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Advice on Heat Treating

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William Chitham17/03/2021 11:23:24
125 forum posts
52 photos

messuhrhalter 01.jpg

I am making a DTI holder from drawings by Stefan Gotteswinter. He specifies that some of the parts should be hardened and, presumably, tempered. These bits (not finished yet) are made from 01 Tool Steel, I am pretty confident I can harden them by heating with a torch then quenching in oil but not so sure how to go about tempering - or if that is even necessary? I understand the principle of reheating and watching for colour change but that usually seems to be done on larger parts where the heat can be applied at one end and the colour change observed as it migrates up the part. These are only 30mm long overall and besides I think I want the whole thing at the same state. Any advice would be welcome.

Edited By William Chitham on 17/03/2021 11:23:55

Mick B117/03/2021 11:28:26
2023 forum posts
117 photos

You should be able to take them down to about 60 Rc in a domestic oven on max (c. 250 C)?

Peter G. Shaw17/03/2021 11:33:17
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1314 forum posts
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Tubal Cain (T.D. Walshaw) in one of his books, Workshop Practice Series No.1 on Heating Tempering etc suggests that one could boil the parts in water: this would achieve some grain refinement, achieve some minor tempering and presumably reduce the brittleness.

Another idea would be to heat in a tray of sand until the items achieve the same colour as the sand.

I should point out that I haven't tried either idea - they are "parked" pending a suitable opportunity to try them.

Also, as Mick suggests, you can stick them in the oven! Again Tubal Cain has some pertinent remarks as to the best time to do it, eg when the household cook is making scones, roast beef, or whatever!

Peter G. Shaw

 

Edited By Peter G. Shaw on 17/03/2021 11:34:31

Edited By Peter G. Shaw on 17/03/2021 11:36:16

Ramon Wilson17/03/2021 12:03:28
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1198 forum posts
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Hii William, I've done a fair bit of heat treatment in my time and would definitely recommend the use of hot sand.

I have a tin of dry sand which I heat before the parts are introduced. The sand does need to be quite hot throughout beforehand otherwise there will be a temperature difference which, depending on the shape of the part may induce a colour variation .

I heat mine on one of those small gas camping stoves. Once hot the part, previously heat treated and cleaned back to parent metal and devoid of any oil (most important) is pushed down in the sand and constantly turned within it to ensure even heating throughout. Depending on the size of container you should be able to do both parts at once to get a degree of similarity but personally I would do them singly keeping a very wary eye on the colour changes. Drop the part in oil as soon as the colour desired is reached

One thing to particularly note is - if you are not happy with the degree of colour once quenched eg not dark enough the part will not change colour further if reheated unless the colour as first quenched is removed back to shiny metal again. It will just remain at that colour but it will continue to temper further.

Hope that helps some

Ramon (Tug)

Rod Renshaw17/03/2021 12:04:35
336 forum posts
2 photos

Some good ideas on tempering above.

I have tried boiling parts in water as Peter suggested but found the tempering effect to be rather small ( only 100 C ) and parts of any complex shape still broke. It might work okay on "blocky" shapes not subject to shock loading. The oven ( up to 250 C ) works better.

There are Youtubes on making simple muffle furnaces from baked bean tins and fireclay, and powered by a propane torch which should give more controllable results, up to maybe 650C, which might give a tough result rather than a brittle one. Might more consistent for the initial hardening, at a higher temp, as well as the tempering. Not tried this myself.

For occasional amateur use, the parts might work for a long time even if left soft.

Rod

pgk pgk17/03/2021 12:45:10
2324 forum posts
293 photos

A question:

For this application I'm guessing the concern is wear at adjustment points. Hardening normally requires the part to be heated and soaked at that heat long enough for full 'penetration' . If in this case the parts were just heated to a surface cherry red before quenching would that be akin to a thick case hardening?

pgk

Chris Evans 617/03/2021 13:46:20
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1960 forum posts

I doubt if you would ever wear them out if no heat treatment is carried out. O1 is pretty tough.

ega17/03/2021 14:08:51
2267 forum posts
188 photos

A deepfat fryer can be used for tempering.

not done it yet17/03/2021 14:09:12
6350 forum posts
20 photos

I expect quenching in oil would leave them hard enough without being brittle? So probaly no tempering necessay. Tempering in a hot oven is sufficient. If not hot enough in the first place, they will not ‘part-harden’.

