Case Carburising

Definition

Case carburising is a thermo-chemical surface hardening process used on low-carbon steel components to create a hard, wear-resistant outer layer (the "case") while maintaining a tough, ductile inner core. This is achieved by diffusing carbon into the surface of the metal at high temperatures.

Main Content

1. The Diffusion Principle

• The process relies on Fick’s Laws of Diffusion, where carbon atoms migrate from a high-concentration environment (the carburising medium) into the low-carbon steel surface.
• The depth of the case is controlled by the duration of the heating cycle and the temperature of the furnace.

2. Carbon Potential

• Carbon potential refers to the ability of the furnace atmosphere to deliver carbon to the steel surface.
• If the carbon potential is too high, brittle cementite networks form; if too low, the surface will not harden sufficiently during quenching.

3. Core vs. Case Properties

• The "Case" is high in carbon (typically 0.7% to 1.0%), making it hard and wear-resistant after quenching.
• The "Core" remains low in carbon (typically 0.1% to 0.2%), providing excellent toughness and resistance to impact loads, preventing the part from shattering.

Visual Representation of Carbon Gradient:

  Surface (Case)       Core
  [ High Carbon ] [ Low Carbon ]
        |               |
        v               v
  Hard & Wear      Tough &
   Resistant       Ductile

Working / Process

1. Heating and Enrichment

• The steel part is placed in a furnace heated to the austenitic temperature range (usually 850°C to 950°C).
• A carbon-rich medium (gas, solid, or liquid) is introduced to saturate the surface of the steel with nascent carbon atoms.

2. Diffusion Period

• The temperature is held constant for a specific "soak time," allowing the absorbed carbon to migrate deeper into the steel lattice.
• The longer the part remains at this temperature, the deeper the "case depth" will be.

3. Quenching and Tempering

• The part is rapidly cooled (quenched) in oil or water to transform the high-carbon surface into hard martensite.
• A final low-temperature tempering process is performed to reduce internal stresses and improve the toughness of the martensite.

Advantages / Applications

• Significantly increases the fatigue strength and wear resistance of gears, shafts, and bearings.
• Allows for the use of inexpensive, easily machined low-carbon steels while achieving the surface performance of high-alloy steels.
• Essential for automotive components that must endure high surface friction and internal shock loading.

Summary

Case carburising is a vital surface engineering technique that balances surface hardness with internal toughness in low-carbon steel components through controlled carbon diffusion. By combining a carbon-rich atmosphere with high-temperature soaking and rapid quenching, manufacturers can produce durable, impact-resistant mechanical parts.

Important terms to remember: - Austenite: The high-temperature phase of iron that allows carbon diffusion. - Martensite: The extremely hard, brittle microstructure formed after quenching. - Carbon Potential: The concentration of carbon in the furnace atmosphere. - Diffusion: The movement of carbon atoms from the surface into the steel.