NITROCARBURIZING

Salt bath nitrocarburizing is a thermochemical process for improving the properties of ferrous metals. However, some tools and other high-alloy steels are susceptible to reductions in core hardness after standard nitrocarburizing. To prevent such losses, a low-temperature salt bath nitrocarburizing process has been developed. With treatment temperatures as low as 480°C, this process not only maintains core hardness, but also can sometimes increase core hardness.

Processing

During salt bath nitrocarburizmg, the part is immersed in a vessel of molten salt. Nitrogen and carbon in the salt react with the iron on the surface, forming a compound layer with an underlying diffusion zone. The compound layer consists of iron nitrides, chromium nitrides, or other such compounds, depending on the alloying elements in the steel, and small amounts of carbides.

Ranging in depth from 2.5 to 20 |im, the compound layer provides improvements in wear and corrosion resistance, as well as in service behavior and hot strength. Hardness of the compound layer, measured on a cross section, ranges from 700 HV on unalloyed steels, up to 1600 HV on high chromium steels. Note that this layer is formed from the base metal and is an integral part of it, and is therefore not a coating. The diffusion zone can extend as deep as 1 mm, depending on the steel. This diffusion zone causes an increase in rotating-bending strength and rolling fatigue strength as well as pressure loadability.

Salt bath nitrocarburizing may be applied to a wide range of ferrous metals, from low-carbon to tool steels, cast iron to stainless steels. Specifically, the process:

• Improves wear and corrosion resistance

• Reduces or eliminates galling and seizing

• Increases fatigue strength

• Raises surface hardness

• Provides highly predictable, repeat-able results

• Performs consistently, even with varying contours and thicknesses within the same part or load

• Maintains dimensional integrity

• Shortens cycle times

• Offers flexibility and ease of operation

Conventional treatment temperatures are in the range of 580°C, but for highly alloyed steels as well as stainless and tool steels, this temperature can cause a reduction in core hardness. The above benefits, derived both from the nitrogen and carbon diffused into the metal surface, as well as the processing in a liquid bath, are often necessary for applications in which a reduction in core hardness is not acceptable. For this reason, a new low-temperature process was developed.

The low temperature process normally takes place at 480°C, although it can operate at 480 to 520°C. This process has specific advantages:

• Core hardness and tensile strength are maintained in the tempered condition.

• Very thin compound layers can be formed.

• Distortion is extremely low.

• Formation of a compound layer on high-speed steels can be suppressed.

• Hardness of surface and diffusion layers can be customized.

This low-temperature process is beneficial for high-alloy steels such as stainless, tool, die, and high-speed steels (see Table N.5).

TABLE N.5

Suitable Steels for Low-Temperature Nitrocarburizing