How can I tell if I've mucked up the heat treating when (re)grinding a blade ?
And what, if anything, can I do to fix it ?
Barney
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How can I tell if I've mucked up the heat treating when (re)grinding a blade ?
And what, if anything, can I do to fix it ?
Barney
If there was any color change to the steel, the heat treat has been altered.
You can try to scratch it with another piece of hardened steel, to see just how soft it became. Or just hone it up and see if it still holds its edge like it used to.
There is probably nothing that can be done to save it if the steel has become unusably soft. You would have to reheat it to critical temp, ~1,600 degrees F, and quench it in an oil of some sort to regain the hardness it once had (which may warp or crack the thin edge).
Funny, I was just having a think about this; Do anyone soften the blade before regrinding... or do you risk warping it?
Why would you want to do that? There is so little material removed in a regrind that it wouldn't be worth the extra procedure or risk in re-heat treatment.Quote:
Originally Posted by mastermute
You would have more risk of warping (or cracking) in the re-heat treating stage that would be necessary to return a softened blade to it's original condition. And a soft blade won't hold an edge.
I don't know, just popping the question! :rolleyes:Quote:
Originally Posted by Russel Baldridge
Thanks, that answers my question... :pQuote:
Originally Posted by Russel Baldridge
The blade in question was a wapi that I'd tried to hone out a warp - this ended badly, ie still warped and major uneven hone wear.Quote:
Originally Posted by Russel Baldridge
I was going to even it out, make a narrower and shorter blade (you can see where this is going can't you ? :hmmm:), experimenting to try and get my metal working skills back - its been a long long time.
In particular I had a couple of spots on the edge kinda flare and go black yet they were not to hot to touch if you know what I mean. These sections have since been ground away as the blade has got narrower so assuming this did damage the temper how localised would that damage be ?
Barney
I agree with Russel. Annealing and re-hardening most blades would be a lot more trouble than it's worth. For one thing, we don't know exactly what the steel is, so you'd be guessing at the proper heat-treatment recipie. Heat-treating damages the outer layer of the steel, so you'd have a lot of cleanup work to do on the tang, spine, etc. The chances of warpage or cracking are high.
Unfortunately, grinding the hardened steel is the most efficient thing to do. :)
Josh
At the risk of aiding and abbetting PCR.... :)Quote:
Originally Posted by barneycg
Yikes, black is bad. If you have ground away all the discolored steel, you're probably OK. Trouble is, you can easily remove the surface oxidation (that's what makes the colors) long before you get rid of all the soft steel.
Josh
Well it's not exactly PCR, it's for her legs - for the moment ;)Quote:
Originally Posted by JoshEarl
I kinda thought black was bad ... what got me was that the blade wasn't too hot to hold - even in these areas. Unfortunately I'm not that well equipped at the moment and a cheap 6" bench grinder is all I have (a KMG style belt grinder is in the plans but it will be some time yet), I'd been moving the blade over the 1/2 inch grinding wheel at an even pace (about 2 seconds per pass tops) and this area on the blade about 3 mm long and about 1mm high just suddenly upped and turned black.
I think it is all ground away since the blade is now about 2-3 mm narrower than when I started and should get narrower still by the time I'm finished.
Cheers
Barney
about that warped wapi: You can sometimes fix this by clamping it in a vise with 2 pieces of wood at the toe and the heel on one side of the blade, and 1 piece of wood in the middle on the other side of the blade.
Then gently but firmly clamp it to bend it the other way.
How do you know how hard to clamp? It's easy. There are 2 ways. I prefer b).
a) turn the handle until the blade snaps, turn it a quarter of a turn back, and that is the ideal possition :D;)
b) turn until the blade is straight again, and then give it a tiny bit more.
If the blade snaps before the point where it is straight... :shrug:
It wasn't that gret a blade anywa. :w
Sounds like you'll be able to salvage it if you continue to grind without anymore over heating.
What I do is keep a bucket of cold water next to the grinder and dip the blade in the water after every pass. It's probably overkill but decidedly better than the alternative.
You could also use one hand to hold the tang and the other to support the opposite side of the blade with your fingers (or thumb) touching the steel where it is being abraded on the other side. If your fingers start to burn, take the blade away from the grinder. You'll never overheat the blade this way because your hand will burn at 200 degrees lower than it takes to harm the steel.
Exactly what I was doing ... even had ice in the water.Quote:
Originally Posted by Russel Baldridge
Again exactly what I was doing - fingers didn't burn but the metal still turned black... this is what surprised me. I did everything I could to prevent overheating and I though I was doing an alright job but a couple of spots just appeared to spontaneously flare and go black without feeling excessively hot to the touch.Quote:
You could also use one hand to hold the tang and the other to support the opposite side of the blade with your fingers (or thumb) touching the steel where it is being abraded on the other side. If your fingers start to burn, take the blade away from the grinder. You'll never overheat the blade this way because your hand will burn at 200 degrees lower than it takes to harm the steel.
Still you live and learn. I should get some time to continue my experiments next week.
Thanks for everyones input.
Barney
The fact that your fingers didn't burn might be caused by the fact that you were holding the spine / tang and not touching the blade enar the edge.
When I grind away pitting on my low speed home made sanding wheel, I keep my fingers of one hand near the edge, with my thumb on the spine.
My other hand holds the tang.
That way I can feel the edge heat up. When it gets to the point of being uncomfortable, I simply take it away for a couple of secs. it'll cool soon enough.
The blade near the edge is very thin, so it will heat up to damaging temperatures a lot quicker than the spin, which has a lot more mass to it.