Thread: Need to temper again, Safest bet?

Originally Posted by Magpie

Charlie, how long would you recommend? Will the fact that its now ground to finished size affect the temper at the bevel? Should I assume that the super thin cutting edge will need to be honed out quite a bit to get back to hard steel after the reheat? I really REALLy don't want to mess this up!

It should be ok tempering while ground thin. I would temper for an hour each time. It is not getting hot enough to burn carbon out, honing should take off where you left it, no need to remove extra metal.

For me, getting the initial bevel is always a bear, it is like working on a bread-knifed razor.

Charlie

Here is a picture after another 45 minutes, this time at 425F Is the color about right? Can you even use color as an indicator (other than "if it turns purple you got too hot")?

Charlie's correct about time. At least one hour if you are doing split cycles at tempering each time. Me, I'd leave it for two hours and call it good. he's also right about the finished quality not affecting the tempering to a great degree, but there is no absolute here either. If enough crystalline relaxation occurs, there might be a little movement in the material, but you won't know until you try. Unless you were grinding this O-1 until it was red (really too hot to hold) I don't expect a lot of internal stressors. If you forged it and didn't thermal cycle it afterward, you might see some change.

Systematic increases in tempering temperature of 25 degrees until the desired end performance is reached are the rule. 25 degrees F usually will bump the temperature enough and will usually give a little increase in temperature that is outside the range of error for the set point of the oven you are using. However, some ovens have a notorious lack of precision at that set point range so care should be exercised as it's very easy to swing too far and you wind up with Rc 58 instead of Rc 59. Or worse.

Which makes me want to ask what kind of oven, and controls you have. A heat treatment oven may have only a 5 degree plus or minus range of error around a set point. A home oven maybe 25-50 degrees. If you have a thermocouple/thermometer/pyrometer you will have a more accurate idea.

Edit: just saw the photo. I can see a little gold wash to the edge and I like that. You could go even into the darker gold - more bronze color and still be safe.

Thanks Mike, No doubts that the oven is a not very accurate thing.
I'm using my kitchen stove (propane) and a ridiculously expensive analog thermometer I get the oven up to temp in advance, and back it down to where I want it. The controls will say 350 degree, the thermometer says 425. I sit and listen to it, and wait for it to fire up, and when I hear the flames quit, I know the temp is at its high point, and I check it then. That way I wont be taking a reading at the low ebb in the cycle.
I'm sure I am just like many other beginers when I shoot for the shorter soak times, because the various information out there says "1 hour per inch of thickness" and we are dealing with 1/8 in thick, so our brains tell us it should not be in there that long.
But who needs google when we have Mike and Charlie? am I right?!?

I temper at least an hour and half at 392. Usually 2 hours, after which I turn off the oven and leave it there to cool off.
I've found anything less than an hour too short.

And as Mike says, beware of oven setpoints. Buy an oven thermometer so that you can read out the actual temperature in the oven, in the location where you put the blade.

Also, since you mention gas: if possible, temper in an electric oven. With gas you get burn cycles than means you either spend half the time below your desired temperature, or above your desired temperature.

A little time below temp is alright by me, I can always put it back in for longer (like I'm doing right now!) I always keep this valuable lesson in my mind....

Now matter how many times I cut this 2x4, its still TOO SHORT!

For O1, I usually temper twice for two hours at 400 for ~62 HRC.

I have a high-end toaster oven in my shop that is really accurate and has a timer and stuff on it. It works like a charm and uses very little energy compared to my gas kitchen oven. My blades are usually a brownish yellow when they come out. I cool them in still air, then put them back in for another cycle.

Question for Mike: does it matter how the steel cools between tempering cycles? I've seen it recommended to water cool 52100 between cycles, which I've had good results doing, but don't understand this metallurgically. Is it like putting cold water on spaghetti noodles?

I have another question too for the metallurgists amongst us.... I kind of LIKE the golden bronze color that results from the temper. If I placed the shave ready razor back into the oven to turn the bevel that color, will it "un-ready" the edge?

Originally Posted by PapaTony

...Question for Mike: does it matter how the steel cools between tempering cycles? I've seen it recommended to water cool 52100 between cycles, which I've had good results doing, but don't understand this metallurgically. Is it like putting cold water on spaghetti noodles?

The al dente of 52100 might crack teeth...I suppose a tomato sauce including some nitric acid would soften it up nicely!

I like the use of a still air box myself but there should be no real dynamic motion from tempering temperatures down to room temp water if you used it. For the most part rapid cooling simply reduces the time in between cycles and allows more work to get done in a shorter period.

The bronze color is an oxide formation and will not affect the edge to any degree. It's a nice looking color to my eye too. It will get erased with honing though. No surface treatment survives honing or abrasion.

Originally Posted by Mike Blue

The al dente of 52100 might crack teeth...I suppose a tomato sauce including some nitric acid would soften it up nicely!

I like the use of a still air box myself but there should be no real dynamic motion from tempering temperatures down to room temp water if you used it. For the most part rapid cooling simply reduces the time in between cycles and allows more work to get done in a shorter period.

The bronze color is an oxide formation and will not affect the edge to any degree. It's a nice looking color to my eye too. It will get erased with honing though. No surface treatment survives honing or abrasion.

Glad you liked the cooking metaphor Mike I thought it was just some more voodoo.