element) by N. The evolution of the microstructure of a Cr19Mn19Mo2N0.7 stainless steel is investigated after solution treatment at 1010, 1060, 1200 or 1250ºC for 30min. A complex multilayer structure has been found under a negative pressure vacuum. A white ferritic layer at the surface is formed, and a subsurface layer with full austenitic structure and a bulk microstructure comprising of austenite and ferrite are detected. With increasing solution temperature, the surface layer thickness increases. The formation of the multilayer structure is attributed to an outward diffusion, a diffusive retardation and an abnormal accumulation of nitrogen during solution treatment.
INTRODUCTION High nitrogen stainless steel (HNSS) with an austenitic structure stabilized by nitrogen instead of nickel, has been widely applied in many industrial fields, due to its low cost, harmless to human body, a good combination of mechanical properties and high corrosion resistance [1, 2]. Various second phases may be formed within austenite in HNSSs, including δ ferrite, Cr2N, χ phase and σ phase. This is due to the segregation of Cr, Mo, N and Mn in the manufacturing process [3-5]. This can result in a depletion of Cr and N from the austenite, and further degradation of performance [6, 7]. Solution treatment is a well-established valid process to change the grain size [8], control the
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relative phase proportions [9], and retard the precipitation of harmful second phases [10]. The possibilities to improve the mechanical properties of stainless steels through solution treatment have been widely researched and discussed. Ren et al. [11] found the joint shear strengths of both Cr22Ni5Mo3MnSi and Cr30Ni7Mo3MnSi stainless steels decreased when the solution temperature was higher than 1050ºC, both because of the formation of σ phase and the reduced volume fraction of austenite. It was mentioned by Lo et al. [12] that a deleterious erosion effect of CrMnNi austenitic stainless steel was manly associated with higher martensite content, as the temperature was raised from1100 to 1300ºC. It was reported by Wang et al. [13] that the σ phas
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