Case-hardening

Case-hardening or surface hardening is a process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal at the surface. This process is used to increase the wear resistance of a piece while maintaining the toughness and strength of the core. This technique is applied in various industries, particularly in the manufacturing of components that are subject to high levels of stress and wear, such as gears, cams, and fasteners.

Process

The case-hardening process involves adding carbon or nitrogen to the outer surface of a metal. The most common methods include:

• Carburizing: This involves introducing carbon into a steel surface. The steel is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide, at temperatures where the steel is austenitic.
• Nitriding: Nitriding introduces nitrogen into the steel and is carried out at lower temperatures than carburizing, which results in a case with good hardness and superior resistance to wear without the need for quenching.
• Carbonitriding: A process that involves both carburizing and nitriding simultaneously to produce a hard, wear-resistant surface.
• Cyaniding: Cyaniding is a case-hardening process that involves the addition of carbon and nitrogen to the surface of the steel using a sodium cyanide bath.

Applications

Case-hardened components are essential in various applications where durability and resistance to wear are required. Some common applications include:

• Gears and sprockets in automotive and machinery
• Bearing surfaces and raceways
• Camshafts and crankshafts
• Fasteners like bolts, nuts, and screws

Advantages and Disadvantages

Advantages:

• Increased surface hardness and wear resistance
• Retention of softer, more ductile core
• Improved fatigue strength

Disadvantages:

• Complex and time-consuming process
• Requires precise control of temperature and environment
• Can introduce stresses and distortions in the material

Environmental and Safety Considerations

The case-hardening process, especially methods involving chemicals like cyaniding, can pose environmental and safety risks. Proper handling, disposal of chemicals, and adherence to safety regulations are essential to minimize the impact on the environment and ensure the safety of workers.

See Also

• Heat treatment
• Quenching
• Tempering (metallurgy)
• Metalworking