Understand the Primary Differences between Hard and Soft Nitriding

08 August 2022

Before metal parts can become strong and reliable, they must be heat treated first. Heat treatment subjects metal workpieces to heat below their melting temperatures. Subsequently, the heated materials are cooled controllably, making sure that they attain the desired mechanical properties of specific industries or users.

Tons of heat treatment processes can be carried out to alter the properties and qualities of metal parts. One of these processes is nitriding.

Nitriding is a heat treatment process that disperses nitrogen elements into metal surfaces, providing them with a case-hardened surface. Many metals can be subjected to nitriding. Some of them are titanium, molybdenum, aluminium, and low-alloy steel. To date, nitriding can be classified into two: hard nitriding and soft nitriding.

Hard Nitriding

The conventional way of nitriding is known as hard nitriding. This process refers to the placement of a workpiece in a furnace gas containing nitrogen or ammonia. The temperature, gas flow rate, ammonia concentration ratio, and other elements are then regulated to a specific level in time, ensuring that the nitrogen can infiltrate into the workpiece’s surface. After a specific period of time, the workpiece is expected to develop a surface nitride phase, making it more resistant to fatigue and wear.

Hard nitriding is often conducted at temperatures of between 480 and 540 degrees Celsius. These temperature levels are necessary to sustain the hardness and other qualities of the workpiece’s core. They are likewise significant in ensuring the hardness of the nitriding layer. As for the time, a workpiece is expected to undergo the nitriding process for around 15 to 70 hours, making sure that it obtains the required depth of a specific application.

Workpieces that undergo hard nitriding can expect higher hardness and wear resistance, better fatigue strength, and higher seizure and corrosion resistance. This process is best for workpieces made from steels that contain chromium, molybdenum, titanium, aluminium and other alloy elements, stainless steel, and die steel.

Soft Nitriding

Soft nitriding, also known as nitrocarburizing or cyanidation, is the process of subjecting workpieces to not only nitrogen but also carbon. It makes the process work similarly to a low-temperature carbonitriding, which is then based on nitriding. This process can be grouped into two: gas soft nitriding and liquid soft nitriding. Gas soft nitriding is a nitriding process that adds low-temperature carbon and nitrogen into the workpiece. Liquid soft nitriding, alternatively, maximises a cyanide salt mixture in a bath in processing a workpiece.

The temperatures used for gas soft nitriding may range between 560 and 560 degrees Celsius, with the whole process lasting around 2 to 3 hours. Liquid soft nitriding, alternatively, may be done between 540 and 590 degrees Celsius.

What is great about soft nitriding, in general, is it can infiltrate workpieces in a short time. It can also provide good toughness and minimal brittleness to the involved metals. So, with metals that possess small load and shallow infiltration layer requirements, soft nitriding would be the best option to maximise. Most steel grades and cast iron can take advantage of this process.

To know more about these nitriding options, you can contact us at Alpha Detroit Heat Treatment.

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