Inexpensive Induction Heater for Hardening

[Clive FosterParticipant @clivefoster55965]

[Clive FosterParticipant @clivefoster55965]

Further to my previous query about case hardening steel sources I see that small, naked board, induction heater units can be got very inexpensively. One of those looks to be a lot less mucking about when doing 50 pins than my Sievert gear.

Are they sufficiently repeatable for reliable case hardening and what size would I need to handle the 2" long, 1/4" diameter pins. I'm thinking the nominal 1000 W or thereabouts jobbies would be about right.

Standard coils look to be too fat and short for my job. Presumably I can wind a longer, thinner one from copper tube. I have some copper brake tube in stock (supplied wrongly when I ordered Kunifer so need to use it for something else less critical).

Are they capable of surface hardening a steel such as 708M40 (EN19) if used for an appropriate time before oil quenching? Obviously I'd need to start with annealed material first. Which may be an issue.

Clive

[Joseph Noci 1Participant @josephnoci1]

Posted by Clive Foster on 29/07/2020 19:10:27:

Are they sufficiently repeatable for reliable case hardening and what size would I need to handle the 2" long, 1/4" diameter pins. I'm thinking the nominal 1000 W or thereabouts jobbies would be about right.

Standard coils look to be too fat and short for my job. Presumably I can wind a longer, thinner one from copper tube. I have some copper brake tube in stock (supplied wrongly when I ordered Kunifer so need to use it for something else less critical).

Are they capable of surface hardening a steel such as 708M40 (EN19) if used for an appropriate time before oil quenching? Obviously I'd need to start with annealed material first. Which may be an issue.

Clive

Clive, most of those heaters available 'cheaply' are a Royer oscillator type – they work quite well. One downside is that the output power on those units depends solely on the input voltage – the 500watt or so units are typically 12 to 15 volts, and the 1-2KW seem to work from 30 to 50 volts . Including losses in both the heater and the power supply, you would need a supply of around 40 to 50 volts at 25 amps or more…

The pins you describe , with a long thin induction coil, would take 30 to 40 sec to get to red heat with a 1KW unit.

The second issue is that even for a 500 watt unit, you will need to water cool the induction coil. Also, the coil must be made from good quality copper as least – not copper plated steel brake piping..

The coil gets VERY hot – in the KW unit there are many hundreds of amps flowing in the coil ( its part of a resonant circuit) and due to the high frequency, the current flows mainly close to the surface of the coil. This means the resistance is higher so the losses are big.

However….I do not think these simple heaters will work for you – they will not be able to case harden as the frequency they work at is too low.

For a ferrous steel ( as opposed to non-magnetic stainless steel, for example) the depth of penetration of eddy currents is very dependant on the frequency of said current. Typically, frequencies of 20KHz to 60KHz are used and this results in full almost full penetration of eddy currents in a steel bar of 25mm diameter. That means that the material will heat up evenly, all the way through. Quenching that will give a fully through hardened work piece.

The higher the frequency, the less the depth of penetration. The workpiece then heats up close to the surface, where the eddy current concentrates, and that heat is then conducted ( more slowly) through the work piece.

This would then harden the outside when quenched. The frequencies typical for this method are from 200KHz, all the way up to 13MHz – the MHz ranges are often used for non-ferrous metals, copper, aluminium, etc, in small workpiece volumes.

A second However, is needed…If the heater ( assuming an appropriate frequency) has to low a power capability, the rate of heating the workpiece surface could be too slow, and be the same as the rate at which the heat is conducted from the surface to the inner core of the workpiece. So, the whole workpiece gets hot through again…

I suspect that for 6mm pins you would need a 2 to 4 KW unit, to get the outer casing red hot, with the inner approaching but not reaching annealing temp – that would probably need to be done in less than 6 to 8 seconds…

Joe

EDIT – those common Royer type induction heaters are typically 30KHz to 50KHz – they are not fixed and change frequency depending on the coil's inductance and whats in the coil to be heated.

Edited By Joseph Noci 1 on 29/07/2020 21:08:54

[Clive FosterParticipant @clivefoster55965]

Thanks for that Joe

Confirms what I thought that these things are basically a cleaner alternative to propane for heating up smaller components.

I did see a You Tube video of a 1.2 KW one running at 95 KHz which suggested it might be marginally viable with a smaller diameter coil for good coupling efficiency. I agree with that 200 KHz and up is more appropriate for surface hardening.

Still might be worth considering as a heat source for use with case hardening compound though.

The tube I have is copper, not copper plated steel, so if I get one I'll wind something up and give it a try. But if its going to take approaching a minute to get the pin hot water cooling the coil will be essential if the job is to be done quickly.

More to arrange and probably more futzing about than what will turn out to be a break even job is worth. I don't mind spending so much on kitting up that I only break even on a job for another guy if I end up with something I'll use in the future. Which, given the limitations, seems unlikely with this.

Clive

[Joseph Noci 1Participant @josephnoci1]

Those Royer units are quite useful for loosening rusted nuts, but not really for prolonged use, not without good cooling, both the coil and the electronics. Unfortunately the higher you go in frequency, so goes the cost as well – requires better switching devices since the switching losses go up as well. If you do go for one of those type units, try go as big as you can. Big ones can do small jobs, not really the other way..

Joe

[Clive FosterParticipant @clivefoster55965]

Yep. Standard rules apply.

You can do very well for surprisingly little money if you happen to be in the sweet spot.

If you don't know where the sweet spot is or don't manage to hit it the choice is between mega frustration and hello big spender.

Everything is more complicated than it looks at first sight!

Clive

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