Enhanced chromizing kinetics of tool steel by means of surface mechanical attrition treatment

Abstract

A nanostructured surface layer of about 20 μm in thickness was fabricated on a hot-working tool steel (AISI H13) plate by means of surface mechanical attrition treatment (SMAT). Chromizing behaviors of the SMAT sample were investigated in comparison with its coarse-grained counterpart. Relative to the coarse-grained form, the Cr-diffusion depth was obviously enhanced by SMAT at various chromizing temperatures, with a maximum increment at 600 °C. A duplex chromizing treatment at 600 °C followed by at a higher temperature led to the formation of a much thicker chromized surface layer (up to 30 μm in thickness) on the SMAT sample. This might be attributed to the formation of a large amount of nano-sized chromium compounds in the surface layer at 600 °C, which retarded growth of nano-sized ferrite grains and improved the Cr diffusion at higher temperatures. Gradient distributions of Cr concentration, microstructure and hardness were formed in the chromized surface layer in the SMAT steel after the duplex chromizing treatment, leading to an obvious enhancement in the wear resistance.

Introduction

Hot-working tool steels are widely used in the metal-working industry for applications such as metal die casting, hot extrusion and hot forging. In such systems, they are subjected to high loads and high temperatures, which cause heavy damages of tool surfaces because of wear, plastic deformation and cracking [1], [2]. Due to the enormous quantities of products and the relatively short service life of tools, even small improvements in their durability would bring a large economic benefit. In the past two decades, various surface modification processes have been developed to improve their surface mechanical and tribological properties, for example gas and plasma nitriding [1], [3], [4], electroplating hard chromium [5], physical and chemical vapour deposition (PVD and CVD) of hard ceramic coating [5], [6] and duplex treatments [2], [6], [7], [8].

Among these processes, duplex treatments were emerged as novel approaches to produce the most effective coatings. This is mostly because they combine the hardening of the substrate by plasma nitriding with the subsequent PVD or CVD of a very hard coating (i.e. CrN or TiN, with a hardness of ∼20 GPa [5], [9]), reducing the hardness gradient between the coating and the substrate. As a result, the mechanical and tribological properties of the tools are improved and their durability is increased significantly relative to those treated by using the traditional processes, such as the most commonly used gas nitriding (with a surface hardness of ∼10 GPa [1]). However, duplex treatments usually require the use of rather complicated processing routes and expensive apparatus for plasma nitriding and PVD or CVD, as well as more downtime. These disadvantages hinder their wide applications.

The surface thermo-chemical treatments (STCT), such as chromizing and niobizing, have also been developed to form wear resistant coatings on tool steels by using simple equipments at relatively low costs [10], [11], [12]. In STCT, the compound surface layer is mainly formed by reaction between the carbide-former (such as Cr and Nb) deposited on the surface and carbon in the substrate. It shows similar properties to those produced by PVD and CVD. However, the formed coating by a traditional STCT is usually very thin, and with a distinct interface (or with a large microstructure/hardness gradient) between the coating and the substrate [11]. These are resulted from the limited diffusion kinetics of the carbide-former atoms at the treating temperature.

The combination of a recently developed technique, surface mechanical attrition treatment (SMAT) [13], [14], with the conventional STCT, may provide us an alternative approach to economically improve the surface properties of hot-working tool steels. SMAT enables to substantially refine grains in the surface layer of various metals into the nanometer scale via repeated plastic deformation [15], [16], [17], [18], [19]. Some flexible and low-cost surface plastic deformation processes have been employed to perform SMAT, such as high-velocity balls impacting [13], [14], [15], [16], [17], [18], [19] and mechanical grinding [20]. Due to the significantly enhanced diffusion and chemical reaction kinetics in the formed nanostructured surface layer by SMAT, a surface layer with gradient microstructure and mechanical properties, from the very hard cemented coating to the hardened diffusion layer with disperse compounds and to the substrate, has been fabricated on several ferrous alloys after the subsequent gas nitriding [21], [22] or packed powder chromizing [23], [24]. The hardened surface layer bonds strongly to the substrate because there is no distinct interface. In addition, temperatures and/or durations of the employed STCT processes have been decreased evidently relative to those of the conventional ones without SMAT.

In this study, a commercial hot-working tool steel plate was subjected to SMAT. The chromizing behavior in the nanostructured surface layer formed by means of SMAT was studied. Accordingly, the chromizing process of the SMAT sample was optimized for achieving enhanced mechanical properties of the chromized surface layer.