Discolouration after quenching and tempering

J

JimDiGritz

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Oct 18, 2021
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I use a MAP gas forge and then quench in Rye quenching oil (https://www.gfsknifesupplies.com/Products/Heat-Treatment/accessories1/quenching-oil-32-5l1)

My knives all come out of the quench (and subsequent tempering) a discoloured grey/black.

This image isn't great (and yes one of the knives has some burnt paper from a stuck on template still attached) however you can see they are not 'clean' - despite all but one being sanded to 400 grit and cleaned before heat treating.

https://imgur.com/UOv6YIC

They take quite a lot of sanding to get them back to bare metal, especially in plunge lines etc.

Is this normal? Many videos I've seen online show the blades coming out of the oven pristine...
 
That is normal. You are putting in a very hot blade into oil.

If you do not want to deal with that use air hardened steel and aluminum quench plates.

No matter how you quench the steel will always have a change some kind of change in color
 
I send my air hardening blanks off to be treated. They come back looking similar.
Sanding to 320 or higher before, just means hand sanding is easier after.
 
AVigil said:
That is normal. You are putting in a very hot blade into oil.

If you do not want to deal with that use air hardened steel and aluminum quench plates.

No matter how you quench the steel will always have a change some kind of change in color
Click to expand...

Thanks, just wanted to check!!!
 
JimDiGritz said:
sadly a 16floz tub will cost $80 shipped to the UK...
Click to expand...
But you apply it in a thin coat, so it will last for many hundreds of blades, and end up only adding a few cents to the cost of each blade...(size does matter here, I guess ;) )
 
Totally worth it. I dip the tip of the blade in, use a toothbrush to wipe it up the length of the blade, use my wife's hair dryer to dry it (takes about a minute), and it virtually kills all O2 getting to the blade. It says it is rated to around 2300°F if I recall, but that would require a very very thick coating. Seems to work well up to about 1750°F (A2 tool steel temps) with a very thin coating.
 
When I did oil quenched blades I had a lot of luck with PBC powder. You sprinkle it on a heated blade, it melts onto it. After quench it mostly pops off leaving surprisingly clean steel underneath. I tried atp many times and the blades never came out as clean as with pbc.

Definitely worth doing some kind of anti scale.
 
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