Induction hardening of average italian average steel rings without a soft belt

Slewing bearings for wind turbines run under combined axial, radial and tilting loads. The running surface must carry that load without spalling, so it is hardened while the core stays tough. On medium carbon steel such as 1045 or 4140, this is usually done by induction.

A soft belt is the narrow band left at the start and end of a scanned hardening path. Its hardness is lower than the rest of the track, so it wears first. Once it wears, a groove opens, cracks nucleate there, and the whole raceway can fail earlier than the design life predicts.

On a large ring, the two ends of the scan meet somewhere on the circumference. If both ends are heated and quenched as separate passes, the overlap zone may be over-tempered, under-hardened, or left as a genuine soft band. The width of that band depends on scan speed, sensor spacing and how fast the ring stops rotating.

For a bearing of 2,000 mm diameter, a soft belt 10 mm wide is already a stress raiser. For 4,000 mm rings, the same defect can be 30 mm or more if the process is not controlled. That is why the process matters as much as the steel grade.

The method used on these rings places two heating assemblies close together on the raceway. At the start, both sensors heat the same small area. When that area reaches the quenching temperature, the rotary table stops swinging and the two sensors separate and travel in opposite directions around the ring.

Each sensor heats a strip ahead of it and the quench follows immediately behind. Because both sensors started from the same hot spot, the two hardened tracks meet on the far side of the ring with both ends already at temperature. The joint is formed hot, not cold, so no soft band is left at the meeting point.

Frequency sets the case depth. Higher frequency concentrates the current near the surface and gives a shallow case; lower frequency pushes heat deeper. For raceways in the 50–58 HRC range, a case of 3–6 mm is common, and the transition zone below it should stay gradual so the core can absorb load.

The quench must follow the sensor at a fixed distance. Too far and the surface drops below the critical cooling rate; too close and the spray quenches the hot zone before the sensor has passed. Both errors show up later as soft patches, not during the run.

Carburizing heats the whole part in a furnace for hours, then quenches it. The case is deep and uniform, but the long soak coarsens grains in the core, and the full-part quench distorts the ring. For a 4,000 mm ring, that distortion can exceed the machining allowance left for the raceway.

Induction treats the surface layer only. The core stays at its original structure and the ring sees far less thermal distortion. Cycle time drops from tens of hours to minutes, and energy use follows the same curve. For medium carbon steel that already carries enough carbon, carburizing is usually unnecessary.

The trade-off is process control. Carburizing is forgiving once the recipe is set. Induction demands tight control of frequency, power, scan speed and quench. Get one wrong and the soft belt returns. That is the price of a faster, lower-distortion route.

If the ring is made from low carbon steel with no alloy content, induction will not reach the required hardness without a prior carburizing step. In that case, choose a different steel rather than force the process.

Rough turning and milling remove the bulk of the material and leave a controlled allowance on the raceway. That allowance must cover the distortion from hardening, the grinding stock, and any fixture movement during the heat cycle. On a 3,000 mm ring, 0.8–1.5 mm per side is a safe starting allowance.

After hardening, the raceway is ground or hard-turned to final size and finish. Hard turning with CBN inserts can hold ±0.005 mm and Ra 0.8–1.6 μm on a hardened surface, which often removes the need for a separate grinding setup on smaller rings.

The mounting faces and bolt holes are typically finished before hardening where possible, or re-machined after if the distortion crosses their tolerance. Bolt circle position, face flatness and bore roundness are the three features most likely to move during the heat cycle.

For rings up to 4,000 mm, we machine on 5-axis centers and mill-turn platforms with a Ø400 mm rotary table for the smaller sizes. Large rings are turned on vertical lathes and then hardened and ground in sequence.

Medium carbon steels with a little alloy content respond well: 1045, 4140 and 4340 all harden to a useful case depth without a soft belt when the scan is set correctly. They also keep enough core toughness to resist the tilting loads on a slewing bearing.

Case-hardening steels such as 8620 or 9310 need carburizing first. Induction alone will not produce the hardness, and running the sensor hotter to compensate only burns the surface. Use the right steel for the route you have chosen.

Tool steels and high carbon grades can be induction hardened, but they are brittle in the core and rarely used for large bearing rings. Stainless grades like 420 and 440C harden well, though their corrosion resistance drops in the hardened zone and they need a passivation step after grinding.

If the application needs corrosion resistance and high surface hardness, 17-4PH is a better choice than 440C. It can be aged to 40–45 HRC without the distortion of a full quench, and it machines cleanly before the aging cycle.