Advantages of Induction Hardening with No Soft Zone on Large ...

Large bearings are required to carry large axial and radial forces and their resulting torques. Typical applications include general machinery and construction equipment, as well as onshore and offshore energy technologies. These diverse applications have one thing in common: their components are highly stressed mechanically, and are therefore induction hardened to increase their dynamic strength and wear resistance. In the past, conventional progressive induction scan hardening necessarily left a small, unhardened zone in the bearing race. This soft zone compromised the bearing’s load-bearing capacity and smoothness. SMS Elotherm has solved this problem with a new induction hardening technology to fully harden the bearing raceway with no soft zone. This scan hardening with no residual soft zone is essential; where systems do not allow any vibration (e.g. magnetic resonance imaging, or MRI technology), where extremely high mechanical stresses need to be considered (e.g. tunnel drilling machines), where continuous rotation is required, or where challenging environmental conditions mandate maintenance-free systems with high lifetime (e.g. offshore technology such as wind turbines, tidal plants or oil platforms).

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The process of induction scan hardening with no soft zone has been patented [1] by SMS Elotherm and ensures–together with other proprietary systems such as workpiece energy measurement and closed-loop inductor positioning control — an easy integration into customer’s production and quality assurance processes.

The Induction Surface Hardening Process

Induction hardening is comprised of two process steps: inductive heating, followed by rapid cooling (quenching) with a quenching fluid. Inductive heating is caused by alternating current flowing through a coil (sometimes called an inductor) that is sized and shaped according to the workpiece. The alternating coil current creates a corresponding changing magnetic field, which in turn creates eddy currents inside the workpiece piece. These eddy currents produce heat inside the workpiece. The heating depth is inversely proportional to frequency. Higher frequencies produce shallower heating in the workpiece. This phenomenon is known as the “skin effect”. Because the heat originates inside the workpiece, it does not have to be transferred into the workpiece via radiation or convection at the surface. The induction heating period is kept short (e.g. a few seconds) to prevent unwanted heating of the workpiece core (Figure 1 and Figure 2).

Before hardening, the steel workpiece contains a mixture α-iron (ferrite) and cementite (Fe3C) at room temperature. The induction coil heats the surface material to a temperature of at least 723°C (the eutectoid temperature), causing the ferrite to transform into γ- iron (austenite). The steel is thus austenitized. At the same time, carbon from the available cementite is able to dis- solve into the austenite, because carbon has a much higher solubility in austenite than in ferrite. This dissolved carbon is essential. A steel workpiece must contain at least 0.02% carbon to be hardenable.

In the next process step, quenching, the austenitized steel is rapidly cooled at acontrolled rate. This rapid cooling prevents the diffusion of carbon atoms and the reformation of original ferrite and cementite mixture. Dissolved carbon atoms are trapped in the ferrite, causing the ferrite body-centered cubic (bcc) crystal structure to deform into a body-centered tetragonal (bct) structure called martensite. The temperature difference and the cooling rate (which can be controlled by selecting the right quenching medium such as oil, or water with additives) determine the level of martensite formation. Faster cooling below the transformation temperature produces more martensite. The fresh martensite is very hard and brittle. Tempering (controlled heating to prescribed moderate temperatures for defined time periods) reduces this brittleness and gives the steel the desired combination of hardness, strength, and toughness (Figure 3).

Advantages of Induction Hardening

Some conventional heat treat shops use case carburizing to harden rings up to three meters in diameter. This process suffers from five major problems. First, it requires long periods—sometimes several days—in the carburizing furnace. Second, it produces large workpiece distortions, which must be corrected by expensive and time-consuming straightening operations. Third, larger workpieces cannot be hardened due to practical limits in the size of the furnace. Fourth, carburizing necessarily treats the entire workpiece, including surfaces that should not be treated. Fifth, carburizing is energy intensive, with high emissions. Induction hardening overcomes all of these problems.

