Well I wear my armor all the time because I have stood up like a lightening rod a feww too many times on some of these issues. I, think it is obvious by now that I agree that if you do it right the first time you can eliminate the risks of the subsequent hardenings
if you know what steel you are working with. You have mentioned 5160, while chromium is not as greedy with carbon as other elements, there is only .55 to .60% carbon to play with there so you will wnat to free up as much as possible. 1525F-1550F should be your target temp for that alloy and if you can't nail that temp for required ammount of time, then you may find the blade a bit harder with finer grain by quenching it a second time, or perhaps even a third, but as has been pointed out you will also increase your chances of both distortion and decarburization with every subsequent heat.
If you were to switch to a simpler steel to match more basic tools your success would increase, with no decrease in performance. 1080 or 1084 would reach desired temp as soon as the magnet quits sticking and the soak time would not be as critical.
So far all I have observed in the multiple quenching thing tells me is that folks are improvising to make basic tools work with steel that require more advanced equipment, but since they are getting the good end results I must applaud them for a clever way around this dilemma.
Now for both my armor and my body guards, if taking on triple quenching will get you a beating, taking on a mans quenchant I have found will start WWIII. I am surprized you are getting distortion at all in the bacon fat. For the sake of peace, I am not going to dwell on that particular medium, but instead let's discuss how quenching works.
When you heat a piece of steel it will indeed expand until it reaches a red color and then there will be a dulling shadow effect before it can get brighter again. At this point there will be a a drastic contraction of the steel as it gets ready for your quench. The opposite can happen on the way back down, if the cooling is not quick enough the steel will expand at around 1100F -1000F., and if you cool the whole thing quick enough, that expansion will happen at around 400-450F and it will be very dramatic. If part of the blade doesn't quite make it while another does there will be some level of permenant distortion because somethings gotta give. On actual liquid edge quenches this is still true but the distortion can be so subtle on wide blades that is is barely noticeable.
Quenching is a very complicated matter, it is a WHOLE lot more that just tossing a piece of hot steel into any cooling medium and waiting for the blade to get hard, and I will debate anybody who thinks it is, because it is far too important for success to blow it off so easily. The two most significant ways for a meduim to cool your blade is convection and conduction. For convection to work, things must be fluid and even gas has more fluidity than a solid, so for this part of the equation you are better off with air than a solid or semi-solid. This is why viscosity is so important in quench oils- lower viscosity means increased convection, we heat our quench oils to 150F to help this as much as we can.
Conduction alone can do the job with some alloys, you have probably heard of folks that use quench plates with stainless steel. This is just a guess but organic fats may be more of an insulator than a metal plate would be, so it could really use the benefit of convection to pull the heated liquidified medium away from the blade, but then we have already eliminated the convection effect
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Industry has been working for years to come up with quenchants that have great convective and conductive qualities, proper viscosities (mostly very low), low vapor forming points and optimum thermal extraction rates. I may be wrong, perhaps some folks in the industry may have encountered exceptions they could share, but I believe the concept of Goop quenches with simple high carbon steel is fairly unique to bladesmithing.
Under the microsope I have observed plenty of fine pearlite in blades that would skate a file just fine (quenches that were farily quick and totally liquid), but would not hold the desired edge in the long run. But the blades would bend like a noodle in a vice, and as long as this is our top criteria I guess just about any cooling method we try will indeed work fine.
None of this is a criticism of you or your efforts Joe, since you came here looking for ways to improve the process, you seem to be headed in the right direction. Just be very careful to be willing to question everything you read (heck, including this post), and verify the inforamtion with other proven sources. Bladesmithing literature is riddled with information and techniques that has left the big boys in the steel working industry scratching their head, wincing, and sometimes snickering a bit.
Heat that 5160 up to 1525F. hold for couple of minutes if you can and then quench into some automatic transmission fluid and see how it turns out.
Did you folks see that - I said ATF, not Parks AAA! IT was hard, but I said it! So maybe the thrashing I am about to receive will be a little less violent. (I also thought that from goop to Parks AAA may be too large of a step as well
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I think you are right on with the 3/16 x1/8 before heat treat.
) and the flames were out. I looked up and saw several people watching me. I endured a few desered jibes about not having lunch ready and went back to work.