I assume that anybody interrupting a quench is attempting to approximate the martempering operation, otherwise there is little other advantage. To do this you want to get as close to Ms (martensite start) as possible without dipping too far below it. If you interrupt too hot you will risk forming some upper bainite. I am not as giddy as many about lower bainite but just about everybody that knows about the stuff would like to avoid upper bainite in blades.
The danger of interrupting too low is dipping into martensite with very thin areas like the edge and then arresting the cooling of the hot spine and risk bleeding the heat back into the edge and thus over-temepering that fresh martensite. I have also heard of folks doing all kinds of "creative" things to slow the cooling hoping to get the elusive uber-blade made of unobtanium, this will also flirt with the same problem, but even more so. This issue would be more likely with faster oils that have the ability to produce wider gradients in temperature of the blade. Just air cool and leave the cooling process alone, because another thing that can result from messing with the continuous cool is the stabalization of austenite, and then you will be doing things like freezing your simple steel blades just to reach full hardness.
I have not worked much with tough-quench but the oils I have worked with often take around a 7 count to get in the neighborhood.
NEVER compare times of real quenchants and vegi oils or automotive products, since real quench oils are designed with a different curve and will cool very fast in the first couple of seconds and then slow down in the last part of the quench. Parks AAA will hover around 800-500F for a surprisingly long time and #50 does much the same.
Make a practice blade if you need to and then quench it and count off the time. If done right the blade sould be albe to be held with new welding gloves without real pain. The blade should be coated with oil with some light vapors coming off from it, due to the fact that most oils have a vapor point that coincides nicely with the temperature range we are looking for. You can also check it with a magnet, as the test blade approaches 40% martensite the magnet will start sticking better. Also bend the blade and straighten it a bit. Meta-stable austenite is very moveable, but martensite definitely is not, with practice the blades resitance to deformation will be able to tell you how far the hardening has progressed.
All of this is one of the reasons I always have students play around a bit with the blades after interrupting at Ms. It not only helps them get a handle on this process but it definitely drives home a very memorable way what is happening inside the blade. One of my favorite tricks to pull to this day is to help a student quench a blade they are particularly proud of and snatch it out of the quench at 450F. and ask them if it looks straight before bending it and then handing it back them and saying "Good, you have about 3 minutes to get it back that way!"
The next time I talk to them they are a true believer in the underlying process' I described and the lessons seem to stick with them for a longer time than most.