In contrast to conventional carburizing, induction hardening is performed only on the highly loaded surfaces where it is needed, such as bearing races and gear teeth. Resulting advantages are faster process times (for example, about one hour for a ring with a diameter of several meters), substantially lower energy consumption, and better of control of workpiece distortion. Induction hardening benefits from rapid and uniform workpiece heating. The inductors with integral quench sprays can be optimized for the application, producing well-defined and reproducible hardening results. For mission-critical components in offshore technology, maintenance or repair operations would be extraordinarily expensive or technically infeasible. This is exactly the type of application where precise and accurate formation of hardened zone must be achieved and documented with online quality control.

SMS Elotherm’s patented Workpiece Energy Measurement (WEM) [2] continuously monitors the electrical power that contributes to net heating of the part. This system automatically accounts for the total energy loss between the system input power and the inductor, and the remaining net workpiece energy is continuously monitored and recorded. Unlike other methods that simply measure the inverter output, WEM provides 100% online quality control of the hardening process without having to destroy the workpiece. Small changes in the inductor–workpiece standoff distance can lead to significant hardening zone variation. Figure 4 illustrates how the SMS Elotherm Workpiece Energy Measurement detects small inductor position changes. By contrast, the conventional inverter power monitor (blue trace) is largely insensitive to this source of process variation. For critical components where induction hardening with no soft zones is requirement, Workpiece Energy Measurement gives the manufacturer a reliable system to fulfill the stringent requirements of modern quality control audits and part/process traceability.

Overview of Conventional Induction Hardening Methods for Bearing Races Scan Hardening

Scan hardening with a remaining soft zone is the standard process for hardening single raceway and multi-raceway large bearings. Other than a linear axis to compensate for different workpiece diameters, the inductor/spray head assembly is stationary. The ring rotates with a low, constant tangential velocity past the inductor. A small soft zone (unhardened area) necessarily remains at the end of the scan path. With less than 100 kW of power a 3 m diameter bearing race can be hardened in less than one hour (Figure 5).

Complete Surface (Single Shot) Hardening

With single shot hardening the workpiece rotates past one or more stationary inductors, or a complete 360° ring inductor interfaces with the entire workpiece. The ring is heated to the appropriate hardening temperature and then the entire workpiece is quenched.

Quenching may be done by submerging the workpiece in a bath or by using spray nozzles that are integrated into the inductors and tailored for the process requirements. The single shot process is best suited for workpieces with a diameter less than 2 m. The electrical power requirement grows quadratically with increasing workpiece diameter. For example, single shot hardening of a 2 m diameter ring would require about 1.5 MW of power. Compared to scan hardening, the single shot process with its high power is very fast, typically a few minutes.

Inductive Scan Hardening with No Soft Zone

As previously described, traditional scan hardening is inadequate for mission-critical components with stringent requirements for smoothness and heavy loads due to the remaining soft zone. Carburizing is limited as a process alternative by furnace size and long process times. Single shot induction hardening is impractical due to its high power requirement. A better hardening method is needed to efficiently and reliably harden large, high-value rings. In response to this need, SMS Elotherm developed and patented a process several years ago to scan harden arbitrarily large rings with no remaining soft zone [3]. This process differs from conventional scan hardening primarily at the start location and end location of the scan, where special sprays and techniques completely quench the austenitized steel while avoiding unwanted tempering and changes in the hardened zone microstructure.

Comparing this process to single shot induction hardening, we find that even a 6 m diameter ring can be processed with a 200 kW scanning system, which is about factor 7–8 times less power than the single shot process would require. In contrast to carburizing, which would require several hundred hours in the furnace, the time required for the scan hardening process (less than two hours) is negligible. Moreover, the cost-intensive and time consuming straightening operation to clean up the distortion caused by carburizing can be avoided altogether.

This induction hardening system is universal and modular, so in addition to scan hardening with no soft zone, conventional scan hardening and tooth hardening on both the inside and outside diameters of the workpiece can be done on the same machine. Manufacturers of large ring bearings with small production runs appreciate this flexibility and the freedom to process diverse workpieces while holding equipment costs at a minimum (Figure 6).

Characteristics of the Induction Scan Hardening with No Soft Zone

The SMS Elotherm patented process for scan hardening with no soft zone requires only two inductor assembles, each comprised of an inductor and a spray head. This differs from three- inductor systems, which require more complex mechanical and control systems. The inductors are narrow to create a compact hardening zone. Flux concentrators focus the magnetic field for one-sided hardening. Each inductor is accompanied by an independently positioned spray head. A single stationary spray head completes the assembly.

