Hard Surface Layers by Pack Boriding and Gaseous Thermo-Reactive Deposition and Diffusion Treatments

Abstract

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...

Surface characterization of a modified cold work tool steel treated by powder‐pack boronizing

“…In this zone, since carbon does not dissolve in the boride layer during boron diffusion, it will be pushed ahead of the boride layer. As a result, it will react with iron to form cementite (Fe 3 C), iron (Fe 3 (B, C))/(Fe 23 (CB) 6 ), as well as a solid solution of (Fe, Cr, Mn)B as a distinct region between the boride layer and the matrix [20]. This is because boron having a high solubility in Fe 3 C can replace carbon to form borocementite.…”

“… The FeB phase has not been formed for this type of steel, as was the case in other studies carried out on equivalent steels. This fact is probably due to the high carbon content in the steel [16, 20]. No peak relating to the oxide phases was revealed in the x‐ray diffraction patterns because oxygen parameter was not incorporated as an input upon the x‐ray analysis. However, this does not necessarily imply the non‐formation of oxide phases in the material. The diffraction peak positions of the iron phase of the samples treated at the lowest temperature (800 °C): at about 2 θ = 42.50° (111), 49.59° (200) and 73.39° (220), Appendix A, are shifted from those of samples treated at the two highest temperatures: at about 2 θ = 43.90° (111), 50.99° (200) and 74.79° (220) (the current issue of the powder diffraction file PDF, ICDD: card n° 01–076‐4446).…”

“…The FeB phase has not been formed for this type of steel, as was the case in other studies carried out on equivalent steels. This fact is probably due to the high carbon content in the steel [16, 20].…”

“…In particular, no attempt was made to survey corrosion resistance of cold work tool steels. To our knowledge, only two important works in this direction have been undertaken; one on AISI O1 type steel and the other on Uddeholm ARNE® steel (AISI 02 equivalent) [16, 20]. The main objective of the first work was to model the kinetics of the boride layer formation, while that of the second work was to compare four different types of steels combining the boronizing treatment with that of the boro‐titanization treatment.…”

Microstructure and Wear Behavior of Tungsten Hot-Work Steel after Boriding and Boroaluminizing

“…The Fe 2 B layer can be obtained using low-temperature boriding, ultra-fast boriding in molten electrolyte, direct and alternating current field, concentrated energy flows, etc. [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Once the brittleness is reduced, boriding acquires potential to be utilized in dynamically loaded machine parts, for instance, to increase surface durability of dies made of hot-work tool steels, which are widely used in forging, die-casting, bending, among others.…”