Case Hardening: 

Case hardening is a heat treatment process used to increase the hardness of the outer surface (case) of a metal while keeping the inner core soft and tough. This improves wear resistance, fatigue strength, and impact resistance without making the entire part brittle.

🔹 Why is it needed? – Some components (like gears, shafts, and bearings) require a hard outer surface to resist wear while maintaining a tough core to absorb impact.

How Does Case Hardening Work?

• The component is exposed to carbon, nitrogen, or other hardening elements.

• These elements diffuse into the surface, forming a hardened outer layer (case).

• The inner core remains ductile and tough.

Key Features of Case Hardening:

✔ Thin hard layer (0.1–2mm thick)
✔ Tough, shock-absorbing core
✔ Improved wear resistance
✔ Extended component life

Types of Case Hardening Processes

1. Carburizing (Carbon Case Hardening)

🔹 Process:

• The steel is heated in a carbon-rich atmosphere (gas, liquid, or solid).

• Carbon diffuses into the outer layer, increasing hardness.

• The part is then quenched to lock in the hard case.

🔹 Types:

• Gas Carburizing – Uses carbon-rich gas (methane, propane).

• Pack Carburizing – Uses solid carbon (charcoal, carbon powder).

• Liquid Carburizing – Uses molten carbon-based salts.

🔹 Applications:
✅ Gears, camshafts, crankshafts, bearings.

🔹 Advantages:
✔ Deep, uniform hardening.
✔ Cost-effective for mass production.
✔ High wear resistance.

🔹 Disadvantages:
❌ Requires additional quenching.
❌ Possible warping due to high temperatures.

2. Nitriding (Nitrogen Case Hardening)

🔹 Process:

• The steel is heated in an ammonia-rich atmosphere.

• Nitrogen atoms diffuse into the surface, forming a hard nitride layer.

• No quenching is needed (reduces distortion).

🔹 Applications:
✅ Engine components, aerospace parts, injection molds.

🔹 Advantages:
✔ No quenching, low distortion.
✔ High surface hardness (above 1000 HV).
✔ Improved corrosion resistance.

🔹 Disadvantages:
❌ Slower process (can take 10–100 hours).
❌ Limited to specific steel types (e.g., nitriding steels).

3. Cyaniding (Carbon + Nitrogen Case Hardening)

🔹 Process:

• The steel is dipped into a liquid cyanide bath.

• Both carbon and nitrogen diffuse into the surface.

• The part is then quenched for additional hardness.

🔹 Applications:
✅ Small parts like fasteners, pins, screws, gears.

🔹 Advantages:
✔ Very fast process.
✔ Produces a thin, very hard case.
✔ Improves wear and fatigue resistance.

🔹 Disadvantages:
❌ Cyanide is highly toxic and requires strict handling.
❌ Limited to thin case depths.

4. Induction Hardening (Localized Hardening)

🔹 Process:

• High-frequency electromagnetic induction rapidly heats the surface.

• The part is quenched immediately, forming a hard case.

• The core remains soft because only the surface is heated.

🔹 Applications:
✅ Shafts, gears, railway tracks, crankshafts.

🔹 Advantages:
✔ Very fast & precise – only selected areas are hardened.
✔ No toxic chemicals.
✔ Ideal for large, high-load components.

🔹 Disadvantages:
❌ Requires special induction equipment.
❌ Surface hardening depth is limited.

5. Flame Hardening (Surface Heating with a Torch)

🔹 Process:

• A high-temperature flame heats the surface.

• The metal is quenched immediately, forming a hard outer case.

🔹 Applications:
✅ Large machine parts, construction tools, industrial rollers.

🔹 Advantages:
✔ Simple and cost-effective.
✔ Can be used for large components.

🔹 Disadvantages:
❌ Not as precise as induction hardening.
❌ Risk of overheating and distortion.