RaymondRife
Apologies if there's already a thread on this, a search didn't find anything relevant.

I came across a youtube vid the other day from Anthony Bourdain's series  Raw Craft,  in the episode a Blacksmith Bob Kramer makes a Damascus steel knife. It's a fairly standard knife build for the most part until he gets to the stage where he heat treats the blade. So he heats it up to cherry red or there abouts and then shows how a shadow runs through the blade starting at the cutting edge and moves through the blade as it goes through its transitional phases.



It seems I can't use a timestamp to FF to the part of the vid where he explains and demonstrates the moving shadow/phase change, he starts discussing it at about 5m 36 seconds.

He doesn't elaborate on the actual details of what's going on or when the blade would be quenched relative to the "shadow" or visible phase change. I'd really love to know the finer details of this method and understand if it applies to general knife making. I know some Japanese swords were made from a hard outer layer and a softer inner layer in some cases, so would this "shadow" be something specific to blades with a harder outer layer/cutting edge?

It seems like a way to gauge a differential quench in some respects but almost in reverse to the way I'd normally do a differential quench. I'd normally heat the cutting edge more than the thicker part of the blade so the bulk of the blade stays a bit softer than the cutting edge.

Does anyone use differential quenching or watch the shadow ?

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anvil
I differentially quench most all of my tools. It's just a far better way than tempering your whole tool the same color.

As far as the shadow, well, again I've heard that "on the net" a few times, but I'm skeptical. The colors you see are caused by the temp your steel is at and the ambient light conditions, not by phase change.

I put it in the category of "the king who wore no cloths" fable. As far as this vid, well, didn't his wife say he went to clown college? [wink]
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jmccustomknives
There is a visible change in the way the steel looks when the phase change occurs, however it is so slight that one shouldn't depend on it.  Use a magnet, it doesn't lie and can't be altered by ambient lighting.  As far as edge quenching goes, I rarely use it and wouldn't use it on any Damascus since the heat treat is what makes the pattern crisp.  Edge quenching makes for a wimpy blade.  I prefer to full quench with a soft back draw since the steel is fully hardened with a spring temper on the back of the blade.  

One thing about using the magnet.  The point where the steel goes non-magnetic as the temperature is rising is higher than the point where the steel gains it's magnetism back.

Rule #10;  "I can make that" translates to; "I'm to cheap to buy it new."

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RaymondRife
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I put it in the category of "the king who wore no cloths" fable. As far as this vid, well, didn't his wife say he went to clown college? [wink]


I suppose taking his advice is a bit like eating a hamburger from McDonalds - cooked by teenagers and endorsed by a clown [wink]

It seemed like hocus pocus to me too, he gave a vague description of the "shadow" with no details about the implications relative to hardness, temper or when it should be quenched etc. Maybe he's watched to too many Japanese sword making vids and wants to make it seem mystical like a dark art?

@ jmccustomknives - I never used a differential hardening much, I've used a differential tempering by just tempering the spine and leaving the cutting edge as hard as possible. Is that similar to how you do yours ?

I must have made over 50 knives from broken power saw blades when I was working in machine shops over the years, every time one broke I'd whack it on the grinder or belt sander if they had one and make a knife out of it and then give it a quick and nasty heat treatment with an oxy torch.  Tempering the full blade was very hit and miss with an oxy and it was easy to overdo it on the cutting edge.  So eventually I just tempered the spine of the blade and left the cutting edge hard.

I didn't really know what I was doing in those days (before the internet) but they made really good workshop knives, someone must have liked them because they always disappeared within days.
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jmccustomknives
When you heat just the edge and quench it is essentially the same as doing an edge quench or clay backed heat treat.  I sounds like we learned the same way, I just fumbled and bumbled my way through the first 4 years.  lol.  My starter steel was bed frames.  [rofl]  Like you I used the torch a lot at first but now opt to use my forged for most knives but do use the torch for the draw.  

Rule #10;  "I can make that" translates to; "I'm to cheap to buy it new."

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Norseman C.B.
Its called decalesence the shadow moves across when the steel hits critical temp, when it falls below critical
the shadow moves again that is called recalescense this is a better way to judge when to quench for some steels as
non magnetic can be too hot for them and cause excessive grain growth, it depends on the steel type I use both methods ......
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anvil
Jmcustom knives, I reread my post and realize its misleading. I always harden the whole blade, and transition thru the tang. I always do a differential temper. Thanks for the heads up. I aim for a softer spine, spring in the middle and thru the transition of the tang, and a hard edge.

However, I believe, as a general rule, critical temp, when you lose magnetism, is slightly hotter than when you lose magnetism, not slightly cooler.

And you are right,, you can't beat a magnet.
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