View Full Version : hot plates and annealing

03-14-2011, 08:25 AM
I have this drill rod, oil hardened with a horizontal "I" cut into it. Length of I is 0.500 and distal end has been ground to a cutting edge. I'm trying to save some money making my own internal retaining ring groove into 303 stainless rod. Length of tool is 2.5 inches.
If after I heat it up to bright orange and then dunk into some lathe oil I really don't want to put it into my convection oven for 45 minutes at 550 degrees.

Could I use a small hot plate, 6 inch diameter with settings of low medium and high to heat up tool for some amount of time? Would I have to wrap part in Stainless steel or perhaps just let it sit on top plate and turn it every so often.

Would a hot plate work for this?


03-14-2011, 09:27 AM
Do not use lathe oil, mineral oil is what you need for the quench. As for the hot plate it will not work because you will have a gradient profile traversing the cross section. In other words, varying states of transformation in addition to a warped piece. As far as the alloy your using I would do some homework on the heat treat temper process before heating it up, it might be an alloy that is hardened with cold work reduction.

03-14-2011, 09:59 AM
I should think a hot plate will work. Keep turning the part so that it heats evenly or better yet, make a little oven by putting a steel cover of some sort over it while on the hot plate and support the part on a little rack made of a wire coat hanger. I don't think the oil used is extremely important. Last time I made a reamer I dunked in used full synthetic 10W-60 from my BMW M5 with fine results. At $12 a quart, I need to get the most out of that oil that I can.:eek:

03-14-2011, 10:17 AM
As far as the alloy your using I would do some homework on the heat treat temper process before heating it up, it might be an alloy that is hardened with cold work reduction.

The OP has already said he has oil hardening steel. If he really didn't know what it was it would take labratory analysis to to define it which I suspect is completely out of his realm of possibility.

You can actually use a hot plate or a torch for the tempering by using color change to indicate temperature. It requires the steel to be cleaned after quenching to a bright surface. Then as it's heated carefully it will go through a series of color changes indicating its temperature. From one text, this is the sequence:

When a piece of steel is heated from room temperature to a red heat, it passes through several colour changes caused by the oxidation of the metal. These colour changes indicate the approximate temperature of the metal and are often used as a guide when tempering.
Colour Fahrenheit temperature Use
Pale yellow 430 Lathe tools, shaper tools.
Light straw 450 Milling cutters, drills, reamers.
Dark straw 470 Taps and dies.
Brown 490 Scissors, shear blades.
Brownish purple 510 Axes and wood chisels.
Purple 530 Cold chisels, centre punches.
Bright blue 560 Screw drivers, wrenches.
Dark blue 600 Wood saws.

You'll need to be alert and watch carefully because it can run up the temperature fairly quickly on a small section so you should be prepared with a damp rag or something to stop temperature rise quickly if necessary.

03-14-2011, 10:27 AM
How many of these grooves do you have to make? If it's only a few and this is a one-off tool, forget the temper and use it as is. I recently made a boring tool from O-1 quenched in ATF and used it without tempering. May have been just lazy, dumb luck but it worked great.

If it was a long run of parts or an interrupted cut the results might not have been as good. I think I read on Frank Ford's site that he had done the same without any terrible consequences.

03-14-2011, 10:33 AM
How about this. Chuck end of this drill rod in my lathe drill a 3/16" hole about .750 into rod.
Then I make a barbecue spit like holder where I weld or bend an "L" using .750 wide with long of "L" resting on top of hot plate and short vertical leg up I weld or drill a hole for a .250 rod going horizontal in direction of "L" leg resting on hot plate. I'll make this vertical short .250 leg to fit into drill rod about an .750 inch and 1 inch above horizontal "L" leg resting on hot plate.

Now I need to turn something upside down to cover all this.

Like maybe a small steel or cast iron pot or I could get a 4" or 5" diameter steel pipe, How tall? and have it sit over "L" and then a steel plate to cover it and leave hot plate on medium or high for how long?

03-14-2011, 10:54 AM
Don't overthink this. DBird is right, if only a one time deal, use it as is, just don't force it too much. It will cut just fine.

Watching the color change has worked forever, use in a well lit shop, daylight if possible. For a cutter such as this one, it is the method I use. Polish the cutter and apply heat on the shank and watch as it travels to the cutting edge, quench when proper color is reached. There is no great risk here, test with a file and if sufficient hardness is not achieved, simply do the process over.

