Heat treatment involves using temperature to modify the properties of a specific material, especially in metallurgy. This process takes place through heating and cooling. It is used to change one or more mechanical, chemical or physical properties of a given material in order to achieve desired properties, such as increased strength, hardness, impact resistance, softening or increased ductility. Carburizing and nitriding are two techniques used in the process of differential hardening of metal structures. We examine what connects and separates them, as well as indicate the most important applications of this type of heat treatment.
What is plasma nitriding?
Nitriding is a process of heat treatment, which involves the diffusion of nitrogen into the surface of the metal to increase the material's resistance to wear and corrosion, as well as its fatigue strength. This process utilizes nitrogen and temperature. The nitriding cycle begins by placing the product in a vacuum chamber. Once the appropriate state is reached inside the chamber, it is filled with process gas to start the preheating cycle.
In the standard heating cycle, the nitriding temperature ranges from 450 to 550 °C. After a certain heating time, the process gas is ionized, removing impurities from the surface of the treated material. Once the surface of a given element is sufficiently cleaned, the nitriding cycle begins. Anti-corrosive nitriding can last from several to several dozen hours. This technique is appreciated for its almost complete lack of distortions and stresses in the treated material. Nitrided steel has excellent resistance to wear and corrosion. Its fatigue durability is also improved.
Gas carburizing - what characterizes it?
Carburizing is a heat treatment process in which the surface layers of steel are saturated with carbon. The process leads to the hardening of the material, which allows for the use of a given element in a wider range of applications. Longer gas carburizing times and higher temperatures of this process usually increase the depth of carbon diffusion. This process is based on carbon monoxide or methane. After it, the material is usually hardened. Gas carburizing is often used in the case of hardening the surface of low-carbon and low-alloy steel. In addition to gas carburizing, we can mention ion (plasma) and vacuum carburizing.
Nitriding vs carburizing - what are the differences between these processes?
Nitriding and carburizing are the two most popular heat treatment practices for hardening the surfaces of functional elements. The basic difference between them is, of course, the saturating element. As the names suggest, nitrogen is used in nitriding, and carbon in carburizing. The temperature to which the material is heated during each process is also important. The nitriding temperature is about 450–550 °C, while gas carburizing is about 850–900 °C.
It is worth mentioning that nitrided steel will be slightly harder (900–1200 HV) than a similar material subjected to carburizing (700 HV). This difference also results from the fact that the first of the mentioned processes usually involves high-quality alloy steels. On the other hand, the second process is rather dedicated to non-alloy materials of lower quality. Importantly, the material subjected to nitriding exhibits anti-corrosive properties, while carburized steel does not.
The order is also important. In plasma nitriding, the steel is first subjected to heat treatment, and only then is nitriding carried out. In the case of gas carburizing, the situation is reversed - first carburizing of the material, and then heat treatment.
Finally, it should be noted that the issue of tempering is crucial in both processes. Nitrided steel requires high tempering to induce the effect of secondary hardness. On the other hand, in carburized materials, high tempering (i.e., at temperatures above 500 °C) should not be used to avoid softening their surfaces.
Nitriding and carburizing - where are they used?
Corrosion-resistant nitriding is a very popular solution when it comes to effectively protecting all kinds of steel moving parts of mechanisms, such as: crankshafts, gears, valve components, clutches or engines. This process can be successfully used for machining components placed in machines that work in a humid environment. Nitriding is also widely used in the case of cold-forming tools and forging dies.
Similar to nitriding, gas carburizing can also be used for heat treatment of many types of steel. Parts made from it are used, for example, in machines where high surface hardness is required in combination with good core ductility and high strength and fatigue resistance of a given material. We are talking about components such as gears, wind turbine components, pump parts or gearboxes.