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...
Thermochemically treated titanium grade 2 and 5 were investigated by light optical microscopy and hardness indentation. Gaseous oxidation in oxygen and N2O containing atmospheres resulted in a diffusion zone of oxygen in solid solution in titanium with a hardness up to 1000HV. A surface scale consisting of oxide can be present depending on the treatment conditions. A new type of carbo-oxidation treatment was applied, where carbon and oxygen are simultaneously incorporated into the surface. This resulted in new microstructural features such as a deep zone of mixed interstitial solid solution, i.e. a diffusion zone, and surface regions consisting of a mixed interstitial compound (TiCXO1-X structure). Carbo-oxidation yields hardness values in excess of 2500 HV in the mixed interstitial compound and values up to 1500 HV in the diffusion zone. Simultaneously, with the surface hardening treatment, core hardening of the material can be obtained.
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