Combinating surface treatments improves surface/subsurface properties by taking advantage of the best attributes of each technique compared with those of the individual method by itself.

Laboratory-scale fluid-bed reactor installed at Deakin University

The process using a combination of two surface engineering techniques is commonly referred to as duplex surface engineering. An example is the thermochemical surface treatment of a steel substrate prior to the deposition of a hard surface coating. Thermochemical treatments such as nitriding and carburizing inherently form a graded hardness from the surface to the substrate, which creates a gradual transition in properties across the coating-substrate interface and provides a tough and supportive subsurface for the hard coating [1, 5]. There are numerous examples in the literature of the benefit of nitriding, nitrocarburizing or carburizing steels prior to the formation of a hard layer by physical or chemical vapor deposition [1, 5-14].

Fig 1 Schematic representation of the fluid-bed reactor used to develop the DST-Cr process

Another aim of duplex surface engineering is combining surface engineering techniques to form particular surface microstructures and compounds; for example, enriching a steel surface with chromium using either a thermochemical treatment [15] or an electroplating process [16] prior to the diffusion of nitrogen by a plasma nitriding process. These duplex processes form a chromium-nitride surface layer having a higher hardness and improved wear and corrosion resistance compared with the untreated substrate and the chromium and nitriding surface treatments alone [15, 16].

Fig 2 Deposition of chromium and the direction of nitrogen diffusion during TRD

The aim of the present work was to develop a new duplex surface engineering process involving a ferritic nitrocarburizing pretreatment of hardened and tempered chromium hot-work tool steel substrates (AISI H13) and a low-temperature (575°C, or ~1070°F) chromium thermo-reactive deposition (TRD) surface treatment of the nitrocarburized surface. This duplex process is designed to form a hard chromium carbonitride surface layer by a diffusion alloying reaction between the nitrogen from the nitrocarburized surface and the deposited chromium metal. By processing at a low temperature, the hardness of the core material is retained and distortion of the treated part is minimized. The developed Duplex Surface Treatment (DST) is proprietary to HRD Technologies and is recognized by the trade name DST-Cr.

Fig 3 Comparison of nitrocarburized microstructure (a) and duplex treated AISI H13 (b) microstructures (500X) after 8 hours of chromium TRD treatment at 575C; microstructure of DST-Cr (c) at higher magnification (1,000X); 2% nital etch

Fluidized-bed processing

The DST-Cr process was performed in an industrial-scale fluidized-bed reactor (Fig. 1), which is a vessel containing a bed of dry, finely divided ceramic and metallic particles (1). A gas flow introduced though a diffusion plate (2) at the bottom of the bed results in each individual particle becoming microscopically separated [17]. This imparts fluid-like properties to the bed, such as high heat-transfer rates and uniform mixing of species in the bed, which are desirable properties for a surface engineering process. By processing in a bubbling or aggregative fluidization state (three times the minimum fluidization velocity), the constituents of the bed are thoroughly mixed and the reaction between the gases and bed material is maximized [18, 19]. The gases used in this study were ammonia (NH3), carbon dioxide (CO2), nitrogen (N2), and hydrochloric acid (HCl).

A layer of coarse grit alumina (3) is used to separate the fine bed material from the diffusion plate. This also allows the atmosphere to mix and reach the processing temperature before entering the reaction zone. The bed was heated by external resistance that consisted of ceramic elements (4) in an insulating refractory housing (5), and the processing temperature was thermostat controlled and monitored externally by a calibrated N-type thermocouple (6). The samples (7) were fixed to a centrally located jig, which was attached to the furnace lid. The output gas was extracted, treated by a scrubber system (8) and burnt by a pilot flame.

Fig 4 Quantitative depth profile of a DST-Cr treated AISI H13 surface

The DST-Cr Process

The nitrocarburizing pretreatment enriches the steel surface with nitrogen and forms a compound layer above a diffusion zone (Fig. 3a). During the second-stage TRD process, chromium is deposited on the nitrocarburized surface by the reaction between chromium powder in the bed and hydrochloric acid added to the fluidizing flow (Fig. 2). This results in the diffusion of nitrogen from the compound layer and diffusion zone toward the surface and a diffusion alloying reaction occurs between the nitrogen and the chromium to form a distinct surface layer.

A 2.5-um thick surface layer (see Fig. 3b and c) is formed typically after 8 hours of chromium TRD treatment at 575 C. The original compound layer has decomposed due to the outward diffusion of nitrogen. Additionally, the diffusion zone increased in depth by approximately 40 um due to the inward diffusion of nitrogen (compare Fig. 3a and b). The original tempered martensite core structure is retained.

Quantitative depth profiling performed by glow discharge optical emission spectrometry (GD-OES) (Fig. 4) reveals that the surface layer is rich in chromium and nitrogen, with smaller quantities of carbon, iron and oxygen present. X-ray diffraction profiling shows that the major phase of the surface layer is chromium nitride (CrN) (Fig. 5). The iron peaks are due to the penetration of the X-rays through to the underlying substrate. Due to the presence of carbon, the surface layer is referred to as chromium carbonitride.

Fig 5 X-ray diffraction profile of the surface layer

The layer hardness is 1,520 +/-43 HV using nanoindentation with a load of 10 uN, and the hardness profile directly beneath the surface layer to the core was measured using micro-Vickers with a 25 g load (Fig. 6). The retention of the nitrocarburized diffusion zone hardness after the chromium TRD process forms a gradual transition in hardness across the layer-substrate interface, which should provide a supportive subsurface for the hard layer and result in good layer adhesion. Additionally, the core hardness of the hardened tool steel is maintained after the duplex process.

Current research into the DST-Cr process aims to form chromium carbonitride layers of greater thickness and to determine the performance of DST-Cr treated AISI H13 tooling used in aluminum forming operations. A state-of-the-art laboratory scale fluidized-bed furnace has been installed at Deakin University to conduct further research into this and other surface engineering processes for ferrous and nonferrous materials (Fig. 7). The authors would like to acknowledge the assistance of the Australian Research Council (ARC). IH

Surface-to-core hardness profile of DST-Cr treated AISI H13

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