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Heat Treat Oven Accuracy / Precision?

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  • #16
    Originally posted by Franz© View Post
    The mercury filled tubes you're so scared of are likely timers if they're sitting inside magnetic coils. They were equal to Hayden clocks in accuracy and superior in #cycles to replacement.
    Who said anything about being scared? I'm an old curmudgeon at heart so I know where you're coming from and won't take offense. No, so far I've only found one mercury tilt switch and an actual mercury filled thermometer, no mercury based time delay relays or anything of the sort. There is a synchronous motor and some cams that provide the timing. It's a really neat bit of mechanics but my reason for replacing the controls is that these parts are no longer readily available. I could spend several hours diagnosing exactly what's wrong and then a few more hours trying to make a new fiddly little gear or a new miniature cam-follower, and then an hour or so tinkering and calibrating - but why? It's still a bang-bang type controller and if anything else breaks on it, I'll have to spend more hours tinkering. Instead, I can make a much more reliable solid-state device that can do a better job controlling the temperature.

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    • #17
      Originally posted by Fasttrack View Post
      Dan Dubeau, keep us updated on your build and especially what you learn looking through those old metallurgy books! I'm sure that's where the answer lies; I'm just trying to mooch and get the info from someone who already knows without having to do a bunch of research myself!
      I plan on starting a build thread. My PID's and some other parts arrived today, but still waiting on a few more things. I wouldn't hold your breath for progress though, this build will probably surpass VPT's roller build in time frame from start to finish . As far as the old texts, it's interesting reading now, but I wish I remembered more specifics from the actual classes and labs.

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      • #18
        Originally posted by MattiJ View Post
        Average expectation could be somewhere between those two extremes but it could be also worse. 3 Celcius at 1200 Cel HSS hardening oven would really tough and hard to archieve.
        agreed and it is not of interest to me. Only mentioned as its the metalworking op with the greatest required precision I could think of off hand.
        .

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        • #19
          Originally posted by Fasttrack View Post


          But if you really need +/- 5 degree absolute accuracy and control, well I highly doubt you're going to get that with a $20 controller. Just reading out a thermocouple accurately and compensating it correctly is not an entirely trivial matter. You're probably looking at an uncertainty of +/- 2 degrees right there, if not worse. There are just lots of places to go wrong reading a thermocouple... you're talking about microvolts per degree signal to read, which means it's susceptible to electrical noise, then you've got issues with long-term stability and corrosion at the weld, cold junction compensation, etc. A carefully designed system can maybe get to 0.5 degree accuracy with thermocouples, but that requires great care in the design and implementation. Setting aside issues with the sensors, you then have gradients inside the oven as Old Mart mentioned. The insulation in the oven, placement of the heaters, location of the sensors, etc. all become critical design choices when you're talking about really tight control.
          Control or electronics part is the easier. 5 degree(C) accuracy with 20 usd controller is pushing your luck but totally homebrew system based on arduino or such wouldn’t be that bad. 24-bit sigma-delta ADC, pt1000 for cold junction and some code slapped together would do it. Hardest part is interfacing the ADC, actual temperature conversations are easy if you dont need blazing fast performance. Arduino was able to compute the ugly 13th degree polynomial thermocouple reference function in about 7 milliseconds by brute force without any tricks or tables.
          HARD part would be the thermocouples itself. All the base metal thermocouples are nasty buggers for any attempts at accuracy and R/S/B platinum based thermocouples get crazy expensive real fast. S type thermocouples we use in cal lab are close to 1000usd per metre.

