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Need help heat treating a custom blade


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A friend of mine asked me to design and make a knife for him. It's a survival type with saw teeth in the spine. How would I go about tempering the blade to keep the edge and saw teeth hard and give a little flex via a softer center? Its forged from a leaf spring and is 8"" from point to plunge cut. Approx 3/16 thick. I'll can only email pics. My phone won't let me post pics on here. Ill send a picture to anyone who wants it. Thank!

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after the quench, Use your normal tempering oven at 280F to 300F for 1 to 2 hours then allow to cool. This is a stress relief, and leaves very little flex, but good cutting edges.

Second stage, now where for a normal blade I would re-temper another 1 to 2 hours at 320F to 350F, in this case we polish to see the colors run, and use a heated rod (red is too hot, use a black heat) and place the rod against the center area you want tempered softer. watch the colors run in this middle section and quench to stop the spread when desired color appears in that area and before it can reach the cutting edge(s) Repeat this a few times alternating sides to even out the added tempering.

This is the same way I do it for a differential tempered sword in case anyone was wondering.

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That is a good method for differential hardening, and will probably serve well enough with any of the many steels used for leaf springs.

Please remember though, flex is dependent on the geometry, not the heat treat. a softer blade may bend when flex is exceeded, a hard one may break, but at a much higher force.

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That is a good method for differential hardening, and will probably serve well enough with any of the many steels used for leaf springs.

Please remember though, flex is dependent on the geometry, not the heat treat. a softer blade may bend when flex is exceeded, a hard one may break, but at a much higher force.


I was explaining tempering not sure it will work for hardening. With hardening we need to slow the quench selectively. normally clay of some sort is used. Tho I am sure there are some that control the hardening with selective heating as well, I have not tried that way.

I do agree that the choice of steels and geometry is just as important as the HT for flexibility.
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Correct me if I'm wrong please. Its my understanding that a clay coated blade tempers it's self somewhat leaving the spine softer and the edge very hard. The hot rod idea suggested is used after a tempering heat then soften the middle section of the blade. Essentially the rod idea leaves a blade softer the the middle with a saw edge and standard edge slightly softer than quenched brittle/hard like a clay coated blade.

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Ok you're wrong. (slightly)

Clay hardening does not "temper itself". It is used with shallow hardening alloys and prevents the clay covered part from hardening in the quench as it misses the nose to form martensite. Note that it doesn't work well or even at all for deep hardening alloys.

Tempering tongs make it a lot easier to temper the center of a blade than just holding a rod on it----think of a pair of tongs with rods welded on the end so the oppose.

Heat in forge and apply to both sides of the center at once. (for more fun you can forge or mill a center shape to cover a central ridged blade like a double edged blade.)

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Ok you're wrong. (slightly)

Clay hardening does not "temper itself". It is used with shallow hardening alloys and prevents the clay covered part from hardening in the quench as it misses the nose to form martensite. Note that it doesn't work well or even at all for deep hardening alloys.

Tempering tongs make it a lot easier to temper the center of a blade than just holding a rod on it----think of a pair of tongs with rods welded on the end so the oppose.

Heat in forge and apply to both sides of the center at once. (for more fun you can forge or mill a center shape to cover a central ridged blade like a double edged blade.)

I had never heard of tempering tongs but had though about welding some round to vise grips so it would heat evenly on both sides. I suppose I was headed in the right direction but it never hurts to ask. Thanks!! And by the way I never take offense to being wrong although there's always "what's right vs what works". Thanks again.
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No problem; it's just sometimes stuff "creeps"; one of the functions of jargon is to be able to precisely describe what's happening and we have a lot of problems with folks using tempering when they mean the whole heat treat process; or just quenching; or quenching and tempering, etc. Just like to keep the jargon straight so's they young'un's can learn it! (or us old cantankerous coots for that matter!)

I've been thinking of having a friend mill some heavy round stock so I could heat those sections in the forge and then drop them in some sort of holder with the blade between them and have another pair heating while the first was "working", hmmm must think some more on this...

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