Large components, such as gears, are used in large structures like ships, wind power plants, rolling mills, transportation, railways, airplanes, cement crushers, mining, or the oil industry. There are three important methods of surface hardening used to improve and extend the technical application of such gear components. Material designers and engineers must decide which method to use. The advantage of surface hardening is a large depth of hardening in a short time, local hardening of large elements, and good flexibility. However, such strengthening also comes with challenges that need to be met. Which heat treatment of metals is the best? Why does heat treatment require maintaining the appropriate ambient and material temperature? What are the types of heat treatment? We write about this below!
Comparison of three heat treatment methods
Below we discuss each of the most popular types of heat treatment.
Carburizing
The first method is carburizing. It is usually carried out at high temperatures from 880°C to 980°C, and hardening temperatures range from 780°C to 860°C. The standard procedure is gas carburizing. By diffusing carbon into the surface and hardening, a strong, hard martensitic surface layer up to 10 millimeters thick is formed. This thermochemical method uses carbon-rich gas, namely methane (CH4) or propane (C4H8), to add a certain amount of carbon to the workpiece. After carburizing, the elements are hardened and tempered to the required surface hardness to reduce internal stresses. In addition to high surface hardness (up to 850 HV) and wear resistance, elements subjected to such treatment also show good resistance to reverse bending and fatigue due to residual compressive stresses. During carburizing, hardening and tempering, specific changes in time and temperature can be introduced to optimize the material properties and minimize dimensional changes associated with the appropriate loading techniques.
Nitriding through heat treatment
The second method is nitriding. This type of heat treatment is also often used. The range of temperatures for nitriding treatment ranges from 500°C to 580°C for gas nitriding and 400°C and above for plasma nitriding. Nitriding is a method of enriching the surface layer of iron material with a certain amount of nitrogen or, in the case of carbonitriding, with nitrogen and carbon. This not only increases hardness, but also wear resistance, fatigue strength, corrosion resistance, and anti-friction properties. Moreover, there is no microstructural transformation from austenite to martensite, which ensures high dimensional stability. Usually, the maximum nitriding penetration depth is 0.8 mm. Depending on the material, the penetration depth is greater than 1.0 mm. Distortion-free nitriding is in many cases a realistic alternative to surface hardening (surface quenching), provided that the appropriate steel is used. Nitrided steels are listed in DIN 17211 and EN 10085 standards.
Carburizing
The last of the heat treatment methods is carburizing by heating with a flame, induction, laser, or electron beam. It is carried out at processing temperatures 50°C - 100°C higher than the proper hardening temperature for the material. As a result of these processes, a hard surface of martensitic structure is formed. These methods can also be used for hardening large elements or complex geometries. Inductive or flame heating is used for areas with high stress of the processed object. Optimization of methods and finding solutions based on flame and induction hardened elements requires a lot of experience. Therefore, the evaluation and consistency of test samples is essential and can be significantly improved by detailed definition of machine parameters. In addition, hardness, depth, and core strength are important factors when the key criterion is the load capacity of the material. If there is a risk that the load capacity of such a gear will reach a critical level, diffusive hardening is preferred, even if the resulting deformation is smaller after nitriding hardening. This type is preferred for materials such as hydraulic cylinders, where a small hardness is sufficient. Due to the highest load capacity, diffusive hardening is the first choice for processing large gear elements.
There are three main areas to consider when surface treating large gear components:
- Hardness
- Weight
- Dimensional changes and distortions
Hardening
Larger elements are usually hardened in oil. Cooling below the melting temperature typically takes longer than an hour, depending on the size of the elements. During this time, heat is conducted through the surface. Therefore, ferritic-pearlitic steel is preferred. Otherwise, the hardness of the surface and the depth of the structure will be significantly reduced. The larger the cross-section, the more necessary it is to use high-alloy steels. Alloying elements Cr, Ni, Mo, Mn, and V improve hardenability, and the addition of V, Ni, and Mo improves strength. If steel with insufficient hardenability is chosen, despite intensive oil hardening with very good cooling and circulation, the result will be an unacceptable decrease in hardness and depth of hardness for large elements. This decrease is visible in gear elements, especially below the pitch circle to the tooth base, where the hardness is significantly below 52 HRC. Elements can be damaged in a short time due to pitting or side fractures. Therefore, the shape of the specimen should be such that, in addition to non-standard dimensions, it also has greater geometric similarity. Cooling rates and steel hardenability have a significant impact on shape change (shrinkage and expandability). Steel mills for carburizing with modern computer-controlled melting can precisely adjust hardenability. The cost is independent of the size of the lower 2/3 or upper 2/3 of the hardenability zone. Even more advantageous for later changes in shape, but for an additional fee, is to agree on tightened hardenability limits. In summary, when choosing materials for large gear elements, geometric effects should be taken into account and the following goals should be achieved:
- The material of sufficient hardness must be chosen
- The cross-section of the material must be as small as possible
- The geometry must ensure dimensional stability.
