Effects of Hardening/Tempering

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asallwey
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Joined: Sun Dec 31, 2006 6:49 am
Location: N. Virginia

Effects of Hardening/Tempering

Post by asallwey »

One of the things I have not done until last week was harden & temper something, a small marking punch I for a project. Many projects I read about talk about H & T items, so I've started to pay more attention to it. They typically follow the process with grinding to final dimension. My big question is, what is the typical effect of H & T on a part?

Say a 1/2" x 3/8" sleeve with a 1/4" thru hole. Does it grow? Shrink? or ???
Or, a 1" x 1/4" solid pin. Does it grow? Shrink? or ???
Or, a 1/4" thick x 1" washer with 1/4" hole. Does it grow? Shrink? or ???

These would not be parts for a space shuttle, so extreme precision is not required. Just wondering from a home shop perspective on what to generally expect.

Alex
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Harold_V
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Re: Effects of Hardening/Tempering

Post by Harold_V »

Because heating and quenching usually causes distortion in parts, you can expect any of the conditions you described. Some materials are known to shrink at a projected rate when heat treated, while others may not. In order for the developed conditions to be addressed, it is generally accepted that material be left for grinding, so the final product meets the desired specifications. Alternately, if the hardened condition isn't excessive, you can do finish machining after heat treatment, but the parts won't respond well to that unless they are under 50Rc. Carbide or ceramics would be required, plus a rigid machine. The net result would NOT rival that of (proper) grinding.

How parts are heated tends to make a huge difference, too. Without a controlled atmosphere, you can expect decarburization as well as scaling of the parts. You can limit the amount of scaling by enclosing the work in stainless foil, or by applying rather expensive protective coatings, which isolate the part from oxygen while at high temperature. To one's advantage, something as simple as a propane torch can be the heat source, but the better the heat control, the better the results that can be expected.

Heat treatment is a complicated process, as there are several ways parts can be hardened. Some methods rely on the carbon cycle, while others rely on grain growth (precipitation hardening). Which method that is employed is reliant on the material one intends to harden.

You would be well served to procure a book on heat treatment to help you better understand the processes involved. My knowledge is limited.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
asallwey
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Re: Effects of Hardening/Tempering

Post by asallwey »

So I guess if the dimensions were to be rather precise, the wise person would make a test piece of the material and just a tad larger, then heat treat it to determine the effects. I'll keep that in mind. I haven't really needed the capability, but now that I've done one I may look for other things.

Thanks for advice,

Alex
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10KPete
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Re: Effects of Hardening/Tempering

Post by 10KPete »

Go to boobtube and look up Clickspring. He uses boric acid paste to cover small parts and there is almost no oxidation. Try it, it works very well.

Pete
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johnfreese
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Re: Effects of Hardening/Tempering

Post by johnfreese »

I wonder if adding powdered charcoal to the boric acid would prevent all de-carb.
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Harold_V
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Re: Effects of Hardening/Tempering

Post by Harold_V »

johnfreese wrote: Sat Apr 24, 2021 12:17 am I wonder if adding powdered charcoal to the boric acid would prevent all de-carb.
That's an interesting thought. It would be much like pack hardening. Sure worth a try. The boric acid, alone, should be helpful, I'd think.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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tornitore45
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Re: Effects of Hardening/Tempering

Post by tornitore45 »

So I guess if the dimensions were to be rather precise,
......
There are alternatives to the wasteful and cumbersome approach of making a test piece
In the home shop precision fitting is most likely required for piston. Anything round can be brought to size with laps, either internal or external.
Flat surfaces can be lapped against a flat surface if possible.
Mauro Gaetano
in Austin TX
asallwey
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Location: N. Virginia

Re: Effects of Hardening/Tempering

Post by asallwey »

I finally have something to add. I read about the oxidation problem that can occur, and possibly leaving scale. If you want to avoid either I have something for you to try. Head over to your local big box or auto parts store and get a 5.5 oz spray can of: Blaster Industrial Graphite Dry Lubricant.

Spray a light coat and let it dry a few minutes, then hit it again with another light coat. Do not spray any more coats on the part. This will dry within minutes, handle carefully as it can be removed. Do your heating and tempering, oven or direct flame as convenient. The part will have a dark grey color, which can be removed without major effort. I enclosed a picture of my punch where I used this. I did touch up a couple of small marks from handling with Brownells bluing, but most of what you see is the result of the spray, which I like. The finish is hard by the way.
The other item is an old lathe tool just for comparison of something that somebody blackened, and is for comparison.

Give it a try,

Alex
Punch_rs.jpg
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