Surface condition and quench speed

[DevinT]

I first read about this in “Tool Steels” forth edition. In testing quench speeds of different quenchants, they polish the material to be quenched to assure uniformity in testing.

They note that scale on simple carbon steels can insulate the material and slow the speed at which the steel cools and a coarse polished finish cools fastest.

I yell at the TV every week during FiF when the smith forges for an hour and then goes straight to the quench without removing the scale.

There is a lot of discussion here about decarb, I think we need to include some talk about surface condition for optimum heat treating.

Hoss

 

[Grayzer86]

What you are saying makes sense. There is basically a layer of scale on the blade, that can behave like a clay coating of sorts. I would imagine this problem is made even worse when we look at things like puffy or partly attached scale, where there could be trapped air pockets. If the quenching can't get to the steel fast enough in those situations there could very well be soft spots.

 

[kuraki]

I agree and have found that to be the case when I would develop scale from normalizing and quench without cleaning it first, my as quenched hardness was not what it should have been.

 

[scott.livesey]

in the same book, you can see pictures of steel cracking or breaking from deep scratches, engraved areas, and number stamps. i have seen pictures on the forums where the blade cracks along a deep scratch left by very coarse belts. finishing the pre-HT blade to 120 seems to be the minimum and 220 would be better. this is for stock removal. I would hope when forging one would cool the blade enough so you could at least clean it with a fine wire brush then do your hardening heat.

 

[samuraistuart]

I have heard that a very thin clay "slip" (satanite or similar) speeds up the quench slightly, compared to a non coated blade. The little imperfections on the surface help to prevent, or at least break up to some degree, the vapor jacket, allowing for better oil/blade contact. The clay comes off quickly/easily in the quench, whereas scale does not. Same with ATP-641, it blows off in the quench during those first seconds, and is a very very thin layer as well. I never thought about the FiF heat treatments that don't bother removing the scale after forging. Heck, many of them don't even normalize/cycle, and seem way too hot going into the quench. Some do, most don't. I always make sure I have a good 120 grit tip to tang scratch pattern, and a thin coating of ATP. Normalizing and cycling get a thicker layer.

Thanks to user "Sandgrouper" for the following link:

https://www.jsme.or.jp/tsd/ICBTT/conference02/TatsuoINOUE_p07a.html
And the following picture from that link showing how quench speed is accelerated when a thin layer of clay is used (as opposed to no clay or thicker clay) and the measurements of the clay are in millimeters:

 

[Russ Andrews]

Devin......did the writer comment on what a "coarse polished finish" was...?

 

[DevinT]

“Roughness of the surface in some instances appears to be an advantage in that it favors wetting of the surface by quenchig medium and thus tends to break down the vapor blanket that forms.”

My take is that it needs to be rough-smooth, too rough might cause stress risers.

Hoss

 

[Willie71]

I have found a 120g finish works best in my shop. I don’t break blades, heat treat comes out consistent, and I don’t waste time on extra possibly unneeded work. The clay sticks well, and it was less consistent at 220g or finer. I find 120g works well for epoxy glue ups as well.