I bought a VERY old heat treat oven quite a long while back...
I'd like to check that it is reasonably calibrated before heat treating this part. Is there some sort of thermocouple and high temp wire I can buy and take a voltage reading? I only know of this concept, not any specifics.
I made a custom bolt out of 4340 for my 30 ton shop press. Looks like I should heat to 1525 and quench in oil. Then temper to 450. correct? I'll use SHMBO oven for tempering (when she goes shopping)
I expect he would be far better off getting a metal sheathed thermocouple with the wire connections well away from the hot end. They can be had in various alloys and good to 1100C or more depending on alloy and the hot end can be poked in while the wire connections can be at ambient so don't need to be anything special. They are available in many lengths and diameter in that configuration. I had a quick look at Omega but they had so many options I though it was pointless to point out one when the OP can chose one to suit his needs. They are not expensive although I thought a few I looked at on the Omega site were and they were in the US, I'm in the UK and thought the prices expensive.
I have various CAL temperature controllers and use them with K and N type thermocouples, I also have a DVM with K type input and they all tally very well with one another.
Well ... Omega makes a series of thermocouples designed to handle a range of temperatures, and they make both readout boxes for them or very nice controllers which will work with the thermocouple to produce the desired temperature.
I picked up a heat treat furnace with no controller at a tool flea market some years ago. I already had a nice Omega controller from an overstock for a project that another poster on this newsgroup had left over. He was asking $100.00 for unused ones which sold new for $200.00, so I got one. I already had some Type-K thermocouples in stainless steel housings which I could use there.
The controller can be told to work in either degrees C or Degrees F -- whichever works for your project.
It will pause at about the 2/3 or 3/4 point on the way up to measure how much overshoot there is and compensate for it. I set 1800F as my target, and I got a total overshoot of 1 degree F when it reached the set point.
Note that the output voltage of a thermocouple is non-linear, and a *very* low voltage compared to most multimeters. I think that somewhere in the range of 35 mV is a reasonable value. And to get absolute readings you need to compare the output of the thermocouple with a standard (in the old days, a second thermocouple of the same type dipped in a container of water and ice for a 0 C (32 F) reference. There are now electronic reference junctions built into such controllers.
The controller I got had the ability to directly control fairly low current, or to put out a 5VDC signal. I used that 5V signal to control a solid state relay (Crydom 24025 IIRC, capable of switching 240 VAC at up to 25 Amps), and let *that* control the heating element in the oven.
The controller is a proportional controller -- it sits with the heating element on until it gets within a certain range (determined by the test on the way up), and then starts turning the element on for perhaps 90% of a 10 second cycle, then 80, then 70, on down until it stabilizes at some percentage on time which maintains the target temperature.
The same oven and controller can be used for the lower temperature tempering if you first let it cool off enough. I've done it that way with some D2 air hardening steel.
I w ====================================================================== Heat treatment for strengthening is done at 1525 F followed by an oil quench. For high strength (over 200 ksi) the alloy should first be normalized at 1650 F prior to heat treatment. See "Tempering" for strength levels. ======================================================================
So the first question is whether you need over 200 ksi strength? I don't know what a 30 ton shop press needs, and I suspect that it varies a lot with design.
The same site says (in the "Tempering" section):
====================================================================== The temperature for tempering depends upon the strength level desired. Before tempering the alloy should be in the heat treated or normalized & heat treated condition - see "Heat Treatment". For strength levels in the 260 - 280 ksi range temper at 450 F. For strength in the 125 - 200 ksi range temper at 950 F. Do NOT temper the alloy if it is in the 220 - 260 ksi strength range as tempering can result in degradation of impact resistance for this level of strength. ======================================================================
So your 450 F tempering suggests that you want to be in the 260-280 ksi range, which also suggests that you need to normalize it first.
And how long you hold it at temperature is a function of the thickest part -- with (IIRC) about 60 minutes per inch of thickness.
The site which I've quoted above is:
Why? Turn off the oven while you quench, and wait until it cools off enough to set to the lower temperature. Bound to be more precise than the typical home oven.
FWIW -- when I built the controller housing, I included a small switch to open the wire from the controller to the SSR (Solid State Relay), so I could be sure that it was not heating, but could use the thermocouple to monitor the temperature in the oven. Note that if you leave the door open while quite hot, the corrosion of the heating elements will increase, and when you think that it is cool enough and close the door, the temperature will shoot back up quite a bit. Better to just be patient. It takes my oven (quite small, and 120 VAC operation) about an hour to make it up to 1800 F, and longer to make it back down to reasonable temperatures.
Also -- note that the dimensions of hardened steel are larger than those of the annealed steel, so make sure that your thread and shank diameters are made with this under consideration. You certainly won't be able to use a die to fix the size after hardening. :-)
If this is an ordinary bolt in a reasonable size, what is wrong with buying one of the proper size and strength off the shelf, instead of going to all this work? Or do you want to get some experience at the hardening process?
I thought about suggesting a probe too. The wire could be snaked in around the insulation while a probe would need a hole or thermowell, drilled from outside and avoiding any heating elements. The thermocouples in my woodstove are wires and sometimes I move them around to see if another placement gives a better reading. If the tool being heat-treated is in a closed box with charcoal only flexible t/c wires work well.
These panel connectors fit into modular audio/video wall plates with only a little whittling:
have wood stove and cooking pot thermocouples, phone, TV antenna, and two speaker (solar panel) banana jacks in the one behind the computer.
Not sure it makes any difference as I have used probes that were 1mm diameter and could be bent to fit but the same applies to bare thermo couple wire and junction . I think the off the shelf probes are more suited to this application of poking in through insulation. I assume we are talking about the same sort of beast , a metal sheathed probe covering a thermocouple junction.
Shoot, I didn't allow for it growing. The only way I had to test the
1.5 by 16TPI threads was to put the whole 30 ton cylinder in the lathe and try to thread it on. I ended up a bit tight. Took fiddling with the old triangle file but finnally got it to spin all the way in real nice.
This bolt holds the block on the end of the cylinder to bolt press beak and other dies to. I just wanted it as good as i could get it. I guess "as is" is good enough. I've never trusted my old heat treat oven, guess it won't get used again this time.