use decalescence to ht?

C

Callum.D

Joined
Jul 17, 2015
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Up until now I have mostly used a magnet to estimate the temperature of my steel however I'd like to work out a more reliable system using visual indicators. I read about decalescence at some point and I observed it in a knife i was normalizing the other day.

As I understand it decalescence occurs because of the bond energy within the steel crystals so what I'd like to know is how can I use the visual effects of decalescence to work out at what point to quench. Should I quench just before I see the re-brightening of the steel or as i see it? Or am I completely on the wrong track?
 
If you wait until decalescence occurs you will have Pearlite in the matrix and that aint good for knives. You should always go from the heat source to the quench as fast as safely possible.
It would be best to use the magnet and train your eye to take the steel a little brighter than it is after it stops attracting the magnet.
Actually, a cheap pyrometer in your forge would be best. By using both the magnet & pyrometer you should be able to do a decent H/T after actually doing it a few times.
Good luck.
 
I believe you are mixing decalescence and recalescence. Decalescence ( dark shadows)happens as the energy is absorbed when the conversion to austenite occurs. That is somewhere around 1420F. Recalescence (Brightening) occurs as the austenite releases energy as it converts into pearlite. This is around 1350F. You never use recalescence as a HT indicator, as that would show the steel was cooled too much to quench. Decalescence can be used, but it is subjective to being able to see the dark shadows well enough to know the exact moment they occur. Then you heat about 50degrees more. Using a magnet will show the same point with no error in viewing.

Non-magnetic point plus one shade of red brighter will work almost every time.
 
The time decalescence makes sense is if going for a clayless hamon.
 
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