Hello and "decarb" Question

[NorCal Nate]

First off let me say hello to all you gents. My name is Nate and I live in Trinidad, CA. I kind of became infatuated with making knives after watching a couple youtube videos and reading David Boye's book and Wayne Goddards revised "The Wonder of Knife Making". Then I found THIS PLACE ( you guys make some purdy blades)! Im a little short on the tools department, but I gotta lotta drive so I've slowly been able to pump out a few knives. Anyway, here is the question.

I read a post last night from a guy who was getting wierd pattens in his steel after HT and quench. A few of you guys said that it was "decarb'' and was probably on the knife during the HT process. Question is does this "decarb" pattern effect performance of the blade? Does it promote rust? Good/Bad? any info will be absorbed

A few days ago I HT'ed 2 blades in a series im working on and after HT and quench bath they looked very similar to the pic the other guy had in his post (ATC customs?? i think his handle was) anyway I liked the rustic look of it and left it on the blade all but the bevel. Handles glued on one last night so no going back. Couldnt sleep much last night thinking i'd done a no-no.

Ill just tell everyone its poor mans demasscus or something!

Thanks for your time and any insite will be greatly appreciated! ~Nate

[Doug Lester]

Decarb removes carbon from the steel and lowers it's ability to harden. It always happens to some extent in forging but the decarb layer is usually ground away. The big issue when you see a pattern in the steel left by the decarb in the finished blade it means that the steel has been subjected to very high temperatures for extended periods of time. That leads to another problem, grain growth. Course grain caused by large crystals in the steel promote brittleness in the steel and makes a blade more subject to breaking. This is a far worse problem than the actual decarberation.

Doug

[NorCal Nate]

I see. I didnt think I left the blades in HT for to long..... once they lost magnetism I put them back in for a minute or so and quickly quenched them. Although I did do the HT on a bbq with hard/soft wood mix so I'm unsure of actual tempurature reached, but the steel was glowing orange if that matters, and it is what I beleve to be L6. Thank you Doug!

~Nate

[Ray Rogers]

QUOTE; what I beleve to be L6

That's part of the problem with salvaged steels - you can't be sure how they should be heat treated. Then, you said "put them back in for a minute or so" .... we don't know how hot your forge was but if the steel was at non-magnetic then an additional 60 seconds could easily pump the temp far above non-mag. I haven't checked the HT specs on L6 (if, in fact, that's what you have) but for most steels you go about 50 to 100 degrees beyond non-mag and then quench. Further than that and all the bad stuff Doug said starts to enter the picture.....

[Doug Lester]

There are other things that can cause a wierd pattern on steel besides decarb. I've heard of stuff in motor oils that can do strange things. If you took it up to non-magnetic and then got it a little hotter and then held it there for about a minute, you probrably did ok. L6 is an AISI designation and the numbers have nothing to do with carbon content. L6 can have anything from 70-90 points of carbon. If the carbon content of that melt, if the steel is indeed L6, was much over 80 points of carbon you would have been better to hold the steel at just above non-magnetic for about 5-10 minutes to get a good distribution of carbon in the austinite. With mystery metal you can only guess.

Judging temperature by color is hard but those of us who cannot afford regulated molten salt tanks or heat treating ovens just have to do our best. If you can put a piece of black pipe into your forge of a length and inside diameter to hold your blade you can do so and build your fire up around it. That will help even the heat out in you charcoal forge. You might want to consider one day building a gas forge with a pyrometer to do your heat treating in. You're still going by magnetism, or lack there of, and color but at least you can keep the temperature regulated in the forge.

Doug

[NorCal Nate]

The steel im working with is a big circular saw blade stamped Henery Disston& Sons Crucible cast steel. My limitied research into that company, which changed names many times led me to beleve the saw blade was from the 30's or 40's. Wayne Goddard states in one of his books that "Saw blades are made from a variety of steel types, however most are the special purpose "L" classification". This is why I figured it was L6.

Doug- It was motor oil it got quenched in. Very dirty diesel oil outta my dodge. I reckon Ill try a diffrent quenchent on the next couple blades it see if I still get wierd patterns. I really cant be truly certian of anything because of my limited experiance with steels and metal working.

Thanks guys ~Nate

[Doug Lester]

Another reason to avoid motor oil is that there are often additives that are released into the air when red hot steel is plunged into it that you really don't want to be breathing. It is also slower than vegetable oils and might not work as well with some of the shallow hardening steels.

Those charts that say what items were made of what steels are at best lists of what they might be made of. What means that they might not be made of them. The newer your item, the less likely it is that those lists are accurate. Used items, especially springs, also have the problem if developing microscopic crack in them during use that will come back to bite you in the hiney later. I strongly recommend that beginners start out with a known steel. I know it sounds like Wayne Goddard advocates the use of salvaged steel, aka mystery metal, in his book The $50 Knife Shop but in his other book The Wonder of Knife Making he specifically recommends against it, at least for beginners.

Doug

[NorCal Nate]

Originally Posted by Doug Lester

Another reason to avoid motor oil is that there are often additives that are released into the air when red hot steel is plunged into it that you really don't want to be breathing. It is also slower than vegetable oils and might not work as well with some of the shallow hardening steels.

Those charts that say what items were made of what steels are at best lists of what they might be made of. What means that they might not be made of them. The newer your item, the less likely it is that those lists are accurate. Used items, especially springs, also have the problem if developing microscopic crack in them during use that will come back to bite you in the hiney later. I strongly recommend that beginners start out with a known steel. I know it sounds like Wayne Goddard advocates the use of salvaged steel, aka mystery metal, in his book The $50 Knife Shop but in his other book The Wonder of Knife Making he specifically recommends against it, at least for beginners.

Doug

Doug I understand completly. But being the hard headed beginner that I am I'm going use up that mystery metal saw blade until there is nothing left. I'll try a few diffrent quenchants and see if anything changes as far as the decarborization goes. I just use what I got and I do love a good mystery, keeps thing interesting!

[Crex]

Good advice to listen to here Nate. If you read Wayne G's books inductively you will see that he enjoys working with recycled steels, but does not actually recommend them for serious work or beginners. There is a world of difference between working unknowns as a beginner and as an experienced and seasoned smith.
By all means use what you have, you can learn from it all. However, (just going by your post here) you do not have a good grasp on thermal cycling steels. I would focus my initial research there. Open wood/charcoal fire is hard to regulate with experience, the black pipe will help, but you still need to know what's going on with the steel/heat. It sounds very much like you got the steel way too hot. Bright orange is too hot for most forgable knife steels. Non magnetic is below critical as mentioned and is usually right around dark red for most. There's alot more to this and color perception is very subjective to say the least. As Doug said the overheating will give you large grain growth = brittleness and lousy edge holding charateristics.
Not really sure you can get L6 to fully recover from over heating like you can 1084. At best, normalize a few times and then reheattreat more carefully.

Do some serious looking and asking....there are sure to be some experienced knifemakers near to you. See if they will let you visit and learn a bit from them. It will greatly enhance your learning experience and speed up the process. There is a knifemaker in every town, usually many more once you start looking.