Nitriding is a case hardening operation that diffuses nitrogen into metal parts and components to create a strong and durable outer layer. Unlike other coatings and plating techniques, there is no risk of delaminating with nitriding, as the nitrogen is actually fully alloyed or incorporated into the surface of the metal.
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Applications of Nitriding
While steel and its alloys have long been treated through nitriding, it is gaining popularity for applications involving other metals, such as cast iron, aluminum, molybdenum, and titanium. Metallurgical, construction, shipping, tool and die, and other high stress machining applications frequently use nitriding, which is faster, more precise, and more readily reproduced than many other case forming heat treatments.
Materials Used in Nitriding
Cold wall and hot wall furnaces are used to heat parts and nitrogen-rich gases, liquids, or solids to enact case hardening. With either type of furnace, temperature regulation is integral to the success of nitriding, as it is with many other forms of heat treating. Non-uniform temperatures can lead to increased distortion and uneven stresses, which could result in unpredictable product performance and even potentially hazardous mechanical failure. Uneven surface hardness, thickness, and case depth may also result from inadequate temperature regulation during the nitriding process.
Technological advancements allow for careful temperature monitoring of both the furnace atmosphere and the part itself to ensure the most effective treatment. Progressively more advanced, the nitriding process allows for a broad range of materials to be case hardened via the introduction and diffusion of nitrogen into the surface layers of industrial parts and components.
Notable Types of Nitriding
There are three main methods used to effectively infuse parts with nitrogen for increased hardness and durability. Each process is named for the technique used to implant nitrogen into the substrate. Gas nitriding is a popular technique in which the gas ammonia is put in contact with a heated work piece. The heat allows the nitrogen present in ammonia to dissociate from the hydrogen and instead bond chemically and physically with the metallic component.
Another technique is what is known as salt bath nitriding or cyaniding. Unlike other processes, which may take hours, cyaniding takes as little as 30 minutes, depending upon the metal and part being hardened. The process for salt bath nitriding is simple. The part is placed in the bath of sodium cyanide, which is heated to a temperature between 550 and 590 °C (1,022 - 1,094 °F). The component remains submerged for a period of time, which is determined by the desired effect of nitriding. Though it was once exceedingly common, the use of cyanide salt baths continues to diminish as more environmentally friendly materials gain preference over the highly toxic cyanide.
The third and final type of nitriding is known as plasma nitriding. This process, also known as ion nitriding or glow discharging, is quickly becoming the most popular avenue for case hardening with nitrogen. At high voltage, nitrogen atoms are dissociated and accelerated to diffuse in the part. This heats the working surface, which cleans and hardens it. In addition to improved strength and structural integrity, each of these nitriding methods offers enhanced performance, strain limits, fatigue strengths, and product longevity when performed properly.