Short Answer:

Case hardening is a heat treatment process used to harden only the outer surface of a metal while keeping the inner portion soft and tough. This method is mainly applied to low-carbon steels, which are soft inside but need a hard, wear-resistant surface. The process increases the surface hardness by adding carbon or nitrogen to the outer layer of the metal.

This treatment gives the metal a tough core that can absorb shocks and a hard surface that resists wear and tear. Case hardening is widely used in parts like gears, shafts, cams, and other components that face friction and impact during operation.

Detailed Explanation :

Case Hardening

Case hardening is a surface heat treatment process that improves the wear resistance and fatigue strength of metals, especially low-carbon steels. The basic idea behind this process is to make the outer surface (called the case) hard and strong, while keeping the inside (called the core) soft and ductile. This combination gives the part both hardness for wear resistance and toughness for impact resistance.

The process is widely used in industries because it increases the life of components that operate under conditions of friction, load, or impact. For example, in gears and shafts, the outer layer resists wear while the inner core can absorb shock loads without breaking.

Case hardening involves heating the metal in the presence of a hardening medium, such as carbon-rich gas or solid materials. During heating, carbon or nitrogen atoms diffuse into the surface of the metal, forming a hard surface layer. The depth and hardness of this layer depend on the duration of heating, temperature, and type of process used.

Purpose of Case Hardening

The main objectives of case hardening are:

1. To increase the surface hardness and wear resistance.
2. To maintain a tough and ductile core.
3. To improve fatigue strength and reduce the risk of surface cracks.
4. To increase the lifespan of components subjected to repeated stress or sliding contact.

By hardening only the surface, the process ensures that the component remains strong without becoming brittle throughout its section. This makes case-hardened parts suitable for mechanical systems where both toughness and surface hardness are required.

Common Methods of Case Hardening

1. Carburizing:
   In this process, steel is heated in a carbon-rich environment (like charcoal or gas) at temperatures around 900°C to 950°C. Carbon diffuses into the surface layer of the steel, making it rich in carbon. After sufficient carbon penetration, the steel is quenched to harden the surface. The core remains soft because carbon diffusion is limited to the surface.
2. Cyaniding:
   Cyaniding involves heating steel in a bath of molten cyanide salts. Both carbon and nitrogen enter the surface layer, resulting in a thin but very hard case. The process is quicker than carburizing and operates at lower temperatures (around 800°C–870°C).
3. Nitriding:
   In nitriding, nitrogen is introduced into the steel surface by heating in an atmosphere of ammonia gas at 500°C–550°C. Nitrogen combines with metal elements like aluminum or chromium to form hard nitrides. This process produces an extremely hard surface without the need for quenching, which reduces distortion.
4. Carbonitriding:
   This process is a combination of carburizing and nitriding. Steel is heated in a mixture of gases containing both carbon and nitrogen. The resulting surface layer is hard, wear-resistant, and has improved fatigue strength. It is suitable for parts requiring precise dimensions since it causes minimal distortion.
5. Flame and Induction Hardening:
   In these methods, the surface of the metal is heated rapidly by a flame or electrical induction and then quenched immediately. The quick heating and cooling harden the surface while the inner portion remains unaffected. These methods are suitable for large or specific areas where selective hardening is needed.

Advantages of Case Hardening

• Produces a hard, wear-resistant surface and a strong, ductile core.
• Increases component life by reducing wear and fatigue failures.
• Improves resistance to impact and surface stress.
• Maintains dimensional stability, especially in nitriding and carbonitriding.
• Allows treatment of selected areas of a component.

Applications of Case Hardening

Case hardening is commonly used in machine parts and automotive components where surface hardness and internal strength are both important. Examples include:

• Gear teeth and camshafts in engines.
• Crankshafts, piston pins, and connecting rods.
• Cutting tools, bearings, and fasteners.
• Rollers, spindles, and valves.

These components experience heavy loads and friction during operation, and case hardening provides the necessary strength to withstand such conditions.

Conclusion:

Case hardening is a vital heat treatment process that enhances the performance of machine components by giving them a hard outer layer and a tough core. It combines wear resistance with impact strength, making it ideal for parts exposed to high stress and friction. Various methods like carburizing, nitriding, and carbonitriding are used depending on the application. Overall, case hardening ensures a long service life, reliability, and better mechanical performance of engineering materials.