Did It Get Too Hot?

DonL

Well-Known Member
I tempered two knives recently in my little toaster oven. Set the temp to 400, verified by two different oven thermos. When I went to take them out after two 1 hour cycles, I had some purple coloring mixed with the straw coloring covering the blades.

Let me say that I try and temper just as quickly after HT as I can. I normally carry the still hot blade into the Garage, grab some old sandpaper and clean down to clean metal and pop it in the oven. I don't worry about getting all the scale off at this time. I just want some clean metal so I can see the temper color.

These knives both had the purple coloring on the "dirtier" sections of the knife. Where I'd sanded down to clean metal, I had the pretty straw coloring.

I'm wondering if it was the scale that turned purple or did I get the knife too hot? The knives in question are 1/8 1084 and neither had been ground as I prefer to grind the bevels after HT.
 
Very probable that it's the scale. Tempering oxide colors are "cumulative" meaning that the pile up with each successive tempering cycle.....the color will often appear different on a "clean" area, versus and area with scale. Very often my 1084 blades will come out of the first tempering cycle with the "straw" color, the second with "vermilion", and the third will have vermilion/purple mix. After many times of verifying the hardness between the colors, I discovered the "cumulative" effect of the oxide colors.

Long story short....as long as your toaster oven doesn't "spike" up and down when it cycles (which most do), your likely OK.
 
What he said. Even more so if you had some oil left on the blade. I get purple all the time when I don't get all of the quench oil off, even with a kiln.
kc
 
Thanks for the response guys, I appreciate it. I figured that it was okay because the clean areas were straw colored and both thermometers read right at 400, but I wanted to be sure before I started grinding.
 
Somebody else mentioned using a pan filled with dry sand to put your blade(s) into if you think your oven spikes the temp. From Ed Caffrey's comments it sounds like "not-to-worry" - but the idea of using sand as a heat buffer seems like a good one to me. You'd want to pre-heat the sand before putting the blade(s) in. I'm interested if any of the more experienced makers have feedback on this idea.
 
Thermal mass can have a great impact on how an oven performs (how steady it holds temp). Personally I prefer a high quality kitchen type oven versus any other for tempering. Why? I've found that "heat treat" ovens will very often over shoot the set point at tempering temps, as do most toaster ovens. It's difficult to explain without common references, but most folks tend to set the ramp control on their heat treat ovens to the max, and what I've observed is that even though the oven will shut off when the readout reaches the set point, blades will often come out 1-2 "colors" more that what was intended.....telling me that although I'm not able to put my finger on it yet, something isn't working as it should.

Toaster oven will very often "spike" much higher and lower, due to the poorer insulation used in the them, and the cheaper sensors/parts used to produce them.

I purchased a brand new higher end "in wall" oven at a garage sale several years ago......after installing it in the shop and testing it, found it to be very accurate from set point to actual temp. As an experiment, I laid four fire bricks across the bottom rack.....then started playing with time to figure out how long I would need to let the oven cycle, in order for it heat all that thermal mass (the bricks), hoping that I could hold the oven closer to it's set point. Eventually I found that after allowing the oven to pre-heat (it has an alarm on it when it decides it's pre-heated), and then letting it sit and "idle" for about 40 mins, it will hold within 10 degrees of it's set point.......I believe the addition of those fire bricks thermal mass, had much to do with it. I really don't know how to explain why I like the kitchen type oven better than a heat treat oven or toaster oven for tempering, beyond the fact that I feel it's a more "even" and "gentle" heat.

I think the idea of the sand could work, but it would require some experimenting to figure out how long the oven would need to idle with the sand inside to ensure the thermal mass of the sand was at the temp you desired. I'm also a firm believer that 1 hour simple isn't sufficient for tempering......more often than not with a single hour, you'll get only a "tempered skin" on a blade, with the core remaining at, or close to as quenched. My rule is 2 hours minimum, for 3 cycles.
 
Hey - I again want to concur with Ed.

I have a stacked kiln for heat treating from Evenheat. However, I put a 70lb cast iron pipe that is half of the high-pressure end assembly from a fire hydrant into my kiln. It is 3/8" thick walls with 6" diameter at the top and 7.5" at the bottom and 36" long. This thing has made my kiln a MUCH better tool. Even on full power, this mass drags the increase speed back. It takes a long time to warm up, but it is STABLE once it does.

kc
 
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