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Carburizing

Carburizing

 

Carburizing refers to a manufacturing process which hardens the exterior of steel or iron parts by impregnating the surface with carbon atoms. This process contributes to "case hardening" or "surface hardening" in which the interior of the metal remains comparatively soft but the external layer hardens into a tough, protective shell. Carburizing typically relies upon highly controlled heat treatment processes to speed the transfer of carbon atoms. 

Metallurgists in this century use a variety of methods to obtain hardened steel surfaces. Along with induction hardening and nitriding, carburizing has become one of the most popular manufacturing processes.

 

Carburizing Process

A manufacturer performs carburizing by endeavoring to harden the exterior of iron or low carbon steel through the addition of carbon. This process transforms the chemical composition of the surface of iron or steel parts. Work pieces typically undergo this process after machining. A carburized tool will break less easily and will resist scratches and other abrasions more effectively during the course of normal use. Manufacturers use case depth measurements to help evaluate the thickness of this outer protective layer.

During carburizing, carbon atoms diffuse from a carbon source into the surface of a metal work piece. They remain permanently attached within the structure of the metal as the part cools. The carbon will increase hardness in one of two ways: either it becomes a part of the exterior structure of the metal itself, forming an outer martensitic or pearlitic steel layer, or in some cases it will form carbides after combining with other elements in the alloy. Both situations produce permanently hardened steel surfaces.

 

Carburizing Methods

Four carburizing methods have gained popularity in manufacturing settings. Each offers advantages and disadvantages:

 

Liquid Carburizing

Manufacturers sometimes introduce carbon atoms to the surface of low carbon steels by submersing the metal at high temperatures in baths of high-carbon liquids. 

Process Details

In the past, the liquids used for this process often included cyanide, a strong acid which would help abrade the surface to prepare the metal for carbon deposition. Today, manufacturers usually employ more environmentally friendly, less toxic acids during the process of liquid carburization. Mesh baskets or wire enable the entire work piece to remain suspended in the liquid bath for a desired period of time, ensuring uniform carburization.

Benefits

This process facilitates the production of exterior metal cases with a high carbon content. It usually requires comparatively little time to complete. Liquid carburizing proves very efficient for strengthening the surfaces of small and medium sized parts; manufacturers can automate this process easily.

Examples

Manufacturers producing small items such as ball bearing frequently employ liquid carburizing.

 

Gas Carburizing

This widely popular method of carburizing uses gases such as methane or carbon monoxide to cause the deposition of carbon onto the surface of low carbon steel alloys or irons.

Process Details

Today manufacturers carefully control temperatures to achieve optimum levels of carburization. This process usually takes place in large furnaces. It frequently occurs in conjunction with automation, and offers excellent control over the depth of case hardening.

Benefits

Gas carburizing enable manufacturers to work on very large work pieces more easily. Additionally, it may permit a company to case harden high volumes of material effectively.

Examples

This process permits the carburizing of commercial safes and vaults, as well as large industrial blades and other heavy items.

 

Pack Carburizing

This manufacturing process, discovered many years ago, involves packing low carbon steel or iron with charcoal or coke and heating the material at very high temperatures.

Process Details

Carbon monoxide gas will form and help transfer some carbon into the surface of the metal. Manufacturers often experience challenged maintaining uniform temperatures across the surface using this method, so carbon levels in the carburized surface may vary. This process tends to generate somewhat thin (1mm to 1.5mm) case hardened layers.

Benefits

Pack carburizing methods enable fabrication companies to carburize both large parts and high volumes of small parts.

Examples

Manufacturers sometimes produce inexpensive consumer tools, such as gardening trowels and rakes, using this method.

 

Vacuum Carburizing

Typically used in conjunction with oil or high pressure gas quenching techniques, vacuum carburizing takes place in the absence of oxygen.

Process Details

Companies often utilize methane as an agent for diffusing carbon, injecting this gas into a carburizing oxygen-free vacuum chamber at very high temperatures.

Benefits

This flexible, expensive process allows manufacturers to carburize a variety of differently sized work pieces effectively. It produces case hardening comparatively rapidly.

Examples

This method proves ideal for the production of many aviation, aerospace and drilling components.

 

Applications

The surface hardening caused by carburizing enjoys widespread applications in a variety of industries. For instance, today carburized drilling bits assist people in many situations, from woodworking to oil and gas exploration. Carburization also contributes significantly to the machining industry, enabling manufacturers to extend the useful lifespan of many items of metal equipment. Some industries which rely heavily upon carburized metals include: the maritime and aerospace industries, agribusiness, industrial manufacturers, and the automotive industry.

 

Key Advantages

Carburizing offers advantages because it supplies case hardening for the exterior surfaces of low carbon steel and iron alloys. Metal parts which undergo heat treatment and carburizing tend to resist abrasions more effectively, for instance. The hardness of a carburized exterior also protects sharp tools against chipping easily during normal use. Carburizing also typically enhances wear resistance, extending the useful lifespan of tools and metal products.


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