Tony Pratt 117/03/2021 14:10:22
1704 forum posts
8 photos

I'm positive tempering at 100 C will have no effect on the component hardness.

Tony

Rod Renshaw17/03/2021 14:12:42
336 forum posts
2 photos

Pgk

Hmm.. it's not a normal process AFAIK, but it should work if the process could be controlled. The thicker parts might be surface hardened while the thinner parts might be hardened (and brittle) all the way through. Difficult to get any consistent results on a small component I would have thought. On a larger object one might try local heating with acetylene, and then quenching, on those bits needing to be hard.

Rod

William Chitham17/03/2021 14:41:10
125 forum posts
52 photos

Thanks for all the suggestions. I don't suppose I would ever wear them out if I didn't harden them but I am using the project as a way to learn new things and I haven't tried heat treating before so... I think I will go ahead and harden them then heat treat in the oven, not least because that will be an excuse to cook a pizza this weekend. In the oven is it a case of holding them at temperature for a certain time then letting them cool gradually?

Of course I might be remaking the parts as tonight's challenge is to mill the dovetail slot in the end of the threaded one with a home ground cutter.

William.

Ramon Wilson17/03/2021 15:50:22
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1198 forum posts
298 photos

William - your original post suggests you wanted to 'colour' them so I suggested hot sand as the best method of controling that in the home workshop. You can indeed put it in with your pizza but the temperature required to get the part to a decent blue black will give you a very crusty edge and topping!!

Colouring as you describe, by heating and watching the colour rise up is usually carried out when making tools - quenching usually occuring as the 'pale straw' reaches the tip. That is not tempering in the true sense in that the part has not had time at temperature to temper fully. I first saw that done as an apprentice on the shipyard by the blacksmith heat treating my new chipping hammer made from an old file!

At one time In my working life I made, on a daily basis many, many small parts from various tool steels but mainly B01 Arne (gauge plate equivalent). These were heat treated correctly and tempered accordingly - usually for two hours per inch of ruling section and twice at that ie quenched after two hours and then repeated. Colour of course was none existent just a manky oily black but hardness was checked throughout the batch to ensure uniformity.

To achieve a colour, and a uniform one at that, the temperature has to be even and specific to have any degree of control (of the colour). Kitchen ovens aren't usually hot enough to reach a dark colour.

You could, as said, leave them as they are - unhardended. I have collets for my mill that I made circa 1984 from the same unhardened 01 tool steel and they are still in use today. Colour or even blacking could then be done by gentle heating and quenching though it will not be hard wearing or by any of the cold blacking methods which will fare much better

The above and previous comment is based on personal experience of heat treatment of tool steels and offered as such - hope it's of use

Regards - Tug

William Chitham17/03/2021 15:58:44
125 forum posts
52 photos

Sorry Tug, thanks for your informative replies but my reference to colour was just in terms of using it as a guide when tempering. I plan to cold blue these parts when finished, I have some Phillips bluing solution which works well.

William.

SillyOldDuffer17/03/2021 16:03:43
Moderator
7572 forum posts
1681 photos
Posted by William Chitham on 17/03/2021 14:41:10:

... I think I will go ahead and harden them then heat treat in the oven, not least because that will be an excuse to cook a pizza this weekend. In the oven is it a case of holding them at temperature for a certain time then letting them cool gradually?

...

William.

Wouldn't be the end of the world if the items were left unhardened because they're unlikely to wear much in ordinary use. No harm in hardening of course, but it wouldn't be the end of the world if you didn't bother to temper them either! Hardening tends to leave metal too hard and a bit brittle, so tempering sacrifices some hardness in exchange for improved toughness. Hard working components benefit from this trade-off, but a DTI holder doesn't sound critical.

