View Full Version : Questions on Re-treating O1 blades?
08-20-2011, 04:15 PM
I have a couple of O1 stock removal blades that I have hardened and tempered, but I feel like I tempered them too hot and want to heat treat them again and shoot for a lower tempering temperature.
What time and temperature should I program my kiln to for re-treating these blades?
Can I just put them into the kiln and ramp up to hardening temperature, or do I need to anneal or normalize them first? If so, what time and temperature do I need to use in order to accomplish this?
Thanks for your help! :5:
08-20-2011, 05:53 PM
I would just reaustinize them without worrying about annealing or normalizing again. This is unless you think that you have had grain growth, then I would normalize a couple of cycles to reduce the grain size. The advantage in starting out a little low when you're trying to find the correct hardness is if you feel the steel is a little too hard all that you will have to do is retemper at a slightly higher temperature. Once it's too soft all you can do is reharden and temper again.
08-21-2011, 01:21 AM
I ordered a hardness testing file that has a nominal hardness of RC60 but can vary from 60 to 62. My plan is to harden some test pieces of steel, then temper at 325 degrees and test with the file. Then I can just keep tempering hotter in 25 degree increments until the file starts to bite. It should at least get me in the ballpark this way. I'm shooting for an RC of 58-60 and I think this method will get me close.
08-22-2011, 07:57 PM
What temperature did you have set for the first tempering cycle that you believe you may have gone too hot? 400 - 425 will get you an HRC of 58 or 59 with 01. The files are not a very good indicator of hardness value. Your programmable kiln can be relied on to give you the desired hardness if the other parts of your heat treating is correct.
08-22-2011, 10:14 PM
My plan is to harden some test pieces of steel, then temper at 325 degrees and test with the file. Then I can just keep tempering hotter in 25 degree increments until the file starts to bite. It should at least get me in the ballpark this way. I'm shooting for an RC of 58-60 and I think this method will get me close.
Start at 450f and save yourself some time.... O1 needs a pretty high temp on the temper, 500f should give you 59-60 hrc according to my info.
08-22-2011, 10:17 PM
I have considered getting a hardness tester in the past and, for testing something like a knife blade, you have the choice of the files you are looking at or an instrument that will cost the best part of $1000. That's for a benchtop unit from Harbor Freight that weighs over 200lbs so it would have to be shipped by common carrier. There are portable units but they run even more. An Ames portable hardness tester that measures in the HRC scale are frequently found used on Ebay for over $1200. There are cheaper HRC testers but they will not measure anything as thin as a knife blade. That's why most of us rely on performance testing like cutting soft iron wire or brass rods. A properly tempered blade should be able to cut into either without chipping, indenting, or rolling over.
08-23-2011, 10:57 AM
Thanks for the replies. I tempered a blade at 425 degrees, then cleaned it up and re-tempered at a higher temp, 450 degrees. When it came out of the oven it was blue all over. I was concerned that the blue color meant it had gone too soft.
08-23-2011, 01:05 PM
.... I was concerned that the blue color meant it had gone too soft.
Actually, while oxide color is of limited use for other than plain carbon steel, that blue color would suggest they may have reached a higher target than you aimed for - perhaps something in the mid 500s F?
08-23-2011, 03:30 PM
About all you can do is to test the edge and see how it comes out. If the edge rolls or goes dull quickly you will have to try again by re-heat treating and tempering at a lower temperature.
08-23-2011, 08:21 PM
I have a friend on another forum who offered to have one of my blades tested by a friend of his, so I'm going to send him one that I tempered at 425. I'll have to grind/sand off any scale and decarb layer to make sure he gets an accurate reading. And of course I understand that a reading on one part of the knife might not be representative of the hardness elswhere on the knife.
But I'd like to get a rough estimate within a Rockwell C unit or two of what the hardness is so I can at least know in which direction to take the tempering. I think 425 should be about optimum according to the manufacturer's data (Precision Brand O1) but I would be much more confident if I had some way to quantify my results. At least the file will let me know if my steel is harder than or softer than RC 60-62.
I figure if I raise the tempering temperature until the file bites, then back off 10-15 degrees that should put me right on, assuming the file's hardness is correct.
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