The Application of Vanadium Carbide-
The TD Process

Introduction

This paper describes the Thermal Diffusion (TD) Process, a surface modification technology long used in Japan but applied only since 1988 in the United States. The TD Process is briefly described to show how a diffusion layer is formed on treated materials, making the process superior to a simple hard coating. Case studies are provided to demonstrate actual examples of TD applications in tool and die treatment. The resulting performance and economic benefits experienced are given as well as an explanation of the limiting considerations of the TD Process.

Technology

Thermal Diffusion (TD) is a high-temperature surface modification process that forms a carbide layer on carbon-containing materials such as steels, nickel alloys, cobalt alloys, cemented carbides and carbons, dramatically hardening the surface of the materials treated. The diffusion layer formed in the TD Process has shown itself to be superior to other coating processes. The diffusion layer is thin, 2 to 20 µm (.00008 - .0008 in.), but extremely dense and thoroughly bonded to the substrates.

TD-processed materials exhibit properties of carbides and nitrides:; high hardness and excellent resistance to wear, seizure and corrosion. These properties impart substantial life improvement to sheet metal dies and roll form tooling with significant increases in machine uptime, reduced maintenance costs and elimination or reduction in die lubricating costs.

Carbide coating by salt bath immersion was developed in Japan and has been used in that country's industries for 20 years as the Toyota Diffusion Process. Developed by Toyota Central Research and Development Laboratories in the '70s, TD was little more than a laboratory curiosity at first. But the Japanese soon realized its potential and moved it from the lab to practical industry applications. In 1987 Arvin Industries, Inc. signed a license agreement to use and offer the process throughout the United States and built its own TD treating center in 1988. The range in size for parts treated has been from 1.2 mm-diameter (.047 in.) punches to 160 kg (352 lbs.) rolls for forming. In many cases, tool life improvements of 30 to 50 times have been achieved after TD treatment.

The Process

The TD Process is performed by immersing parts in a fused salt bath kept at temperatures of 871 to 1037° Centigrade (1600 to 1900° F) for one to eight hours. This temperature range is suitable for quench-hardening many grades of low alloy steels, carburized steels and tool steels.

Carbide constituents dispersed in the salt bath combine with carbon atoms contained in the tooling substrate, which must contain a carbon content of .3 percent or greater. A carbide, carbonitride or nitride layer is formed into the surface of the substrate by diffusion of carbon and nitrogen from the substrate. The carbide layer produced has a fine, non-porous composition metallurgically bonded into the surface through diffusion rather than by coating.

Parts to be processed are pre-heated to minimize distortion and to lower processing time. They are then TD-processed at the austenitizing temperature for the grade of steel being treated. After processing, the parts are quenched in air or salt to produce the hardened substrate. Then tempering is carried out. High-speed steels and ether steels that have austenitizing temperatures greater than 1900° Fahrenheit may be beat treated in a vacuum, a gas or a protective salt to achieve full substrate hardness after TD treatment.

Carbide layers commonly produced include vanadium carbide, niobium carbide and chromium carbide, depending on the carbide forming elements and nitride forming elements used in the salt bath. Tantalum, titanium, tungsten, and molybdenum can be used besides those mentioned. Vanadium and niobium carbide layers exhibit superior peel strength and resistance to wear, corrosion, and oxidation when compared to other processes. Chromium carbide has light wear resistance and high resistance to oxidation.

Because U.S. applications emphasize hard surfaces, vanadium carbide has been most often used. Vanadium TD-treated materials show surface hardness in the range of 3,200 to 3,800 on the Vickers hardness scale, for comparison, cemented carbide registers only up to 1,800 on the Vickers scale.

Characteristics of TD-Treated Materials

Hardness - Extreme surface hardness results when a vanadium carbide layer is produced, figure 1 shows the comparison of hardness of surface layers in Micro Vickers hardness among various methods of treating. Vanadium carbide retains exceptional hardness of Hv 1,000 even at 800° Centigrade. Furthermore, hardness will be returned to previous levels once the layer is cooled to room temperature after exposure to high temperatures.

Figure 1. Surface hardness of TD-treated materials.

Wear-resistance — Carbide layers from the TD Process show high wear-resistance against materials such as steel, non-ferrous metal, plastics, and rubber. Figure 2 shows results obtained by measuring the abrasion of the dies after continuous forming of cold-rolled mild steel plates not TD-treated.

Hardened and tempered steel show considerable abrasion loss. Little abrasion is recognized on the VC-treated steels from the TD Process.

Figure 2. Wear-resistance of TD-treated materials.