All good experience though, but the timing is important.

  1. Pre-heat the tempering oven to about 200°C
  2. Heat the items slowly* to about 800°C, and hold them at that temperature for 1 hour per inch of thickness, plus 15 minutes per additional inch. (*Slowly means raise the temperature by 200°C per hour per inch of thickness.)
  3. Plunge the items into oil and then immediately transfer them into the tempering oven. Don't let the temperature of the items drop below about 50°C, so work quickly!
  4. Leave in the oven for 1 hour per inch of thickness, and always for at least for 2 hours.

This complicated recipe is needed to harden and temper for best results and it's a good deal more scientific than what I do with a blowlamp! In practice, small items don't seem particularly fussy, but I don't have any way of measuring how hard and brittle my results actually are. All I want is the metal to be a bit better than the un-heat-treated version, i.e. harder, and not too brittle. It mostly works.

My limited experience for what it's worth: 800°C seems important, over cooling in the quench is definitely bad, and not letting metal soak long enough at 200°C may explain some disappointments. I'm not sure about getting the item into the oven before it drops below 50°C because I always bung it in as soon as possible, and in any case have no way of confirming if the metal really is still hot enough. Results are mixed, most common problem is too brittle.

Dave

Grindstone Cowboy17/03/2021 16:59:37
714 forum posts
58 photos

For a DTI holder, would brittleness be a major issue anyway? Hopefully it's not going to be subjected to much stress (unless you knock it off the bench)

Rob

William Chitham17/03/2021 17:20:15
125 forum posts
52 photos
Posted by Grindstone Cowboy on 17/03/2021 16:59:37:

For a DTI holder, would brittleness be a major issue anyway? Hopefully it's not going to be subjected to much stress (unless you knock it off the bench)

Obviously I will be knocking it off the bench with grim regularity like everything else! Impact shocks aside I think the most vulnerable bit in normal use will be the "ears" of the dovetail which will only be about 0.75mm thick and will tend to be spread by the force of the locking collar screwed up from beneath.

William.

Ramon Wilson17/03/2021 18:57:19
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1198 forum posts
298 photos

Hi William,

If you are going to cold blue them unless things have changed from a chemical perspective I'm not so sure hardening would be a good idea anyway. Do you know if your Phillips solution will work on hardened steel?

Long time ago at work we tried the KoldBlak system. It worked really well on unhardened parts but would only produce a brown colour on heat treated and ground steel. The recomendation was to treat hardened parts with 'Aldecon C' if I remember the name correctly. I believe that was Hydrochloric acid and very strong. When we came into the toolroom after the first time we used that every steel surface and I do mean every was covered in a fine patina of rust -. Our benches where we serviced the press tools were steel topped and shiny with use - not any longer ! Ha! I was a popular boy as you can imagine.

Just another thought

Given your description of the marginal area at the end of the dovetail I would be inclined to leave the hardening experience for something later down the line

Good luck with it however you approach it thoughyes

Tug

Martin Kyte17/03/2021 20:24:39
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2570 forum posts
45 photos

If you are going to worry abot surface wear then I would have thought that the items shown along with their use would suggest case hardening as the most suitable process. This would achive a hardwearing surface to the adjustment joints. The treads however should be cut after the casing takes place but before heat treatment so that the threads are left soft and non brittle. Tempering is not required with case hardening.

If you do want to blue afterwards then clean up after heating to cherry red and quenching and then warm gently in a flame untill the parts are blue and chuck them in a container of oil.

regards Martin

Edited By Martin Kyte on 17/03/2021 20:25:00

Edited By Martin Kyte on 17/03/2021 20:25:13

Nigel Graham 217/03/2021 20:54:11
1712 forum posts
20 photos

Polishing the quenched parts then heating them in lead kept just on melting-point will produce a bluish-purple effect, but only on the surface exposed to the air. That still submerged will stay bright.

(I found this when making new leaf-springs for a 7.25" gauge locomotive.)

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