Reducing fleet consumption in the automotive industry or service-free lifetime operation of components in offshore oil rigs all depends on the quality of the components used. The heat treatment – here the surface hardening – plays an important role for keeping geometrical dimensions as small as possible and boosting the component’s resistance to ever-increasing loads at the same time.
An intelligent manufacturing technology is an essential asset in times of growing international competition among the manufacturers of such components. This article compares the traditional process of case hardening with induction hardening and proves that the induction technology is advantageous when it comes to the integration into manufacturing lines, high productivity, energy- and resource-saving production, flexibility of the material selection and reproducibility of the hardening results.
Cast, hot- and cold-formed steel parts frequently do not have the necessary microstructure properties to satisfy the operating demands. Different heat-treatment methods can be used to increase or optimize the material properties, such as wear resistance, strength or ductility.
Heat treatment can be defined as “a series of operations in the course of which a solid ferrous product is totally or partially exposed to thermal cycles to bring about a change in its properties and/or structure. The chemical composition of the ferrous product may possibly be modified during these operations.”
While induction hardening belongs to the thermal processes, case hardening is a thermochemical process.
A precondition for the hardenability of the respective material is a corresponding carbon content in conjunction with the content of alloying elements. Furthermore, a careful examination of the right combination of workpiece geometry, hardening specification and heat-treatment process is necessary.
Heat-treatment processes such as case hardening are used to prolong the service life by increasing the surface hardness and vibration resistance while maintaining a ductile, elastic microstructure at the core. Steels suitable for case hardening have a carbon content of approximately 0.1-0.3% weight percent. For a high surface hardness – for example, 60 HRC – a carbon content of 0.1-0.3% is not sufficient. The part has to be carburized.
Carburization takes place by diffusion of the carbon into the workpiece surface. A mixture of carrier gas and additive gas forms the basis for the carburization atmosphere in the furnace. Crucial factors for the right choice of the carburization process are the material-specific parameters, hardening demands in conjunction with the gas composition and a continuous, homogeneous furnace atmosphere.
Hardening is achieved by heating to austenitizing temperature with a sufficiently long holding time and subsequent quenching process. What is crucial here is that the carbon in the austenite is brought into solution. The amount of carbon is dependent on the material composition and the state of the initial microstructure. Excessive holding times or excessive temperatures during the austenitizing process can have a negative impact on the grain growth and material microstructure.
The hardening process can be followed by a low-temperature cooling process or direct tempering process. Both processes result in a reduction of the residual austenite and of the hardness and distortion properties.
Tempering is performed in different temperature ranges. The tempering temperature of parts made of low-alloy or unalloyed steels generally lies between 180-250°C (356-482°F). A higher temperature leads to a greater drop in hardness.
In contrast with conventional furnace technology, during induction heating the metal workpiece is subjected – partially or completely – to an electromagnetic alternating field using a current-carrying coil. This alternating field creates eddy currents in the material flowing in the opposite direction to the original current generating heat.
In the context of induction hardening, the expression short-cycle austenitization is also used, since by comparison with furnace processes the austenitizing temperature is reached for just a few seconds. The hardening temperatures are generally 50-150°C higher than with conventional furnace hardening. The process sequence during hardening consists essentially of heating, holding, quenching and possibly a subsequent tempering process. Thus, it is significantly shorter than the process sequence for conventional case hardening.
The process is monitored by an appropriate control system so that the hardening results are reliably reproduced. The microstructure properties can be set to the required depth in carbon-based materials by varying the frequency employed, the energy input, the quenching method and the constant coupling distance between workpiece and inductor. The hardening process is tailored specifically to the hardening requirements (Fig. 2). Cylindrical workpiece geometries, for example, are hardened using the scan hardening or single-shot hardening process (Fig. 3). Depending on the modulus and geometry, gears can be hardened at the tooth flanks, at the tooth root or in the tooth gaps or completely by single-shot gear hardening using a ring inductor. On the workpiece in Figure 2, the middle rib was not hardened for strength reasons.
Advantage of Induction
The technical and economic benefits of induction are of enormous significance, particularly considering energy efficiency. On one hand, machine concepts tailored specifically to the customer’s requirements offer the possibility of integrating the induction heating unit into a production line and, on the other, of having it operate as a separate, stand-alone system.
