Unbluing a blued mortise chisel

[PSobon]

I was happily mortising away last night with my trusty mortiser when the bit started smoking. I really should have stopped to see what was going on but only had two more do and it was late so I blithely continued. The chisel/bit combo seemed to be adjusted properly but I managed to blue it up pretty good. The chisel is a cheap one that came with the machine ( General ). It seemed to blue very quickly. Does the quality of the chisel have anything to do with how much heat ( i.e. abuse ) the chisel will take? Also does anyone have any good ideas on how to do backyard retempering? The hottest thing that I have is a basic plumbing propane torch. I DAGS on this subject but couldn't find anything that would suggest a plan. TIA.

[Fly-by-Night CC]

If the chisel's made of high-speed steel you wouldn't have to worry about bluing and ruining the temper. Bluing happens at a temp much lower than would affect the hardness of high-speed steel.

[Andy Dingley]

It's dead.

[Fly-by-Night CC]

Well, he could bring it back to life by rehardening and tempering it if it did indeed lose the hardness. Shouldn't be that difficult to do with a smallish chisel and a propane or MAPP torch.

[Chipper Wood]

Retempering can be easily done with your propane torch and a quenching agent. A bit of time and work is involved. However if you are curious and want to experiment some, here is a rather in depth information site that should help. The blue color can be sanded away. Your drill bit is probably in worse condition than the chisel. As you need only to harden the cutting end of the chisel, It is an easy task. You can resharpen with a Dremel type tool and fine grain pointed grinding stone. The drill bit is another story as it would require specialized controlled heating equipment and quenching compounds to achieve the proper hardness throughout. It would be easier to just replace. :

[bridger]

IIRC high speed steel is untemperable in the home shop. it takes some pretty sophisticated equipment....

[RB]

Lee Valley describes the cause of your problem quite succinctly and aptly in their current catalog.

"The standard problem with these sets is that users ignore the bit setting instructions and burn both the bit and chisel because they fail to leave the correct gap between the two. Just a little warning that if anything turns blue you should look in the mirror to see the cause."

's time for a replacement chisel.

RB

PSob> I was happily mortising away last night with my trusty mortiser when

[Fly-by-Night CC]

Right. My first post told him that if it was HSS, then bluing it wouldn't harm the hardness. Bluing happens at a much lower temp than would affect the hardness of HSS. I hadn't heard back from the OP on what the steel is - I'd almost assume it is HSS as most tooling is nowadays, especially something like a mortising bit and chisel.

My reply to propane it was in response to Andy who said it was a dead chisel. If bluing it did kill it - it would be high-carbon steel and that *can* be easily rehardened and tempered at home.

[PSobon]

I am definitely guilty of tool abuse. ( stop me before I blue again!) I'm not really sure what kind of steel was used to make the chisel. I was hoping that I could just throw it in the oven or hit it with a torch to retemper it but apparently there is much more to the process than that. I guess in the final analysis it really doesn't matter. If bluing it ruined the chisel then it was cheap to begin with and not much of a loss. If it is good chisel made of HSS then bluing it doesn't matter. This fact was something that I was not aware of. Thanks for all the info on this question.

[-linux_lad]

If you can heat the chisel to a dull cherry-reddish color (usually right about the time there is a sudden change in it's magnetic properties), and then quench the tool properly, you'll be able to restore the hardening. The quicker the steel cools, the harder it will be, because the atoms are bouncing around like crazy and suddenly stopped before they can align. To reharden a mortise chisel, you should dip the whole thing wet clay repeatedly, then pare away the clay around the tip at a step angle. This will allow the steel to cool slower as the tip is quenched.

Heat the tool with a torch just until the tip is no longer magnetic, then plunge into cold motor oil. You may have practice a few times before you get the right cooling speed. You can do scratch testing on the chisel tip to determine an acceptable level of hardness. It's really not too difficult. If your chisel doesn't cool fast enough in in oil, try water instead. Some people like to use dry ice, but if you try it, be prepared for some blood curdling howling as the CO2 is converted from frozen to gaseous state quickly.

The colors you see when the tool gets hot is caused by oxidation. Different gasses in the environment produce different colors of oxidation at different temperatures. If the tip is too brittle, you can drive off some of the hardness by heating and quenching at when the color is yellowish straw.

Good luck!

--

-linux_lad To verify that this post isn't forged, click here:

[Andy Dingley]

It's dead. It's a cheap and lousy chisel, made from cheesemetal. If it's not capable of retaining its temper when blue (which is likely) then it's probably already showing microcracking. No attempt to re-harden this is going to work usefully. You might get it "hard" again, but you aren't going to turn it back into a chisel.

If it's real HSS, you need to normalise it before trying to re-harden it. You need an electrically controlled furnace to do this, because you need to cool it from red heat over about 5 hours (which means gradually reducing the heat, not just wrapping it up and letting it cool slowly).

[Fly-by-Night CC]

But if it is real HSS, then bluing it wouldn't have affected it's edgeholding ability anyway so no remedies are necessary. I agree that if it's of the high-carbon variety, then the OP might be better served in replacing it with a HSS version just for longevity's sake. But that's not to say that a softened high-carbon chisel can't be brought back to useable condition in the home shop. (I agree HSS is a different and much more difficult animal to tame with home shop tools.)

If the chisel is not marked as HSS, then to settle this, the OP would need to do a spark test on the upper section of the square portion to really see what he's got and if any remedies are in order.

[Fly-by-Night CC]

I guess I should have read your reply before I again responded to Andy.

If you have a grinder (or maybe a belt or disk sander) you can do a quick test to get an idea of the type of steel in the chisel. The characteristics of the sparks thrown off the tool give an indication as to the steel. After leaving the abrasive, if the sparks leave in a line and then branch and behave similar to a 4th of July sparkler then it's likely a high-carbon steel. If the sparks are an orangish and don't branch out - meaning they streak off the abrasive in a line and just disappear with no additional separation then the steel is likely high-speed. To do this test, pick an out of the way area that isn't going to be needed for an edge or for mounting in the machine.

If it were mine, I'd just try using the chisel and seeing how the edge holds up. -- If it dulls easily or rolls then the hardness has definitely been compromised and you'll have to reharden it or replace it. If it appears to behave normally and works as well as before then it's likely HSS and you can go on your merry mortising way.

[Hock Tools]

Probably more info than you bargained for (and much of it less than accurate). If the tool is high speed steel, it should be able to be "blued" without damage (though I'd consider the coloration to be a hint that something's not right with the operation at hand -- your cutter's dull or you're pushing it way too hard.) Resharpen it and see if it still works. If it won't hold an edge, it's probably not HSS and will need to be rehardened and tempered (or replaced). The problem here is that, without knowing the exact alloy it's made from, you may or may not succeed as the procedure differs for different steels. More at:

heat treatment of simple carbon steels. Good luck.

Ron Hock HOCK TOOLS