Heat Treatment of Tool Steel

[JellyParticipant @jelly]

I’m nearing completion of a punch and die set for cutting the teeth in sawplates, and am wondering if there is an alternative to using a commercial heat-treating firm.

They’re made from AISI D2 (DIN X155CrVMo121), which I believe would suggest a salt bath method with a 1hr soak at >1000°c and a ~250°c quench, before air cooling.

(Tried to get D6 which can be flame heated and oil quenched satisfactorily, but no luck).

Has anyone got suggestions for or advice on attempting this sort of procedure?

[JellyParticipant @jelly]

[Tony Pratt 1Participant @tonypratt1]

I would suggest it is false economy to try heat treating D2 at home.

Tony

[JellyParticipant @jelly]

The thinking I’ve done around the issue suggests that far from any kind of economy, it might well be more expensive!

But, the facility to carry out such work would undoubtedly be useful for more than just this task… At which point it might be kind of economic, if I squint and try very hard to justify it to myself.

And, all that still discounts your precient observation that there is every chance of adding yet more expense by horribly distorting or even cracking the parts if I cock it up even slightly.

I still can’t quite escape the desire to try though…

[Jeff DaymanParticipant @jeffdayman43397]

Are you concerned primarily about cost?

If you have a lot of work in the parts, I would not risk cracking or distortion on quenching by trying to save a few pounds by trying it at home. Unless you have some very sophisticated temp measuring gear, a good furnace, and excellent safety and fire supression gear I would not recommend trying a DIY salt bath and quench for D2 tool parts.

If commercial places give you high quotes or a rough time, you might try going through a middleman tool shop – many mould and die shops have HT work done every week and so they get better rates and good cooperation. They also might spot potential problems with your parts if there are any, and suggest fixes pre-HT. JD

[JellyParticipant @jelly]

If anyone is interested in watching my eventual success or failure at this on the forum, I have an approach worked out now.

At least two heated containers for salts (NaCl/CaCL2 mix for treating, NaNO2/KNO3 for quench, I don’t really want to have cyanides anywhere near me if I can help it) are required, in principle mild steel vessels should be sufficient but I need to do some calculation to verify the load they could support when at 2/3 of their melting point (there will be an upper limit to the size of bath feasible with a given wall thickness, which may not be a linear relationship due to the amount of salt needed to provide sufficient thermal mass)… if not Inconel is just about feasible, (again, this imposes a size restrictions, this time through cost, as it rapidly becomes prohibitive as the scale increases).

Heating would be delivered using water cooled induction coils driven by the slightly dubious 1.1kW solid-state ocilator kits available on eBay and elsewhere.

Temp control I can cover using one of the spare PLC’s kicking about, 12 I/O’s ought be sufficient for several thermocouples per vessel and on-off switching of both driver circuits.

The two major challenges are both related to heat-flow.

In an ideal world each vessel would be in a refractory jacket to minimise energy use, but this increases the risk of accidentally melting the vessels on startup due to the crude on-off method of temp control, I need to better understand the thermal conductivity and convective flow characteristics of molten salts before making a judgement on this.

Secondly, the parts themselves are liable to be inductively heated if the frequency of the ocilator circuits is not high enough… I can either put a thermocouple directly on them and allow that to take over control once the part goes into the bath (requires making suitable probes), increase the frequency of the drivers (requires more complex electronics), or try to model the heat flow from the part to the salt bath and programme the PLC to account for this and add energy to the system in pulses that can be equalised with the salt bath.

Without doing all the maths, and at least one trip to the library, I’m not sure this approach is workable… But it still might be at this stage.

[John ReeseParticipant @johnreese12848]

Joshua,

How much is your time worth?

It sounds like you are willing to walk through hell barefoot rather than use a commercial heat treat shop.

I would not want to risk my investment in the tools by trying to do the heat treating myself. I admit that I am fortunate to have access to two heatreating firms that do not gouge for one-off tool jobs.

[JellyParticipant @jelly]

It’s not so much an unwillingness to use a commercial shop (there are more than enough to choose from in Sheffield), as a question of “can I do this myself”.

A little bit like building a Quorn T&C grinder, the value is at least as much in an interesting challenge as it is in the end result…

Admittedly I would like to use the punches eventually, but I’m not in any great rush.

[not done it yetParticipant @notdoneityet]

Remember to remove any jewellery, not wear anything with metal buckles or buttons, etc and turn off the pacemaker, if you have one!

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 06/05/2017 04:22:09:

… and turn off the pacemaker, if you have one!

.

"interesting" advice ^^^

MichaelG.

[Clive HartlandParticipant @clivehartland94829]

Maybe wrap yourself in tin foil and be a Faraday shield? Might help if you get splashed with salts.