Seizure-resistance — VC-coated steel from the TD Process resists seizing at any temperature. In the case where the mating material is stainless steel, the seizure resistance of a TD-treated VC layer is considerably better than that of cemented carbide. VC also shows superior score-resistance, regardless of mating materials.

Impact-resistance — In the Izod impact test, TD-treated steels are equivalent in impact values to hardened and tempered steels, regardless of the substrate. Therefore, if a material having high impact resistance is selected for the substrate, it will be effective against breaking and chipping after TD treatment.

Corrosion-resistance —- No corrosion is shown in test pieces immersed in a 36 percent hydrochloric acid solution that corrodes stainless steel.

Peeling-resistance — Unlike plating, the treated layer produced by the TD Process will not easily peel off. The VC layer Is metallurgically bonded versus deposited or mechanically bonded. In tests, various surfaces where repeatedly struck on the same spot with an acuminated hammer. A chromium plated layer was cracked after a small number of strikes and peeled off after about 50,000 strikes. The TIC layer produced by the CVD method or PVD method is cracked after 30,000 strikes and peeled off after 100,000 strikes. The TD-treated VC layer suffered neither cracks nor peeling after 200,000 strikes.

Cutting performance — Figure 3 shows the cutting performance comparison between hardened tempered M2 steel and VC-treated D2 steel produced in the TD Process.

Figure 3. Cutting performance of TD-treated materials.

The TD Process is best used on tools that have high wear and galling problems. TD has been used on tooling and dies for the following industries: sheet metal, cold forming dies, hot forming dies, powdered metal production, glass, textile, pump parts, machine parts, engine parts and wire and tubing production. TD has also been used on production parts having stringent wear resistance and corrosion requirements. Treated parts can be re-treated up to eight times.

The TD Center has treated a variety of air-hardening tool steels. These include A2, D2, M2, and all of the high-speed powdered steels such as the CPM series and the ASP series. Other materials such as Ferro-Tic and cemented carbide also have been treated with great success.

Underhardened high speed (RC 57 to RC 59) steel that is TD treated sometimes outperforms fully hardened steels. However, substrates must be selected to withstand operating surface pressures or shock inherent in the conditions under which the specific tool operates. Where higher substrate hardness is required, a cemented Tungsten carbide substrate is recommended. For best dimensional stability, use cemented Tungsten carbide or properly heat treated D2.

In figure 4 the part is made from 3.17mm hot rolled mild steel. The part is produced in a progressive die then bent and seam welded along the front edge. Punches must notch through the doubled metal thickness 6.34mm thick and along the weld area. Prior to TD treating of the tool, the notch and pierce tooling had to be sharpened every 6,000 hits. The tooling only had a useful life of three sharpenings.

After TD treatment, 260,000 parts were produced before It was necessary to sharpen. About 750,000 parts were produced from the same punches. The tool steels used in this application were A2 and M2.

Savings were realized not only in the reduction of downtime for maintenance, but also in the reduction of rejected parts and the cost of tooling replacement.

Figure 4. Evaluation of TD-treated notch and pierce tool.

The die in Figure 5 is used to make a stainless steel bracket (1.35mm thick, 300 series stainless steel) is produced on a progressive die. The tool steel treated was D2.

Prior to TD treatment, this tool had been treated with TIN by the PVD process. Even with the TIN-treated die, galling and scoring would occur after 4,000 pieces, causing substantial equipment downtime. After the initial treatment and diamond polishing, quality of the part was improved and the die produced 110,000 pieces without servicing. To date, this die has been re-TD-treated three times with the same positive results.

Figure 5. Evaluation of TD-treated bracket die.

The tool in Figure 6 is used to expand tubing for fuel systems used in the automotive industry. The tool is 19.05mm in diameter.

Initially, the tool was made from cemented Tungsten Carbide. Due to galling, breakage would occur about every 400 pieces. TD Center engineers selected A2 as a replacement tool steel. After TD treating the A2 material and diamond polishing, tool life was improved to 22,000 pieces on average. Savings realized were significant, especially in tool replacement.

Figure 6. Evaluation of TD-treated expander nose.

The product shown in Figure 7 is the inside liner of a microwave oven. Two deep draws are required which are very difficult due to small radius requirements. The part material is draw quality, aluminum killed. The tool steel used in this application was D2.

Prior to TD treatment, galling would occur every 650 pieces. This resulted in costly equipment downtime and polishing costs, along with rejected parts for scratches and cracks.

The two draw caps, weighing a total of 115 kg, were TD treated and diamond polished. 58,000 parts were produced with no polishing to date, and the part rejects due to galling and fractures have been reduced to almost zero.

Figure 7. Evaluation of TD-treated draw cap.