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          • #20
            Originally posted by Dan Dubeau View Post
            I plan on starting a build thread. My PID's and some other parts arrived today, but still waiting on a few more things. I wouldn't hold your breath for progress though, this build will probably surpass VPT's roller build in time frame from start to finish . As far as the old texts, it's interesting reading now, but I wish I remembered more specifics from the actual classes and labs.
            Sorry to hear you caught the Pelvic Inflamitory Disease, I'm told it can be painful.
            I'm told by the fellow who invented Gardasil the PID thing can lead to multiple additional diseases as time goes on.
            Suggest you see a doctor ASAP

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            • #21
              I work with 17-4 PH stainless a lot and heat treat it myself; most sources I've read recommend no more than +/- 10° F variance (temps are 900-1150° F for scale), treatment times for 17-4 are typically 1-3 hours so the system needs to stabilize and hold that temp accurately for a while. The control box I use was built by a friend at work, it has a cheap PID controller with a SSR (40A I think?) and uses a K type thermocouple in a molten salt bath. I have the system set for a 3 second cycle time; recommendations were 1-3 seconds for SSR relays.

              If the thermocouple readout is accurate, once the system stabilizes (and is properly tuned) it does hold temp to within a few degrees, I'd have to watch it again to quote exact numbers but less than +/- 5° F. Some of that may be due to the salt bath though; a heat treat oven may fluctuate faster with wider extremes. I also use the same system for bullet casting, but usually don't bother tuning it as closely; it'll usually vary +/- 10° F in that application.

              FWIW, I've had heat treated samples tested for hardness in our materials lab, and the results seem to be spot on as predicted in the heat treat charts.

              Edit - as a side note, heat treating 17-4 this way in a salt bath can result in some very interesting gold/bronze colors on the part that are consistent enough to be decorative if one chooses, and are resistant to handling and moderate use. Using different salt compositions seems to affect the color, as does as the presence of iron oxide in the salt. I've been able to achieve everything from bright gold to dark brownish-bronze.
              Last edited by Yondering; 01-14-2020, 08:38 PM.

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              • #22
                Does 17-4 require a salt bath heat treat? Is that to keep oxygen out? I've no clue bout salt baths, but do use 17-4 pre hard the odd time at work, but have never had the need for further heat treatment. One of my favourite steels to work with, especially turning.

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                • #23
                  Chinese PID and a cheapie SSR and my furnace keeps +-10f, easy. Long as the heating elements are in good shape, you shouldnt have any issues getting the same. Id shoot for +-20f from setpoint as a minimum

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                  • #24
                    Quick theory question about this - does it make any difference in temperature stability if a sizeable piece of metal is kept in the oven during the heat treating process? In the refrigeration world a full refrigerator is much more stable temperature wise because the thermal mass helps reduce fluctuations. My thought is this - if there is only a small part in a heat treat oven most of the heat gain and loss will go into the air, which can quickly change temperature. Add in a few pounds of metal to make a 'thermal bank' and heat gain and loss is buffered somewhat as the metal takes more time to change temperature. Does that make sense? Does that happen in practice?

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                    • #25
                      I expect you would want to know what you are measuring.... If you had a goodly chunk sitting there, and put your part on it, then measuring the temp of the chunk might be best.

                      Thing is, that may affect your times to temp, time to cool, etc. The chunk might add a significant lag.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #26
                        I retrofitted the one we have at work a few years back. I bought a PID control and type K thermocouple from AD-

                        https://www.automationdirect.com/adc...ies)/sl4848-rr

                        I used a 50amp AC contactor to control the elements (the furnance only draws 22 [email protected] 220vac,but overkill never hurts)

                        Also added an analog pyrometer and thermocouple to the furnace for a reference, both thermocouples are in stainless steel wells.

                        https://www.axner.com/analogue-pyrometer.aspx

                        IIRC I used a hysteresis value of 5*F (many alloys have a 25-50 degree window, so keeping it as narrow as possible is best within reason) One steel I like to heat treat is 1045, the furnace takes about 1hr 45 m to reach my set point value of 1530F. Once the SV is reached the control triggers a soak timer which counts down the selected soak time at which point it rings a bell telling me the cycle is finished and it's time to quench.

