Friday, December 17, 2010

Blacksmithing: Carbon Migration in Forging Blades

Is there Carbon Loss In Forging Blades in a Gas Forge?

Hi David Merry Christmas and thank you for an ongoing 'live' tutorial. It is very generous of you and I want you to know that I appreciate the time you put into it for us!

I do have a question at this point. One issue that you haven't addressed (perhaps you just haven't gotten to it yet and if so I apologize for jumping the gun) is that of carbon loss with a gas forge. When I was training in Japan, I watched a Bladesmith produce kitchen Kata-ha blades with a gas forge. He explained, however, that he had to work extremely fast and could only afford one heat for the forging process (he uses a spring hammer) or he would lose too much carbon. When I say fast, I mean it. I took a video of him forging 4 - 5 blades every 7 minutes. He is not an amateur, but rather the person who is considered to be (within Japan) the best kitchen knife forger in the world, so I trust his word.

It would be useful to hear your thoughts from a Western perspective.

This is an advanced question for sure.

I do make knives but there are many people much more skilled than I. I am sure the fellow in Japan is one of them but here is my view point.

My take on is a bit different. Yes I do agree if the gas forge is running too lean (too much oxygen then yes at high temperatures there is carbon loss but really only in the outer 1 mm of the steel. The carbon migrates to the lower carbon areas and if low carbon in the atmosphere of the forge then it can jump to the atmosphere, causing a net carbon loss in the blade.

There is an old saying to "forge thick and grind thin" once you start grinding you are getting back into the high carbon steel.

By keeping the forge running neutral or a bit rich there is extra carbon available and carbon migration is less of a concern but you don't have infinite heats to work with. I agree completely that you should forge out the blade in the minimum heats possible as a best practice.

Different steels will behave differently as well. Some steels need to soak a long time at high temperatures to allow the carbon to jump to a face centered cubic crystal. Think here S7 or H13. Neither would I use for knives but the high alloys do change things a lot. So in part the alloy of his knives would make a huge difference in the final quality of the piece as well as the number of heats he takes.

One heat is pretty fast! And shows a great deal of control on his part.

When You get a chance look up the article on heat treating in the members area. There is a pdf at the bottom of the page that you should download and read as it goes into much more detail than I possibly could.

Link To Blacksmith Members Sign In

I hope this helps but do check out the pdf as it goes into great detail.

Merry Christmas and Happy Holidays to all!
David Robertson
Artist Blacksmith