Heat Treatment Guide
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Carburising Process

Carburizing (or Carburising) is a case-hardening process that diffuses carbon into the surface of low-carbon steels to create a hard, wear-resistant outer layer (case) while maintaining a softer, tougher core.

Fundamental Principles
Carburizing works by enriching the surface of steel with carbon at high temperatures.

Carbon Diffusion

The core principle. Low-carbon steel is heated to an austenitic temperature (typically 900-950°C) in a carbon-rich environment. At this temperature, carbon atoms from the atmosphere diffuse into the steel's surface.

Creating a Case

The diffusion process creates a high-carbon surface layer, or 'case'. The depth of this case is controlled by time and temperature.

Hardening the Case

After diffusion, the part is quenched. The high-carbon case transforms into hard martensite, while the low-carbon core remains softer and tougher.

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Types of Carburizing
Different methods to introduce carbon into steel.
Gas Carburizing
The most common and precise method. Parts are heated in a sealed furnace with a controlled atmosphere of endothermic gas and a hydrocarbon gas (like natural gas or propane) to provide carbon.

Advantages:

  • Precise control over case depth and surface carbon.
  • Suitable for high-volume production.
  • Clean process, often integrated with quenching.
Pack Carburizing
The traditional method. Parts are packed in a steel box with a solid carbon-rich compound (e.g., charcoal mixed with an energizer like barium carbonate) and heated for a long period.

Advantages:

  • Simple, requires no special atmosphere furnace.
  • Good for large parts or small batch sizes.
  • Protects parts from scaling.
Liquid (Salt Bath) Carburizing
Parts are immersed in a molten salt bath containing cyanide compounds. This process is very fast but involves highly toxic materials.

Advantages:

  • Very fast heating and carbon diffusion rates.
  • Uniform heating minimizes distortion.
  • Provides both carbon and some nitrogen (carbonitriding effect).
Vacuum (Low-Pressure) Carburizing
A modern, clean method where parts are heated in a vacuum and then exposed to pulses of a hydrocarbon gas (like acetylene or propane) at low pressure. The vacuum ensures a perfectly clean surface for rapid carbon absorption.

Advantages:

  • Excellent uniformity and control over case depth.
  • No intergranular oxidation (IGO), leading to better fatigue properties.
  • Highly repeatable results.
  • Can be combined with high-pressure gas quenching for minimal distortion.
Related Processes

Carbonitriding

A similar process to carburizing, but with the addition of nitrogen (from ammonia gas). The nitrogen improves hardenability, allowing for a less severe oil quench, and increases wear resistance. It is often done at slightly lower temperatures than gas carburizing.

Vacuum Carbonitriding is an advanced version that offers even cleaner results and better process control, similar to vacuum carburizing.

Decarburization

The opposite of carburizing. This is the loss of carbon from the surface of steel when heated in an oxygen-rich atmosphere. Decarburization is almost always an undesirable defect, as it leaves a soft skin on a part that should be hard. It is prevented by using a controlled, protective atmosphere or vacuum during heat treatment.