I posted this early this morning, but it got caught in the site maintenance. I just showed up.
If you are set on doing stainless steel in a forge (not the correct choice), the best results will come from trying to imitate the conditions in a HT oven.
1) Preparation - The blade should be ground clean at 120 grit and to about 70% finished dimensions.
The edge should be around .050".
2) Oxygen exclusion - The blade will need to be wrapped in a stainless foil packet and the edges double crimped tight with a mallet. You want the packet as flat and air-free as possible. No need for a piece of paper as many suggest. Dusting the blade with talcum powder may help if your foils tends to stick to the blade. Using the higher temperature
321 foil helps eliminate sticking. If doing the HT in a forge, a wash coat of satanite on the blade may be wise. Let it dry overnight before sealing in the foil packet. I have heard that thisprevents sticking and somewhat evens out heating.
3) Temperature reading and control - A forge often runs far hotter than a HT oven. Many can't be regulated down to carbon steel HT temps. Stainless steel is HTed in the 1000°C range, so the temperature should be controllable with careful adjustment of the air/gas flow. HT is best done with a blown burner, as venturi burners don't regulate as well.
You need a
pyrometer to read the temps of the blade. You can make your own high-quality pyrometer with a
type-K thermocouple, type K wires, and a readout. For a forge, a simple PID that reads to around 1250°C will work fine as the readout. The TC needs to be type K, 8-gauge bare leads, with ceramic insulator beads. You will also need about three feet of type K polarized TC wire and a TC mounting block (Make sure the TC, wires, and block are all correctly connected + to + and - to -. They all are polarized due to the different metals in the TC.). Amazon and eBay sell these parts as a "kiln" kit or separate for very little money. For the additional price of a propane solenoid and some valves, you can convert the forge to auto-temperature controlled - ***See note below.
4) Obtaining even heating - The foil packet will need to be isolated from the flames. Get a heavy walled pipe (schedule 80) or 1/4" wall square tube. Close one end off with a wad of Kao-wool, piece of soft firebrick, or weld a steel plate on it. You place the blade packet in this tube after the forge has been regulated for a good 15 minutes and holding as close as possible to the desired heat. **** See note below.
Place the TC on the inside bottom of tube, with the tip in the middle, and plug the open end with a wad of Kao-wool. Regulate the forge to attain an even reading in the HT range desired. The forge needs to be fully pre-heated to soak the refractory, so 15 minutes is a minimum soak time.
When the forge has soaked for this time, place the blade packet in the muffle tube and put the Kao-wool plug back in. When the readout returns to stable again in a minute or two, start the timer. When the time is up and you are happy with the temperatures, remove the packet for quenching.
5) Quenching - Oil quench and/or quench plates is best. Quench plates are thick aluminum plates (usually 2" thick and a couple inches longer and wider than the foil packet) that the foil packet is placed between and weighted or clamped. The plates both cool the blade plus prevent warping. If oil quenching, cut the end off the packet with heavy scissors and remove the blade with tongs or needle nose pliers. Quench in medium speed oil for about 20 seconds and then place in the quench plates. If not using quench plates, just keep the blade in the oil and hope for the least warping.
6) Post Quench cooling - Stainless steel needs a lower cooling temperature than carbon steel. Room temperature works for carbon steel, but stainless needs to drop to around -100°F/-75°C. Dry ice and alcohol bath will do that. If you can't do a DI bath, at least place the blades in the freezer overnight.
7) Tempering - Temper as soon as the blade has finished its cooling. If it was a dry ice bath, you can place the blade in the oven after a few minutes in the bath. If you used the freezer, it is good to let it sit for at least 4 to 6 hours. Temper in the kitchen oven at 170 0176C/350°F for two hours, cool in running water, temper again for an additional two hours.
NOTE:
In the stickys there is a thread about my two-stage PID forge control. It allows running a forge with a fairly constant temperature. This is used for carbon steel HT by many folks. It would work for stainless steel if you can take care of the other parameters. The whole conversion costs around $100 and makes a forge much better for many tasks. You will already need the TC, wires, and readout. The solenoid and a few valves is all the extra needed to make itself regulation.
Here are some threads on the controller build: (some photos and videos links may be dead)
So after going through the stickys a couple weeks ago and finding all of the amazing plans for different PID controlled forges, tempering ovens, salt pot etc, I've switched gears fully to creating a really high-functioning PID controlled forge & heat treating oven. First off, thanks to other...
www.bladeforums.com
Here is a sample of what parts are needed. The TC is 11 gauge, which is the absolute minimum. 8-gauge is better, and why many get all the parts separately.
Search "
PID Temperature Controller Kiln Probe SSR Relay 40A HS ABS Box Pottery Glass F C"
Sometime next year I will start to offer plug-and-play units of my two-stage controller. I have a lot to do in remodeling the house and finishing the shop mods before then, but I'll let everyone know when I get ready to start selling them and other shop equipment and plans. Probably summer of 2025.
