Re Hardening

[tornitore45]

Suppose I have a Carbon Steel tool previously hardened and want to modify it.
Can it be hardened again after a deep Draw to make it machinable again.
Would it be better to Draw it down (at what color?) or do a full anneal?

[Harold_V]

There's two issues with dealing with hardened material. One of them is the risk of decarburizing. Carbon near the surface can be eliminated, so the piece can't be hardened a second time. That's assuming one doesn't have a controlled atmosphere furnace, and few, if any, home shops do have.

The second issue is scaling. Again, if one has a controlled atmosphere furnace, that wouldn't be an issue.

There are paints available that prevent scaling, and I'd have to assume that they also prevent decarb. You can also consider the idea of pack hardening, whereby the item is surrounded by a source of carbon (coked peach pits are commonly used), which limits scaling and replenishes carbon. It's the method commonly known as case hardening. Doing so would ensure that the piece was hardened on the surface, where it most likely matters.

It might be helpful to just draw down the hardness, assuming you have just carbon steel. The lower temperature required would limit scaling, but you won't be able to avoid heating to a higher temperature for re-hardening, where you may experience scaling.

Harold

[kkgilkey]

I am preparing to make a small furnace for hardening/tempering O1 and the like. What are the issues in building in a "controlled atmosphere" into the furnace? Which inert gases are most useful and affordable? Any safety issues? Thank you for your help

M'G

[earlgo]

The guy on Clickspring just uses a boric acid paste to prevent scaling. It seems to work for him.
Recently I needed a spring made from 1074 steel, and I hardened it with a torch, the part being dipped in boric acid after heating a bit.
Then I tempered it in a lead pot at 660°F using a bullet casting thermometer to verify the temperature. Worked great and I didn't even have to get out the breechclout and tortoise shell rattle.
--earlgo

[10KPete]

Though I've never tried it I suspect that the boric acid method could be used in the anneal to prevent carbon loss..

Pete

[kkgilkey]

I have heard of various coatings (like the boric acid tip, thanks!) and of course SS wraps that prevent scaling and decarb, but I am still intrigued by a controlled atmosphere approach to preventing such problems. Especially as I don't have a surface grinder (yet), and preserving dimension and finish will be important to me. My interest in heat treating is mostly for tooling I make which includes metrology fixtures down to the good-ole center punch. This would include both basic hardening/tempering as well as case hardening (NOT for decorative purposes).

M'G

[choprboy]

I have a small electric kiln that I was going to use for annealing/hardening when I get a better thermocouple and temperature controller on it. I figured I'd just stick a bit of pipe in thru the vent hole, connected back to my TIG tank, running argon at a few CFH, to make it an inert atmosphere. That should prevent scaling/decarb and doesn't require coatings (or using stainless foil wrap, which is the another method). Haven't tried it yet... but should theoretically work.

Edit: Ninga'd out of the mention of using stainless wrap....

[tornitore45]

My operating environment is a selection of Propane torches of various size and fire brick stack in the driveway open air.
SS foil is out. Not considering the cost, the time to unwrap is too long for a quench.
No gas, No Kiln.
No case hardening, the box size/mass is beyond my heating and safe transport capability.
My only option are "paints".
There are notions on the web that liquid soap or chalk and water paste work, I wander whether they do or are old wife's tales.

[10KPete]

Mauro,
Check out the Brownells web site. I think they have some anti-scale stuff.

Pete

[Harold_V]

I have a small electric kiln that I was going to use for annealing/hardening when I get a better thermocouple and temperature controller on it. I figured I'd just stick a bit of pipe in thru the vent hole, connected back to my TIG tank, running argon at a few CFH, to make it an inert atmosphere. That should prevent scaling/decarb and doesn't require coatings (or using stainless foil wrap, which is the another method). Haven't tried it yet... but should theoretically work.

That may be a great solution to the problem, although it might pay to explore other gasses, as a matter of economy. Could be nitrogen would work, but I don't know that it would, nor do I know the cost difference.

Industry tends to control atmosphere simply by controlling the fuel. They can case harden the same way, introducing an excess of fuel (often just natural gas), providing the necessary free carbon.

As far as the furnace atmosphere is concerned, providing the precise amount of fuel (rules out electric furnaces) can yield a neutral atmosphere, where there's no decarb, nor scaling. When I was operating my humble shop, I did both tooling and product, often requiring heat treat. As certification was generally required, I sub-contracted that work to a certified heat treat shop. Parts returned were never scaly, although they were discolored from the process.

I'm familiar with the stainless foil method, but have not experienced its use personally. Seems it could be somewhat cumbersome, depending on quench requirements. It also appears to me to offer considerable risk of getting cut badly. Any thoughts you'd like to share in regards to those two issues?

Harold

Edit: Comment to Mauro
If you aren't familiar with heat treating, make sure that if you quench that you do NOT present a flat side to the quenchant. as that results in a badly bowed object. Insert from an edge, and plunge deeply, moving up and down, not sideways, nor swirling.
I learned this lesson by ruining a nice set of parallels I made in my apprenticeship. I now have one parallel as a reminder. The first one quenched bowed an honest 1/16"---and it was just 6" long.

[tornitore45]

Yap I learned the easy way, from other people mistakes.
Not really, I learned from the shop teacher in 8th grade. Quench the tool tip, polish and watch the residual heat in the shank heat the tip to straw yellow and re-quench.