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Since you are not getting consistent results it would seem that there is a problem of some sort. The best way to get consistent results is to do the process exactly the same way each time - in this case that means being as repeatable as possible with temperatures each time. So, what process/tools/technique do we have available that might help accomplish this goal?
You have your pipe, good. You are catching the temp at non-mag on the way up, good. You put the steel back for 20 seconds after non-mag, good (since you say you're getting fine grain but might not hurt to try a shorter time like 10 seconds to see what difference you get). You pull the steel from the quench after 15 seconds, good for the fact you are consistent with the time but your results are not what they should be. That is too short a time, might work some times but might not be enough another time (which happens to be consistent with the type of results you're getting). 1084 is not one of the steels recommended for an interrupted quench which is what you're doing. Let it stay in the oil until it is dead cold or barely warm. That may not solve all your problems but it should provide more consistent results. One step at a time .
Something else just occurred to me: you grab the coupon with tongs. That has to be sinking a lot of heat. My guess is that you also are holding the coupon with tongs when it gets quenched, bad idea. Coupons are small, tongs have lots of mass, screws up the heat. Drill a hole in the coupon and put a loop of baling wire through it, handle it only by the wire. I'd almost bet money that will improve the consistency of your results dramatically ...
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