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Carburizing is one of the most widely used carburizing processes. It has been used for a long time. The process involves the diffusion of carbon into mild steel to create a high-carbon steel surface. The carburizing process is also known as hardening. Heat treatment provides a wear-resistant surface while maintaining flexibility and core strength. The carburizing process has evolved with advances in heat treatment techniques that increase the hardness and durability of products such as carbon steel wire springs and carbon steel forgings. Carburizing is a heat treatment process in which carbon is released when steel is heated to 850 to 950 degrees Celsius in the presence of carbonaceous material, such as charcoal or carbon monoxide, to harden the steel. 


Steel heated at this temperature has an austenitic structure, which has a high solubility for carbon and is stable. Depending on the time and temperature, the carbon content of the affected area may vary. Longer carburizing times and higher temperatures generally increase the depth of carbon diffusion. When steel is quenched quickly by quenching, the higher carbon content on the outer surface is hardened by the transformation from austenite to martensite. At the same time, the core retains a soft and hard ferritic and perlitic microstructure. What is carburizing of steel and low carbon steel? What is carburizing of steel and low carbon steel? How does it work? We read about it bellow!

Carburizing method

Steel-carburizing is the heat treatment of metal surfaces using a carbon source. Carburizing can be used to increase the surface hardness of mild steel. Early carburizing involved the direct application of charcoal wrapped around the workpiece, but modern techniques use gas or plasma containing carbon. The process is highly dependent on the composition of the surrounding gas and the temperature of the furnace. It must be carefully controlled because the heat also affects the microstructure of the rest of the material. In applications requiring high gas composition control, carburization can be carried out in a vacuum chamber at external pressures. Plasma carburizing is increasingly being used to improve the surface properties of various metals, especially stainless steel. The process is environmentally friendly. It also provides uniform processing of parts with complex geometries, making it very flexible for machining parts. The carburizing process involves the diffusion of carbon atoms into the surface layer of the metal. Since metals are composed of atoms that are tightly bonded to the metal lattice, carbon atoms diffuse into the metal’s crystal structure and either remain in solution or react with elements in the parent metal to form carbides.
If the carbon remains in a solid solution, the steel is heat-treated to harden it. Both mechanisms strengthen the surface of the metal, the first by forming pearlite or martensite and the second by forming carbides. Both materials are hard and wear-resistant.
Gas carburizing is generally carried out in the temperature range from 900 to 950°C.
In acetylene-oxygen welding, the carburizing flame is a flame with very little oxygen that produces carbon black, a lower-temperature flame. It is often used to anneal metals to make them more pliable and flexible during welding.

Types of carburizing

Depending on the source of coal, there were three methods of carburization: solid carburization, liquid carburization, and gas carburization. Charcoal, molten salt, and carbon-containing gases such as natural gas and propane are used.
There are three commonly used carburization methods:

  • gas carburization
  • liquid carburization
  • solid carburization

All three processes are based on the transformation of austenite to martensite during hardening. The increase in the carbon content of the surface must be significant enough to produce a martensitic layer of sufficient hardness, typically 700 HV, to provide a wear-resistant surface.
The carbon content required for a diffusion surface is typically 0.8 to 1.0% C. These processes can be carried out on various carbon steels, alloy steels, and cast irons with carbon contents of up to 0.4% by weight and usually less than 0.25% by weight. Improper heat treatment can lead to oxidation or decarburization. Although a relatively slow process, carburizing can be used as a continuous process for expensive surface hardening.

Carburization processes and methods

While the basic principles of carburizing have changed little since its inception, the technology for introducing coal has improved. A standard carburizing process used in industry is described below.

Pack carburizing

In this process, mild steel components are packed in a high-carbon environment, such as iron chips or carbon powder. These components are heated to produce carbon monoxide, an agent of.
Reduction occurs at the surface of the steel, releasing carbon that has diffused to the surface due to the high temperature. The steel component hardens as the carbon inside the component is absorbed. Depending on the process environment, the carbon content at the surface ranges from 0.7% to 1.3%. The depth of the case is about 0.1 mm to 1.5 mm. Controlling the carburization of the package is problematic because it is difficult to maintain an even temperature. Encapsulation carburization is effective for introducing coal, but this method is prolonged.

Gas carburization

During gas carburization, the part is surrounded by a constantly replenished atmosphere containing carbon to maintain a high carbon potential. Although the rate of carburization is greatly improved in a gaseous atmosphere, this method requires a multi-component atmosphere, the composition of which must be strictly controlled to avoid harmful side effects such as oxides on the surface and in the grain boundaries. In addition, different equipment is used to create the atmosphere and maintain its composition. The gas carburization process is theoretically similar to the plated carburization process, except that carbon monoxide (CO) is supplied to the heated furnace, and the carbon is decomposed. This process eliminates many of the problems associated with bag carburization. CO gas must be safely shut off. Despite the added complexity, gas carburizing has become the most efficient and widely used method for bulk carburizing steel parts.

Liquid carburizing

Steel parts are immersed in a carbon-rich, liquefied environment during this process. The main ingredient in this bath is cyanide. However, non-toxic baths have been developed for safety reasons to achieve similar results. The elements are held in a molten salt that introduces carbon into the steel. The carbon diffuses inward, forming a hardened shell through rapid hardening. The shell produced by carbon diffusion is similar to that produced by gas carburization. The liquid carburized case is characterized by low nitrogen and high carbon content.

Vacuum carburizing

The process involves carburizing in an oxygen-free, low-pressure environment. Although the furnace shell is more complex, the atmosphere is greatly simplified. A single-component environment containing simple gaseous hydrocarbons such as methane is used. Since the domain used for heating is oxygen-free, the carburization temperature can be significantly increased without oxidizing the surface or grain boundaries. Higher temperatures increase the solubility and diffusion rate of carbon. This reduces the time required to deepen the crust. While vacuum carburization overcomes some of the complexities associated with gas carburization, it introduces a significant new problem that must be solved. Because vacuum carburizing is carried out at external pressure and the flow rate of carburizing gas into the furnace is meager, the carbon potential of the gas in deep cavities and blind holes are quickly depleted. If this gas is not replenished, there can be large crust depth inhomogeneities on the part’s surface. If the gas pressure is increased significantly to overcome this problem, another problem arises, such as free carbon formation or soot deposition. Therefore, to achieve a reasonably uniform depth on parts with complex shapes, the air pressure must be periodically increased to replenish the depleted atmosphere in the cavity before reducing it to operating pressure. It is clear that there is a delicate balance in the vacuum carburizing process: the process conditions must be adjusted to achieve the best balance between the uniformity of the shell, the risk of soot deposition, and the carburization rate.

Plasma carburizing

In the carburizing method, plasma applies positive carbon ions to the surface of the steel part (cathode). The main difference between conventional carburizing and plasma carburizing is the reduced carburizing time of the plasma method. The rapid surface saturation achieved also leads to faster diffusion kinetics. In addition, plasma carburizing provides a very uniform carburizing depth, even for parts with irregular surfaces. Plasma carburizing is increasingly used in large industrial plants to improve the surface properties (such as wear and corrosion resistance, hardness and load-bearing capacity, and quality-dependent variables) of various sheets of steel, especially stainless steel. The process is used because it is environmentally friendly (compared to gas or solid carburizing). It also provides uniform parts processing with complex geometries (plasma can penetrate holes and tight crevices), making it very flexible for machining parts. Due to the lack of oxygen in the furnace atmosphere, plasma carburizing was used.