Figure 1. Typical diamond block (Photograph courtesy of Sentry Company)

Question:
How do you protect the surface of M-series high-speed steel from decarburization? Is there a coating or paint that can be applied?

Answer:
There are several ways in which you can either eliminate (in the case of vacuum) or minimize (in the case of protective atmosphere) the amount of decarburization occurring on the surface of your M-series high-speed steel. In vacuum, after pumpdown below 1 x 10-3 torr, use of a partial pressure (e.g., nitrogen, argon) in the range of 1,000 microns (1.33 mbar). Protective atmospheres suitable for processing M-series high-speed steel include hydrogen, dissociated ammonia, nitrogen/hydrogen mixtures or (properly controlled) endothermic gas.

For small operations there is a product called a “diamond block” that has been reported to work well for the heat treating of all types of tungsten, molybdenum and cobalt high-speed steel as well as for high-carbon, high-chrome and air-hardening steels. Another method for small operations is the use of a foil wrap.

I am unaware of a coating or paint that works well at the high temperatures required to harden M-series HSS. Perhaps our readers can recommend a product? Remember, there are advantages and disadvantages to each method.

Diamond blocks (Fig. 1) are composed of a carbonaceous material and are usually designed to encompass the part on several sides (equivalent to a muffle effect). These products are typically used in air, at hardening temperatures from approximately 800°C (1475°F) up to a maximum temperature of approximately 1300°C (2375°F). They are designed to be slowly consumed in 40-60 minutes, generating a protective atmosphere that is mainly carbon monoxide (CO). When used for hardening tool steels, the atmosphere generated minimizes oxidizing effects responsible for scale and decarburization. They are simple to use and reportedly do not employ chemically active materials that attack furnace refractory or internal components. Tools hardened by this method are claimed to retain their initial sharp, clean-cut edges with minimal scale, decarburization or reduction in size.

Figure 2. Typical foil wrap (Photograph courtesy of Sentry Company)

Foil wraps (Fig. 2) are good up to 2200°F (1200°C) and have the advantage of being ultra-thin, in some cases only 0.002 inches (0.05 mm) thick, and can be cut to size from a roll and folded entirely around the workpiece. Other types of foil wraps have welded edges and form a protective pouch into which the part is placed. The packed workpiece is then brought to temperature in the furnace. Afterward, the workpiece and the foil are quenched into oil, water or air.

Some protective coatings in the form of pastes are available, but they are typically limited to around 1550°F (840°C). These are suitable for low and non-alloyed types of steel that must be annealed or hardened. They are (relatively) easy to use and can be applied like paint. Removal is typically with hot water.