Thermoreactive Deposition / Diffusion Process

The thermoreactive deposition / diffusion process (TRD) is a method of coating steels with a hard, wear-resistant layer of carbides, nitrides, or carbonitrides. In the TRD process, the carbon and nitrogen in the steel substrate diffuse into a deposited layer with a carbide-forming or nitride-forming element such as vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. The diffused carbon or nitrogen reacts with the carbide- and nitride-forming elements in the deposited coating so as to form a dense and metallurgically bonded carbide or nitride coating at the substrate surface.

The TRD process is unlike conventional case-hardening methods, where the specific elements (carbon and nitrogen) in a treating agent diffuse into the substrate for hardening. Unlike conventional diffusion methods, the TRD method also results in an intentional buildup of a coating at the substrate surface. These TRD coatings, which have thicknesses of about 5 to 15 µm (0.2 to 0.6 mil), have applications similar to those of coatings produced by chemical vapor deposition (CVD) or physical vapor deposition (PVD). In comparison, the thickness of typical CVD coatings (usually less than 25 µm, or I mil) has about the same range as TRD coatings.

The hard alloy carbide, nitride, and carbonitride coatings in the TRD method can be applied to steels by means of salt bath processing (Ref 1-4) or fluidized beds (Ref 5). The carbide coating by salt bath immersion was first developed in Japan and used industrially almost 20 years ago under the name of the Toyota Diffusion (TD) coating process (Ref 1, 2). The TD method uses molten borax with additions of carbide-forming elements such as vanadium, niobium, titanium, or chromium, which combine with carbon from the substrate steel to produce alloy carbide layers. Because the growth of the layers is dependent on carbon diffusion, the process requires a relatively high temperature, from 800 to 1250°C (1470 to 2280°F), to maintain adequate coating rates. Carbide coating thicknesses of 4 to 7 µm are produced in 10 min to 8 h, depending on bath temperature and type of steel. The coated steels may be cooled and reheated for hardening, or the bath temperature may be selected to correspond to the steel austenitizing temperature, permitting the steel to be quenched directly after coating.

Salt bath temperatures can also be lowered to the tempering range of steel (Ref 3). In order to lower salt bath deposition temperatures, techniques to produce alloy carbonitride coatings are used. Such coatings are applied to hardened and nitrided steels in vanadium-containing chloride baths at temperatures of 550 to 600°C (1020 to 1110°F). This section deals exclusively with coating at a high temperature.

Coating Procedure and Mechanism of Coating Formation. The high-temperature salt bath TRD process is performed in a molten borax bath at 850 to 1050°C (1560 to 1920°F). Immersion time ranges from 0.5 to 10 h to obtain an optimum carbide layer thickness of 5 to 15 µm (0.2 to 0.6 mil) for most applications. This temperature range is suitable for quench hardening many grades of low-alloy steels, carburized steels, and tool steels.

Before parts are TRD processed they are first preheated to minimize distortion and to lower the TRD processing time. They are then TRD processed at the austenitizing temperature for the particular grade of steel. After TRD processing, the parts are quenched in air, salt, or oil to produce a hardened substrate. After quenching, tempering is carried out. Figure 1 shows a schematic of a typical cycle. High-speed steels and other steels that have austenitizing temperatures greater than 1050°C (1920°F) may be post-TRD heat treated in vacuum, gas, or protective salt to achieve full substrate hardness.

Figure 1. Schematic of typical TRD processing cycle

When substrate materials containing carbon and nitrogen are kept in contact with treating agents at appropriately elevated temperatures, carbon and nitrogen chemically combine with the carbide- and nitride-forming elements of the treating agent due to their small free energies for carbide and nitride formation. This formation of carbides, carbonitrides, and nitrides on the substrate results in the growth of a layer, as shown in Fig 2 for vanadium carbide and chromium carbide coatings. Carbide layers are formed in the following steps:

Reagents Used. The carbide-forming elements (CFE) and the nitride-forming elements (NFE) must be in an active state to combine with carbon and nitrogen. Typical reagents have the CFE and NFE dissolved into molten salt in the salt bath immersion method and those in halide vapor produced through reaction between CFE- and NFE-containing powders and halide at elevated temperatures in the powder-pack and fluidized-bed methods. Therefore, borax with additions of CFE and NFE contained in ferro alloy powder or with oxides of CFE and NFE and their reducing agents, such as boron carbide and aluminum, are successfully used as bath agents. Mixtures of ferro alloy powder containing CFE and NFE and halide powder, such as ammonium chloride, often added with alumina powder, are used in packed boxes (Ref 6 and 7) or in a fluidized bed (Ref 5).

