Carburizing – Process, Applications, Alternatives

Technique and agents

Carburizing can be carried out with four different carburizing agents. The aim of the process is to prepare metals for subsequent case-hardening. Typically, workpieces are exposed to a high temperature during carburizing with the carburizing agent. This causes the carbon to diffuse into the surface of the workpiece, where it reaches a carbon content of at least 0.25 %, which is crucial for hardening the steel.

After carburizing, the actual case-hardening can take place. Case-hardening is divided into three different processes: direct hardening, single hardening and double hardening.

The following carburizing agents can be used for the process:

Solid agents

Solid carburizing agents include carbon powders and granulates. The carburizing agent usually consists of a mixture of a carbon carrier, an activating agent and a binding agent. The components are held with the agents in boxes at temperatures of 870°C to 930°C for a predefined time. The carburizing time depends on the material and the carburizing agent used. Carburizing with solid media is difficult to automate and is therefore rarely used. There is a risk of coarse grain formation due to the long carburizing times caused by the poor heat conduction of the powder/granulate.

Liquid agents (Liquid carburizing)

With liquid carburizing agents, the workpieces are held in a cyanide-containing molten salt at temperatures between 900 and 930°C (max. 950°C). However, the workpieces must be preheated to ensure a completely dry surface. Preheating also minimizes distortion, as the temperature difference between the workpiece and the molten salt is reduced. Carburizing with liquid media impresses with its fast processing time and high reproducibility. Nevertheless, liquid carburizing agents are not widely used today, as the disposal of salt solutions is difficult and expensive due to their poor environmental compatibility. In addition, these molten salts are very harmful to health.

Gaseous agents (Gas carburizing)

In principle, all carbon-containing gases can be used as gaseous carburizing agents. The carburizing atmosphere basically consists of carbon monoxide, carbon dioxide, water vapour, hydrogen, oxygen and methane (concentrations vary). The release of carbon can be attributed to the Boudouard reaction, heterogeneous water gas reaction or the methane reaction. Due to the easy and inexpensive processing as well as the harmlessness for the environment, gaseous carburizing agents are the most widely used method today.

Low pressure (Vacuum carburizing)

In the low-pressure process, workpieces are treated with hydrocarbons in a vacuum furnace. As the name suggests, low-pressure carburizing takes place at low pressure. In this method, carbon transfer from gases containing carbon, such as propane and methane, is strongly favored. At pressures between two and 25 millibars, diffusion and carburization cycles are controlled. Unlike gaseous carburizing agents, no gas is enriched in the low-pressure process.

Suitable media and areas of application

Steels with a low carbon content are best suited for carburizing. Due to the diffusion of carbon into the material, martensite can form in the surface layer after further processing. This gives the workpiece a hard surface while retaining its soft, tough core.

Carburizing is suitable for case-hardening steels. Examples of this are:

• 1.6587
• 1.7147
• 1.7131
• 1.1121

Applications

Carburizing finds applications across multiple industries, each benefiting from its unique properties:

• Maritime: In the maritime industry, carburized components are used in ship engines and gear systems, offering resistance to saltwater corrosion and mechanical wear.
• Aerospace: Aircraft components, such as landing gear and engine parts, benefit from the high strength and fatigue resistance of carburized steel.
• Agribusiness: Agricultural machinery requires durable parts that can withstand harsh conditions, for which carburized steel is ideally suited.
• Industrial Manufacturing: Carburized gears, bearings, and tools are common in manufacturing equipment, where they provide enhanced longevity and performance.
• Automotive: In the automotive industry, carburizing is used for critical components like gears, shafts, and bearings, which demand high wear resistance and strength.

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