Hardening Steel Surfaces

February 08, 2023

One of the most popular methods for case hardening steel, carburization is a sophisticated, cost-effective process that consistently creates excellent results. This post explains what carburization is as well as the various techniques used in the process.

Carburization is a case hardening process that uses carbon absorption to develop a hard exterior shell, or “case,” on a steel workpiece. Case hardening is often used interchangeably with carburization, but this is inaccurate. Case hardening is the overarching term for any process that hardens a steel part’s surface, while carburization is a specific type of case hardening.

The process transforms low-carbon steel, also called mild steel, into high-carbon steel by soaking it in an extremely hot, carbon-dense environment. This treatment enables loose carbon atoms to attach to the workpiece’s surface, resulting in a rigid surface with a soft inner core. Carburizing is sometimes used to fix undesired decarburization that occurred in prior manufacturing processes.

In general, the hotter the furnace and the longer the workpiece soaks, the more carbon will permeate the steel and the harder its surface.

The Carburizing Process

Generally, the carburizing process is relatively simple:

1. A steel workpiece is heated in the presence of a carbon-dense medium in any form — gas, liquid or solid.
2. The carbon from the medium diffuses into the workpiece surface, forming a hard case.
3. The steel is removed from the furnace and quenched, hardening the carbon in the case.

The result is a piece with a hard, wear-resistant outer shell and a soft core.

Early carburization required the steel workpiece to be in direct contact with a solid carburizing medium during the procedure. While similar techniques are still in use today, most modern methods use high-carbon gases or liquids.

The composition of these mediums and the temperature of the furnace must be carefully controlled, as different temperatures and mixtures can change which microstructures will form in the case and impact the final result. For example, low-pressure vacuum chambers provide excellent control over gas composition in vacuum carburization procedures.

What are the Different Types of Carburizing

Many different carburizing techniques are used today, each with various advantages and disadvantages. The following are the four most common methods for industrial applications.

Gas Carburization is one of the most popular carburization techniques used today because it produces a highly uniform result, making it an advantageous step in mass production.

In this process, you heat mild steel in a furnace filled with a carbon-rich atmosphere. This atmosphere is typically a mixture containing:

• A hydrocarbon, such as methane, propane, or chemical acetylene
• An inert carrier gas, such as nitrogen or hydrogen

When the furnace reaches the carburizing temperature, which is approximately 950 degrees Celsius, the hydrocarbon gas decomposes into carbon and hydrogen atoms. The crystalline microstructure of the steel workpiece’s surface shifts, allowing the piece to absorb the carbon atoms from the air. This process can last anywhere from two to 36 hours, depending on the workpiece and the desired case depth.

Once the steel has been carburized, you remove it from the furnace and quench it in oil, gas, water, or a caustic soda. This step rapidly cools the metal, permanently locking in the carbon.

Vacuum Carburization, or low-pressure carburizing (LPC), is a specific method for gas carburization that takes place in a low-pressure, oxygen-free vacuum chamber.

The lack of oxygen in the environment allows you to use temperatures up to 1,000 degrees Celsius without the risk of oxidation, which can cause corrosion. Higher temperatures increase carbon solubility and diffusion rate, accelerating the process and minimizing the time it takes to reach the desired case depth.

The atmosphere used for LPC must be highly controllable and reproducible to ensure consistent results. Advanced nitrogen and methanol technologies, combined with a precise atmosphere control system, provide a high degree of control over the process.

Liquid Carburization requires placement of the workpiece in a carbon-dense salt bath and heat it to the temperature required to change its microstructure to austenite, which is around 900 degrees Celsius. As it soaks, the workpiece absorbs carbon from the bath and hardens.

The liquid bath typically contains a mixture of sodium carbonate, sodium chloride, and silicon carbide. Cyanide salts are another common ingredient due to their nitrogen content — the nitrogen can help prevent oxidation and further increase surface toughness.

Although liquid carburization is generally the fastest technique, it is difficult to use for small batches. Mass production is often the best application for this process.

Solid Carburization, or pack carburization, involves packing your workpiece in a container with a solid carburizing medium and heating that container in a furnace at approximately 900 degrees Celsius.
Common materials used include:

• Container: The container is often carburized steel, aluminum-coated carbon steel, or heat-resistant iron-nickel-chromium alloys.
• Carburizing medium: Common carburizing mediums include coke, sodium carbonate, or a charcoal-barium carbonate that supplements carbon absorption.

As the container heats up, the carbon monoxide from the medium dissociates and decomposes into carbon and carbon dioxide on the workpiece’s surface.
Solid carburization is the simplest technique out of the four, but it is ultimately unreliable for producing consistent results — uniform carburization is difficult to achieve across the entire workpiece.

The main advantage of steel carburization is that it is a practical and affordable process for steel products in various industries. Other benefits include:

• Cost-effective production: Carburization is significantly less costly than other steel hardening techniques, making carburized steel pieces more affordable than other case-hardened steel parts. As a result, carburized steel is an ideal material for mass production.
• Increased hardness: Carburized steel gains a highly wear-resistant surface with improved fatigue strength. These properties allow it to handle more force without wearing down as quickly as un-carburized steel.
• Improved corrosion resistance: Carburizing mild steel creates a protective, carbon-dense outer layer on the workpiece, making it more resistant to corrosion.
• Malleable core: Although the surface of a carburized workpiece hardens during the treatment, the core remains soft and ductile, enabling manufacturers to create more intricate pieces. In contrast, steel that has been hardened through other methods is more rigid, making it difficult to shape.

These advantages make carburized steel an excellent material for various products, particularly delicate machine parts such as gears, bearings, piston pins, and sprockets.

Contact Linde for Gas Carburization Atmospheres

At Linde, we are committed to making the world more productive every day, which is why we provide our customers with highly repeatable carbon-dense atmospheres for use in gas carburization and LPC processes. You can count on our industrial atmospheres for your steel carburizing heat treatment applications.