Two questions: repeated heat treatment and excessive carburization

[overmodulated]

Tonight I attepted to harden a 15" 5160 blade in a coal forge that wasn't quite big enough. I moved the blade slowly to distribute the heat. But I failed to achieve uniform heat across the blade. (some parts started to burn, while others still had a "shadow" in the orange. Quenched it in peanut oil, and the file test releaved uneven hardening. So I desire to try this again, if I can get away with it.

Problem is: there's a serious amount of carburization on the surface, that will be a **** to clean up. (picture)

So the two questions are: can I repeat the quenching process and still end up with a hard edge, and is there a way to minize the carburization in the future? Thanks much!!

Edited December 22, 2009 by mod07
Foul Language removed

[Rich Hale]

I think what you are seeing is scale. If I were you I would normalize the piece and reheat treat. But not in the same forge. Or figure a way to make the fire long enoough for the blade. You can build a ground trench..and start a forge in it. I think you can google directions for that. When you are starting to heat treat blades you should develop a method of testing each and every blade. That is the only way you will know what you are doing to a blade and see results. I also do destructive testing on a couple of blades each time I try a carrbon steel I have not used before. I feel there is just no reason to put a knife together and part with it unless I truly trust wot it will do as a knife. The critical thing on testing is a record of wot youi have done and how it worked. There is a link on the forum for the testing that ABS suggests for blades. That may help you figure out wot to do. Also read the stickies on heat treat and bp0078.

[pkrankow]

JS Test

Rich mentioned this.
Phil

[ThomasPowers]

More likely to be decarburization than carburization.

Had a fell yesterday tell me about how the japanese swordsmiths did the folding and welding to put carbon into the blade. I then explained that they started with nealry 2% C and ended at around 0.5% C and so I don't think that was the reason...A lot of folks don't understand that in general forging you can scale off steel faster than you can soak carbon into it. Takes skill and care to up the carbon content during forging.

[E Schroeder]

if You can lay your hands on a piece of steel pipe(non-Galvanized, very very important if you like being alive) that just a bit bigger than your knife heat the whole pipe in the fire and the heat will bleed through and very evenly heat your knife which you can then quench. Its really good at keeping your blade clean. If you have an OA torch you can heat treat with that too. Use a welding tip and hold the blade with the belly facing up. heat from the spine to the belly going back and forth across the blade, you have to be careful that the knife is roughly the same temp on both sides or you can get a wicked warp or crack in the blade.

Edited December 22, 2009 by E Schroeder

[overmodulated]

interesting heating method: lay the knife inside a steel pipe and have the whole thing in the coal fire - do I understand that correctly? Seems like this could take a while, but iof it works I'll try it.

This blade is way too long to use a torch, I believe, but if I were to use a torch wouldn't I concentrate the (moving) flame along the edge rather than the spine? Thanks aagin

[E Schroeder]

yes you do understand the pipe method correctly, and as for the AO method no you don't start on the edge you start about a third of the blade off the spine as heat travels up and you are trying to heat the blade not melt it, if you start on the edge and move back you risk melting the edge, done it a couple times had to learn by ruining several blades. Also I do all of my work on charcoal, so if I can get it the pipe method to work you should be able to do it pretty easy using coal.

Edited December 22, 2009 by E Schroeder

[overmodulated]

Rich Hale: you were right - it's scale! Found that out tonight. The stuff actually brushed off when reheated to sufficient temp.


E Shroeder: great tip with the pipe. Although I had to use a square pipe to fit the blade, it provided a nice little "oven" within which the monster blade got evenly heated. Got a good quench on thuis. I think this 5160 blade has been salvaged. Thanks for the good tip.

[ThomasPowers]

Put an end on one end of your pipe and stuff some crunched up real charcoal in your pipe to scavenge O2 and have even less scale!

Also you generally hold the blade in the pipe so the edge is not touching the sides to prevent un-even heating. I usually rest the back edge of single edged blades on the bottom of the pipe to soak heat in faster into the thicker parts for through hardening.