Nitriding

Nitrogen diffusion case hardening process

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.

Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow discharge nitriding, is an industrial surface hardening treatment for metallic materials. In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of nitriding with ammonia). There are hot plasmas typified by plasma jets used for metal cutting, welding, cladding or spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low pressure regimes. Usually steels are beneficially treated with plasma nitriding. This process permits the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only is the performance of metal parts enhanced, but working lifespans also increase, and so do the strain limit and the fatigue strength of the metals being treated. For instance, mechanical properties of austenitic stainless steel like resistance to wear can be significantly augmented and the surface hardness of tool steels can be doubled. For instance, at moderate temperatures (like 420 °C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties. In the plasma nitriding processes, nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or argon are also used. Indeed, argon and hydrogen can be used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process hydrogen gas is also added to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance).

Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding to take place at a lower temperature than the last tempering temperature. A fine turned or ground surface finish is best. Minimal amounts of material should be removed post nitriding to preserve the surface hardness. Nitriding alloys are alloy steels with nitride forming elements such as aluminum, chromium, molybdenum and titanium. In 2015, nitriding was used to generate a unique duplex microstructure in an iron manganese alloy (martensite austenite, austenite ferrite), known to be associated with strongly enhanced mechanical properties.

Systematic investigation into the effect of nitrogen on the surface properties of steel began in the 1920s. Investigation into gas nitriding began independently in both Germany and America. The process was greeted with enthusiasm in Germany and several steel grades were developed with nitriding in mind: the so called nitriding steels. The reception in America was less impressive. With so little demand the process was largely forgotten in the US. After WWII the process was reintroduced from Europe. Much research has taken place in recent decades to understand the thermodynamics and kinetics of the reactions involved.