Low-pressure carburizing (or known as vacuum carburizing) offers a clean, modern method, the so-called carburizing process. This thermal treatment requires true precision, ensuring minimal distortion of the original part during the carburizing process. This method can also serve as carburizing of steel.
Carburizing process
Vacuum carburizing (low-pressure) is an alternative to traditional gas carburizing (atmospheric). The carburizing process involves heat-chemical treatment, which consists of heating steel elements to a temperature typically in the range of 900-1000°C, and then placing them in a carburizing atmosphere. The purpose of carburizing is to enrich the surface layer with carbon through carbon atoms, increasing the carbon content for many types of steel increases its hardness and wear resistance while maintaining a plastic core. Quenching and tempering immediately after carburizing is most common, however, unlike conventional carburizing, it has many advantages, both from the side of the carrier gas used - carburizing (chemical composition C2H2 - acetylene), and from the environment - vacuum furnace. This process is environmentally friendly - no CO2 emissions and positively affects the core's durability.
Carburizing process - areas of application
Tool steels particularly benefit from the method of low-pressure carburizing, as this process creates exceptionally long wear surfaces, ideal for machining operations and improving the properties of certain materials:
The main advantages are:
– no grain boundary oxidation (no IGO),
- narrower tolerance range for the CHD carburized layer - Less result dispersion,
- stability and reproducibility of results,
- precise carburizing of narrow holes, including blind holes,
- minimization of hardening distortion,
-increased productivity due to the ability to perform processes at temperatures higher than conventional, i.e. 980°C and higher,
– clean surface of details (silver metal) – without scales,
Vacuum carburizing
During machining, special attention is paid to achieving and maintaining critical dimensions. Gears are then sent for heat treatment. Exposing parts to high temperatures, changing the chemical composition of the material, and rapid cooling in oil for hardening are necessary to achieve high strength and long life expectancy from large gearboxes. Heat treatment, although crucial for gearbox performance, can cause significant stress on parts and must be performed correctly to produce an acceptable part.
One of the most common questions is "How much will this process distort the raw material?" It's hard to answer this question. Factors such as the chemistry of the material, heat treatment and processing history, geometry, equipment, uniformity of the casing, and hardening processes play a role in determining how far the gears shift. Heat treatment controls the last three variables and helps minimize the variability of part dimensions. Vacuum carburizing can effectively solve these problems, as it provides a reliable alternative to conventional endothermic gas carburizing in integrated hardening furnaces or in-depth.
Process repeatability
The first stage of the procedure is heating to the process temperature, which takes place under vacuum. Before starting carburizing, increase and hold the load until the part reaches a uniform temperature. Vacuum carburizing is a non-equilibrium process that uses a series of charging and diffusion cycles. During the charging cycle under low pressure, a unique mixture of carburizing gases is injected. Carbon additives are based on carburized surfaces. The duration of the charging cycle is relatively short, while the longer diffusion time is used to distribute carbon on the parts. After carburizing to the desired depth, the part is cooled with nitrogen and then reheated to the hardening temperature. When the part has a uniform temperature, the charge is quenched in oil.
Distortion Control
The most challenging task in the heat treatment of gear wheels is minimizing distortions. Four main factors contribute to this: residual stress, changes in the dimensions of martensite, heat dispersion, and creep at high temperatures. The most important are the martensitic transformation and heat dispersion.
Residual stresses in materials cannot be controlled by heat treatment, but they can cause significant problems during hardening. The best way to solve the problem of residual stresses is heat treatment before processing. Typical processes include a combination of normalizing, annealing, hardening and tempering, and full annealing.
During hardening after carburizing, substances with a higher carbon content have a greater volume increase after conversion to martensite than other materials and undergo higher saturation of the surface layer. This can cause a large load on the material and cause its deformation or size change. The most effective way to minimize this effect is to maintain the uniformity of the shell depth across the entire part profile. The stress distribution caused by hardening is more uniform, and the movement of parts is reduced. Some size changes may occur.
If heat dissipation is not uniform, it can cause significant displacement of parts. When the gear comes into contact with oil, it begins to transform into martensite, and the material structure is complete. As the hardening of parts progresses, more material changes and internal stresses increase. If heat dissipation is asymmetric in the cross-section of the gear wheel, the stress caused by the formation of martensite can deform the part. The tool must be designed so that the gear enters the oil with minimal turbulence and is oriented so that the hardening flow is uniform from all sides.
Creep at high temperature occurs when a part is not properly supported during the carburizing process. Cycle times are very long - over 100 hours for some coatings. At this temperature, the metal moves or creeps, which results in increased ovality. Again, the key is proper tool design.
The vacuum carburizing process carried out in a blast furnace resulted in significantly better deformations compared to parts processed in a shaft furnace. When the deformation is small, a correction of the treatment can be made before heat treatment to reduce grinding after heat treatment. In addition, the depth of the ground shell can be adjusted, resulting in shorter heat treatment cycles and lower costs.
Microstructure
The flexibility of vacuum carburizing process variables allows for the design of each cycle to optimize the properties of the alloy subjected to heat treatment. The size and distribution of carbides in the carburized coating can be precisely controlled. The hardness distribution and depth can be predicted and adjusted to any specification.
Due to the fact that the process takes place in a vacuum, there is no oxygen. This eliminates the occurrence of intergranular oxidation (IGO) typical for endothermic atmospheric treatments. The presence of IGO requires heat treatment operations after such processes as full profile grinding or ballizing. Eliminating this reduces costs.
Summary
The vacuum carburizing process offers solutions to some of the challenges gear manufacturers face during heat treatment of parts. Innovative fixture design, uniform shell formation, reduced distortion, and elimination of IGO allow for better parts. Repeatable processes can be used to solve some of the toughest production problems. Gas carburizing can serve many functions (e.g. gear construction).