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Preventing scaling

Heat treatment of gear cutters

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Brian Wood20/08/2014 10:10:25
2579 forum posts
39 photos

Some advice please.

I have made two multi tooth gear cutters in boron steel of about 60 mm diameter, 1 inch bore and 6 mm in section thickness; the next stage is to harden the cutting teeth.

I want to restict the risk of scaling in the bore and location faces next to the bore and as I don't have suitable grinding facilities to finish those they are machined to size.

Can anyone recommend an effective coating that will do this for me please?

Many thanks


Michael Cox 120/08/2014 11:02:56
551 forum posts
27 photos

Soft soap smeared on the part before heating limits the scaling on steel quite well. In the past you could by soft soap from Boots but I do not think they stock it any more. It can be bought on ebay. I have experimented with ordinary soap shaved into small pieces. It is best to heat the part gently and then dip it in the soap that will then melt and cover the surface. Then you can heat the part to red heat and quench with little scale forming.


JasonB20/08/2014 11:06:08
23076 forum posts
2771 photos
1 articles

When I have used Tipex correction fluid to stop silver solder spreading there is no sign of scale under the tipex, may be worth having a try on some scrap as the hardening temperatures will be a bit higher. Use the solvent based one.


speelwerk20/08/2014 11:21:45
447 forum posts
2 photos

I use "Condursal Z0095" which works up to 850C, for higher tempertures they recommend "Condursal Z1100" with which I have no experience. You cannot use it in a flame since you have to be precise with your temperature. Niko.

Brian Wood20/08/2014 12:28:00
2579 forum posts
39 photos


Thank you for your suggestions

Michael. I'll see what the guvner can provide in the way of soaps to try out.

Jason. I tried your suggestion as I have Tippex in the house. Testing was done on a small sample of the steel used for the cutters, painted on one side and open on the other. At red heat I could see scale forming on the unprotected side and after quenching the painted side was a lot cleaner, not spotless it must be said. Out of interest I took a sample up to gas welding temperatures, here the Tippex failed as you would expect in a severe test like that.

Niko. Duffy Condursal is probably excellent but buying and shipping from Illinois for a small job like mine is completely unrealitic; if the job is a disaster it is easier to make new parts and try again. If I was doing it every day that might be a different matter.

On balance I might try a combination approach with soap and Tippex, the Tippex is visible which helps a lot in monitoring the effects

Thank you all again


speelwerk20/08/2014 12:42:33
447 forum posts
2 photos

Brian, it is a bit far fetched to get it from Illinois, I bougth it locally here in NL, as far as I know it is a German product. Niko.

Brian Wood20/08/2014 13:39:55
2579 forum posts
39 photos

Hello Niko,

I looked the product up and was taken straight to the Duffy page in the USA. I would need to look for it from a UK stockist in Yorkshire, next move if the other methods fail me.

Thank you again for you interest


Neil Wyatt20/08/2014 14:15:55
19079 forum posts
736 photos
80 articles

I've heard powdered french chalk, mixed to a thick paste with meths, suggested specifically as it helps stop the steel losing carbon.


Brian Wood20/08/2014 16:26:46
2579 forum posts
39 photos

Hello Neil,

I have seen whiting (what ever that is) mixed with meths as a recommendation. Checking out French chalk I find it is actually ground talc and that of course is talcum powder, so I will experiment with that as well. At least the perfume will inprove the smell of the job! Another experiment would be to mix plaster powder (gypsum---calcium sulphate) with meths.

Just for reference, my wife laughed when I asked about soft soap-----unobtainable she tells me. It is based on potassium stearate rather than the much cheaper sodium stearate, the basis of ordinary soap, and that difference is where the 'soft' bit comes from. Today those more gentle soaps have all been replaced by pump action oil based creations, probably completely unsuitable for this purpose.



Neil Wyatt20/08/2014 16:35:19
19079 forum posts
736 photos
80 articles

I think I meant ordinary chalk (calcium carbonate) non-carbonates won't help prevent de-carburisation.


