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The wonderful reaction between ammonia and metal: Do you know how the nitridation process changes the properties of matter?
Nitriding is a heat treatment process that creates a case-hardened shell by diffusing nitrogen into the metal surface. This technology is mainly used for low-alloy steels, but is also suitable for materials such as titanium, aluminum and molybdenum. Typical applications for nitriding include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die casting tools, forging dies, extrusion dies, firearm parts, injectors and plastic molds, etc.
The nitriding process can make the material stronger and more wear-resistant, which is crucial to improving the performance of production tools and mechanical components.
Nitriding treatment method
Nitriding methods vary according to the media used, and mainly include three methods: gas nitriding, salt bath nitriding and plasma nitriding.
Gas nitriding
In gas nitriding, the medium that supplies nitrogen is a nitrogen-rich gas, usually ammonia (NH3), so it is sometimes called ammonia nitriding. When ammonia comes into contact with a heated workpiece, the ammonia decomposes into nitrogen and hydrogen, and the nitrogen diffuses on the surface of the material to form a nitride layer. Although this process has been around for nearly a century, in recent decades the associated thermodynamics and reaction kinetics have been intensively studied and processes that can be precisely controlled have been developed.
The advantages of gas nitriding include precise control of the chemical potential of nitrogen in the nitriding atmosphere and the ability to process large batches of workpieces.
Salt bath nitriding
In salt bath nitriding, the nitrogen supply medium is salt with nitrogen content, such as cyanide salt. These salts also donate carbon to the surface of the workpiece and make the salt bath a nitrogen-carbon co-process. However, due to the extremely toxic nature of the salts used, this process is gradually being phased out under recent environmental and safety regulations.
Salt bath nitriding is fast and easy to operate, but its environmental risks and costs have made it no longer commonly used in many countries.
Plasma nitriding
Plasma nitriding, also known as ion nitriding, is an industrial surface hardening treatment. This technology uses a high-intensity electric field to generate pre-nitrided gas and form a highly reactive gas called plasma. Plasma nitriding has a wide temperature control range and can enhance the performance of metal parts, greatly improving wear resistance and fatigue strength.
Parts that have been plasma nitrided usually do not require further processing or polishing because they are already up to standard for use.
Nitrided materials
Examples of steel suitable for nitriding include SAE 4100, 4300, 5100, 6100, 8600, 8700 and other series. Ideally nitrided steel should typically be quenched and tempered and maintain a finely machined surface to remove as little material as possible to retain surface hardness. Nitrided alloys are alloy steels containing nitriding elements.
The history of nitriding
In the 1920s, systematic research on the effect of nitrogen on the surface properties of steel began, and research on gas nitriding was carried out in Germany and the United States at the same time. In Germany this process gained popularity and a variety of steels were developed designed to be nitrided; however in the United States the process was almost forgotten due to low demand. After World War II, the technology was reintroduced from Europe, and nitriding technology gradually improved in the United States.
In summary, the nitriding process not only changes the surface properties of materials, but also has a profound impact on the production operations of various industries. As technology continues to advance, can we expect nitriding to have greater potential in future applications?
