Nitriding Process and Its Purpose

29 April 2016

Atmospheric Ammonia

Although classed as chemistry, the nitriding process identifies closely with heat treatment technology. In describing the stages, we see the alloyed steel component enter an oven-hot realm, an environment where metal is heated until its microcrystalline structure is on the cusp of transformation. Gaseous ammonia is added to the atmosphere at this point, and a complex interplay of temperature-derived wizardry takes place on the surface of the hot metal. Filled with ammonia, the gas is a cloud of short-chain molecules, one that binds nitrogen to hydrogen. As the gaseous compound comes into contact with the hot surface, it breaks down, disassociating into pure nitrogen and hydrogen.

Nitride Diffusion

As the controlled atmosphere splits into its basic atomic constituents, a fine case hardened surface develops on the object. This isn’t a separate film, it’s a part of the metal, a case hardened layer that leapfrogs quenching. And, since quenching is oftentimes a source of structural distortion, the chemical stage has achieved hardening without the introduction of quench-induced stress. Nitriding furnaces add surface wear resistance, improve fatigue resistance, and the layer provides a substantial corrosion barrier. Meanwhile, all of this case hardened proficiency comes without a price. The temperatures used are below the transformative thermal barrier, so base mechanical properties remain unchanged.

Controlled Workflow

Computer monitored atmospheric injection combines with a thermostatically managed workstation to ensure the workflow is optimally conducted from start to finish. The uniform application of the ammonia gas reaches into every obstructed corner, every bore hole and recessed geometrical outline to react with the hot metal of the alloy. Special tapes are available to mask sections of the part if this is a desirable feature. This intelligent masking practice blocks off areas that require further machining.

Finite control of the nitriding process strengthens steel alloys without adding stress. The highly desirable case hardened feature requires no quench work, is atmospherically controllable, and added diffusion stages can easily eliminate a chalky-white layering effect without affecting productivity. In addition to gaseous methodologies, there are also salt bath and plasma nitriding facilities to consider.

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