Carburizing is one of the most commonly used processes. It involves the diffusion of carbon into mild steel to create a high-carbon steel surface. The carburizing process is also known as hardening. It is a heat treatment method that provides a wear-resistant surface while maintaining the ductility and strength of the core. The carburizing process has evolved with advancements in heat treatment techniques that increase the hardness and durability of products and are used to produce items such as carbon steel wire springs and carbon steel forgings. During carburizing, the steel is heated to 850 to 950 degrees Celsius in the presence of a carbonaceous material, such as charcoal or carbon monoxide. This behavior is performed to harden the steel.
Steel heated at this temperature has an austenitic structure, which has a high solubility for carbon and is a stable structure. Depending on the time and temperature, the carbon content in the affected area may vary. Longer carburizing times and higher temperatures generally increase the depth of carbon diffusion. When the steel is rapidly quenched, the higher carbon content on the outer surface is hardened by transformation from austenite to martensite, while the core retains a soft and hard ferritic and/or pearlitic microstructure. What elements are subjected to the carburizing process? Does it affect the strength of the core? How many carburizing methods do we have? We write about all this below!
Carburizing method
Carburizing of steel is a thermal treatment of metal surfaces using a carbon source. Carburizing can be applied to increase the hardness of the surface of soft steel. Early carburizing involved the direct application of charcoal wrapped around the sample being processed, but modern techniques use gas or plasma containing carbon. The process largely depends on the composition of the surrounding gas and the furnace temperature and must be precisely controlled, as heat also affects the microstructure of the remaining part of the material. In applications requiring high control of gas composition, carburizing can be carried out in a vacuum chamber at very low pressures.
Increasingly, plasma carburizing is 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 geometry, making it very flexible in part processing. The carburizing process involves the diffusion of carbon atoms into the surface layer of the metal. Since metals consist of atoms tightly bound to the metal lattice, carbon atoms diffuse into the crystalline structure of the metal and either remain in solution or react with elements in the parent metal, forming carbides.
If carbon remains in solid solution, the steel is subjected to heat treatment to harden it. Both mechanisms strengthen the surface of the metal, the first by creating pearlite or martensite, and the second by creating carbides. Both materials are hard and wear-resistant. Generally, gas carburizing is carried out in the temperature range from 900 to 950°C. In oxy-acetylene welding, the carburizing flame is a flame with a very small amount of oxygen, which produces soot. It is often used for annealing metals to make them more plastic and flexible during welding.
Types of Carburizing
In the past, depending on the source of carbon, there were three methods of carburizing: solid carburizing, liquid carburizing, and gas carburizing. Charcoal, molten salt, and carbon-containing gases such as natural gas and propane are used respectively.
There are three commonly used methods of carburizing:
- gas carburizing
- liquid carburizing
- solid carburizing
All three processes are based on the transformation of austenite into martensite during hardening. The increase in carbon content on the surface must be large enough to produce a martensitic layer of sufficient hardness, usually 700 HV, to provide a wear-resistant surface. The carbon content required for the diffusion surface is usually from 0.8 to 1.0% C. These processes can be carried out on various carbon steels, alloy steels, and cast irons with a carbon content of up to 0.4% by weight and usually below 0.25% by weight. Improper heat treatment can lead to oxidation or decarburization. Carburizing, although a relatively slow process, can be used as a continuous process for costly surface hardening.
Carburizing processes and methods
While the basic principles of carburizing have changed little since its inception, the technology for introducing carbon has improved. Below is a description of the carburizing process commonly used in the industry.
Pack carburizing
In this process, elements made of soft steel are packed in a high-carbon environment, such as cast iron chips or coal powder. These ingredients are heated to produce carbon monoxide, which is a reducing agent. Reduction occurs on the surface of the steel, releasing carbon that diffused onto the surface due to high temperature. The steel element hardens when the carbon inside the element is absorbed. Depending on the process environment, the carbon content on the surface ranges from 0.7% to 1.3%. The casing depth is about 0.1 mm to 1.5 mm. Controlling the carburizing of the package is difficult because it is hard to maintain a uniform temperature. Sealed carburizing is effective for introducing carbon, but this method is very slow.
Gas carburizing
During gas carburizing, the part is surrounded by a constantly replenished atmosphere containing carbon to maintain a high potential of this component. Although the rate of carburizing is significantly improved in a gas 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 oxides on the surface and at grain boundaries. Additionally, separate equipment for creating the atmosphere and controlling its composition is needed. The process of gas carburizing is theoretically similar to the process of plated carburizing, with the difference that carbon monoxide (CO) is supplied to the heated furnace and carbon is decomposed. This process eliminates many problems associated with bag carburizing. The CO gas must be safely cut off. Despite the additional complexity, gas carburizing has become the most efficient and widely used method of mass carburizing of steel parts.
Liquid carburizing
In this process, steel parts are immersed in a carbon-rich, liquefied environment. The main component of this bath is cyanide. However, for safety reasons, non-toxic baths have been developed that can achieve similar results. These elements are kept in molten salt, which introduces carbon into the steel. Carbon diffuses inward, creating a hardened shell through rapid quenching. The shell produced by carbon diffusion is similar to that produced by gas carburizing. The liquid carburized casing is characterized by low nitrogen content and high carbon content.
Vacuum Carburizing
The vacuum carburizing process involves carburizing in an oxygen-free environment at low pressure. Although the furnace shell is more complex, the atmosphere is significantly simplified. A single-component environment is used, containing simple gaseous hydrocarbons such as methane. Since the environment used for heating is devoid of oxygen, the carburizing temperature can be significantly increased without oxidizing the surface or grain boundaries. Higher temperatures increase the solubility and diffusion rate of carbon. This way, the time needed to deepen the shell is shortened. While vacuum carburizing overcomes some of the intricacies associated with gas carburizing, it introduces a new serious problem that needs to be solved. Since vacuum carburizing takes place under very low pressure, and the flow rate of the carburizing gas to the furnace is very low, the carbon potential of the gas in deep recesses and through holes quickly depletes. If this gas is not replenished, large inhomogeneities of shell depth may occur on the surface of the parts. If the gas pressure is significantly increased to overcome this problem, another problem arises, such as the formation of free carbon or soot deposition. Therefore, to achieve a reasonably uniform depth on elements with complex shapes, the air pressure must be periodically increased to replenish the depleted atmosphere in the recess before reducing to the working pressure. It is clear that there is a delicate balance in the vacuum carburizing process: process conditions must be adjusted to achieve the best balance between shell uniformity, the risk of soot deposition, and carburizing speed.
Plasma carburizing
In the plasma carburizing method, positive carbon ions are applied to the surface of a steel part (cathode). The main difference between conventional carburizing and plasma carburizing is the shortened carburizing time with the plasma method. Rapid surface saturation also leads to faster diffusion kinetics. In addition, plasma carburizing provides a very uniform carburizing depth, even in the case of parts with irregular surfaces. Plasma carburizing is increasingly used in large industrial plants to improve surface properties (such as wear and corrosion resistance, hardness and load capacity, and quality-dependent variables) of various types of steel, especially stainless steel. This process is used because it is environmentally friendly (compared to gas or solid carburizing). It also provides uniform treatment of parts with complex geometry (plasma can penetrate holes and tight gaps while saturating the surface layer), making it very flexible in part processing. Plasma carburizing was applied due to the lack of oxygen in the furnace atmosphere.