Nitriding

Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum. Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, firearm components, injectors and plastic-mold tools. The processes are named after the medium used to donate. The three main methods used are: gas nitriding, salt bath nitriding, and plasma nitriding. In gas nitriding the donor is a nitrogen-rich gas, usually ammonia (NH3), which is why it is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated work piece it dissociates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface of the material creating a nitride layer. This process has existed for nearly a century, though only in the last few decades has there been a concentrated effort to investigate the thermodynamics and kinetics involved. Recent developments have led to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. The advantages of gas nitriding over other variants are: Precise control of chemical potential of nitrogen in the nitriding atmosphere by controlling gas flow rate of nitrogen and oxygen. All round nitriding effect (can be a disadvantage in some cases, compared with plasma nitriding) Large batch sizes possible - the limiting factor being furnace size and gas flow With modern computer control of the atmosphere the nitriding results can be closely controlled Relatively low equipment cost - especially compared with plasma The disadvantages of gas nitriding are: Reaction kinetics heavily influenced by surface condition - an oily surface or one contaminated with cutting fluids will deliver poor results Surface activation is sometimes required to treat steels with a high chromium content - compare sputtering during plasma nitriding Ammonia as nitriding medium - though not especially toxic it is harmful when inhaled at a high concentration.