Preventing scaling

[Chris Pattison 1Participant @chrispattison1]

I have a copy of Henly's 20th Century Formulas, 1942 edition. There is a chapter entitled Hardening without Scaling. It suggests preparing equal parts common salt and fine corn meal. Wet the part and dip into the mixture and then heat. Take out and roll in the mixture to increase the layer thickness. The mixture forms a flux over the surface, and comes easily off later when dipped into water. Borax might be as good, but more difficult to remove. Over to you.

All sorts of interesting stuff in this gem of a book.

[VicParticipant @vic]

You can buy small tubs of ready mixed fire cement for sealing fires in many DIY stores, I wonder if that would work?

[Brian WoodParticipant @brianwood45127]

Hello Chris and Vic,

I did the heat treatment on these cutters in the end painted up with talcum powder in meths which worked perfectly satisfactorily, scaling was negligible and the cutters have done their job.

So thank you to everyone who has added to the sum of knowledge on the subject, it has been appreciated

Brian

[Jon LightParticipant @jonlight78570]

Posted by Chris Pattison 1 on 15/03/2015 19:58:31:

I have a copy of Henly's 20th Century Formulas, 1942 edition. There is a chapter entitled Hardening without Scaling. It suggests preparing equal parts common salt and fine corn meal. Wet the part and dip into the mixture and then heat. Take out and roll in the mixture to increase the layer thickness. The mixture forms a flux over the surface, and comes easily off later when dipped into water. Borax might be as good, but more difficult to remove. Over to you.

All sorts of interesting stuff in this gem of a book.

Interesting – seems to be along the same lines as "salt ceramic"

**LINK**

Japanese sword makers have been known to add charcoal to clay mixes used for anti-scale, perhaps charcoal or hydrocarbons in the form of starch serve a sacrificial role taking oxygen that would otherwise become iron oxide. Tried charcoal in various clay mixes I have been experimenting with but felt results were either neutral or negative – one thought was that if charcoal is playing a sacrificial role then when it has burnt away the clay is even more porous on account of the voids left by the charcoal.

Common salt is an interesting one – Wiki says MP 801 deg C – my process is 830 deg C. I like the idea of salt as a flux, I would hazzard a guess that if it coats well then it would be an effective physical barrier for oxygen ( not porous like a clay ) and it should wash off afterwards easily enough. Being a compound of highly reactive sodium I would have thought that even in its molten state it would be very unlikely to combine or react chemically with anything else so possibly an ideal, non-reactive impermeable molten coat that washes off trivially.

Excessive amounts of hydrocarbons in some electric kilns could be a problem as many filament types dont like reducing atmospheres but I would have thought a thin coat of corn meal might not be too much of a problem.

I think I will give it a go – thanks – Jon

[Jon LightParticipant @jonlight78570]

Posted by Keith Long on 20/08/2014 16:58:58:

Brian

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.

Hi – copper plate is likely to oxidise just as quickly as steel – the scale is not as difficult to remove as steel scale but clean up can still take a while and acids are usually used – see jewelry enamel forums and communities for moans and groans on their copper scale problems. Copper plating has to be done to a very high standard to resist elevated temperatures (any less than this will blister) and plating copper directly onto steel is not easy for the amateur who will usually resort to a preliminary flash coat of nickel which is probably not the kind of metal you want to have in oxide form hanging around your workshop. Most of the above from direct practical experience (850 deg C) although with a different purpose in mind.

[Chris Evans 6Participant @chrisevans6]

It is a shame you do not have any local engineering or toolmaking companies to put these kind of jobs in with their batch.

When I ran a mould making tool room we specified "Vacuum Hardening" for parts we wanted to come out clean. I was always willing to put odd bits in with our work as most times we where no where near the minimum order value.

[Tim StevensParticipant @timstevens64731]

Copper plating is used to prevent carburising an area to remain soft, so I see no reason why it should not do a protecting job for you, too. It must stand red heat to work at all.

Tim

[duncan webster 1Participant @duncanwebster1]

If using a muffle furnace Tubal Cain ecommended putting a lump of charcoal in with the job, the idea being that the charcoal mopped up the oxygen before it had time to react with the iron and cause scaling.

