Low-pressure carburizing (or known as vacuum carburizing) offers a clean, modern carburizing method. This heat treatment requires true precision, ensuring minimal deformation of the original part during the carburizing process.
Carburizing process
Vacuum (low-pressure) carburizing is an alternative to traditional gas (atmospheric) carburizing. It is a thermo-chemical treatment that involves heating steel parts to temperatures typically in the 900-1000°C range and then placing them in a carburizing atmosphere. The purpose of carburizing is to enrich the surface layer with carbon, thereby increasing its hardness and wear resistance while maintaining the plastic core. Hardening and tempering immediately after carburizing is most common, but unlike conventional carburizing, it has many advantages, both from the carrier gas used – carburizing (C2H2 – acetylene) and from the environment – the vacuum furnace. The process is environmentally friendly – no CO2 emissions.
Areas of application
Tool steels particularly benefit from low-pressure carburizing because the process produces exceptionally long wear surfaces, ideal for machining operations.
The main advantages are:
– no oxidation of grain boundaries (no IGO),
– narrower tolerance range for carburized layer CHD – Less scattering of results,
– 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 temperatures, i.e. 980°C and above,
– clean workpiece surface (silver metal) – no scales.
Vacuum carburizing
During processing, special attention is paid to achieving and maintaining critical dimensions. The gears are then sent for heat treatment. Exposing parts to high temperatures, changing the chemical composition of the material and cooling rapidly in oil to harden are necessary to achieve the high strength and long life expected of large gears. Heat treatment, while critical to gearbox performance, can cause significant stress on parts and must be done correctly to achieve an acceptable part.
One of the most common questions is „How much will this process distort the raw material?” This question is difficult to answer. Factors such as material chemistry, heat treatment and machining history, geometry, fixtures, case uniformity and hardening processes all play a role in determining how far the gears move. Heat treatment controls the last three variables and helps minimize part size variation. Vacuum carburizing can effectively solve these problems because it is a reliable alternative to conventional endothermic gas carburizing in integrated quench or plunge furnaces.
Repeatability of the process
The first step in the procedure is heating to process temperature, which is done under vacuum. Before carburizing begins, increase and hold the load until the part reaches a uniform temperature. Vacuum carburizing is a non-equilibrium process that uses a series of charge and diffusion cycles. During the charge cycle, a unique mixture of carburizing gases is injected at low pressure. Carbon additives are based on carburized surfaces. The duration of the charge cycle is relatively short, while a longer diffusion time is used to distribute the carbon to the parts. After carburizing to the desired depth, the part is cooled with nitrogen and then reheated to the quenching temperature. When the part is at a uniform temperature, the charge is quenched in oil.
Controlling distortion
The most difficult challenge in heat treating gears is minimizing distortion. Four main factors contribute to this: residual stress, dimensional change of martensite, heat dissipation and high-temperature creep. The most important is martensitic transformation and heat dissipation.
Residual stresses in materials cannot be controlled by heat treatment, but can cause significant problems during hardening. The best way to address residual stresses is through heat treatment prior to processing. Typical processes include a combination of normalizing, annealing, quenching and tempering, and full annealing.
During post-carburization quenching, substances with higher carbon content have a greater increase in volume after conversion to martensite than other materials. This can put a lot of stress on the material and cause it to deform or change size. The most effective way to minimize this effect is to maintain uniformity of shell depth throughout the part profile. The stress distribution caused by hardening is more uniform, and part movement is reduced. Some size changes may occur.
If the heat dissipation is not uniform, it can cause significant movement of the part. When the gear comes into contact with oil, it begins to transform into martensite, and the material structure is complete. As the hardening of the part progresses, more material changes and internal stresses increase. If the heat dissipation is asymmetrical in the gear section, 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 quench flow is uniform on all sides.
High-temperature creep occurs when the part is not properly supported during the carburizing process. Cycle times are very long – more than 100 hours on some coatings. At this temperature, the metal moves or creeps, resulting in increased ovality. Again, the key is to design the tool correctly.
The vacuum carburizing process carried out in a stack metallurgical furnace resulted in much better deformation compared to workpieces processed in a shaft furnace. When the deformation is small, machining adjustments 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 each cycle to be designed to optimize the properties of the heat-treated alloy. 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 each specification.
Because the process takes place in a vacuum, there is no oxygen. This eliminates the occurrence of intergranular oxidation (IGO) typical of endothermic atmospheric treatments. The presence of IGO requires heat treatment operations after processes such as full profile grinding or peening. Eliminating this reduces costs.
Summary
The vacuum carburizing process offers solutions to some of the challenges gear manufacturers face when heat treating parts. Innovative tooling design, uniform shell forming, reduced warpage and elimination of IGO produce better parts. Repeatable processes can be used to solve some of the most difficult manufacturing problems. Keep in mind that heat treatment of tool steels is also carried out in vacuum furnaces with high pressure gas quenching. Gas quench is possible for carburizing gas used in the furnace chamber. Using low-pressure carburizing has many pros. For example, Your product will have tough core, complex shapes and high hardness.