Oct.2023 16
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What is surface hardening of metals, and what are the common processes?

Wear resistance and resistance to indentation are vital properties of metals. When deciding which metal to use, hardness is an important consideration.

To ensure the appropriate hardness of metal products, surface hardening processes are often employed to alter the surface hardness.

So, what is surface hardening?

Surface hardening refers to a method that hardens the surface layer of metal components while retaining toughness in the core through appropriate means.

Surface hardening can significantly improve the wear resistance and fatigue resistance of components. Today, I will introduce several common surface hardening processes.

  1. Quenching

Quenching involves heating the metal to a very high temperature and then rapidly cooling it, typically by immersing the material in oil or water or exposing it to a cold air stream.

The rapid cooling "locks in" the microstructural changes that occur when the material is heated, resulting in high hardness in the components.

Quenching is a cost-effective and versatile process widely used in modern mechanical manufacturing industries. Critical components in machinery, especially those used in automobiles, airplanes, and rockets, typically undergo quenching.

For steels or irons with sufficient carbon content, quenching has a pronounced effect. If the carbon content is low, additional steps such as nitriding or carburizing can be performed.

  1. Nitriding

Nitriding is a chemical heat treatment process that involves introducing nitrogen atoms into the surface of the metal at a specific temperature and in a particular medium. Nitriding is typically carried out in a nitriding furnace, forming a high-hardness and corrosion-resistant nitride layer on the surface.

There are generally three methods of nitriding: gas nitriding, liquid nitriding, and ion nitriding.

The key feature of the nitriding process is minimal distortion during heat treatment and a shallow hardening layer. This makes it suitable for parts operating under cyclic loads, such as shafts. It reduces the likelihood of distortion compared to quenching.

  1. Carburizing

Carburizing involves heating low-carbon steel components in a carburizing medium to introduce carbon atoms into the surface. Afterward, the surface carbon content increases, leading to higher hardness after quenching.

The external carbon source for carburizing can be gas, liquid, or solid, depending on application requirements.

Carburizing is mainly used for components that experience wear, bending stresses, and impact loads, such as shafts, gears, and camshafts. These parts require a high surface hardness while maintaining sufficient strength and toughness in the core.

Similar to nitriding, carburizing typically involves lower heating temperatures, reducing the likelihood of distortion.

After metal surface hardening treatment, a wear-resistant surface is created, significantly improving wear resistance and fatigue resistance of components. Meanwhile, the core remains ductile, providing good toughness and strength and excellent resistance to impact loads.