Start Sequence

The inductor assembles are brought together back-to-back and then energized with independent power supplies. Both assembles travel side-by-side in the same direction for a short distance. One of the assemblies reverses directions, so the inductor assemblies travel in opposite direction, each accompanied by its spray head. This technique avoids the formation of a soft zone at the start location (Figure 7).

Figure 8 depicts the hardening result with the etched case (below) and with two hardening passes along the workpiece axis at 0.5 mm and 5 mm depths after tempering. The start location is still recognizable and one can see that the entire area has been hardened with a relatively constant case depth (Figure 8). With this special process technique for the inductors and sprays we create uniform hardening with no soft zone at the start location. The next critical process is at the end location where the two inductor assemblies come together.

End Sequence

Like the start sequence, the end sequence relies on precise control of the quenching sprays and the tight motion control of the inductors and sprays to achieve a uniform case depth with no soft zone at the end location (Figure 9 and Figure 10). The resulting bearing surface meets the requirements for high loads and smooth operation.

Conclusion

Induction hardening has proven itself in the manufacturing of high-value, large ring bearings. Wind power certainly owes it success in part to surface hardened bearing races and gear teeth. Construction equipment would wear out quickly, and aerospace would entail unacceptable risks without the benefits of induction hardening. Alternative hardening methods for large ring bear-ing often struggle with furnace dimensions and extraordinarily long process times. Conventional scan hardening with its remaining soft zone and single shot induction hardening with its high power requirement are ill suited to meet the growing demand for very large ring bearings. The SMS Elotherm patented process for scan hardening with no remaining soft zone bridges the gap between conventional hardening methods and the growing demand for very large bearings with high load ratings, low noise, and longer service lives. The patented technique of using two inde- pendently controlled inductor/quench heads to achieve uniform hardening at the start and end of the scan is essential to the success of this method.

Future developments in renewable energy such as onshore and offshore wind power and photovoltaic systems equipped with positioning systems to track the sun will accelerate the trend toward larger, more durable rotary joints. These high-value components will be economically, reproducibly, and traceably manufactured with uniformly hard wear surfaces created by induction scan hardening with no soft zone.

“Induction hardening is a non-physical treatment unless it uses an electrical current for rapid treatment. It increases the metal’s strength and hardness.”

Induction hardened sheet metal is a metal heat treatment process in which metal parts with sufficient carbon content are heated by electromagnetic induction and quenched rapidly. This technique is mainly applied to different types of steel and steel alloys in order to enhance mechanical characteristics such as surface hardness, cyclic strength, and wear resistance in certain zones. Some uses are in power trains, suspension, engines, and stamping parts. 

Induction heat treatments are relatively fast and accurate processes of hardening surfaces on parts as compared to conventional heat-treating processes. Here, we will give a step by step guide on induction heat treatment, its benefits, drawbacks, uses, and contrast with case hardening. So, let us take a closer look.

What is Induction Hardening? 

Induction hardening

It is a non-contact heating method that uses electromagnetic induction to produce heat at the surface layer of a metal part. The metal is put into a strong, varying magnetic field, and an electric current is generated in the metal, causing heat. The metal is generally heated to a temperature in the transformation range or above and then rapidly cooled. 

The quenching process is carried out using water, oil, or air. This rapid cooling leads to a martensitic transformation in the metal, which increases its hardness and brittleness. It’s suitable for surface hardening of a part or assembly where only a certain area of the part requires hardening without altering the properties of the part. This makes it particularly appropriate for components that need high surface wear resistance while they may not need other mechanical properties. In general, carbon and alloy steels with a carbon content between 0.40% and 0. 45% are most suitable for steel heat treatments. Induction hardening has the following advantages over other methods of hardening. It is easier to control since it is an electrical process as opposed to a combustion process. Also, it warms only the outer layer of the metal and not the inner layers of the metal. This makes it possible to have a very good control, and this will lead to a very good hardening of the surface. The depth of the hardened layer can also be easily controlled.

2 Stages In the Induction Hardening Process

Usually, it consists of two primary stages: Induction heating and quenching. The first process is the use of electromagnetic induction to heat electrically conductive metals, and the second process is quenching to change the surface characteristics of the material.  

