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Carburizing is the process by which low-carbon steel is transformed into high-carbon steel. This is done by exposing it to carbon-rich conditions. Typically, the components are carburized in furnaces, ladles, and other closed systems. By heating a steel sheet in an atmosphere of dense carbon, the object will attach these atoms to its surface at the molecular level. During this process, the steel gains both hardness and strength.


Advantages of carburizing precision components 

Carburizing is one of the most popular forms of hardening. It can provide components with varying degrees of hardness. The higher the carburizing temperature and the longer the time, the harder the item will be.

The main advantages of carburizing:

  • Creates a hard steel surface by increasing the carbon content of the surface.
  • The increased surface hardness results in increased wear and fatigue resistance.
  • Steel cores largely retain their elasticity.
  • In some cases, it can remedy unwanted decarburization that occurs early in the manufacturing process.

The depth of the carburized steel layer is a function of the carburization time and the carbon potential available at the surface. A longer carburizing time is used to achieve a greater depth of the hardened layer, and a high carbon potential results in a high carbon content at the surface, which can result in excess of residual austenite or free carbides. These two microstructure elements hurt the stress distribution in carburized parts. 

The carburizing atmosphere must be capable of transferring carbon into the steel to provide the desired surface hardness. This transfer must be strictly controlled in terms of the concentration of carbon on the steel surface to meet hardness tolerance requirements. The ratio (% vol CO) ²/ (% vol CO2) in the furnace atmosphere can control the concentration.

The process of carburizing precision components 

While the fundamentals of carburizing have changed little since its inception, techniques for inserting carbon have improved. Standard carburizing processes used in the industry are outlined below.

  • Batch carburizing – In this process, mild steel components are surrounded by an environment with high carbon content, such as iron chips or carbon powder. These elements are heated to produce carbon monoxide, a reducing agent. The reduction occurs at the surface of the steel, while the release of carbon diffuses to the surface when exposed to high temperatures. The steel hardens as the carbon is absorbed into the component, and the carbon content on the surface varies from 0.7% to 1.3%, depending on the process environment. The depth of the coating is about 0.1 mm to 1.5 mm. Batch carburizing is challenging to control because it is problematic to maintain a uniform temperature. 
  • Gas carburization – During this process, the part is surrounded by a carbon-containing atmosphere that is constantly replenished, thus maintaining a high carbon potential. Although the carburization rate is greatly increased in a gaseous atmosphere, this method requires a multi-component atmosphere, the composition of which must be very strictly controlled to avoid harmful side effects such as surface and grain boundary oxides. In addition, different equipment is required to create the atmosphere and handle its composition. The gas carburization process is theoretically similar to the batch carburization process, except that carbon monoxide (CO) is fed into the furnace, and carbon decomposition occurs. The CO gas must be safely contained. Despite the increased complexity, gas carburizing has become the most efficient and widely used method for carburizing steel parts in large quantities.
  • Liquid carburizing – Steel parts are immersed in a liquid, carbon-rich environment in this process. The main ingredient in this bath is cyanide. However, non-toxic baths have been developed for safety reasons that achieve similar results. Parts are preserved in molten salt, which introduces carbon into the steel. It diffuses inward through rapid hardening, forming a hardened shell. The shells produced by carburizing are similar to those produced by gas carburizing. The case made by liquid carburizing has low nitrogen and high carbon content.
  • Vacuum carburizing involves carburizing in an oxygen-free environment and low pressure. The atmosphere is much simpler, although the furnace shell is more complicated. A single-component environment containing simple gaseous hydrocarbons such as methane is used. This heating application is oxygen-free, so the carburization temperature can greatly increase without oxidizing the surface or grain boundaries. Higher temperatures increase carbon solubility and diffusion rates. Although vacuum carburization overcomes some of the complications associated with gas carburization, it introduces a significant new problem that must be addressed. Because vacuum carburizing is carried out at shallow pressure, the flow rate of carburizing gas into the furnace is very low, and the carbon potential of the gas in deep cavities and blind holes is left negligible. If not replenished, the depth of the casing on the surface of the part can vary greatly. If the air pressure is increased significantly to overcome this problem, another problem appears in the form of deposits of free carbon or soot. Therefore, in order to achieve a fairly uniform depth on parts with complex shapes, the air pressure must be periodically increased to replenish the depleted atmosphere in the cavity and then lowered again to operating pressure. Clearly, there is a delicate balance in vacuum carburizing: process conditions must be adjusted to achieve the best compromise between casing uniformity, risk of soot formation, and carburization rate.
  • Plasma carburizing – In this method, plasma deposits positively carburized ions on the surface of the steel component. The main difference between conventional carburizing and plasma carburizing is the reduced time with the plasma method. The quickly achieved surface saturation also leads to faster diffusion kinetics. In addition, plasma carburizing makes achieving a very uniform carburizing depth possible, even on parts with irregular surfaces. It is increasingly used in large industrial plants to improve the surface properties (such as wear and corrosion resistance, hardness and load-bearing capacity, as well as mass-dependent variables) of various steels, especially stainless steel. The process is used because it is environmentally friendly (compared to gas carburizing). It can also uniformly machine parts with complex geometries (plasma can penetrate through holes and narrow slots), making it very flexible for machining parts.

Carburizing steel 

The carbon content of carburized steel is generally around 0.2%, and the carbon content in the carburized layer is usually controlled in the range of 0.8% to 1%. However, the carbon content at the surface is usually limited to 0.9%, as too high a content leads to residual austenite and brittle martensite.

Most carburized steels are calcified steels (deoxidized by adding aluminum), which retain fine grain sizes up to about 1040°C. Coarse-grained steels can be carburized if double quenching provides grain fineness. This usually involves direct quenching followed by re-hardening at a lower temperature.

Selection of steel grades 

When selecting steel grades, alloy and carbon content are first required to meet aspects of core hardness after austenitizing, quenching, and tempering. For a specific core hardness requirement, the required alloy content increases as the size of the workpiece increases. The hardenability of carburizing steels must be good enough to achieve a martensitic surface coating to the desired depth. Steel for carburizing must contain a certain amount of alloying elements. Another requirement is that the steel used for carburizing should be fine-grained. This means the steel should contain an alloying element, mainly aluminum, which forms deposits. These act as a barrier to grain growth until a certain maximum temperature is reached, usually around 950°C.

Core hardenability

Many alloy steels for surface hardening are now determined by core hardenability. While the same considerations apply to selecting non-carburized grades in general, there are some specifics in carburized applications. First and foremost, the hardenability of the core and shell must be considered for carburized steels. Due to the difference in carbon content, the shell and core have completely different hardenability, and for some steels, the difference is much greater than for others. In addition, these two elements perform different functions during operation. Before the introduction of low-alloy steels containing boron, there was no need to worry about the hardenability of the shell because the alloy content combined with the high carbon content always provided sufficient hardenability. Keep in mind, however, that the steel is hardened immediately after carburizing to dissolve carbon and alloying elements in the austenite of the case, and for parts hardened by reheating, as well as pieces with large cross sections – shell and core hardenability requirements must be carefully evaluated.

The relationship between the thermal and carbon gradients during the hardening of carburized parts can be a measurable difference in pitting depth and hardness. This means that for a given level of carbon, an increase in the hardenability of the substrate can lead to a higher proportion of martensite, resulting in a greater measured pitting depth. Therefore, a shallower carbon profile and shorter carburizing time can achieve the desired result in properly selected steels.