Short Answer:

Case hardening is a heat treatment process used to harden only the outer surface of a metal part while keeping the inner core soft and tough. This makes the part strong on the outside to resist wear, but flexible inside to absorb shock. It is mostly used for low-carbon steel parts like gears, cams, shafts, and other components that need a hard surface with a strong core.

There are different types of case hardening, such as carburizing, nitriding, cyaniding, flame hardening, and induction hardening. Each method involves a different way of hardening the surface while protecting the inside of the part from becoming too brittle. The choice of method depends on the material and the application.

Detailed Explanation:

Case hardening process

Case hardening, also known as surface hardening, is a very important technique used in mechanical and manufacturing engineering. The main goal is to improve the wear resistance of components without making the whole part brittle. In this process, only the outer layer (called the “case”) of the metal is hardened, while the inner core remains soft and tough.

This type of treatment is ideal for components that need to handle surface friction but also must not break easily under load. It combines hardness on the outside and toughness on the inside, which is a perfect combination for many machine parts.

The process usually involves:

1. Heating the metal to a high temperature.
2. Adding hardening elements like carbon or nitrogen to the surface.
3. Cooling the part at a controlled rate.

The surface absorbs the added element and transforms into a hard structure, while the inner metal remains unchanged.

Types of case hardening

There are several popular methods of case hardening, each with different techniques and results:

1. Carburizing:
   • In this method, the metal is heated in the presence of a carbon-rich gas or solid.
   • Carbon enters the surface of the metal.
   • After soaking, the metal is quenched to harden the carbon-rich layer.
   • Commonly used for gears, pins, and camshafts.
   • Creates a deep hard case (1 mm or more).
2. Nitriding:
   • Nitrogen is introduced into the surface by heating in a nitrogen-rich gas like ammonia.
   • This is done at a lower temperature than carburizing and without quenching.
   • Results in a very hard and wear-resistant surface.
   • It also improves corrosion resistance.
   • Common for automotive and aerospace parts.
3. Cyaniding:
   • The part is heated in a bath of cyanide salts, which provides both carbon and nitrogen.
   • It is faster than carburizing or nitriding.
   • The process gives a thin but hard surface.
   • Due to toxic chemicals, it is less commonly used today.
4. Flame hardening:
   • A high-temperature flame is applied directly to the surface of the metal.
   • The surface is heated quickly and then quenched.
   • Only specific areas are hardened, and depth is controlled by the flame speed.
   • Used for large machine parts like rails or shafts.
5. Induction hardening:
   • An electromagnetic coil creates heat through induction on the surface of the metal.
   • The heated surface is then quenched.
   • The process is quick and very controlled.
   • Mostly used for axles, bearings, and gears.

Benefits of case hardening

• High wear resistance on the surface
• Shock-absorbing inner core
• Improved lifespan of machine parts
• Suitable for low-carbon steels
• Surface properties can be customized as per need

This method is especially useful when the part is subjected to high surface stress but also needs to handle impact or bending.

Applications of case hardening

• Automotive parts: gears, shafts, pistons
• Tools and dies: punch tools, cutting edges
• Industrial equipment: cams, bearings, rollers
• Military and aerospace: firearm parts, landing gear
• Agricultural machinery: blades, sprockets, levers

These parts experience high contact stress and need both durability and toughness.

Conclusion

Case hardening is a useful heat treatment process that increases the hardness of the metal surface while keeping the core soft and tough. This makes parts more durable and resistant to wear without becoming brittle. Different methods like carburizing, nitriding, cyaniding, flame hardening, and induction hardening are used depending on the type of metal and the application. Case hardening is widely used in automotive, mechanical, and industrial sectors for making long-lasting and reliable components.