Carburizing Facts for Kids

A modern computerised gas carburising furnace

Carburizing (say: car-buh-rye-zing) is a special process that makes metal stronger. It's a type of heat treatment where iron or steel soaks up carbon. This happens when the metal is heated up with something that contains carbon. Things like charcoal or carbon monoxide can be used.

The main goal is to make the metal's surface much harder. This also helps it last longer and resist wear. How much carbon the metal absorbs depends on how long it's heated and how hot it gets. More time and higher heat usually mean more carbon goes deeper into the metal.

After heating, the metal is quickly cooled down. This quick cooling is called quenching. The outer part of the metal, which now has more carbon, becomes very hard. But the inside of the metal stays softer and tough. This makes the metal strong on the outside and flexible on the inside.

This process is used for metals that don't have much carbon to begin with. It creates a hard outer layer, often called a "case." This case can be quite deep, up to about 0.25 inches (6.35 mm). Sometimes, carburizing is even used to fix metal that has lost carbon during other manufacturing steps.

How Carburizing Works
What Materials Are Used?
Changes to the Metal
What Metals Are Carburized?
Choosing the Right Method
See also

How Carburizing Works

Carburizing makes the surface of low-carbon steel harder. It involves heating the metal with a source of carbon.

Early and Modern Methods

In the past, people would pack charcoal around the metal they wanted to treat. This was called case hardening. Today, modern methods use gases or plasmas that contain carbon. Examples include carbon dioxide or methane.

The process needs careful control of the gas mix and the furnace temperature. This is important because the heat can also change the metal's inner structure. For very precise control, carburizing can happen in a vacuum chamber.

Plasma Carburizing

Plasma carburizing is a newer method that's becoming more popular. It helps improve how metals act, like how well they resist wear or corrosion. It's especially good for stainless steel. This method is also better for the environment. Plus, it can treat parts with tricky shapes evenly. This is because the plasma can get into small holes and tight spaces.

How Carbon Strengthens Metal

Carburizing works by letting carbon atoms spread into the outer layers of the metal. Metals are made of atoms tightly packed in a crystalline lattice. The carbon atoms move into this structure. They either dissolve into the metal or combine with other elements in the metal to form carbides.

When carbon dissolves, the steel is then heat treated to make it hard. Both ways make the metal's surface stronger. They create hard materials like pearlite, martensite, or carbides. These materials are tough and resist scratching and rubbing.

Gas carburizing usually happens at very high temperatures, around 900 to 950 degrees Celsius (1650 to 1740 degrees Fahrenheit).

Carburizing in Welding

In oxy-acetylene welding, a "carburizing flame" has less oxygen. This flame is sootier and not as hot. Welders often use it to anneal metal. Annealing makes the metal softer and more flexible during welding.

Ensuring Good Contact

For good carburizing, the metal surface needs to touch the carbon-rich materials as much as possible. In gas and liquid carburizing, metal parts are often placed in mesh baskets. Or they might be hung by wires.

In "pack carburizing," the metal and carbon are put together in a sealed container. This makes sure they stay in contact over a large area. These containers are often made of carbon steel. They are coated with aluminum or a special heat-resistant alloy.

What Materials Are Used?

Many different materials can be used to add carbon. These materials must have a lot of carbon in them. Some common ones include:

Carbon monoxide gas (CO)
Sodium cyanide and barium carbonate
Hardwood charcoal

In gas carburizing, carbon comes from propane or natural gas. For liquid carburizing, the carbon comes from a melted salt mixture. This mixture mainly contains sodium cyanide (NaCN) and barium chloride (BaCl₂). In pack carburizing, carbon monoxide comes from coke or hardwood charcoal.

Changes to the Metal

Carburizing will always cause some small changes to the metal's size. How much it changes depends on the type of metal, the carburizing method, and the original size and shape of the part. However, these changes are usually small compared to other heat-treating processes.

Here are some ways carburizing changes metal:

Mechanical changes: The surface becomes much harder and resists wear better. It also gets stronger against bending or breaking.
Physical changes: The tiny grains inside the metal might grow bigger. The overall size of the metal might change a little.
Chemical changes: The amount of carbon on the surface of the metal increases a lot.

What Metals Are Carburized?

Usually, metals with low carbon content are carburized. These are often low-carbon and alloy steels. They typically start with about 0.2% to 0.3% carbon. The metal's surface must be very clean. Things like oil, rust, or cleaning solutions can stop the carbon from spreading into the metal.

Choosing the Right Method

Different carburizing methods are good for different situations.

Pack carburizing can handle larger metal parts.
Liquid carburizing and gas carburizing are faster. They are also easier to use with machines for moving parts. Liquid carburizing is often used for small and medium parts.
Gas carburizing is generally used for larger parts.
Vacuum carburizing (also called low-pressure carburizing) can be used for many different parts. It works well with oil or high-pressure gas cooling.

Carburizing has some advantages over a similar process called carbonitriding. It can create a deeper hard layer and causes less bending or twisting of the metal. It also makes the metal stronger against impacts. This makes it great for things that need to be very strong and resist wear, like scissors or swords. However, it can be more expensive, needs higher temperatures, and takes more time.

Carburizing facts for kids

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