title = "Nitriding: Gas, Salt Bath, and Plasma Methods"

content = """
Nitriding is a thermochemical case hardening process that introduces nitrogen into the surface layer of steel at temperatures below the lower critical temperature (Ac1), typically 900–1100°F (480–595°C), producing a very hard, wear-resistant, and corrosion-resistant case without requiring a post-treatment quench. The absence of quenching minimizes distortion, making nitriding particularly suitable for precision components that have already been machined to final dimensions.

Gas nitriding exposes the steel to an atmosphere of dissociated ammonia (NH3) at temperature. Ammonia decomposes on the steel surface to produce nascent (atomic) nitrogen, which diffuses into the steel and reacts with nitride-forming alloying elements (aluminum, chromium, molybdenum, vanadium). Aluminum-containing steels (e.g., Nitralloy 135M with 1.0% Al) achieve the highest case hardness (70+ HRC surface) from nitriding because aluminum nitride is extremely hard and fine. Chromium steels (4140, 4340) also nitride well with surface hardness of 55–65 HRC.

The nitrided case consists of two layers: the compound layer (also called white layer)—a thin, brittle iron nitride layer at the very surface (0.0002–0.002 inch thick)—and the diffusion zone beneath it where nitrogen diffuses and forms dispersed alloy nitrides. The compound layer provides excellent wear and corrosion resistance but is brittle; it may be removed by light grinding or lapping after nitriding if impact loads are present. The diffusion zone provides core compressive residual stresses that improve fatigue strength.

Salt bath nitriding (ferritic nitrocarburizing in a cyanide-cyanate salt bath—trade names Tenifer, Tufftride, Melonite) is performed at approximately 1050°F (565°C). It adds both nitrogen and carbon to the case simultaneously and produces excellent wear resistance and corrosion resistance on low-carbon steels. It is widely used for automotive crankshafts, camshafts, and tools.

Plasma (ion) nitriding uses a plasma discharge in a nitrogen-hydrogen atmosphere at reduced pressure. Nitrogen ions are accelerated onto the part surface by a high-voltage DC field. Plasma nitriding offers better control of compound layer composition and thickness than gas nitriding, lower temperatures (enabling treatment of precipitation-hardened stainless steels without sensitization), and the ability to mask areas selectively using simple metal shields.
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version = "0.0.1"
