Wear resistance improvement of austenitic 316L steel by laser alloying with boron M. Kulka ⁎, D. Mikolajczak, N. Makuch, P. Dziarski, A. Miklaszewski Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M.Sklodowskiej-Curie 5, 60-965 Poznan, Poland abstract article info Article history: Received 2 October 2015 Revised 22 February 2016 Accepted in revised form 25 February 2016 Available online 27 February 2016 Austenitic 316L steel was known for its good resistance to corrosion and oxidation. Therefore, this material was often used wherever corrosive media or high temperatures were to be expected. However, under conditions of appreciable mechanical wear, this steel had to be characterized by suitable wear protection. In this study, laser boriding was used in order to improve the wear behavior of this material. The composite boride layer consisted of hard ceramic phases (borides and boro-carbides) and a soft austenitic matrix. The significant increase in wear resistance of laser-borided layer was observed in comparison with the untreated 316L steel. The predominant abrasive wear was accompanied by adhesive and oxidative wear evidenced by shallow grooves, adhesion craters and the presence of oxides. © 2016 Elsevier B.V. All rights reserved. Keywords: Laser-boriding 316L steel Composite surface layer Microstructure Hardness Resistance to wear 1. Introduction AISI 316L austenitic stainless steel is well-known for its most effec- tive balance of carbon, chromium, nickel and molybdenum for corrosion resistance. Therefore, this material is often used for high temperature conditions, aggressively corrosive environment and nuclear reactor applications. However, a relatively low hardness (200 HV), resulting in the poor wear resistance, is an important disadvantage of this steel, that is the reason for its limited use. Under conditions of appreciable mechanical wear (adhesive or abrasive), this material should be charac- terized by suitable wear protection. An austenitic structure, which cannot be hardened by the conventional heat treatment, causes that there is no easy way to improve the wear resistance of this steel [1]. Processes, usually used for wear protecting the constructional or tool steels, such as nitriding, carburizing or boriding, were also developed in order to produce the surface layers which could improve the wear behavior of austenitic steel [2–14]. Glow discharge assisted low- temperature nitriding was often studied [2,3]. The process, carried out at 440 °C for 6 h, resulted in the formation of a thin layer (4 μm) consisting of chromium nitrides (CrN) as well as of austenite supersat- urated with nitrogen [2]. The layer, produced at 550 °C (823 K) for 6 h, was characterized by the thickness about 20 μm [3], and iron nitrides (Fe 4 N) were additionally observed in microstructure using the higher process temperature [3,4]. The chromium nitrides Cr 2 N were also identified in the nitrided layer [5]. Low temperature plasma carburizing was a thermochemical treatment designed so as to achieve a good combination of wear and corrosion resistance in stainless steels [6–9]. At the temperature below 520 °C (793 K), the process produced the layer consisting only of the austenite supersaturated with carbon, and characterized by an expanded lattice [6–9] whereas the chromium carbides, expanded austenite and martensite occurred after carburizing at higher temperature [6]. The layers obtained the thickness up to 50 μm. Austenitic steels were also pack-boronized [10–12] in the temperature range of 800–950 °C (1073–1223 K) without sacrificing corrosion resistance. The microstructure consisted of two-phase boride layers (FeB + Fe 2 B). Diffusion process required a relatively high tem- perature and longer duration in comparison to typical boronized con- structional and tool steels. Pack-boronizing of 316L steel at 950 °C (1233 K) for 8 h resulted in the production of a layer with thickness up to 90 μm [10]. Surface mechanical attrition treatment (SMAT) of 304 SS steel, carried out before pack-boriding, caused the increase in the growth kinetics of boride layer [11]. Factors which were responsible for the increase in case depth of borided layers were as follows: the de- crease in grain size, formation of nanocrystalline structure with high free energy state close to the surface, the increased volume fraction of non-equilibrium grain boundaries, and accumulation of high density of dislocations at the grain boundaries as well as within the grains [11]. Even for relatively thin boride layer (up to 25 μm), the corrosion re- sistance of the pack-borided 316L steel was acceptable [12,13]. Hence, this process could be applied for steel implants made of this steel [14]. Titanium nitride (TiN) coatings were also applied in order to im- prove tribological properties of 316L steel [15,16]. They were deposited by physical vapor deposition (PVD) which resulted in producing thin Surface & Coatings Technology 291 (2016) 292–313 ⁎ Corresponding author. E-mail address: michal.kulka@put.poznan.pl (M. Kulka). http://dx.doi.org/10.1016/j.surfcoat.2016.02.058 0257-8972/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat