Basic how to use an oven questions

theWeatherman

Well-Known Member
So I have to ask because I have been looking for a while and it doesn't seem to come up a lot. I have been doing as much reading as I can about metallurgy and heat treating. I specifically look at O-1, 1095, and W-2 because that is what I want to use.

Now.... I snagged a copy of the ASM Heat Treater's Guide book and looked over what other people do and I need some guidance and some people to lend me some understanding. You look at how to harden the steels, thermal cycles, temps, soak times, quench speeds, but!!!! you don't hear about some really small specifics. Like what does preheat entail, or what does preheat slowly to a specific temp mean? Like for D2 it says preheat slowly, or for O-1 it says preheat slowly, and preheat to 1200F. Does that mean you put the knife in the oven, turn the oven on, heat to 1200F, sit for a minute or two and then hit the HT temp and soak?

Also, you don't hear about how to use the ovens. Do you place the knives in while at room temp and then turn the oven on? Or do you hit the HT temp and then place the knives in?

I know these are weird questions but when you search for it I don't find much. Depending on the metal one way or the other could change your outcomes.

What you guys think?
 
some oven basics...

the vast majority of heat treat recipes are for 1" or thicker blocks of steel. Some soak times may be adjusted accordingly. Also, some soak times are necessary for the carbon in steel to go into solution. I generally soak at full temp any blades I am heat treating for 20 to 25 minutes. My hardness testing afterwards has always given me the RC numbers I am supposed to expect so to me, that seems long enough. I know a lot of guys soak only for 15 minutes but I don't hear anyone soaking much less than that.

Never, put anything or take anything out with the coils charged (electrified). One quick touch to the coils with your metal blade or foil pouch and you will have a very serious chance of killing yourself. This is really easy to overlook when you are in the middle of something.

Most will put a stainless air cooling blade in the oven while it is cold and ramp up to temperature, pause around 1450F for 5 to 10 minutes and then continue ramping up to full temperature, soak for 20 minutes or so and quench. Heating carbon steel is a bit different and varies to the type of steel. Look up the recipe as for each type. You will probably have to interpret more than one recipe.

Get and use a hardness tester. I understand that is simply beyond the reach for many people but using a hardness tester takes away a lot of the voodoo. There are lot of guys that say they don't need a tester to know when their steel is "good n hard". These are experienced guys and if that's how they roll, I am all for them. They learned through trial and error. Using a hardness tester takes years off a learning curve and will help you dial into your steel quicker and easier.

Place any blade in the center of the chamber. Keep as far away from the coils as you can. An oven heats full blast for a few seconds and shuts off, then repeats that until it gets to the programmed temperature. Those coils are nearly 2400F or more. The cycling on and off can be heard as the relays in the oven click on and off. This is assuming a digital control. An analog control just goes full blast until it hits the programmed temperature until the sensor says stop.

Allow the oven to cool slow as you can by keeping the door closed. If you leave an oven door open, or worse, blow a fan inside to cool it down quicker, you are going to stress the insulating bricks loosen everything up or even possibly (unlikely but possibly) crack a brick. To test this, leave the door open and listen to it creak as it cools down unevenly.

If you experiment with inert gas atmosphere such as Aargon and then go back to standard atmosphere, expect to significantly shorten the life cycle of your coils. The owner of one of the major oven company's explained this to me: The coils are basically molten metal when hot held in place by surface oxidation. If you use an inert atmosphere it thins or removes that oxidation coating and that's fine if you continue with that environment but if you switch back you will have an issue with premature failure of the coils.

Heat Treating Law:Elements fail at the worst possible time. Have a spare on hand. Some ovens take two elements.

Kiln shelf commonly found in pottery supply places or even bricks can be used to stand up a blade in the center of the oven. I used to lay mine down on the floor of the oven until I started using a blade rack.

Some guys that process a lot of knives, can load an oven up with a dozen or more at a time. Most new guys might be better off keeping it down to 1 or 2 at first. It's terrifying to drop a 1950F blade on the floor for the first time, especially if it touches something it shouldn't, like your foot, or the cat or something flammable.
 
Tracy, wouldn't having the blade inside the oven before you turn it on overheat the blade because the oven is overheating to reach the temp? So if you have a pre heat temp, set the temp let it hit that pre heat temp and then place the blade in the oven, then continue to the soak temp?

Same would go for Carbon steel. Set the temp to the HT temp, once it has reached the HT temp, place the blade in and let the blade hit the HT temp, soak and then quench?