Clive

[Chris Evans 6Participant @chrisevans6]

Last time I used a heat treatment plant they charged me their minimum order of £35. If I can wait I put it in to where I used to work and get it sent in with theirs for free !

[CirclipParticipant @circlip]

Obviously never seen Yellow/red glowing molten salt? Even doing it commercially, it's NOT to be p***ed about with. Swallow you pride and let a professional do it, you know it makes sense.

Regards Ian.

[Neil WyattModerator @neilwyatt]

You might find this link useful:

**LINK**

Neil

[JAParticipant @ja]

I admire your courage (or foolhardiness).

Don't do it alone, have someone some distance away with a mobile phone. Make sure your will is up to date. In industry no one would do such work alone.

JA

Edit – Neil, you beat me to it.

Edited By JA on 06/05/2017 12:07:08

[JellyParticipant @jelly]

Circlip, you’re right I’ve never seen incandescent molten salt (except during exothermic production of nickel-chromium alloys, which is a sight to cause concern in even the fool-hardiest soul).

I have been in the melting and casting depts of several large foundries however, and whilst molten liquids certainly deserve respect… It’s not something which I find scary, with your outer PPE, multiple layers of FR clothing, seperation distances and tools for handling hot items it’s all easily controlled.

You’ve just raised a major issue that might stop me though, For safety the environment needs to be indoors, sand floored and still open enough to allow quick escape and a safe distance.

I have several friends who would happily help/act as lookout, as I certainly wouldn’t dream of doing it alone (on doing some preliminary reading, i was alarmed to realise quite a few knifemakers and other specialist blacksmiths posting online, do work alone with molten salt baths).

[Ian S CParticipant @iansc]

Here in NZ it was(?is)not uncommon for there to be salt bath facilities at secondary schools/technical collages in their metal work shops, last time I was in the metal work shop in the local HS they had a small unit, used for case hardening mainly.

Ian S C

[CirclipParticipant @circlip]

Obviously NZ doesn't have our breed of Elfins, our kiddies aren't allowed to strike a match without full face masks and welders gloves. Having said that, wouldn't trust some with a cup of water.

Does a white Lab coat constitute FR clothing Joshua? Was part of Labs job to change 3" square electrodes in 4ft x 2ft high temp tool hardening salt bath. Wind it up to 1200c and go like bu***ry to change them.

Regards Ian.

[Neil WyattModerator @neilwyatt]

Posted by Ian S C on 06/05/2017 12:49:14:

Here in NZ it was(?is)not uncommon for there to be salt bath facilities at secondary schools/technical collages in their metal work shops, last time I was in the metal work shop in the local HS they had a small unit, used for case hardening mainly.

Ian S C

There's a bit of a difference between a case-hardening bath at about 500C and one at 1000C for hardening HSS!

Neil

[Nick HulmeParticipant @nickhulme30114]

https://www.speedymetals.com/information/Material11.html

D2 Tool Steel

For D2 I went the higher temperature, air-cool then temper route, this allowed it to be done in a small temperature controlled electric furnace, no hot salts, no quench, I used a stainless foil packet for surface protection.

– Nick

[Neil LickfoldParticipant @neillickfold44316]

We used to do all our own heat treatment years ago. But as the gear got old and required upgrading the decision to have it all done by an outside company happened. The advantage of the inhouse setup was control of the process and saved time for the part being made. The downside was cost and the time being spent to monitor the process. In this regard, the commercial heat treatment company is cheaper, but is a 3 day turn around or sometimes more. Now days most parts are single tempered, but double or triple tempered can be asked for. They do have inert atmosphere ovens which reduces scale etc and leaves the parts looking nice and sharp corners being still sharp.

Neil

[JellyParticipant @jelly]

Chatting to a metalurgist friend this afternoon I discovered that there are a number of high temperature oils available for marquenching* at the kind of temperature I require (250?c or thereabouts).

Combined with the suggestion of using a stainless foil packet above this seems like a much less hair-raising method than molten salt.

* There are some unavoidable sharp corners in the die, where the potential deformation associated with forced-air cooling might well give rise to cracking.

[Bill Davies 2Participant @billdavies2]

I used to heat treat HSS tools in molten salt baths, well over 1000 degrees. It was uncomfortable to look at the baths. We removed any moisture or oil by placing the components in Wild Barfield electric furnaces, dry heat, any residue would have caused the salt bath to explode as any contamination vaporised.

The salt baths were 5/8" or 3/4" steel vessels, heated by gas, and would burn through fairly regularly. I have an itch, yet to be scratched, to do some home casting, but I wouldn't entertain this. Good luck with getting some of the salts. I don't think I could get to handle kilogram quantities of sodium cyanide these days. We also had antidote knocking around if we had a whiff of the fumes.

Good luck,

Bill

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