Short Answer:

Case hardening is a heat treatment process used to harden only the outer surface of a metal while keeping the inside soft and tough. This makes the surface wear-resistant while the core remains flexible and can absorb shock without breaking. It is mainly used for steel parts that need a strong surface but also require toughness inside.

Case hardening is commonly used in manufacturing of gears, cams, crankshafts, fasteners, and other machine parts that experience friction or impact. These parts need to resist surface wear but also handle internal stress, which is why case hardening is ideal for them.

Detailed Explanation:

Case hardening process

Case hardening is a special type of surface heat treatment in which the outer layer (case) of the metal is made hard and wear-resistant, while the inner part (core) remains soft and tough. This combination gives the part a long life with strength and durability on the outside and flexibility on the inside.

The process of case hardening involves adding carbon or nitrogen (or both) to the surface of a low-carbon steel part. The part is then heated to a high temperature so that the carbon or nitrogen atoms can enter the surface. After this, the part is cooled rapidly (quenched), which makes the outer surface very hard.

There are several types of case hardening methods, including carburizing, nitriding, cyaniding, flame hardening, and induction hardening. Each method uses different techniques to harden the surface, but the goal remains the same — to increase surface hardness and improve resistance to wear and fatigue.

Where case hardening is used

1. Gears
   Gears are often used in machines and vehicles, and they face constant friction and pressure. Case hardening helps gears last longer by making their teeth hard enough to resist wear.
2. Camshafts and Crankshafts
   These engine parts rotate at high speed and are under continuous stress. Case hardening ensures that the surface can bear the load without damage while the core absorbs shock.
3. Pins and Shafts
   In mechanical assemblies, pins and shafts are subject to repeated movement and impact. A hard surface resists wear, while a tough core prevents bending or breaking.
4. Machine Tools
   Tools like drills, punches, and dies must maintain sharp edges during cutting or pressing. Case hardening makes sure their surfaces stay hard and usable for longer time.
5. Bearings and Rollers
   These parts rotate continuously under load. Case hardening increases their surface life by making them wear-resistant.
6. Lock Components and Fasteners
   Small mechanical parts like bolts, screws, locks, and keys are case hardened so they don’t wear out quickly or get deformed under repeated use.

Types of case hardening

1. Carburizing
   The part is heated in a carbon-rich environment. Carbon enters the surface, and after quenching, the outer layer becomes hard. It is one of the most commonly used methods.
2. Nitriding
   Nitrogen is added to the metal surface using ammonia gas. It is done at lower temperatures and produces a very hard and wear-resistant surface.
3. Cyaniding
   The part is heated in a bath containing cyanide salts. It’s a fast and effective process, mainly for small parts.
4. Flame Hardening
   The surface is heated using a high-temperature flame and then quickly quenched. It is useful for large or specific surface areas.
5. Induction Hardening
   The surface is heated by electromagnetic induction and then quenched. It is fast and precise, used in automated systems.

Conclusion

Case hardening is an important heat treatment process that helps improve the wear resistance and durability of metal parts without losing their inner strength. It is highly useful in mechanical engineering, especially for machine parts that must withstand continuous friction, pressure, and stress. By creating a hard shell with a soft core, case hardening ensures that components work effectively and last longer in tough working conditions.