The only thing that kind of worries me is you hear about the control of the temps and times, when you place the blade into the oven to reach temp how do you determine it has reached the correct HT temp, if you are placing the blade into the oven after the oven has reached the temp?
 
If you haven't done so, go to Tracy's site and watch the videos on using the two different types of computer controllers on the kilns he sells. They will answer many of your questions. As far as ramping at a certain rate, most H/T protocols listed by steel companies suggest a starting temp. then a ramp to temp. x at a rate no greater than y degrees per hour. X & Y vary depending on the steel and its requirements. The programmers on the kilns allow this to be done properly. I hope that all made sense and helped some.
 
Just like heat treating, you'll get varied advice on how use a kiln. Tracy is bang on about manufacturer data sheets usually relating to thicker work pieces. The idea of ramping slowly and sometimes presoak is to try to keep the inside of the steel at more or less the same temperature as the outside, so the whole piece goes through phase chages more or less together - thereby avoiding some stress. With something as thin as a knife blade - in something as slow as an electric kiln, it's kind of a moot point. It took your oven about 1/2 hour to get to 1400 - and at that slow rate, the outside and inside are already at about the same temperature - and will continue to be all the way up to your austenizing temperature. For the most part, slow ramping and presoak meet the test of "do no harm", but with knives, I'd suggest they have little real value.

As for overheating to get to temperature, well the coils are hotter, but the area where your knife blades are is near the thermocouple, so that area doesn't get overheat. If you were to put a well sealed foil envelope in a 1950F kiln, the air inside would expand so quickly, it would blow up the envelope like a baloon popping and let the atmospere in. That's why we start from cold with envelopes (or at least below 500F)

With carbon blades there is no foil envelope to worry about, so most makers do put the blade in a hot kiln - in large part because the blade just gets uglier the longer it is exposed. Some HTers use a decarb protection paint like ATP641 or Turco to keep oxygen from their carbon blades. Your post makes an excellent suggestion for carbon steel in that - if protected from decarb - it might make sense to do a presoak at 1400, so the whole thing goes through phase change together when you ramp it up to austenizing temperature.

I believe I just might grab a glass of wine and head for the hot tub to think on that a bit. :)

Oh yes - Tracy makes an excellent point about live coils. 220V at 16 amps will really blow the wax outa' your ears. Evenheat kilns are available with a door switch that cuts power when you open the door and restores it when you close. It is an oustanding option for something like $50 extra, that may save your life. We won't sell a kiln without it.

Rob!
 
I cant afford a hardness tester just yet but I did purchase RC files and find them very helpful as well as the brass rod.

I use PBC powder to coat the carbon steel and love it. It cleans up easy under hot water but most of it pops off in the quench. Tracy has it and one pound will do many blades.

I purchased one of the last sugar creek 24" ovens from Tracy and added a PID controller to it myself for a couple hundred less than if the unit had come with the PID already on it. I highly recommend either purchasing the oven with a PID or building your own controller box that can run your tempering oven as well for around 100 bucks.

Rick Marchland mentioned adding 1/4 metal plates to the bottom of his oven, same model as mine, to add thermal mass. Since doing so I now turn the power to the oven off when opening it via the analog switch, do what I got to do placing or removing a blade then when I close the door back I watch the temp as it will rise back up to the intended temp that I am austenizing at due to the added thermal mass. Once its back at temp I turn the analog switch back on and never go over the austenizing temp. Again Rick told me this and it has worked very well for me since and is much safer as the sugar creek unit did not have a kill switch for the door.
 
Hi,
i've been using a saw cut soft brick to keep the blades upright. I've used a fairly wide cut, just to hold the blades with little else touching. I start with the blades at room temp and bring the heat up with blades in place. When etching, the blades etch differently at and near where they are held. Any ideas, suggestions?
Thanks,
John
 
Hi,
i've been using a saw cut soft brick to keep the blades upright. I've used a fairly wide cut, just to hold the blades with little else touching. I start with the blades at room temp and bring the heat up with blades in place. When etching, the blades etch differently at and near where they are held. Any ideas, suggestions?
Thanks,
John

Hey John,

I would guess that where they are being held isn't getting to the same temp as the rest of the steel. Look at this:

http://usaknifemaker.com/heat-treat...ade-fixture-mk1-designed-for-knife-ovens.html

this holder should take care of that problem because they were be no recess to "insulate" the holding area.
 
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