Did I lose all my carbon?

[lyuv]

I forged a san-mai blade with 1.25% carbon steel core (125sc), and wrought iron clad. After quenching, the core is soft. Tried twice - oil and water.

Forging time was about 3-4 hours, and at very high temperature. The core started at 4mm thickness, and got down to about 1mm.

Is it likely I lost more than half the carbon to carbon migration? Any other explenation/solution?

I hesitate to just keep quenching, because I hear it's bad for the grain structure.

[Latticino]

Hard to diagnose as I've never had this problem with san-mai.  Carbon migration is certainly a possibility.  A couple of suggestions for review:

1. Your core thickness seems very thin at 1 mm.  My target would be a final core thickness of closer to 3 mm.
2. Did you follow the normalizing guidelines I spent time writing up for you after your last post? (you never responded to that post, so most likely this will be my last feedback for you)
3. Are you sure that you have the high carbon steel on the edge (core)?  How have you tested this?  Consider etching your ground billet to check.
4. Did you grind in bevels to expose the hardenable core (see # 3)?
5. How are you ensuring that you are reaching the austinizing range before quenching?  Have you ever hardened the 125SC steel without a cladding?  Did you use the same procedure, including soaking (if required)?  How about your quench media (preheat, age, agitation...)?
6. Are you getting into the quench fast enough?  I can't find any TTT charts for your 125SC steel, so have no idea how fast you need to quench

[ThomasPowers]

3-4 hours at high temps is quite long enough for massive amounts of carbon migration.  If that is your typical process you may want to look into adding a carbon migration blocker like pure nickel between the high and low C materials.

[lyuv]

1 hour ago, Latticino said:

Hard to diagnose as I've never had this problem with san-mai.  Carbon migration is certainly a possibility.  A couple of suggestions for review:

Laticcino,

The normalizing I asked about was for this blade. Only now I got to do the normalizing (and quenching). I kept my phone by the forge, and followed your instructions to the letter.

My appologies for not responding. I got upset by the hardening thing (and other issues like delamination), and forgot to thank you. My bad.

I did etch the blade to see the layers. And to be absolutly sure, I ground 1/2cm off the edge, and filed across all the edge. All soft. Also - A delaminarion exposed the core at a point, and I tested there too.

I did harden that steel before, but can't be absolutly sure about the temp. I use a magnet and soak at the non-magnetic temo for several minutes. But I cant tell how high above that point I am. It IS possible I"m too close, and dropping too much before the quench.

BTW, the blade is for a kitchen knife. the thickest point will be 3mm, tappering down to 1-1.5mm. And the clad should be seen on most of it. So I need the core to be very thin.

[Latticino]

Sounds like you did all you could to make it work.  Very possible carbon migration is a problem.  I would follow Thomas's suggestion to make future blades with the nickel layer.  For this one you might as well experiment, though I would stick with the heated oil quench and forgo water.  See if you can figure out how to see the decalescence during the heating process before your quench to ensure you are fully austenized before you go to quench.  I still struggle with that, but working in a darkened room can help.

I understand the desire to make kitchen knives thin, but would try to bias the cladding as thin as possible while keeping the core close to a minimum of 2 mm if possible.  At 1 mm normal decarb can take a toll of your steel, even without san-mai.   You should be grinding down to that final thickness after you heat treat.

I've only forged a handful of kitchen knives, but when I do I try hard to work at lower heats once I get down to 3 mm thickness and not overwork the steel.  Extremely high carbon steel, like you are using, is a real bear to work well.  I have a san-mai billet I got from Rick Furrier that is wrought with a 1075 core that has done very well for me as regards hardening, but I'm sure he welds it together very quickly with a hydraulic press and I worked the blades pretty fast thereafter.

Good luck.

[ThomasPowers]

The high heats that wrought iron likes working at is not helping things either. I would suggest working down the materials a lot before doing the weld so there is less time spent at the higher temps for the core material.

[templehound]

 

I guess You bought that steel from A.Wirtz(Germany), because he is the only one  who makes C125Sc.This is the reason why it is not found on any charts anywhere

(I work on some blades with that steel and in my experience it is not very edge holding but very tough compared to the amount of carbon.Very good steel for the forest and the field, it is very! easy to sharpen.)

This is a very fine steel with some manganese adding to 1,25%C but it is on absolute file-level.The manganese makes it suitable for quenching in oil instead of water.

This kind of steels do not forgive mistakes...like forging 3-4 hours on a single knife blank(wow!) at very high temperature(ouch!)...xxxx, I wonder if there is something left on carbon ...this steel requires accurate control of temperature, rate of cooling down should be very!! fast...if You quench a bit too slow it wont be get hard...and besides You have a grain as coarse as aquarium perlite.

If quenched properly every blade shows a natural Hamon.

The most common mistakes are that fellows do not use a thermometer and hold too long before quenching....or forge to hot and excessively long...

the solution of C in file steels, 125SC and the like is mostly completed after reaching quenching temperature....there is nothing in it that delays the solution of the carbon.After C is in solution, grain growth will start immediately.....

one of the aspects is,  some guys are interested in forging, not in making knives...and as a "simple specification"they try to make a knife....and adding some forge welding thinking it will go easy, just like this.....besides 3layers are more advanced to weld than You think...because the first weld is double over the full length....Having 40 layers and making them 80 layers is half the weld and easier

Compare it making a blade of O1 or O2 via stock removal....heat treated with control and information from all over the net....that results the more or less in a quality knife blade....if You want a knife

But making a knife by forging and no help of accurate control and information is like a beginner on the guitar wants to start learning with some Flamenco....

well, speaking honestly....throw it away and start a new blank but get yourself more educated on the subject....those words are not meant to be harsh and discouraging they are meant to stimulate Your thoughts on the subject.

good luck

 

Edited November 28, 2018 by Mod34
Edited for inappropriate language

[lyuv]

Thanks all,

Templehound - you are correct about the source of the steel. I had good success with it in other blades, including san-mai, but never with such high temp or thin core.

Seems I got to the limit., that so far I only knew about as a theoretical issue. Yes, I lost (one more) blade, but learned a valuable lesson.

BTW - I never thought this would be easy, but the opposite. Every work I did so far (mostly knives) was first of it's kind. I like the research and the challenge. In this case, working with wrought iron was a first. The motivation for making the blade was to overcome the difficulty, and not to have a knife. I still dont know what to do with it if I"ll get it done :-).