Induction Surface Hardening and Tempering

Induction hardening is primarily used for surface hardening. The heating process does not affect the core structure. It is possible to heat a material locally where it is functionally desired. Other sectors of the material remain untreated and it is easy to machine them.
Main advantages of induction hardening are: Low distortion, Low risk of scaling, Localized hardening, Good reproducibility of hardening process, Easy integration in production line, Fully automatic process easily attainable, Easy to operate machines, Less harmful to the environment compared to other hardening processes, Use of unalloyed steels.

Applications for Induction Hardening

Induction hardening is primarily used for surface hardening. The heating process does not affect the core structure. It is possible to heat a material locally where it is functionally desired. Other sectors of the material remain untreated and it is easy to machine them.

Induction hardening is in most cases more economical compared to other heating processes. In some cases it is the only possible heat treatment process.

Main advantages are:

  • Low distortion
  • Low risk of scaling (These two advantages may allow final machining before hardening)
  • Localized hardening
  • Good reproducibility of hardening process
  • Easy integration in production line
  • Fully automatic process easily attainable
  • Easy to operate machines
  • Less harmful to the environment compared to other hardening processes
  • Use of unalloyed steels.
The surface hardness depends on the carbon content of the steel. Surface hardening increases the wear resistance and can be used to increase the strength of highly stressed components. It is advisable to design a new component suitable for induction hardening. The following applications are a small selection of induction hardened components and applications.

The frequency range of power sources for induction hardening is as follows:

  • High frequency above 100 kHz (HF)
  • Intermediate frequency between 10kHz and 100 kHz (ZF)
  • Medium frequency from 3 kHz to 10 kHz (MF)
Small components with low hardness depth are normally hardened with HF or ZF; bigger components with higher hardness depth are normally hardened with ZF or MF. An exact definition of the frequency application range in advance is not always possible.

Machine types used for induction hardening are:

  • Vertical hardening machines
  • Horizontal hardening machines
  • Indexing table machines
  • Special machines

Component Group: CV-Joints

The tendency for light components with increasing torque values leads to thin-walled joints with high requirements on the hardening process. The rolling paths in the bell area as well as the shaft (if there) are hardened. Depending on the requirements it is done with static rotation, progressive, progressive rotation or single shot hardening. The huge number of different joints requires flexible hardening machines, quick and reliable change of tooling for different components.

The machine and hardening parameters have do be controlled continuously. Hardness and crack detecting machines can be integrated into the hardening installation.

Outer and inner races are mostly single shot hardened. Thus process requires advanced power sources with high power output, short and precise heating time and a quick ramp up time to achieve a reproducible quality. Some of these components can be tempered by residual heating.

Interconnecting shafts are hardened in single shot or progressive with rotation. The hardening process is horizontal or vertical clamping between centers. Induction tempering is also possible. The change of length during heating can be registered and corrective action can be taken.

Components Group: Steering Parts

The shaft can be hardened progressively with rotation of the workpiece. The tooth part can be hardened with three different methods:
  • Combined induction-conduction hardening
  • Progressive induction hardening with rotation and ring-inductor
  • Progressive induction hardening with form-inductor and pre-positioned tooth section
Pinion racks and power valves of power steering systems can be induction tempered after hardening. Indexing table machines with automatic loading/unloading systems reduce the cycle time. A suitable heating process and the correct frequency are essential for the achievable quality.

Hardening and tempering without scale is done in a protective atmosphere. Polishing and cleaning of the component after hardening is not needed.

A patented quenching curtain system at in- and outlet of a chamber minimizes the gas consumption. The oxygen concentration in the chamber is controlled to ensure the process reliability. The process is suitable for integration in production lines and it can be retrofitted in appropriate older equipment.

Components Group: Engine Parts

The demand for lower emissions, lighter and smaller engines influence the design of camshafts drastically. Precise hardening of camshafts in one operating sequence requires precise CNC controlled hardening machines with special inductors and quench rings to position each cam very precisely. A low distance between cams may require the simultaneous hardening of several cams at once. Separate control of power and heating time is an advantage. It is nowadays standard that all essential process parameters are automatically controlled by CNC.

Induction hardening of valves requires precise positioning between inductor and work piece. Frequency, work piece rotation and energy input into the work piece have to be controlled in small tolerances. A fast floor-to-floor time requires fully integrated indexing table machines often with hardening of two valves simultaneously.

Hardness testing equipment can be integrated. Valve hardening with lower production rates can be done with a good reproducibility on universal hardening machines.

The hammer area of rocker arms is hardened progressively or by oscillation. Exact and reproducible coupling between work piece and inductor are also essential for this work piece. Rocker arms in cast quality need a special heat treatment with pre-heating and special timings. Tempering with residual heat is possible-also bore hardening for bigger diameters.

Components Group: Drive Shafts

Rear axle shafts are hardened progressively with rotation or in single shot. Horizontal and vertical hardening machines with one or more hardening stations are used for this process. Hardening of the radius increases the component strength. Depending on the design and length it is recommended to use a steady rest to reduce the run-out.

These shafts are single shot hardened. In comparison with a structural steel hardened in a furnace cheaper carbon steel can be used with higher strength after induction hardening.

Components Group: Axles and Rods

The high production volume of motor shafts requires horizontal in-line machines and progressive hardening with rotation. The very high feed rate requires precise and fast control for the power source and machine to achieve exact and reproducible hardening zones at the beginning and end of the component. A tempering process can follow.

Hardening requires a horizontal machine with continuous feed and work piece rotation. The low wall thickness of the hollow shaft calls for precise power and frequency to reduce bending and to avoid through hardening. The machine is ideal for integration in the production line. Process control in connection with a rejecting chute ensures that only good work pieces leave the machine.

Piston rods are preferably hardened on a horizontal machine independent of the machining condition of the end face. In-line or separate machines with loading magazine can be realized -induction tempering after hardening is possible.

Components Group: Gears

Induction hardening increases the flank durability and tooth ground strength like furnace hardening but grinding may be skipped or reduced which results in cost advantage.

November, 2005
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