Stage 1: Induction Heating  

In this stage, the induction hardened material is usually inserted in a water cooled copper coil and subjected to an alternating magnetic field. This field is produced by an electromagnet and an electronic oscillator. The oscillator passes the alternating current through the electromagnet and as a result, the magnetic field that passes through the material is also alternating. This process creates circular electric currents known as eddy currents that warm the metal in the coil to its transformation temperature. Induction hardening is a type of surface hardening and can go up to a maximum of 8mm depth. The depth of penetration depends on the frequency of the alternating magnetic fields; the higher the frequency, the deeper the penetration of the currents.  

Stage 2: Quenching 

After the metal is induction hardened to its required temperature by induction, it must be cooled quickly or quenched. This is usually done by submerging the part in a bath of oil or water, although cold air can sometimes be used. Tempering makes certain that the outer layer of the material is made harder and that the heat does not penetrate the inner layers of the material so as to cause unwanted phase transformations. The rapid cooling results in the formation of the martensitic or ferritic-martensitic structure in the surface layer. These structures have higher tensile strength and lower initial yielding stress as compared to the structures of pure ferrite. Furthermore, quenching decreases the grain size, which is important for increasing the hardness of the material.

Material Choice For Induction Hardening

Induction hardening is used on medium to high carbon steel, alloy steel, cast iron, and powder metallurgical materials. There are also some stainless steels that can be used in different manufacturing sectors.

Medium to High Carbon Steels

It’s an ideal technique for medium to high carbon steels because the carbon content is sufficient for the required martensitic transformation for hardness. Such steels normally contain carbon above 0.40%, and the hardness level of the material ranges between 56 and 65 HRC. However, lower carbon steels such as can also be used but to a lower hardness of around 40-45 HRC. Low carbon steels like , , 12L14, and are not used because they do not show much increase in hardness.

Alloy Steels

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This is because alloy steels have additional elements that make them more suitable for induction heat treatments than plain carbon steels. Out of these, steel is the most used because it can be easily machined, cheap, and can be quenched to achieve a hardness of 58 HRC and above with a carbon content of 0.45%. Also, steel has less chance of cracking during the hardening process as compared to other materials. Some other alloy steels that are frequently used are /, , , and ETD150.

Cast Irons

Another group of materials that are commonly used in the induction treatment process includes cast irons. They are selected based on their capacity to provide high hardness and wear resistance on the surface but are more ductile at the center.

Powder Metals

Powder metals can also be induction hardened, which provides a more flexible method for producing parts with intricate geometries and mechanical characteristics suited to their use.

Stainless Steels

In some industries, certain grades of stainless steel are used in induction hardening. Although not widely used compared to carbon and alloy steels, these materials can be improved through the process to improve surface hardness and wear.

The choice of materials for induction hardening is determined by the content of carbon and other alloying elements that affect the hardenability of the material and the required hardness after the treatment.

Benefits of Induction Hardening

Induction hardening has several advantages, but at the same time, it has certain disadvantages as well. Its major advantage is that it is highly efficient and fast, which is ideal for organizations with large production lines. Moreover, it’s less time-consuming as compared to other heat-treating processes such as case hardening or nitriding. Also, it is cost-effective since it does not require extra materials like salt baths or gas tanks and provides an even finish on the whole part. 

Another advantage is the flexibility of the hardness of parts after the process of induction hardening through tempering. It also increases the hardness, durability and ability of the metal to withstand wear and tear, hence suitable for parts under pressure or friction. In addition, it increases the endurance limit, thus lowering the chances of failure under cyclic loads. 

However, there are some limitations of Induction hardening. It is primarily used for ferrous materials, which restricts its use with other materials. There is also a risk of warping or distorting of the parts if they are not handled properly or if they are not cooled properly during the process. Compared to other methods, the cost of acquiring and maintaining the hardening equipment may not be very high, but it is still a cost.

Further, due to high rates of heating and cooling during induction, hardening cracks may sometimes appear on the surface of the workpiece. Therefore, the process parameters have to be well controlled. Operators also need training and skills to guarantee proper handling and control from the start to the end of the process. However, the advantages usually outweigh the disadvantages, and therefore, the method is widely used in various manufacturing processes.

