The ideal way to do this is with martempering (I like to call it marquenching) which uses special quenchants designed to hold temperatures around Ms (400F+) this cools the blade to exactly the desired temperature and then hold it here while the temperature of the blade equalizes throughout before removing and allowing the cooling to continue at a slower pace in air.
However martempering requires proper equipment ($$$) and mediums designed to do it correctly, and it just does not work well with shallow hardening steels (1075, 1080, 1084, 1095, W1, W2, etc…
due to the inability of a 400F+ medium to extract the heat fast enough to avoid pearlite. Thus the topic of this thread- interrupted or timed quenching.
Problem: You have a blade made of 1095 and wish to get some of the benefits of martempering, but your steel will not respond to low temp salts even if you could afford the setup.
Solution: Quench your blade into oil like you normally would but remove it from the oil before just before it forms martensite and allow it to air cool. The catch is that you really need to get as close to Ms as you can without going too far below it.
If you interrupt way too hot you could form upper bainite which plain sucks for almost anything, or at just a bit too hot you could make a little lower bainite, which will not be the martensite you are shooting for. In this cooling all the phases you could make other than martensite are, in part, diffusion dependant, which means time to make them increases exponentially with the lowering of temperature. With 1095 at 800F you have around 3.5 seconds before you start losing austenite to another phase if cooling is stopped. At 600F you have around a minute and forty seconds to avoid making anything else when holding. At 450F you have around 10 minutes to avoid making lower bainite after all cooling ahs stopped. But at 425F martensite will instantly begin to form.
So if you shoot for a temperature around 430F and then pull the blade out of the quenchant things should work out well. But how the heck do you know if you got near this temperature? You could go with the inferred thermometer trick that has been mentioned or you could use some convenient indicators in the oil. When all the hissing and rolling subsides in the oil you count a second or two and pull out the blade. Most oils have a vapor point right around 400F so you observe the oil coated blade. If it is lightly coated with oil and faint wisps of vapor come off of it you are probably in the neighborhood. I it bursts into flame you were WAY too hot, if the blade is dry and smoking you are too hot and if the oils is covering the blade with no vapor at all you may be too cool. But you can also use the magnet, and this time it will be even more accurate than at the high heat. Above Ms the blade will be austenite which is non magnetic still, as martensite begins to form the magnet will gain more and more power over it.
With 1095 at around 315F you will have 50% martensite formed, at 235F you will have around 90%. This means that if you see a kink or bend that you don’t want at 300F or better you can easily straighten it because half the blade could still be soft austenite. It also means that as much as 50% of your martensite will exist at temperatures capable of giving a quick snap temper to help stabilize things.
