Induction hardening
Horizontal centerless scanning hardening. The first coil hardens to a depth of 1.5mm. The second, visible on the right, performs in-line tempering. A hardened and tempered 400mm-long shaft exits the machine every 40 seconds.

Differences between scanning and single-shot induction hardening.

There are two alternative methods of induction hardening: conventional ‘scanning hardening’, and the less common ‘single-shot hardening’. It is sometimes the case that workpiece characteristics determine which method must be used .

 A long, largediameter shaft, for instance, requires scanning, as the power needed for single-shot hardening would simply be excessive. Then there are workpieces whose irregular shapes or complex geometries makes  single-shot hardening the only viable alternative.    

Scanning hardening involves movement between the workpiece and the the induction coil. Scanning is divided into two processes: vertical and horizontal hardening. In the former, the workpiece is held in a vertical position between two centers. An induction coil then moves upwards across the length of the workpiece. The speed at which the coil moves can vary, but is typically in the range of 5 - 25mm/sec.

A major advantage with vertical scanning is that the induction coil is relatively easy to make, as it is normally a single-turn round ring. Another advantage with vertical scanning is that the quench assembly is placed below the induction coil. This means the quench medium flows downwards without interfering with the heating. It is possible to control the depth of hardening in different zones of the workpiece by adjusting the coil’s speed and the power fed into it.

Less deformation

With horizontal scanning hardening, a horizontally held workpiece is fed through a coil and quench. One benefit of horizontal scanning is that it can reduce deformation. Minimal deformation is achieved by having the workpiece in a concentric position in the coil and quench. This results in symmetrical heating and quenching, which  minimizes deformation. Another benefit is the possibility  of hardening large workpieces. It is, for example, possible to harden 6m long tubes with this method.

Single-shot hardening means the complete hardening zone is first heated and then quenched. Such hardening can be achieved with a multi-turn coil that encircles the entire hardening zone. But for workpieces with rotational symmetry  it is more usual to use a coil that follows the workpiece’s contour, combined with rotation. Coils can be designed to ‘push extra heat’ into areas such as fillets on flanged shafts, where it is often difficult to obtain sufficient hardening depth.

The benefits of single-shot hardening include minimized deformation, and optimal results for workpieces with complex geometries and/or large diameter changes. The method also reduces deformation. And the method’s relatively long heating times (compared to scanning) benefit the workpieces’ microstructure and residual stresses. But even if single-shot’s heating time for each grain is longer compared to scanning, the total heating time is shorter since the entire heating zone is heated at the same time.     

Single-shot hardening typically requires more power than scanning. This extra power is needed to achieve the required temperature increase in the complete hardening zone. Moreover, the coils used in single-shot hardening are more complicated and expensive than those used in scanning. And if the power demand changes somewhere on the workpiece, it will be necessary to physically modify the single-shot coil. With scanning, such changes can usually be handled by adjusting the control program.