In the past decades, high nitrogen steels (HNS) have been regarded as substitutes for conventional austenitic stainless steels because of their superior mechanical and corrosion properties. However, the main limitation to their wider application is their expensive production process. As an alternative, high-temperature solution nitriding has been applied to produce HNS from three commercially available stainless steel grades (AISI 304L, AISI 316, and EN 1.4369). The nitrogen content in each steel alloy is varied and its influence on the mechanical properties and the stability of the austenite investigated. Both hardness and yield stress increase and the alloys remain ductile. In addition, strain-induced transformation of austenite to martensite is suppressed, which is beneficial for subsequent low-temperature nitriding of the surface of deformed alloys. The combination of high-and low-temperature nitriding results in improved properties of both bulk and surface.

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Thermo-reactive deposition and diffusion (TRD) and boriding are thermochemical processes that result in very high surface hardness by conversion of the surface into carbides/nitrides and borides, respectively. These treatments offer significant advantages in terms of hardness, adhesion, tribooxidation and high wear resistance compared to other conventional surface hardening treatments. In this work 4 different materials, ARMCO, AISI 409, Uddeholm ARNE® (AISI O1 equivalent) and VANADIS® 6 PM steel representing different classes of alloys, i.e. pure iron, stainless steel and tool steels, were subjected to TRD (chromizing & titanizing) and boriding treatments. For the steels with low carbon content chromizing results in surface alloying with chromium, i.e. formation of a (soft) 'stainless' surface zone. Steels containing higher levels of carbon form chromium carbide (viz. Cr23C6, Cr7C3) layers with hardnesses up to 1800HV. Titanizing of ARNE tool steel results in a surface layer consisting of TiC with a hardness of approximately 4000 HV. Duplex treatments, where boriding is combined with subsequent (TRD) titanizing, result in formation of hard TiB2 on top of a thick layer of Fe-based borides. The obtained surface layers were characterized with X-ray diffraction, scanning electron microscopy, reflected light microscopy and micro-indentation. Keywords:Boriding; Thermo-reactive deposition and diffusion (TRD); Chromizing; Titanizing; TiB2 1.IntroductionThermochemical surface engineering of metallic materials is widely applied for applications where components are exposed to (excessive) wear or where other surface characteristics are desired, e.g. fatigue and corrosion resistance [1]. Conventional and commonly applied methods include carburizing, carbo-nitriding and ferritic nitriding/nitrocarburizing in many different commercial variants. For applications where high hardness is needed, e.g. where excessive wear is encountered, boriding and thermo-reactive diffusion processes can be applied.Boriding is a process where boron is diffused into a material resulting in formation of a compound layer consisting of hard borides of the elements present in the treated substrate. When the process is applied on steels, iron borides are formed with a hardness of more than 1600 HV [2,3,4]. The borides formed are typically FeB and Fe2B as well as borides of the alloying elements, e.g. Cr andMo [2,3]. For tribological applications a single layer of Fe2B is usually preferred since it is less brittle than FeB [3,5]. Different process media are possible, including powder pack, salt bath and plasma.The Thermo Reactive Deposition and Diffusion (TRD) process was originally developed by Arai [6,7] and is also known as the Toyota diffusion process. The substrate to be treated is typically immersed in a molten (borax) salt bath containing strong carbide or nitride forming elements. The carbide/nitride forming element (e.g. V, Nb, Cr etc.) reacts with the carbon or nitrogen in the base material to form a thin reaction layer of (hard) ca...

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This article addresses an investigation of the influence of plastic deformation on low-temperature surface hardening by gaseous nitriding of two commercial stainless steels: EN 1.4369 and AISI 304. The materials were plastically deformed to several levels of equivalent strain by conventional tensile straining, plane strain compression, and shear. Gaseous nitriding of the strained material was performed in ammonia gas at atmospheric pressure at various temperatures. Microstructural characterization of the as-deformed state and the nitrided case produced included X-ray diffraction analysis, reflected-light microscopy, and microhardness testing. The results demonstrate that a case of expanded austenite develops and that the presence of plastic deformation has a significant influence on the morphology of the nitrided case. The presence of strain-induced martensite favors the formation of CrN, while a high dislocation density in a fully austenitic structure does not lead to such premature nucleation of CrN.