Weight
It is crucial in the front line of performance. In every case, one should strive for weight savings, but never at the expense of stability. In the case of large gear elements, welding has proven effective and is now the standard, achieving good dimensional stability and also reducing weight.
Preliminary material processing
From our experience, the following types of pre-processing have proven to be effective for large gear components and should be thoroughly analyzed in terms of costs and benefits:
- Forging 3-D (core deformation by stretching and compressing) results in a more uniform part with less segregation, less porosity, and an improved core structure.
- Do not cool with air after forging, but allow the part's temperature to naturally drop to 840°C. Then, the material should be hardened by immersion in oil and tempered at an ambient temperature of 650°C. If this is not possible, in addition to forging, at least preliminary hardening and tempering at a temperature of 840°C, followed by oil hardening and tempering at a temperature of 650°C to improve impact strength. Measurements before and after the process can provide insights into dimensional changes and deformation behavior.
- Throughout the process, it is necessary to avoid the irreversible fragility range of annealing in the range of 250-400°C, i.e., the rapid displacement of each stage of the process within this range. If possible, cooling should be performed in oil from a specified annealing temperature of 650°C.
- Avoid micro-cracks, losses, voids, dendrites, and impurities to reduce the risk of hydrogen embrittlement and delayed cracking.
- An essential benefit is the high final dimensional accuracy of the machined object, as well as the minimization of rework, dimensional errors, and defects
Dimensional changes and deformations
Depending on the size of the element, the limitation of dimensional changes and deformations becomes increasingly important. Dimensional change means expansion or contraction. Distortion means a certain irregularity - deformation, resulting from specific factors. On a gear wheel with a diameter of 1000 mm (40 inches), the deformation is only 1 mm (0.04 inches) per micron. In the case of a gear wheel with a diameter of 5000 mm (200 inches), a deviation of 1 micron will cause a dimensional deviation of 5 mm (0.2 inches), which is the full depth of CHD surfacing. This means that the total hardness must be ground to maintain geometry. Therefore, the goal is to minimize deformation and predict dimensional changes. Expansion and contraction are dimensional changes caused by microstructural transformations and thermal stresses during heat treatment. These factors certainly determine the behavior of the dimensions of the workpiece, but in most cases they are still inevitable during heat treatment. Various causes contribute to such distortions. They include high residual stresses and differences in alloy concentration, as well as the choice and quality of the material. In addition, geometric asymmetry and uneven temperature distribution during the production process can also cause deformations. However, this can be avoided by taking appropriate actions in the field of steel supply, design and retail production. During heat treatment, the key aspects are: plant engineering, process engineering and the type of raw material. However, these three factors can outweigh the others. Therefore, hardening distortion is crucial for the success of the entire production process of large gear elements. Atmosphere control, temperature and hardening/cooling are key aspects when using these technologies.
Summary
Due to the highest load capacity, diffusion hardening turned out to be the first choice for machining large gear elements. The selection of steel, conditioning heat treatment, hardness, weight, dimensional change, and deformation are the main aspects that need to be met in order to achieve optimal carburizing results during cutting. If these factors are perfectly coordinated, it is possible to go beyond the limitations of existing technologies and open up completely new possibilities for gear wheel manufacturers. During this process, one decisive, key factor is particularly important: Deformation during hardening. In the past, deformations caused by hardening proved to be a critical problem. Thanks to special vertical hardening technology, very low hardening deformations can be achieved even with horizontal loads on crown and bevel gears.