Using PID/thermocoupler to deduct critical temp

[Jon Von Hall]

Ive been working with 5160 as the main steel in my forge for several years, I go buy in leaf spring sizes from a large supplier nearby and cut it down to whatever size I need. I have the hardening and tempering process down so I get consistent results, not that this is surprising since this steel type is very forgiving during heat treating anyways. I want to start making puukkos and smaller fixed blades that require good edge retention so Im wanting to switch steel types, and with that will come a new set of heat treating variables, tools, and skills. To my disappointment my supplier no longer carries high carbon 10xx material, 1080 and 1084 or 1095 specifically, and I don't feel like waiting for orders from the internet, I like to actually go and view the steel and hold it in my hand to make sure im getting the right size. That being said O-1 is in plentiful supply at this warehouse and seems to be a great steel for knives but the downside is that its expensive and heat treating it is more complicated, and I wouldn't be getting the most out of the steel by eyeballing the critical temp from a color chart. Getting to the original point of my post, Im in obvious need of a Thermo coupler/PID for these new steels, that being said how do you use in to measure the temp of the steel. Do you measure the atmosphere of the inside of the forge or do you touch the sensor on the steel itself, this may seem like a no brainer but Ive always needed step by step clarification when there are large margins for error. Thank you in advance.

 

[timgunn1962]

In most cases, you measure the temperature of the forge and leave the workpiece in for long enough to equalize its temperature with the forge before timing any soak. This obviously means that your forge needs to be able to be set to the required temperature and needs to hold the set temperature within close limits for an indefinite period.

You don't actually need PID unless you are intending to automate the temperature control. PID stands for Proportional, Integral, Derivative and is the control algorithm used by the majority of process/temperature controllers. A PID controller can be used as a temperature display only and this is probably a good way to go if you are using a blown forge, since you are already tied to mains power.

If you are using a Naturally Aspirated forge, a handheld pyrometer is probably the easiest way to do things.

I use TM902C handheld pyrometers, bought on ebay for around 5 bucks, delivered (albeit slowly). They take a type K thermocouple input (miniature plug) and will read to about 1370 degC, equal to about 2500 degF (even if they are marked on the front for a lower temperature in my experience). Downside is that they do not have a degF display option. Upside is 5 bucks. They come with a glass-fiber insulated bead-type probe that is good to about 400 degC (about 750 degF) and is flexible enough to use in most ovens, etc: excellent for checking tempering setups.

I use Mineral Insulated thermocouples for HT. I usually source them locally and specify a type 310 stainless steel sheath of 6mm or 1/4" diameter. The diameter gives them enough rigidity that I can measure where I want. The 310 SS sheath is good to about 1100 degC (about 2000 degF) with a decent working life. If necessary, it can go to 1370 degC (2500 degF), but this will dramatically shorten its life.

Omega have a proprietary sheath material, Omegaclad XL, that is claimed to be good to 1335 degC (2435 degF) and this would be my choice if I did not have the local thermocouple source. A good welding temperature seems to be around 1300 degC and I'm not good enough to judge it by eye yet.

http://www.omega.com/pptst/KHXL_NHXL.html

I use handheld probes that are 24" long below the handle, so the part number from Omega would be KHXL-14U-RSC-24. It lets me check the temperature distribution in the forge without burning my hand.

Once you have established how much of the forge is at an even temperature and how long it holds it, you can either just set the forge and pull out the thermocouple, or you can leave it in with the tip in the even temperature zone. If the thermocouple stays in, you can eyeball the workpiece temperature behind the thermocouple: when the colors match, so does the temperature.

Once you have found the size of the even temperature zone, you can fit a cheaper themocouple through the forge wall as a permanent installation.

 

[Jon Von Hall]

Thank you Sir