Heat treating problems

I just started using the KnifeDog oven, working with 1/8" 1095. With 1450 as a target temp I soaked for 10 minutes and then went into a 125 degree quench. I did a file test and the blade was dead soft. So after cleaning up I again tried a HT, this time not relying on the pyrometer, took it to 1500 , which was non-magentic. Back into the stove and took the temp up to 1500, held for 8 minutes and then quench. I was very unsure of the quality of the HT so I tested the blade on a vise, broke like a twig!. The grain was very fine, but the blade had no flexability. I am new to using electric ovens.
 
I see few concerns, but the big note is that you would expect it to break like glass pre-temper. That spring you are looking for shows up after tempering relieves stress.
 
I just started using the KnifeDog oven, working with 1/8" 1095. With 1450 as a target temp I soaked for 10 minutes and then went into a 125 degree quench. I did a file test and the blade was dead soft. So after cleaning up I again tried a HT, this time not relying on the pyrometer, took it to 1500 , which was non-magentic. Back into the stove and took the temp up to 1500, held for 8 minutes and then quench. I was very unsure of the quality of the HT so I tested the blade on a vise, broke like a twig!. The grain was very fine, but the blade had no flexability. I am new to using electric ovens.


We have all heard the story of Goldilocks, and you are living it with the 1095 Tom. You have went to 1450F (the porridge is too cool), and you have tried 1500F (the porridge is too hot); now you need to go to what your simple hypereutectoid Goldilocks is looking for, 1475F (just right). The magnet will let go of this steel at around 1420F so you have 80 extra degrees to deal with in reconciling it with 1500F.

Due to the fact that you undoubtedly got hardness by the second shot, I would say your issues are in one of two areas, insufficient solution or decarb. Unfortunately the two can be at odds in the fix. If you did no normalizing between the two hardening cycles then the solution effects from the two heats are accumulative and that would account for your final hardness. This becomes more of a factor if you are using the steel as it arrived to you and not forging it, since the carbon will be locked up really well in a spheroidized form from the mill and will need time at temperature to dissolve. But this is a very simple carbon steel so that time should not be too excessive and I would expect a 10 minute soak at 1475F to do the trick with no problem. Now for the decarb… the open air of a kiln is a sponge for the carbon in your steel when it is at temperature and you will lose some from the “skin” of your knife leaving a thin layer that can, if it gets thick enough, tell the file that you didn’t make hard steel, but below it is good hardened material. Now for the bad news, soaking presents more opportunity for this problem to occur, so heat treating is always a delicate balancing act.

Now on to the “flexibility” topic, after all of my years working and teaching in this business I have come to the conclusion that this is probably the one area of knifemaking that has the most misconceptions by far. First and foremost is the distinct difference between “flexing” and “bending”. Flexing would imply that the blade was deformed by sideways force and then returned to true when the force was released. Bending is when the blade takes any sort of a set at all and does not return entirely to true. Heat treating will only determine where the action goes from “flexing” to “bending”, it cannot change anything before this point, which is known as the “yield point”. The amount of force needed to flex the blade is determined solely by the cross section of the steel, and the amount of force it takes to bend it into a pretzel is not different enough from the point at which it yielded for you to tell without very precise equipment. Soft steel will yield very quickly and then continue to yield with little more force. Hard steel will resist yielding very much longer but will be closer to the breaking point when it does.

Hope this all helps.
 
I would suggest that you split the difference at 1475° and hold for 10 minutes and see what that does for you. You don't want to put too much carbon into solution even if you don't grow grain. Basically, as Knifemaker pointed out, there's nothing going wrong at this point. Your steel should be as fragile as glass. You need to temper, I would recommend 450° for at least two 2 hour cycles, to release some of that carbon that was trapped in the iron matrix when you quenched the steel.

Doug

Oops! Kevin beat me to it.
 
I'm also going to guess that if you did two back to back heat treating cycles, you not only got more carbon into solution as the cumulative effect like Kevin mentions, but you did not say how long you let the oven cycle at your 1450 target temp on the first go-'round.
If you just got it UP TO 1450 and then stuck your knife in for 10 minutes, that's not gonna float.
You need to let your oven FULLY equalize at 1450 for a good 20 minutes or more before placing the knife in.
That way the ENTIRE oven is up to temp and not just the thermo couple.
If you then did a second cycle at 1500, by this time, your full oven temp actually WAS! up to the displayed temp and you got more of a full effect from the actual heat.
Lots of variables here.
When using my oven, I always let it sit at the desired temp for 30 minutes before using it, whether I be austenizing, sub-critical annealing or post-hardening tempering.
 
Many thanks to KnifeMaker, Kevin, Doug and Karl: Karl you are correct I did not let the oven cycle at temp, I will do as you recommend, Dougs suggestion of splitting the temp is a good idea, I just fell back to my charcoal forge roots and relied on the magnet, I have no idea what the actual temp is, I assume that the pyrometer is calibrated but have no way of knowing this. I did not mention that I did temper after quench 375 for 2 hours. I have just about used up my stock of 1095 and have been using O1 which I have made some good blades with. I will take all our your collective advice and try it again!!
Tom
 
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