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