WD-40 won't work anyways. PB Blaster stands a chance. Useful stuff if you wrench on your own car as well.
Otherwise agreed try harder, LOL. And temp changes. Cold will pull them slightly apart from each other. Heat will actually press them into each other, it doesn't sound helpful and isn't right away, but if it was hot enough to slightly deform the metal, then when cold again it'll be very helpful indeed to have done it. Maybe do heat, then freeze, then cool and try again.
or make your own penetrating oil with mineral oil and acetone in a small spray bottle to keep it food grade.
If you really can't get a box end wrench on a hex nipple and if for some reason you can't use a pipe wrench, then get a pipe nipple extractor (works like a screw extractor for broken bolts) or an internal pipe wrench (similar to an extractor, but it has a cam rather than reverse screw threads).
I'm assuming these parts are SST. That being said, SST pipe fittings are notorious for galling when not assembled with an appropriate coating such as loctite 567 pipe dope or Nickel-impregnated teflon tape or both. Having worked in the nuclear power industry for thirty years where SST pipe and tubing are used extensively I have had a few instances where the galling was so bad that it was impossible to get the fittings apart without destroying one or the other or both. You might try tightening them a little first then spray with CRC Food Grade Penetrating Oil as you back it out. Good luck
Take the valve off what it's on (assuming you can) and put the valve into a bench vise in order to hold it more securely. Then either put a wrench on the hex nipple (the ones I have use a 7/8" wrench, which I have at least a few of) and turn it. Remember, righty tighty, lefty loosy. I've done this when I couldn't move a fitting when on my keggle (or other item) with solid results. Deep sockets (on a ratchet) can also help. More if you have some right sized metal pipe that you can slip over the handle. Or use a breaker bar (doesn't everyone have at least one of those??). Only 'lube' that should be needed is "elbow grease".
Even with galling (if you applied enough pipe tape to the fitting it won't gall) you should be able to remove it with enough force. Of course, one (or both) of the items might have ruined threads by then. Just use it as a learning experience on what NOT to do.
IME, with stainless fittings, even a moderate amount of Teflon pipe tape (found in most hardware stores) is sufficient for keeping fittings healthy and preventing leaks. We're not subjecting things to stresses associated with the nuclear power industry here. At most, we're talking about boiling liquid temps (~212F) and maybe radiant heat from gas burners.
The other month I went through a lot of the fittings on my keggles and plate chiller and changed over the male fittings (from cam lock to ball lock). Some were on rather securely. I had my helper (aka "Brew Bitch" or "Manual Labor") help by holding the keggles so they wouldn't tile as I used wrenches. I was able to remove even the most secure fittings this way.
IF you do end up using penetrating oil (of any kind) I would make damned sure to clean all traces off of the items you're going to use again. If the ball valve is a two, or three, piece, take it apart and clean it completely. I'd even soak in either denatured alcohol or acetone to remove all traces.
Personally, if you're at that point through, I'd just toss the set and replace. IMO, it's simply not worth dealing with things like that either now, or in the future. Especially if the threads have ANY damage to them (from going gorilla when tightening them). When I've tightened fittings, it usually no more than 1/4 turn past snug. Check for leaks and adjust if needed. 99.95% of any leaks I've had were through vessel walls, not at fitting to fitting mating. I'm actually slowly progressing through my brewing vessels to go from no-weld, to welded configuration. Helps when you know a [TIG] welder that works for beer.
Yes, they are SST. I did wrap the fitting with teflon tape. I guess I way overtightened the fitting. I just ordered another ball valve in the meantime. I will keep fussing with this at my leisure. Thanks for all the suggestions.
If it doesn't have teflon in it, not the tape, the ball seat, I'd try heat/torch. Some automotive bolts that is the only answer, other than going back in time and adding anti-seize.
Public service announcement- if it has been 5 years since you've had your alternator off, pull that long bolt and grease it up.
Or overnight in a freezer, pull it out and hit the outside of it with some hot water, that should work and be the least destructive
I would try soaking the whole thing in a can of coke overnight; I've heard that works about as good as PB Blaster, and wouldn't be as much worry to clean.
Side note, as someone who has been building fittings, hoses, and chasing leaks in high pressure fuel, air, and hydraulics for a few decades professionally, I've seen the misuse of teflon tape cause more leaks than prevent. People see it on the shelf and think it is a panacea for fittings. It's best use is for tapered pipe threads. Not compression fittings, jam nuts etc.
So I'm putting together my keg system and taps right now and I see a lot of fittings are metal on metal. I've always used Teflon tape (plumbers tape) in such instances, but wanted to see what everyone on here does before I go putting Teflon tape on all my threads.
I acknowledge that I know less than basic info about all this, it's literally all new to me.
I've freed several stuck nipples (non-hex) by double-nutting the exposed threaded end and using that as the "hex" to get a wrench on. Putting the valve in a vise would make this process easier as you could focus on the nipple without having to hold the valve at the same time. Food grade mineral oil is all that I would use. Maybe with some acetone as that flashes off without leaving a residue (that I am aware of).
That's fine, we're all learning. Metal to Metal= thread tape just shows what I am talking about.
If you can see it, below on the left is a hex nipple hose fitting. On the left of that is a male tapered thread. It is designed to seal on the threads. Sealing water (in your case beer?), air, nitrogen, thread tape is fine. It provides some galling protection and can give some adjustment room. On the right of the same fitting is a straight pipe thread. It is NOT designed to seal on the threads. You can see it better on the top fitting the shiny taper inside; that is where it seals, against another fitting or against a compression ferrule (a hollow sleeve that looks like a wide wedding ring). Using tape on the right side is not fine. You need to be able to feel the fittings tighten, and just get them past snug. Tape on the threads leads to people cross threading, under tightening, or overtightening them. They cannot feel the fittings tighten because they are tightening against the wad of tape they wound on there. If there is not a good, straight, parallel joining at that taper of the fittings themselves, the joint has already failed and no amount of tape will do anything.
I'm not sure what kind of fitting you have, but some NPS fittings will be machined for a gasket or o ring, if they are designed to hold any pressure at all. The second pic to the right is what I am talking about. That is a straight NPS nipple on one side, and an AN flare fitting on the other. Tape should not be used on either side. On the nipple, that's what that aluminum crush washer is for, and on the flare side, that's what the 37 degree angle is for. If you're hoping for the threads, or anything on the threads to seal anything, it's already failed.