Tempering Question

[Hurley]

I lucked out and found a large toaster oven I bought my gf about 4-5 years ago and she put it at the bottom of the walk-in pantry and forgot about it because it took up too much counter space. That being said, I grabbed it and took it out to my shop for tempering. So as I'm new to this tempering stuff, how long and at what set heat do I put the oven on. I think it goes to 450 degrees and has an hour timer. I know I need to get a thermometer because I'm pretty sure the oven won't be correct. Any help would be appreciated!

[jmccustomknives]

Polish a large test piece, something that will go all the way across. Preheat your oven to 375 put it in for an hour and check it for the tempering colors. It should be light yellow all the way across. turn it up 25 deg and do it again through the tempering color range (light yellow, straw, brown and blue). I've had ovens that had hot spots, these can ruin a blade. Get a thermometer after it passes the tests.
My oven was bought from a yard sale for $3.

[Ray Rogers]

How long and at what temp is pretty much tied to which steel you're talking about. In the case of 1084 about 400 or 425 F for one hour would be a good start. You could let it cool in still air after that until it just warm to the touch and then put it back in the oven for another hour if you wish (some guys do, some don't). Testing the oven the way IM suggested is a good idea and definitely get a thermometer involved. Beyond that, test the blade very carefully until you decide what tempering temperature works best for you ...

[Doug Lester]

You could also put a pan of fine sand in the toaster oven to serve as a heat sink to buffer the effect of the heating coils cycling.

As far as how many cycles and for how long goes, that can cause a real argument. Some do three cycles of two hours each primarily because that's what they read somewhere and even if it doesn't do any good, it won't do any harm either. Honestly, there are steel that we use that have a low enough carbon content, hypoeuticoid or euticoid steels, that aren't going to form retained austinite and could probably get by with one two hour cycle.

If you haven't gotten the idea already, heat treating, which includes tempering, is regulated by the exact alloy involved. That's why using salvaged steel, aka mystery metal, is so problematic. It can even cause some problems with tool steels that have rather wide range of elements in their make-up.

Doug

[Hurley]

Ok.....one more thing, I know this sounds weird, but what setting do you recommend broil or bake? I would think bake, but I've been wrong lots before!

[Eli Jensen]

I put one thermometer in the front and one in back and split the difference and wrap the blades in foil, sometimes with some sand sprinkled in to even out the temp.

[jmccustomknives]

Never use the broil setting. You'll get hot spots for sure. I like the idea of using sand or something as a heat sink. A cookie sheet also works well. These are the ones that are made from two pieces of sheet metal with an air space in between to keep from burning the bottoms of the cookies.

[claymoore]

Originally Posted by Eli Jensen

I put one thermometer in the front and one in back and split the difference and wrap the blades in foil, sometimes with some sand sprinkled in to even out the temp.

Hey Eli,
Can you explain this a little more? Regular tin foil? Any special sand? And what steel are we talking about. This caught my eye because i never heard this before and love to try new things. When you get time.
thanks,
clay
.