The process in which low-carbon steel is converted into high-carbon steel is called carburizing. This is done by exposing it to action in carbon-rich conditions. Typically, carburized elements are in furnaces, vats, and other closed installations. By heating a steel sheet in a dense carbon atmosphere, the object attaches these atoms to its surface at the molecular level. During this process, steel gains both hardness and strength.
Advantages of carburizing precision components
Carburizing is one of the most popular forms of hardening. It can provide elements with varying degrees of hardness. Generally speaking, the higher the temperature of carburizing and the longer the time, the harder the carburized object will be.
Main advantages of carburizing:
- Creates a hard surface of steel by increasing the carbon content in the surface.
- Increased surface hardness results in increased resistance to wear and material fatigue.
- Steel cores largely retain their plasticity.
- In some cases, it can act as a remedy for unwanted decarburization, which occurs at an early stage of the production process.
The depth of the carburized layer of steel is a function of the carburizing time and the potential of carbon available on the surface. A longer carburizing time is used to achieve a greater depth of the hardened layer, a high carbon potential results in a high carbon content on the surface, which can lead to an excess of residual austenite or free carbides. These two microstructure elements have a negative impact on the distribution of residual stresses in carburized elements.
The carburizing atmosphere must be capable of transferring carbon to steel to ensure the desired surface hardness. This transfer must be strictly controlled in terms of carbon concentration on the steel surface to meet hardness tolerance requirements. The concentration can be controlled using the ratio (% vol. CO) ²/ (% vol. CO2) in the furnace atmosphere.
Process of carburizing precision components
While the basics of carburizing have changed little since its inception, the techniques for inserting carbon have improved. Below are the commonly used carburizing processes in the industry.
- Periodic carburizing — in this process, elements made of soft steel are surrounded by an environment with a high carbon content, such as cast iron chips or carbon powder. These elements are heated to produce carbon monoxide, a reducing agent. The reduction occurs on the surface of the steel, while the release of carbon diffuses to the surface under the influence of high temperature. The steel hardens when carbon is absorbed into the interior of the element, and its surface content 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. Periodic carburizing is difficult to control, as maintaining a uniform temperature is problematic.
- Gas carburizing - During this process, the part is surrounded by an atmosphere containing carbon, which is constantly replenished, maintaining a high carbon potential. Although the rate of carburizing is significantly increased in a gaseous atmosphere, this method requires the use of a multi-component atmosphere, the composition of which must be very strictly controlled to avoid harmful side effects such as surface oxides and grain boundaries. In addition, separate equipment is required to create the atmosphere and control its composition. The process of gas carburizing is theoretically similar to the process of periodic carburizing, with the difference that carbon monoxide (CO) is supplied to the furnace and carbon decomposition occurs. The CO gas must be safely enclosed. Despite the increased complexity, gas carburizing has become the most efficient and widely used method of carburizing steel parts in large quantities.
- Liquid carburizing - in this process, steel elements are immersed in a liquid, carbon-rich environment. The main ingredient of this bath is cyanide. However, for safety reasons, non-toxic baths have been developed that achieve similar results. The parts are preserved in molten salt, which introduces carbon into the steel. It diffuses inside through rapid quenching, creating a hardened shell. The shells produced by carburizing are similar to those produced by gas carburizing. The casing made of liquid carburizing is characterized by low nitrogen content and high carbon content.
- Vacuum carburizing — The process involves carburizing in an oxygen-free and low-pressure environment. The atmosphere is significantly simpler, although the furnace lining is more complex. A single-component environment containing simple gaseous hydrocarbons, such as methane, is used. Such application for heating is oxygen-free, allowing the carburizing temperature to be significantly increased without oxidizing the surface or grain boundaries. Higher temperatures increase the solubility of carbon and the rate of diffusion. Although vacuum carburizing overcomes some complications associated with gas carburizing, it introduces a new serious problem that needs to be addressed. Since vacuum carburizing takes place under very low pressure, the intensity of the flow of carburizing gas to the furnace is very small, and the carbon potential of the gas in deep recesses and through holes becomes negligible. If not replenished, the casing depth on the part surface can vary significantly. If the air pressure is significantly increased to overcome this problem, another one arises, in the form of free carbon deposits or soot. Therefore, 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 the working pressure. It is clear that in the vacuum carburizing process there is a delicate balance: the process conditions must be adjusted to achieve the best compromise between the uniformity of the casing, the risk of soot formation, and the rate of carburizing.
- Plasma carburizing — In this method, plasma deposits positively carburized ions on the surface of a steel element. The main difference between conventional carburizing and plasma carburizing is the shortened time when using the plasma method. The quickly achieved surface saturation also leads to faster diffusion kinetics. Moreover, plasma carburizing allows for achieving a very uniform carburizing depth, even on parts with irregular surfaces. It is increasingly used in large industrial plants to improve surface properties (such as wear and corrosion resistance, hardness and load capacity, as well as mass-dependent variables) of various types of steel, especially stainless steel. This process is used because it is environmentally friendly (compared to gas carburizing). It can also evenly process parts with complex geometry (plasma can penetrate through holes and narrow gaps), making it very flexible in machining parts.
Carburizing Steel
The carbon content in carburized steel is generally about 0.2%, and the carbon content in the carburized layer is generally controlled in the range from 0.8% to 1%. However, the carbon content on the surface is usually limited to 0.9%, as too high content leads to residual austenite and brittle martensite.
Most of the carburized steel is calm steel (deoxidized by adding aluminum), which retains fine grains up to about 1040°C. Coarse-grained steel can be carburized if double tempering ensures grain fragmentation. It usually involves direct tempering, followed by re-tempering at a lower temperature.
Selection of steel grades
When choosing steel grades, the alloy and carbon content is first required to meet aspects related to the hardness of the core after austenitizing, quenching, and tempering. For a specific core hardness requirement, this means that as the size of the machined part increases, the required alloy content also increases. The hardenability of carburized steels must be good enough to achieve a martensitic surface layer to the desired depth. Carburizing steel must contain a certain amount of alloying elements. Another requirement is that the steel used for carburizing should be fine-grained. This means that the steel should contain an alloying element, most often aluminum, which forms deposits. They act as a barrier to grain growth until a certain maximum temperature is reached, usually around 950°C.
Core Hardness
Many alloy steels intended for surface hardening are currently defined based on the hardness of the core. While the same considerations generally apply to the choice of non-carburized grades, there are certain specifics in carburized applications. First and foremost, when carburizing steel, the hardness of the core and the shell should be taken into account. Due to the difference in carbon content, the shell and the core have completely different hardness, and for some steels, this difference is much greater than for others. Moreover, 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 hardness of the shell, as the alloy content combined with high carbon content always ensured sufficient hardness. However, let's remember that steel is hardened immediately after carburizing to dissolve carbon and alloying elements in the austenite shell, and in the case of parts hardened by reheating, as well as parts with large cross-sections - the requirements for the hardness of the shell and the core should be carefully evaluated.
The relationship between the thermal gradient and the carbon gradient during hardening of carburized parts can be a measurable difference in etch depth and hardness. This means that for a given level of carbon, an increase in substrate hardness can lead to the formation of a larger proportion of martensite, resulting in a greater measured etch depth. Therefore, a shallower carbon profile and shorter carburizing time can be used to achieve the desired result in appropriately selected steel.