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As a retired Mech Eng **2 and QA engineer, heavy industry....
Induction heating depth is sort of shallow. Higher frequencies more so, lower frequencies go deeper. The induction heating of spring wire may not be deep enough for proper transformation [skipping technical wording]. As the wire is heated right before coil winding, the quench of heat by the cold steel mandrel may be a factor is limiting heat soak into the core of the wire and the steel in contact with the steel mandrel will be cooler before the spring is quenched.
From a process control point of view the completed spring needs to be cut and a small section ground and polished then etched and examined with a microscope for grain size. And hardness needs to be sampled across the depth of the wire on the radial direction and circumferential as well. Process specification should be set in advance by engineering and QA should be responsible for determining if the process is specification capable. Each new heat [melt batch] of material needs to be qualified for hardness after a reference heat treatment and should be pulled to yield as well. Each 'heat' of steel requires a mass spectrometer report. As wire is drawn down to size, many wire deliveries could share the same heat.
Steel properties for a given allow depend on TTT, time, temperature and transformation. These spring problems are alloy, TTT or both.
These springs are yielding; the problem may uniformity of properties.
We had a thick very high strength drive shaft in a 170 ton diesel electric hauler. Locomotive traction motor pinion gear on a large bull gear driving that long shaft to a planetary gear wheel hub with 48" drum brakes. The existing shafts, think body hardened quenched and tempered tool steel, were breaking at the Athabasca tar sand operations during spring thaws when the oil sands became wet goo. So an idiot in Engineering spec'd to have those shaft case hardened and they were made and I was asked to check that vendor did this OK, but too late, they were expedited and build. I pointed out that under the case hardening, perhaps 7/16" - 1/2" deep that there is a zone below that is effectively annealed and weaker than the body quenched and tempered shafts, thus weakening the shaft. I predicted that sub surface shear tears would develop then the shaft would open up like a rope as the case hardening fractured. Sure enough, next spring, that exactly what happened. I scolded the supplier who should have seen this in advance as I did.
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