Significant components such as gears are used in large structures such as ships, wind turbines, rolling mills, transportation, railroads, aircraft, cement crushers, mining, and the oil industry. Three important surface hardening methods are used to improve and expand the technical application of such gear components. Material designers and engineers must decide which method to use. The advantages of surface hardening are:
- The great depth of hardening in a short time.
- Local hardening of large parts.
- Good flexibility.
However, such strengthening also comes with challenges that must be overcome. Which metal heat treatment is best? Why does heat treatment require the right ambient and material temperature? What are the types of heat treatment? We write about it below!
Comparison of three heat treatment methods
Below we discuss each of the most popular types of heat treatment.
The first method is carburizing. It is usually carried out at high temperatures of 880°C to 980°C, with quenching temperatures oscillating between 780°C and 860°C. The standard procedure is gas carburization. A strong, hard martensitic surface layer up to 10 millimeters thick is formed by diffusing carbon to the surface and quenching. This thermochemical method uses a carbon-rich gas, methane (CH4) or propane (C4H8), to add a certain amount of carbon to the workpiece. After carburizing, the workpieces 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, parts treated in this way also show good back bending and fatigue strength due to residual compressive stresses. During carburizing, quenching, and tempering, specific time and temperature variations can be introduced to optimize material properties and minimize dimensional changes associated with the respective charging techniques.
The second method is nitriding. This type of thermal improvement is also frequently carved. The temperature range for nitriding treatment is 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 an iron material with a certain amount of nitrogen, or in the case of nitrocarburizing with nitrogen and carbon. This increases hardness, wear resistance, fatigue strength, corrosion resistance, and anti-friction properties. In addition, there is no microstructural transformation from austenite to martensite, which ensures high dimensional stability. Typically, the maximum penetration depth of nitriding is 0.8 mm. Depending on the material, the penetration depth is greater than 1.0 mm. Nitriding without distortion is often a viable alternative to surface hardening (surface hardening), provided the right steel is used. Nitrided steels are listed in DIN 17211 and EN 10085.
Carburizing by flame heating
The last of the heat treatment methods are carburizing by flame, induction, laser, or electron beam heating. It is carried out at processing temperatures 50°C – 100°C – higher than the hardening temperature inherent in the material. These processes result in a hard surface martensitic structure. These methods can also be used to harden large parts or complex geometries. Induction or flame heating is used for areas of high workpiece stress. Optimizing methods and finding solutions based on the flame- and induction-hardened workpieces requires great experience. Therefore, evaluation and consistency of test samples are essential and can be significantly improved by defining machine parameters in detail. Besides, hardness, depth and core strength are important factors when the load-bearing capacity of the material is a key criterion. Suppose the bearing capacity of such gear is in danger of reaching a critical level. In that case, diffusion hardening is preferred, even if the deformation occurs less after nitriding hardening. This type, in turn, is preferred for materials such as hydraulic cylinders, in which low hardness is sufficient. Due to its highest load capacity, diffusion hardening is the first choice for machining large gear components.
There are three main areas to consider when surface-treating large gear components:
- Dimensional changes and distortion
Larger parts are usually hardened in oil. Cooling below the melting point usually takes more than an hour, depending on the size of the parts. During this time, heat is conducted through the surface. Therefore, ferritic-perlitic steel is preferred. Otherwise, the surface hardness and depth of construction will be significantly reduced. The larger the section, the more necessary it is to use high-alloy steels. The alloying elements Cr, Ni, Mo, Mn and V improve hardenability, and adding V, Ni, and Mo improve strength. Suppose steels with insufficient hardenability are selected, despite intensive quenching in oil with very good cooling and circulation. In that case, the result will be an unacceptable drop in hardness and depth of hardness for large components. This drop is evident in gear components, especially below the pitch circle to the tooth root, where hardness is well below 52 HRC. The components can be damaged in a short period due to pitting, or lateral fractures. Therefore, the shape of the preparation should be such that it has greater geometric similarity in addition to non-standard dimensions. The steel’s cooling rates and hardenability strongly influence the shape change (shrinkage and expansion). Carburizing steel mills with modern computer-controlled melting can fine tune the hardenability. The cost is independent of the size of the lower 2/3 or upper 2/3 of the hardenability zone. It is even more advantageous for later shape changes, but at an additional cost, to agree on tighter hardenability limits. In summary, when selecting materials for large gear parts, geometric effects should be taken into account, and the following goals should be achieved:
Select a material with sufficient hardenability
The cross section of the material must:
- be as small as possible
- Geometry must ensure dimensional stability.
It is crucial at the first line of performance. In all cases, weight savings should be sought, but never at the expense of stability. For large gear components, welding has proven effective and is now the standard, achieving good dimensional stability while also reducing weight.
In our experience, the following types of pretreatment have proven effective for large gear parts and should be analyzed in detail for cost-benefit:
- 3-D forging (core deformation by tension and compression) results in a more uniform part with less segregation, less porosity, and improved core structure.
- Do not air-cool after forging, but allow the part temperature to drop to 840°C naturally. The material should then be cured by immersion in oil and tempered at an ambient temperature of 650°C if this is not possible, in addition to forging, at least pre-harden and temper at 840°C, followed by oil quenching and tempering at 650°C to improve impact strength. From pre-and post-process measurements, conclusions can be drawn about dimensional changes and deformation behavior.
- Throughout the process, the range of irreversible tempering embrittlement in the 250-400°C range should be avoided, i.e., the rapid movement of each process step in this range. If possible, perform cooling in oil from the specified annealing temperature of 650°C.
- Avoid microcracks, cavities, voids, dendrites and impurities to reduce the risk of hydrogen embrittlement and delayed cracking.
- A significant benefit is the high final dimensional accuracy of the workpiece, as well as minimizing rework, dimensional errors, and defects
Dimensional changes and deformations
Depending on the size of the workpiece, it is becoming increasingly important to limit dimensional changes and deformations. Dimensional change means growth or shrinkage. Deformation means a specific abnormality – deformation resulting from certain factors. On a gear wheel with a diameter of 1,000 mm (40 inches), the deformation is only 1 mm (0.04 inches) per micron. On a 5000 mm (200 inch) diameter gear wheel, a deviation of 1 micron will result in a dimensional deviation of 5 mm (0.2 inch), and this is the full depth of CHD surfacing. This means that the total hardness must be ground down to maintain the geometry. Therefore, the goal is to minimize deformation and anticipate dimensional changes. Growth and shrinkage are dimensional changes caused by microstructural changes and thermal stresses during heat treatment. These factors certainly determine the dimensional behavior of the workpiece, but in most cases, they are still unavoidable during heat treatment. Various causes contribute to such distortions. These include high residual stresses, differences in alloy concentration, and material selection and quality. In addition, geometric asymmetry and uneven temperature distribution during manufacturing can also cause distortion. However, this can be avoided by taking appropriate measures in steel procurement, construction 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 deformation is critical to the success of the entire manufacturing process of large gear components. Control of the atmosphere, temperature and quenching/cooling are key aspects when using these technologies.
Diffusion hardening has emerged as the first choice for machining large gear parts due to its superior load capacity. Steel selection, conditioning heat treatment, hardenability, weight, dimensional change, and deformation are the main aspects that must be met to achieve optimum carburizing results during machining. If these factors are perfectly coordinated, it is possible to go beyond existing technologies’ limitations and open up new opportunities for gear manufacturers. During this process, one decisive key factor, in particular, is important: Deformation during curing. In the past, deformation caused by hardening has proven to be a critical problem. With unique vertical hardening technology, very low hardening distortion can be achieved even when crown and bevel gears are loaded horizontally.