                        So far it has worked well and once I learned how to set up the controller, it's become a trouble free process. Both the control readouts and analog pyrometer agree with each other within 2 degrees, so I ain't complaining.
                        I just need one more tool,just one!

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                        • #27
                          Originally posted by Dan Dubeau View Post
                          Does 17-4 require a salt bath heat treat? Is that to keep oxygen out? I've no clue bout salt baths, but do use 17-4 pre hard the odd time at work, but have never had the need for further heat treatment. One of my favourite steels to work with, especially turning.
                          It doesn't require a salt bath, but it's one of the better methods and the easiest for me in my shop since I already had the equipment for lead casting. The main criteria for 17-4, being a precipitation hardening steel, is to hold it at an elevated temp for a certain amount of time. Mostly I use the H900 condition, which requires 900° F for 1 hour, then air cool. (One reason 17-4 is so great, heat treat is really simple.)

                          The salt bath is good for maintaining an even temperature on the part, as opposed to a torch or some torch-powered ovens, but the other advantage is protecting the surface from oxygen so there is no scale to deal with (although 900 F is not all that hot anyway); when the part comes out it is finished and ready to use.

                          I use various mixtures of potassium nitrate, potassium nitrite, and sodium nitrate depending on the desired temp range and color. These "salts" look almost like hard white plastic at room temp, but turn liquid somewhere between ~250° F and ~750° F depending on the mixture. The liquid salt bath is clear or yellowish-clear. Precautions should be taken to avoid spills and any sort of water in the bath (can you say steam explosion?) but otherwise are fairly easy to work with. They do need to be removed from the pot after use though, since they are hydroscopic and will cause the pot to rust (although they do not cause severe corrosion like sea/table salt).

                          For some reason, my parts come out darker when using straight potassium nitrate, and more of a bright gold when using a sodium nitrate mixture. Here is an example of one of the gold colored parts; this muzzle brake was bead blasted before heat treating so it's a sort of satin gold color. The color has darkened with use now (picture was taken when it was new) but hasn't worn off. When I don't want the part to be colored, I just bead blast or sand/polish the part after heat treat, or do the finish machining after.


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                          • #28
                            Originally posted by Fasttrack View Post
                            I picked up an old, 5 kW heat treat oven at auction for $40. Figured I couldn't go too badly wrong at that price point.

                            Any thoughts?
                            Very nice score. I would assume with that much potential it has so nice electrical controls. Very nice. JR


                            My old yahoo group. Bridgeport Mill Group

                            https://groups.yahoo.com/neo/groups/...port_mill/info

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                            • #29
                              Thanks for the info. Sounds like an interesting process. I've heard about it before, but have never really been that curious to dig deeper into it. Friend of mine used to work at a rubber extrusion place about 20 years ago, and I can always remember his arms being all burned up all the time from the small explosions in the salt bath. He didn't work there long, sounded like a nasty process. I imagine heat treating with salts would be slightly more contained and less dangerous, but still requires care an attention.

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                              • #30
                                Originally posted by Dan Dubeau View Post
                                Thanks for the info. Sounds like an interesting process. I've heard about it before, but have never really been that curious to dig deeper into it. Friend of mine used to work at a rubber extrusion place about 20 years ago, and I can always remember his arms being all burned up all the time from the small explosions in the salt bath. He didn't work there long, sounded like a nasty process. I imagine heat treating with salts would be slightly more contained and less dangerous, but still requires care an attention.
                                Heat treating steel in a salt bath, the only small explosions you'd get would be from moisture, and that would only be if you'd made a mistake. The rule is that only dry parts go in the bath, and keep water away from the process. For the kind of guys who drip sweat everywhere when working, keep your head and other dripping parts away from the salt bath. (I have watched a couple guys like that drip sweat all over their work and bench; I always thought it was nasty and careless, but around a molten salt or metal bath it could be fatal.) For the most part though, if you avoid putting damp parts in the bath and avoid tipping the whole thing over, the process is relatively safe; no worse than running a lathe or mill IMO.

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