Keith Long20/08/2014 16:58:58
879 forum posts
11 photos


In an old copy of "Fowlers Mechanics & Machinists Pocket Book" it suggests using fireclay to blank off areas that are to remain soft when case hardening. The provisos listed are that care should be taken to ensure good adhesion and that the clay should be quite dry before the item is placed in the case hardening box. Now I know you're not case hardening but Pyruma fire cement is available pretty cheaply and might be worth a go on some scrap material to see if it works. It isn't going to be worried by the temperature and should chip away after fairly easily.

In the next paragraph the possibility is brought up of electroplating copper onto the parts not to be hardened, that might also work to avoid scaling, or possibly easier what about fastening some copper shields in place over the location faces with a bolt through the bore which should be protected at the same time.

Brian Wood20/08/2014 17:21:34
2579 forum posts
39 photos

Thank you Keith-----more interesting possibilies. Pyruma is easy to get hold of and well worth trying.

I could use it to seal copper shields, that combination will keep oxygen away and thus the scaling.



jaCK Hobson20/08/2014 18:06:59
265 forum posts
93 photos

Heating and quenching in heat-treating salts is a way of minimising scale. On a small scale you could try heating in Lo Salt from the supermarket (try on scrap part first). You will need a decent heat source to get the salts hot enough. Be careful.

Low temp (quenching) salts are not so easy to get. Hot blueing salts may do the job. Maybe a normal oil or water quench won't leave too much scale anyway.

In the past I have been advised to put the part in a small container with the end packed with clay. Heat the lot up in a suitable furnace/forge and put the container clay first into the quenchant. The clay should fall out the bottom, and the part with it. This is probably more efficient at minimising decarb rather than scale and for smaller parts that would fit in a tube. So probably not right for you.

I've never tried any of those. I have some fancy anti-scale mixture imported from US which is better than nothing but not perfect for large parts.

speelwerk20/08/2014 19:13:46
447 forum posts
2 photos

Copper will turn very soft at high temperture, it would be better to shield with the same material as the piece is made of. Niko.

JohnF20/08/2014 20:32:12
1175 forum posts
193 photos

Brian, I use Zebo black lead grate polish for stopping scaling on the inside of gun barrels for a particular process but it is "supported" with steel plugs so not sure how it would perform just painted on but maybe you can make a plug with some clearance and a washer either side well lubricated with Zebo.

It certainly stops any scaling for me. Just google Zebo black lead.


Brian Wood20/08/2014 20:32:51
2579 forum posts
39 photos

Hello Niko,

More good advice, thank you.

The cutters will be at temperature for about 1/4 hour using the usual mantra of 1 hour per inch of thickness, that gives plenty of time for copper shields to sag away.


Brian Wood20/08/2014 20:53:20
2579 forum posts
39 photos

Hello John,

I've used Black lead as grate polish in the past, just never considered it for this application. The last time I used it the formulation had given it an almost silvery appearance, certainly not black as it had been. I think in the end I used to add graphite to shoe polish and use that instead.

In your case scaling would be very bad news and the sealed plugs are perhaps doing much of the work, but there will be residual oxygen in the barrels before you seal them.

I will mull over all the contributions and choose a path, at the moment shields and plugs sealed with Pyruma is my favourite

Jack Hobson

I don't have the facilities for heat treatment salts but I do have a small muffle oven, so it will have to be the more traditional methods of heating and soaking at temperature followed by water quenching. My experiments on small samples of this boron steel so far leave them hard enough after quenching, but I have yet to try the effects of stress relief. Local industrial advice was to use a domestic oven at 180 Centigrade


John Haine20/08/2014 21:56:02
4718 forum posts
273 photos

Borax? Doesn't flux stop surfaces oxidising?

John Ockleshaw 121/08/2014 02:55:17
53 forum posts
7 photos

Hello Brian,

I do not know which of the boron steels you have used, but if it has any of the nitriding agents in it you may consider nitriding them. You can do this at home if you do not mind going to a fair bit of trouble or otherwise send them out to commercial heat treaters

Vic21/08/2014 07:36:03
3092 forum posts
16 photos

Not sure if it's suitable for your project but I understand parts can be wrapped in stainless steel foil for heat treating. Don't know where you can buy it though.

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