what makes an oil..fast...madium...or slow for a quench?

[sunny_2_75]

hello evryone on the forum. I was curious as to what makes an oil slow, medium, or fast for a quench. I hope this hasnt been asked numerous times already. I try to read, learn and understand as much as I can in this never ending endevor to make knives. I want to know why something works as apposed to just being told a particular brand of oil to use for a particular type of steel. Im not really sure how to research this. Does anyone have some insight?

 

[Stacy E. Apelt - Bladesmith]

The speed at which the quenchant lowers the steel temperature from austinization to the martensitic start point, and from there to the martensitic finish point is what determines fast, medium, slow. Brine is fast, plain oils are medium, and air is slow. The quenchant companies modify oils to make them cool faster in the first area and slower in the second, and these are the oils that are used by knifemakers. Heatbath/Parks #50 is fast (near brine) and AAA is medium. Canola oil seems to be somewhere between the two.

Quench oils are rated in seconds, which indicate the time for some sort of lab test. The seconds rating is not the time it takes to get past the pearlite nose. Fast 9-11; medium 12-14; slow 15-18.