Modern induction-hardening machines are characterized by great flexibility for a constantly changing and growing spectrum of parts. Relevant criteria are:
- Short cycle time
- Direct integration into production lines
- Reproducibility and process precision thanks to custom-designed inductors
- Low re-machining costs, distortion
As an option, these can be customized with numerous additional functions and hence tailored to the customer’s specific application. For example:
- Number of workstations
- Manual or automatic loading
- Single-shot or scan hardening
- Individual combination possibilities for hardening and tempering processes
- Integration of intermediate work steps (post-cooling, straightening, etc.)
- Special design of coils, with an integrated inert-gas chamber
The induction hardening process is generally followed by a tempering step. This serves to relieve or reduce material stresses caused by the growth in volume of the material microstructure during martensite formation. Furthermore, it allows the hardness after quenching of the workpiece to be adjusted to the hardness required by the customer’s specification. This tempering process can be performed in different ways, for example:
- Tempering from the residual heat
- Induction tempering
- Furnace tempering
Induction tempering can be realized and integrated into the process by using a dual-spindle machine. This allows induction hardening followed by induction tempering for a wide range of different workpiece geometries. The proper machine concepts as well as the necessary periphery for exact parameterization of the whole process (e.g., inverter technology, control unit or main transformer, cooling of the electrical circuits, cooling of the quenching medium, etc.) are essential components for meeting the following demands:
- Fast reaction time for extremely short heating times
- Continuous operation at nominal load
- Variable frequency
- Integrated surge-current and surge-voltage protection
- Standby mode (switching off of all pumps and auxiliary units)
- Accessibility and ease of maintenance
- Small installation space for the machine
Application Example: Cost Comparison
The exemplary calculation refers to the pure energy consumption required for the defined heat-treatment process described. Other costs – such as the acquisition of a shaft furnace, induction hardening system, charging rack and quenching unit; charging and set-up; further work steps such as insulation, straightening and blasting; and the personnel costs – are not included due to the individual design.
Furthermore, the elimination or reduction of additional work steps such as pre-machining (grinding allowance), material procurement, hard machining or the blasting of workpieces offers a great cost-reduction potential compared with a case-hardening process. In view of the comparatively high wear of a shaft furnace and the necessary charging racks, additional costs are to be expected due to the increased maintenance.
Example 1: Case Hardening of 16MnCr5
The example has a case-hardening depth of approximately 2 mm and a hardness of 57-62 HRC, and it is treated in a shaft furnace. The round shaft has dimensions of 30x500 mm (1.18x19.7 inches) with a weight of approximately 3 kg (6.6 pounds). The charge size is 8 tons. The total costs are $1,353, and the process takes 46 hours.
Example 2: Induction Hardening of Quenched-and-Tempered Steel (42CrMo4)
The case-hardening depth is approximately 2 mm and the hardness is 57-62 HRC by single-shot hardening. The shaft is the same dimensions as example 1 (30 x 500 mm). The workpiece weight is also the same, and the 8-ton load corresponds to 2,667 shafts. The cycle time/shaft is 20 seconds (consisting of heating, quenching and tempering). With an energy input of 0.56 kWh/shaft, an 8-ton charge requires energy input of 1,492 kWh. Therefore, the total costs are $150, and the pass time for 8 tons is approximately 15 hours.
The comparison of the pure energy costs alone shows the superiority of the induction process by direct comparison with case hardening for suitable workpieces. In the example, the induction-hardening process time of 15 hours for 8 tons of workpieces (2,667 shafts) is only around one-third of the 46-hour process time for case hardening.
Heat-treatment processes such as induction hardening or case hardening are necessary to meet the enormous industrial demands for quality and load-bearing capability of individual components. As an example, the vibration resistance in the material can be optimized by both methods to such an extent that parts subject to high dynamic loads can be made usable.
The wide variety of different conventional-hardening processes shows the various potential fields of application and should not be generally questioned. The creation of individual microstructures, particularly in steels with specific alloying elements, is made possible by carefully balanced heat-treatment processes in the furnace. In many cases, however, an induction solution is far faster and far more cost-effective for achieving specific surface-hardness parameters or microstructure properties. The suppliers of induction heating technology are ready to share their know-how.
The energy cost calculation alone shows a significant difference in the comparison of the two processes in favor of induction hardening. The high investment costs for heat-treatment furnaces are in contrast to the comparatively low costs for corresponding induction heating facilities. The customized integration of induction heating into complex manufacturing processes ensures cost-effective production.
For more information: Contact Torsten Schaefer, vice president sales & service, SMS Elotherm North America LLC, Tech Induction, 13129 23 Mile Road, Shelby Township, MI 48315; tel: 586-469-8324; fax: 586-469-4620; e-mail: email@example.com; web: www.techinduction.com & www.sms-elotherm.com