Substrate Materials. Most carbon-containing materials such as steels, cast iron, cobalt alloys, cemented carbides, carbide-metal cermets, carbide ceramics (Ref 8), and carbon may be used as the substrates for carbide coating. Carbon-deficient metals, for example, iron and nickel alloys, can be used after carburization prior to application of the carbide coating. Carbonitride coating is applicable to preliminarily nitrided steel. A nitride coating can be formed on nitride ceramics (Ref 8).

Various tool steels are most frequently used for tooling. Low- or medium-carbon constructural steels are used for machine components. The composition and properties of the coatings are almost independent of the substrate materials. Therefore, inexpensive and easily machinable materials should be used.

Effect of Treating Parameters. The coating growth rate is determined by the number of carbon atoms and nitrogen atoms that can be supplied to the coating from the substrate by diffusion, if the treating reagents can supply CFE and NFE in excess of the critical amount required to combine with the carbon and nitrogen supply from the substrate. Excess amounts of material containing CFE and NFE (for example, more than 10 wt% Fe-V, or 20 wt% V₂O₅ and 5 wt% B₄C in molten borax for vanadium carbide coating, or more than 10 wt% Cr and 1 wt% NH₄CI in a fluidized bed for chromium carbide coating) are usually added to maintain this requirement. Therefore, the coating growth rate is determined by factors that affect only the amount of CFE and NFE required for coating: temperature, time, type of substrate, and type of coating.

As in many diffusion treatments, the effect of temperature and time on coating thickness (d) is expressed by the following equation:

d²/t = K = K₀exp(-Q/RT)                  (Eq1)

where d is the thickness of coating (cm), t is time (s), K is the growth rate constant (cm2/s), Ko is the constant term of K (cm²/s), Q is the activation energy (KJ/mol), T is absolute temperature (K), and R is the gas constant.

Figure 3 shows the relation between the thickness of the vanadium carbide layer formed on W1 steel versus salt bath temperature and immersion time in the molten salt bath immersion method. The temperature is usually selected around the hardening temperature of steels, that is, 800 to 1250°C (1475 to 2285°F).

The carbon and nitrogen content in the substrate has a positive effect on the growth rate. However, the total content in the substrate does not have a direct effect. For example, in steels the carbon content in the austenite matrix, not the total carbon content, is nearly linear in relation to the thickness of the carbide coating. This is shown in Fig 4 for the salt bath immersion process (Ref 1, 2). In the case of alloyed steels, an increase of temperature increases the carbon content in the matrix phase, as well as the diffusion rate of carbon in the carbide layer and in the substrate, resulting in a considerable increase of coating thickness. Figure 5 exemplifies the relation between bath temperature and immersion time needed for producing a 4 µm and 7 µm thick VC coating

on four types of steel. In the case of cemented carbides, not only the carbon content but also the amount of cobalt matrix has a large effect on the thickness. The diffusion rate and its temperature dependence in relation to the carbon and nitrogen content are different between coatings. However, the difference in thickness among vanadium carbide (VC), niobium carbide (NbC), chromium carbide (Cr₇C₃, Cr₂₃C₆), and titanium carbide (TiC) is negligibly small.

Control of Distortion. The possibility of distortion is present with the high-temperature process. Distortion entails dimensional change and deformation. Dimensional change is due to phase transitions in heat treatment of the base steel and to formation of the carbide layer. Deformation is a change in shape.

TRD processing usually hardens a material. Therefore, to minimize dimensional change, it is best to start with a part that has been hardened and finish ground. Even then, there will be some dimensional change due to differences in the amount of retained austenite. Cemented carbide is not hardened in the process, therefore it has very little dimensional change.

The amount of retained austenite before TRD processing should equal the amount after processing. The easiest method of controlling retained austenite is to reduce it to 0% before and after the TRD process. This can be achieved in D2 tool steel by tempering at 520 to 535°C (975 to 1000°F) to decompose the retained austenite. Sub-zero treatment is another method of decomposing retained austenite.

