Nitrided surfaces are highly wear resistant and provide anti-galling properties. Fatigue life is improved, and the process improves the corrosion resistance of the part. An additional advantage of nitriding is that the surface hardness is resistant to softening by temperatures up to the process temperature.

The nitriding process involves the diffusion of nitrogen into the base steel. This diffusion takes place at relatively low temperatures (typical process temperature is 975 F) and the hardening occurs without quenching. Core properties are not affected by the nitriding process provided the final tempering temperature for the product was higher than the nitriding process temperature.

Although a wide variety of steels can be nitrided, three are most commonly used:

• AISI 4140 - Most commonly used low alloy steel for nitriding applications, the combination of alloy and carbon allows core hardnesses of RC 28-32 developed by quenching and tempering at temperatures exceeding the nitriding process temperature.
  
  
• AISI 4340 - This higher alloy steel is used when higher core hardnesses (to RC 39) or heavier sections require higher hardenability steel.
  
  
• Nitralloy - This group of steels, specifically designed for nitriding, may be quenched and tempered to typical core hardnesses of RC 20-25. The advantage of the nitralloy steels is their high response to nitriding and the very high (RC 62-65 equivalent) surface hardnesses that result.

Regardless of the steel used for nitriding, one of the two methods is recommended for heat treating:

• Method 1. For minimal distortion.
  - Quench and temper stock to specified core hardness
  - Rough machine
  - Stress relieve
  - Finish machine
  - Nitride
  - Lap or lightly grind as necessary
  
  
• Method 2. For maximum machinability.
  - Rough machine
  - Quench and temper to specified core hardness
  - Finish machine
  - Nitride
  - Lap or lightly grind as necessary

Typical nitrided case depths are .010" to .020". Deeper case depths are possible, but require significantly long cycles because of the slow diffusion rate of nitrogen into steel.

Case depth is generally specified as total case determined by etching a mounted microspecimen, or as the depth at which a specified hardness is obtained. The case depth hardness should be specified in terms of the actual core hardness (eg "case depth at 110% of core" or "core hardness plus 3 RC") as the hardness gradient in a nitrided part depends heavily on the prior hardness.

A typical by-product of nitriding is the white layer, a thin layer of extremely hard iron nitride. Although this layer is not objectionable if it is kept thin, some specifications do require its removal. The white layer is minimized by using two stage nitriding, called the Floe Process. Using this process, a white layer less than 0.0006" may be maintained.

In the event certain areas of a component must be kept soft, this can be readily accomplished by means of a special protective paint.

Milwaukee Gear's heat treat affiliate, Treat All Metals, specializes in quality heat treating, and our nitriding capabilities are no exception. Our process equipment is certified to ensure accurate control of the nitriding process. We have full capabilities of providing quench and temper and stress relieving operations prior to nitriding to insure your product is heat treated at the necessary temperatures to withstand the nitriding operation without softening or distortion.

The Quality Control Metallurgical Lab is well equipped to test your product to your most exacting requirements. When necessary, all process steps and inspections can be certified or substantiated with documentation.

Our experience with the nitriding process can assist you in selecting and successfully applying the processing sequence best suited to your product.