I have heard of using a bed of brass filings or turnings for a bed to use on the hotplate or stove to provide a source of uniform heat when drawing back, I intend to do this with a flat spring I have in the works.

03-14-2011, 10:54 AM
You don't need to be that fancy about it. You can simply treat it like toasting a marshmallow, turning it and moving as needed. If you cover it on a hotplate then you need some feedback telling you when to quit. If it's covered and you have no visual feedback you'll need a thermocouple and readout which is way more trouble than I think you want to go to. There's no way to simply go by timing it. Commercial heat treaters control temperature in the oven and use timing just to be sure the entire part has had sufficient time to come up to temp.

If you're apprehensive about reading color temperatures, get one of the temperature indicating crayons shown at the bottom of this page on McMaster Carr's catalog http://www.mcmaster.com/#temperature-crayons/=bfi7b1

Color your piece in several places then just roast it on your spit over the hotplate until the crayon melts and move on to a different section.

03-14-2011, 11:33 AM
...or fire up your little propane torch and heat it 'til the color changes as described above.:)

03-14-2011, 11:39 AM
550 seems like high temperature for tempering...I'd use more like 375 to 400.

But I agree with the others. As long as it's not subject to shock (e.g. interrupted cuts) you can probably use it as-is.

03-14-2011, 12:42 PM
They way I read the original post the material to be used is 303 stainless, it is not heat treatable......

03-14-2011, 01:13 PM
I think he said he is making the tool from "drill rod" and will be using said tool to cut a groove in some 303.

03-14-2011, 01:38 PM
To make the pins on the spinning dolly I recently posted I made a cutter with a hole in the middle, bit like a grown up hole saw.
I left a little on the teeth, and when hardening only soaked at temp for about 3 mins.
This means the majority of the tool didnt get enough soak time to transform to the state it needs to be in to harden (1 hour soak / inch is a good enough rule of thumb).
The teeth are of course quite thin at the pointy bit, and so would harden right up, as would the outer areas of the thicker bit (bit like case hardening).

Anyhow after quenching it right out I used the final grind to put enough heat into the teeth to temper them back a little. For a low use tool thats a good enough plan IMHO.

So, can you arrange something similar with your tool?



03-14-2011, 02:11 PM
Toaster ovens work great for this. Might not get to 550 but will get pretty close. You can help it along the last little bit with a propane torch.

J Tiers
03-14-2011, 09:43 PM
That's a bit large for my suggestion, but it's worth mentioning.....

For parts that I am afraid may overheat, I just go on in to the kitchen stove..... with the part and a reasonably thickish piece of steel etc, around maybe 0.090 for small parts, and up to 3/16 for larger.....

Part, hardened and polished up goes on plate, plate goes on/in flame.

The plate evens out the heat, so that the part gets a nice een temp source if you let a round part roll back and forth, it will heat evenly. Smaller or non-round, just heat the plate slower.

Watch the colors, the plate will show them first, giving warning, then the part will start to change. When part is OK, dump it off to cool.

You can do anything from small hair-like springs up to fairly large parts this way.

Big parts it is probably better to do by temperature.

03-15-2011, 07:59 AM
Why not use whatever you used to heat it to bright orange to also heat it for tempering. Apply the heat away from the business end and watch the colours creep up toward the business end. when the correct colour reaches the end dunk it. Isn't this kinda the standard procedure for off-hand hardening and tempering.


03-15-2011, 09:30 AM
Why not use whatever you used to heat it to bright orange to also heat it for tempering. Apply the heat away from the business end and watch the colours creep up toward the business end. when the correct colour reaches the end dunk it. Isn't this kinda the standard procedure for off-hand hardening and tempering.


i agree, just go slowly so the part is more less uniformly warming up, torch on, torch off, torch on etc...someone new to it can be surprised by how fast the colours flow. I've also used the toaster oven which has advantages in some cases such as multiple parts at once, doesn't overshoot the mark etc.

an interesting tidbit, mostly academic to what we do in the home shop, but did you know the tempering level achieved is a function of temperature and time? For example if you leave something at the temp producing the yellow oxide for so many minutes/hours you can get the exact same rockwell hardness as if you temperated the way we do ( for seconds) at say a dark blue?