Figure 8 is a valve cover used on a diesel engine. The part is produced in a very large 9-station transfer die (approximately 1.8 meters x 3.7 meters). All wear-related sections of the die, which were made of A2 and D2, were TD-treated. The goal was to extend tool life and to eliminate all die lubricants. Substantial savings are realized by the elimination of lubricants, reduction in maintenance, and cost for part cleaning prior to welding.

Another saving not shown above was in material used to produce the part. Initially, interstitial free (IF) or vacuum degassed steel was required to produce the part within tolerance. After TD treating, common draw quality aluminum killed steel could be used. The tool is operable after 272,000 parts without maintenance compared to 4,200 prior to TD treatment.

Figure 8. Evaluation of TD-treated transfer die.

The tool shown in Figure 9 is used for bending 460 series stainless steel tubing used for automotive exhaust systems.

To produce parts prior to TD treatment, the tool was inserted with wear-resistant bronze to prevent galling and maintain the dimensions on the bend radius. The tool was capable of running an average of only 13,750 pieces prior to servicing. The TD-treated D2 replacement tool has processed 256,000 pieces and is still operating.

Figure 9. Evaluation of TD-treated vector bender die.

In addition to the savings realized from tool maintenance, improved quality was realized through dimensional stability.

In Figure 10 the extrude punch tool is shown in front of the part produced. This punch extrudes a bearing seal mounting area, and the metal is extruded to control very tight dimensional tolerances. The major problem with conventional tooling approaches was galling of the sealing surface.

When the die was initially made, D2 was used for the punch and only about 300 pieces between polishings were possible. Also, large amounts of die lubricant were required. To improve this condition, cemented Tungsten Carbide was tried next. With the Tungsten carbide punch, 4,000 pieces could be produced between die servicing, and lubricant was still required.

Next a new punch was made from D2 and TD-treated. All die lubricant was removed from the operations and the process is still functioning after production of 202,000 units.

Figure 10. Evaluation of To-treated extrude punch.

Lessons Learned from Case Studies

Proper surface preparation prior to TD treatment is key to enhancing the movement or sliding action of metal. Surfaces should have a finish of 5 to 7 RMS (Root Mean Square). Post-treatment finishing, such as diamond polishing, will further improve the quality of the surface adding to surface lubricity.

A trend now being experienced in the U.S. TD market is the increased use of TD-treated cemented Tungsten Carbide in tooling. Although the harder carbide substrate (1200 to 1700 Vickers) alone solves many problems compared to a typical A2 or D2 tool steel (700 Vickers) application, galling still results in many applications. A TD-treated carbide tool (3200 Vickers) yields superior performance in many applications. It has a very high substrate hardness which resists surface pressure and an extremely hard surface which yields superior galling performance. Several producers of cemented Tungsten Carbide are now recommending TD treatment to U.S. toolmakers as a major wear improvement to their products.

Process Considerations

Distortion. With the high temperature used in the TD Process, distortion is possible - either a change in critical dimensions or a change in shape.

Dimensional changes are due to phase transitions in heat treatment of the base steel and to formation of the carbide layer. To minimize changes in dimensions, parts should be hardened and finish ground. Parts with tolerances of plus or minus .04 mm (.0015 inches) or greater make better candidates for treatment.

Deformation is caused by thermal stresses, transformation stresses, creep during heating, anisotropy of the substrate structure and residual stresses. Deformation can be minimized by:

Parts made from air-hardened steels requiring tight tolerances should be double high-tempered before using the TD Process. In making new tooling, it is recommended to leave stock on non-working surfaces and finish only the working surfaces. The non-working surfaces may then be finished after TD processing.

Edge preparation. With cutting and piercing tools, an edge that is too sharp or burred will break. The cutting edge should be rounded to a radius of .05 to .25 mm (.002 to .010 in.) with a stone or emery paper before treatment. After treatment, or when the edge is worn, resharpening can be done. This is not detrimental because performance is governed by the carbide layer on the side surface of the cutting edge.

Surface finish and polishing direction. Due to high carbide layer hardness, a TD-processed tool with a rough surface will perform worse than an untreated tool. The surface should be finished to at least Rmax 3 µm (120 µin) or less. All large scratches and machine marks should be removed. When plated steel, stainless steel, high strength steel and aluminum are worked, a finish of Rmax .5 to 1 µm (20 to 60 µin) is recommended. The polishing lines should be parallel to the metal flow. The abnormal "white layer" produced in electrical discharge machining should be removed before TD processing.

Conclusion

The TD Process is a surface modification process that will enhance wear resistance on all carbon rich steels. This will not only extend the life of tooling or product, but will also maintain a high quality product with reduced downtime and substantially reduced cost.