Disadvantages of Induction Hardening

However, there are some disadvantages of induction heat treatment that one has to consider before opting for this method. The main limitation is that it can only be used on ferrous materials such as iron and steel. This limitation is due to the fact that it uses a magnetic field to produce heat, and only ferrous metals are influenced by the magnetic fields. Induction hardening is not possible for non-magnetic materials such as aluminum and copper which are classified under nonferrous metals.

Another disadvantage is that the process has a high running cost compared to the other methods of hardening. Although it is cheaper in terms of material and manpower as compared to some of the heat-treating techniques, the machinery used in the process increases the cost. Also, using electricity instead of conventional fuel types may result in high long-term operating costs because of the fees for energy consumption.

Induction hardening also has its limitations. The size and shape of the induction coil should match the part to be heated. Standard coils for treating round objects like shafts and rollers are usually obtainable, but for some jobs, special coils may be required, thus increasing the difficulty and the cost.

Also, the requirement of equipment is a constraint because not all heat treatment suppliers may have access to or offer induction hardening equipment. The cost of this equipment may be high initially, and the operators may need to undergo some training to use the equipment properly.

In addition, normally it is suitable for thinner materials because it entails heating the materials’ surface. Thicker materials may not heat uniformly, which may cause problems such as uneven hardness or even cracks. These limitations point to the fact that there is a need to consider and assess whether the process is applicable to a certain process or not.

Induction Hardening Vs. Case Hardening: Key Difference

Induction hardening and case hardening are two identical heat treatment processes used in industries today. Both processes aim at hardening the outer layer of workpieces; however, the two processes are not the same. Here’s a closer look at their distinctions: 

Case hardening

Handling of Parts 

One of the main distinctions can be observed in the way they address parts. Case hardening is done to multiple workpieces at once; in induction hardening, the workpieces are done one at a time. This leads to a “batch by batch” approach for case hardening and a more precise, workpiece-oriented approach for induction hardening. 

Manufacturing Integration 

Case hardening normally involves other organizational activities to move parts between the production line and the hardening process. On the other hand, induction hardening can be easily incorporated into the production line itself, which uses a hardening machine as a part of the production line.  

Processing Techniques 

Case hardening uses heat and chemistry to start a thermochemical reaction, while induction treatments use electromagnetic energy to induce an alternating current in part, heating only the surface. This is a non-contact heating process that is electromagnetically induced and is aimed at the surface of the workpiece. 

Applications 

Induction hardening is suitable for applications where there is a need to harden specific areas of a component, for instance, crankshafts and gear teeth. On the other hand, case hardening is more appropriate for large parts that have to be heat treated in batches and where complex shapes are not as important to the hardening process. 

Flame Hardening vs. Induction Hardening: Key Differences?

Flame hardening and induction hardening are two different methods of surface hardening that are used in metalworking procedures. Flame hardening is the process of heating the surface of a workpiece with oxy-acetylene flame and quenching it with cold water. This method is widely used for large parts like gear teeth, brake drums, axles, cams, and crankshafts. On the other hand, induction heating involves using an electromagnetic field to heat the surface of the workpiece through a coil through a high-frequency current. It is usually used on small components such as gears, crankshafts, and camshafts because of the localized heating characteristics. 

Flame hardening

Another significant difference is that the treatment processes of these methods are different. Flame hardening, on the other hand, makes the workpiece have a uniform hardness on the entire surface, while induction hardening only hardens some areas of the workpiece. This difference makes it a better choice where only some regions of the material need to be hardened, providing better control over the process. 

Further, the process of applying flame hardening involves the use of flame and skilled labor since there is a possibility of damaging the workpiece. However, in the case of induction hardening, unskilled operators can do it as it doesn’t harm the material, which makes it suitable for manufacturing processes. 

Therefore, flame hardening and induction hardening both have their advantages and disadvantages, and the choice between the two depends on the size of the workpiece, the area to be hardened, uniformity, and the level of skill of the workforce. Both methods have their advantages and disadvantages that should be taken into consideration depending on the needs of a manufacturing company.

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