Deformation is caused by thermal stresses, transformation stresses, creep during heating, anisotropy of the substrate structure, and residual stresses. The following are steps that can be taken to minimize deformation:

In making new tooling, it is recommended to leave stock on nonworking surfaces and finish only the working surfaces. The non-working surfaces may then be finished after TRD processing.

General Characteristics. Carbide coatings that are available with the high-temperature salt bath process include vanadium carbide, niobium carbide, and chromium carbide. Vanadium carbide and niobium carbide have high surface hardness and resistance to wear, seizure, and corrosion. Chromium carbide has light wear resistance and high resistance to oxidation. The surface hardness and wear, seizure, corrosion, and oxidation resistance in relation to other surface-hardening processes is shown in Fig 6 to 10.

Figure 6. Surface hardness of carbide layers by TRD process in relation to other surface-hardening processes

The spalling resistance of the carbide layers is very good. Figure 11 shows the spalling resistance of vanadium carbide in relation to other coating processes. In applications with cyclic stresses, the fatigue resistance of steels is often slightly deteriorated by tensile residual stress induced on the base metal, as shown in Fig 12. The residual stress initiates cracks in the base metal. The problem can be solved by proper TRD processing and soaking after TRD treatment to decrease the tensile stress, if necessary. The toughness is usually not affected by the process, as shown in Fig 13.

Tooling Applications. Tool steels that contain 0.3% or greater carbon may be TRD processed. This includes most cold- and hot-work steels, high-speed steels, and some martensitic stainless steels. To achieve full substrate hardness in high-speed steel, it must be rehardened after TRD processing. Cemented carbide is frequently used as the substrate for tooling.

The substrate hardness may be the same or lower than normal in some applications.

In applications where tool chipping or breakage is the problem, a lower substrate hardness with increased toughness can be used. The hard carbide coating provides the surface wear resistance. Under-hardened high-speed steel could be used to provide needed substrate toughness.

In applications with high surface pressures, such as extrude dies and cold-forging dies, the carbide layer has to be supported by a hard substrate. High-speed steels should be post-TRD hardened. Some powdered high-speed steels that contain cobalt can be treated at the maximum TRD processing temperature of 1050°C (1920°F) to give hardnesses of 60 to 65 HRC. The hardest substrate available is cemented carbide, which can be TRD treated very successfully.

Edge preparation of cutting and piercing tools is important. An edge that is too sharp or that contains burrs will break. The cutting edge should be rounded to a radius of 0.05 to 0.25 mm (0.002 to 0.010 in.) with a stone or emery paper. A worn cutting edge may be resharpened. This is not detrimental because performance is governed by the carbide layer on the side surface of the cutting edge.

The surface finish and polishing direction of a forming die prior to TRD processing is very important. Due to the high-hardness carbide layer, a TRD processed tool that has a rough surface finish will perform worse than a regular uncoated tool. This is shown in Fig 14. The surface should be finished to a maximum peak-to-valley roughness height (R_max) of 3 µm (120 µin.). All large scratches and machining marks should be removed. When plated steel, stainless steel, high-strength steels, and aluminum are the materials being processed, a finish of 0.5 to 1 µm (20 to 40 µin.) for R_max is recommended on the tool being used. The polishing lines should be parallel to the metal flow. The characteristic white layer that is produced in electrical discharge machining should be removed before TRD processing.

Figure 14. Influence of tool surface finish on seizure-initiating load for a TRD-coated tool and uncoated tool. Mating material, SUS304; speed, 2.6 m/s (8.5 ft/s); lubricant, none

Tools processed by TRD may be re-treated by TRD. Some tools have been re-treated eight times. After the worn areas are refinished, tools can be re-treated without removing the sound carbide. The difference in layer thicknesses will be insignificant, due to the slower growth rate of the carbide layer on the previously coated areas.

Other TRD Product Applications. The TRD process is very useful for products as well as for tools because the carbide coating formed provides high resistance to abrasive wear, adhesive wear, fretting wear, corrosion, and oxidation, which cannot be provided by other conventional surface treatments. The following are examples of application:

Structural steels such as 10xx series carbon steel, and 41xx series low-alloyed steel are widely used for these applications. Low-carbon steels are often carburized prior to TRD processing. Substrate hardening is done during cooling in TRD treatment or by reaustenitizing hardening, if it is necessary. Attention should be paid to surface finishing and edge preparation for components used in severe conditions. Barrel finishing is often used for surface finishing of small components in large volume.

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