Steel Surface Hardening Process Helps Extend Tooling Life.

A method called the Thermal Diffusion (TD) Process has been developed for surface hardening of steel parts, tooling, and stamping dies.

The TD Center is the only U.S. licensee of the TD Process. According to this company, tooling life extended from 5 to 50 times of normal life was experienced.

TD is a surface modification process that forms a vanadium carbide layer on the surface of steel and cemented carbide. This is done by immersing parts in a fused salt bath kept at temperatures of 1,600 to 1,900 degrees Fahrenheit for one to eight hours.

The 2- to 20-micrometer (.00008-to .0008-inch) thick vanadium carbide layer produced has a fine, nonporous composition metallurgically bonded to the surface through diffusion, rather than by coating.

Testing shows that a TD-treated surface can resist wear, seizure, peeling, and adhesion to the substrate better than other processes, according to the company. The treated materials show surface hardness in the range of 3,200 to 3,800 on the Vickers scale. For comparison, cemented carbide registers 1,200 to 1,800 on the scale.

The company also says one automotive plant was using carbide end forming tooling on stainless steel tubing. That company experienced galling and scratching because when those two metals come into high pressure contact, they try to adhere. Their tooling could be used on only 50 lubricated pieces before it needed repolishing. After being treated with TD, more than 3,000 pieces were turned out without lubricants, according to the company.

TD is a high-temperature process. Therefore, if a tooling part demands very close tolerances, or if it is subject to distortion, the tooling part may not be a candidate for the process.

A critical question fabricators should ask is "Under any circumstances, would I subject the part to a temperature of more than 1,600 degrees Fahrenheit?"

Also, fabricators must be fully familiar with the pedigree of the part to be treated. For instance, steel with less than .3 percent carbon content will not effectively accept the process because the atoms or irons in the salt bath must combine with carbon atoms in the substrate to form the vanadium carbide layer.

A process specification sheet is used that requests such information as material type and application of the part to be treated.

Another limitation is that individual parts cannot exceed 17 inches in diameter by 20 inches in length.

The use of TD-treated cemented Tungsten carbide is increasing in the tooling industry. Although the harder carbide substrate (1,200 to 1,800 Vickers) alone solved many problems compared to a typical A2 or D2 tool steel (700 Vickers) application, galling still resulted in many applications. A TD-treated Tungsten carbide tool ( 3,200 Vickers) yields a high substrate hardness that helps prevent galling.

The process itself requires at least four days to be performed on the tooling.

According to the company, the process has applications in the aerospace industry as well as metal stamping.