[Jon LightParticipant @jonlight78570]

Posted by Chris Pattison 1 on 15/03/2015 19:58:31:

I have a copy of Henly's 20th Century Formulas, 1942 edition. There is a chapter entitled Hardening without Scaling. It suggests preparing equal parts common salt and fine corn meal. Wet the part and dip into the mixture and then heat. Take out and roll in the mixture to increase the layer thickness. The mixture forms a flux over the surface, and comes easily off later when dipped into water. Borax might be as good, but more difficult to remove. Over to you.

All sorts of interesting stuff in this gem of a book.

Hi Chris,

I am actively working on mixes so I tried this salt recommendation out. I am testing mild steel squares 1mm thick – 5 minutes exposure to 830 Deg C, normal atmosphere.

I played around with the mix for a while, eventually replaced corn meal with a high quality clay and milled sand for added physical robustness, corn meal did not seem crucial other than as a binder and thickner.

I was pleased with the result, oxides came off relatively easily and the surface required very little abrasion to come up shiney. I use a USB microscope to inspect pores, some mixes leave hard to get at crud in pores, salt seemed fine no problems. The only issue I had with is was that it did tend to leave the surface of the metal a bit pitted with very shallow pockmarks.

I think for anyone proposing to clean up with a grinder or rotary wire brush the salt mix would be ideal, simple with non-toxic readily available materials but those requiring a fine surface with minimal clean up might wish to compare with alternatives.

I would certainly use a salt flux subject to the caveats about finish, cheap fast non-toxic with easy to find materials.

[Jon LightParticipant @jonlight78570]

Posted by Neil Wyatt on 20/08/2014 16:35:19:

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

Neil

Neil Calcium carbonate is a flux used by potters which makes it promising – intending to try this one out, I will let you know.

[Jon LightParticipant @jonlight78570]

Posted by John Haine on 20/08/2014 21:56:02:

Borax? Doesn't flux stop surfaces oxidising?

As far as I understand it fluxes can perform two roles, iron oxide softens and dissolves or melds into the flux, secondly the flux forms a barrier for oxygen possibly aided by the presence of dissolved iron oxide.

Some antiscale formulas seem to work mainly by stabilising the oxide layer so that it does not flake, an unbroken layer helps to inhibit further oxidation, I suspect others lean more towards dissolving oxides quickly but not necessarily inhibiting their formation. Two mechanisms the balance between the two is likely to vary from formula to formula.

Interesting to note how vitreous enamel ground coats work on steel – the formulation is designed to absorb iron oxide the formation of which is welcomed since the subsequent pitting of the steel is thought to be what allows the ground coat to afix itself to the steel. Seen from this angle it eats its own footholds and handholds by absorbing oxides that in turn are due to oxygen feeding on steel.

When ground coat is detached from the steel after say a sharp blow from a hammer the steel will often be exposed as a silver virgin metal although very pitted and cratered. Borax is one of the ingredients of enamel ground coats and I have heard knife makers moan that despite its abilities as an antiscale they dislike its tendency to pit the steel.

At elevated temperatures oxygen is a savage beast, it will eventually get through most thin layers you care to put in its way so for most diy pastes and mixes I dont think its so much a question of "stopping surfaces oxidising" so much as slowing oxidation, preventing fresh metal being exposed (physical stabilisation of existing oxide layers or fluxes that melt and coat) and making sure that the inevitable oxides lift cleanly or are softened* saving on clear up operations.

* One formulation I have experimented with still left a modest oxide layer although of a slightly lighter grey appearance than usual – I thought the formulation had failed until I tried a very weak acid followed by gentle wire brush and found that its scrubbed off more like a stuborn paint than the usual rock hard and tenacious black scale. The measures mentioned would have had no impact on the scale I usually encounter.

[Peter KroghParticipant @peterkrogh76576]

I use boric acid powder wetted with alcohol to form a paste. This was taught to me by a clock maker. For example,

hardening a tap you've made; wrap the tap quite loosely with some soft iron or steel wire. This will hold the paste.

Coat the tap and it's wire cage generously with the paste. Heat and quench. Unwind the wire. The tap will be

pretty much as bright as it was before heating. Works very well. It's the same boric acid powder used in ant bait.

Pete

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