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Live and Learn - Why Not To Use Oxy Torch for Tempering


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Sad day yesterday. After two days of hard work on this blade I did the HT and it hardened great! A little warpadge that I knew I would be able to straiten out so I procieded to temper and took the blade to a nice dark blue, I wanted the blade to be tough. So then I took the blade to the vise using the three bolt method to straiten and with a slight turn on the vise SNAP!!! :o:blink::(.

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But every experience brings learning and I did learn that temper colors seem to come out different with an Oxy Aceteline torch. If I had tempered this blade with an oven to a dark blue like this it would bend over 90 degrees no prolem! Well maybe not 90 with a blade this short but still.

 

I was happy to see how fine the steel grain is in the break though!

 

Just thought I would share this experience. I have also discovered that forging a guard to fit a blade is a much tougher endever then I was expecting. But I am very happy with how the guard turned out. So I will be forging out a new blade to fit the guard! :)

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Bummer Dave, I know how that feels, that was looking good too but if in every mistake there is a lesson learned and experience gained then it is not waisted effort. Best of luck with the next one though I am sure you won't need it.

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hay dave can you regrind as a divers knife??

~M~

Hey Mike, that is a good idea to reconstitute the blade into something ells! I will probably make it into a shorter style blade. Thanks :)

Hi Dave

 

 

 

How did you use the torch? Back and forth across the blade, or just along the back?

Gerald, I did a kind of circular motion. just a little across the spine then off and around then a little across the spine again.

Bummer Dave, I know how that feels, that was looking good too but if in every mistake there is a lesson learned and experience gained then it is not waisted effort. Best of luck with the next one though I am sure you won't need it.

Thanks for the wise words Rob! haha I will take all the help I can! :)

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Interesting. If you were heating from the spine and letting the heat run across the blade to edge, not sure why it didn't work. Idle speculation would be that the temper was ran too quick. I learned to temper agricultural tools (Bill Hooks, etc) by heating a plate in the forge and setting the blade spine down. The colors would slowly run up to the edge. Using a torch should have given the same result.

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The colours you get using oxy-acet can be very missleading, because the amount of oxygen in the flame can be adjusted, you can get a range of colouring for a given temp, depending on how the oxy has been adjusted and how it influences the oxidation colours on the surface.

 

I also wonder if the surface cooling that happens when the flame isn't touching a particular spot really throws off what you actually get as well.

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But every experience brings learning and I did learn that temper colors seem to come out different with an Oxy Aceteline torch. If I had tempered this blade with an oven to a dark blue like this it would bend over 90 degrees no prolem!

 

Using oxidation colors, to judge temper, can be problematic but, you know that now! Just a little bit of oil, on the blade, will change the color of oxides that form during tempering and that is why a freshly sanded surface is usually recommended. The type of fuel used (if using a torch) and flame type (oxidizing, neutral or, reducing) also seems to change the colors, just not as drastically as oil will. Another drawback of using a torch is that the heat may not penetrate fully through the steel unless you are careful and patient. I suspect this one to be the culprit for your broken blade. Another, less likely, possibility is temper embritlement. This is when the temper cycle triggers retained austenite to precipitate out as fresh, untempered, martensite. The very safest (common) option is to use three cycles in an oven or furnace of some sort with a thermometer to check temperature.

 

~Bruce~

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Thanks Grahm and Norris! It was my suspition that the oxy to aceteline ratio would render different temper colors. Solution... temper in an oven! I have been tempering in an oven for years so I guess I thought it would be faster to use the oxy torch. I have also wanted to make a pare of temper tongs as well. Similar idea to your method Gerald. You heat up the tongs in the forge and then hold the blade with them till the temper colors start to run. Great way to get a soft spine. I actually thought it would be a good process to temper to staw color in the oven then use the temper tongs to soften the spine. I would place the blade into a pan of water that covered the edge to make sure I didn't over temper. B)

Edited by Dave Wood
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I would hazard a guess that the torch method is best left to tempering chisels and punches. You heat, quench, and then several inches back, slow heat the middle of the tool. The colors take a few minutes to run and that gives plenty of time for the temping to happen As you said, live and learn :-)

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I suppose the oxy was too hot? I did something much the same with a piece of scrap (luckily). I assumed that the oxidation appeared before the steel was cooked through, so to speak.

But I second the diving knife option ;)

Edited by Greg C.
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Another spine only temper would be to weld two 1/2 inch pieces of square bar to some tongs, heat them to yellow, grip the spine and hold with the edge in a shallow tray of water until the whole shebang has cooled, test with a blunt file and repeat if needed... I use this for daggers, omiting the water, and obviously gripping the center of the blade.

Edited by Miles Hebbard
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