Parks Oil ? Fast or slow?

For carbon steel blades:
A fast oil like Parks #50 or Houghton K ( 7-9 seconds) will harden a deep hardening ( slower hardening oil type) steel like 5160.
However, a medium speed oil like Parks AAA or Houghton G or canola oil (10-11 seconds) will not harden a shallow hardening ( fast hardening oil type) steel like 1095.

You can use even slower oils for oil quenching some carbon steels, stainless, and high alloy blades. There are 11-15 second oils used by some for 5160 and O-1 ( Drasta 119S, Citgo 0510, and Metaquench 42), and special blends as slow as 45 seconds for oil quenching stainless/high alloy steels..

If you look at a TTT or CCC chart you will see the pearlite "nose" at 1000°F is a fraction of a second for the shallow hardening steels. Deeper hardening steels can cool much slower and still harden completely through the blade. Air hardening and stainless steels have minutes to cool below 1000°F.

You might compare this to driving a nail. You can sink a 4 penny finishing nail ( slow/deep hardening - 5160) with a framing hammer ( fast oil) but you can't sink a 16 penny common nail ( fast/deep hardening - 1095) with a tack hammer ( slow oil) .

All that said, hardening a blade that doesn't need it too fast can induce stresses and warpage. It is best to have two quench oils. One fast and one medium speed. I would venture that most experienced smiths use Parks #50 (or equivalent) and AAA ( or equivalent) in their shops.

Going faster than #50 with water or brine (5-7 seconds) is only done for certain very shallow hardening steel to get a hamon. There is no normal reason for a knife to be quenched in brine/water. Doing so gives a high risk of cracking .. or the dreaded PING as a blade breaks in half in the quench tank.
 
Seved, when Jill mentioned Parks 50 as a 7-9 second oil, this refers to the oil's "nickel ball speed". This is a test that was developed in order to show how fast an oil can quench. A ball of nickel that is 7/8" in diameter is heated to 1625F and then dropped in the oil. The time it takes for the temperature of the ball to drop from 1625f down to 670F is known as that particular oil's "nickel ball speed". Parks 50 is one of the fastest quench oils used by knife makers, along with DT-48. These oils simulate (or get closer to) the speed of water, without the risk of cracking. While fast oils can be used on deeper hardening steels like 5160, it not generally recommended. It is possible that the fast oil causes to much stress in deeper hardening steels, but I don't know the exact extent of such. Ideally, you use the oil that is best for the alloy. 5160 certainly doesn't need Parks 50, but rather a medium speed oil would be a better choice. The slower the oil that still gives the maximum post quench hardness is what should be used.
 
Thank you Samuraistewart . You learn something new everyday .
I read about s test using 50 Parks to 5160 and Cryo. So i got curios. I have use my 1095 oil to 5160 it didnt crack but it warped a little. But i it was harder then with my slower oil.
But i get 63-64 before tempering with my slower oil.
 
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