A wide variety of steels, some stainless steels and certain tool steels can be nitrided using gas, ion (plasma) or salt-bath methods. Three of the most common nitrided steels are:
  • SAE 4140. This low-alloy steel is commonly used for nitriding applications. The combination of carbon and alloying elements allows core hardness in the range of 28-32 HRC developed by quenching and tempering at temperatures exceeding by about 50°F the nitriding process temperature.
  • SAE 4340. A higher-alloy steel used when higher core hardness, up to 39 HRC or heavier section sizes require higher hardenability steel.
  • Nitralloy. This family of steels was specifically designed for nitriding. Typical quenched and tempered core hardness is 25-35 HRC. The advantage of the Nitralloy steels is their excellent response to nitriding and the resulting (very) high surface hardness, typically in the 62-65 HRC equivalent range.
Regardless of the steel used for nitriding, two heat-treat methods are in common use:

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 for steel are between 0.010"-0.020". Shallower or deeper case depths are possible. Significantly longer cycles are required for case depths about 0.020” due to the slow diffusion rate of nitrogen into steel. Stainless and tool steel case depths are typically limited to 0.001”-0.003”.

Case depth is generally specified as total case determined by etching a mounted sample or as the depth at which a certain hardness is obtained. The case-depth hardness should be specified in terms of the actual core hardness (e.g., "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. This layer may or may not be objectionable, but in most cases it must be kept thin. Some specifications require its complete removal. The white layer is minimized by using a two-stage nitriding called the Floe (pronounced “flow”) Process. Using this two-stage process, a white layer less than 0.0005" is achievable.