PHT stands for Peters heat treat, and I think the delta treated 3V will be marked by Survive as 3V∆ as shown on the GSO 7/7 photo on Survives website.
I've only been following Survive since the summer when I stumbled on to the Survive forum after following Nathan's progress updates on his 3V HT tweaks, so take the following with a grain of salt. Maybe one of the folks that have been following Survive longer than I have can chime in.
My understanding is that Survive switched to a 3V low temperature HT protocol a year or so ago as opposed to the then-standard high-temper 3V protocol. The low temp protocol offered slightly better edge retention and corrosion resistance as compared to the former industry-standard high-temp protocol. Nathan Carothers, Dan Keffeler and Guy/Survive collaborated in developing the new delta 3V when they discovered some edge stability irregularities with the low temp protocol, and the delta treatment refers to a tweak to the low temp protocol that offers slightly better edge stability over low temp protocol. According to what I recall from a few of Nathan's posts, the delta treatment requires more steps and costs more than the former low temp protocol, and Nathan was able to use it first on a smaller scale as Peters HT ran into some issues implementing the new delta protocol at a larger scale than Nathan was using. Those issues were overcome and the 4.1 is the first Survive model implementing the delta protocol, and all 3V models being heat treated after the 4.1 will have the delta treatment.
All that said, my understanding is that the former low-temp protocol is still excellent. The new spec GSO 5.1 and 4.7 have the low-temp 3V protocol. The new spec 4.1 and later GSO models will all have the delta 3V protocol. I'm not sure if the older GSO 5, 6, 10, etc. have the old high temp protocol or the initial low temp protocol though.
Well stated!!! I can comment on the original heat treat and the interim heat treat on the new spec 5.1. Long story short the new heat treat is better and a little easier to sharpen. I believe, and please correct me if I am wrong, all the 3v coming out prior to the 4.1 is what I am calling the interim heat treat, that is to say a different heat treat from the original. As to other differences between the original 5.1 and the current 5.1 here are my observations:
new finish
lengthened the gimping on the spine of the knife and it is less rounded (I like the old jimping better, it's softer and doesn't bite my old hands)
the ingraving is inhouse and slightly different
new fasteners are new & now come with a tool to remove them
the sheath is longer and sits farther up on the handle
they increased the size of the opening at the butt for the lanyard
the handles have been lengthened as well as contoured making the new 5.1 longer than the old one without lengthening the blade or the choil & without adding much more weight
the new handles have more palm swell than the old handles if you look at them from above or below
the new knife has a slightly decreased width from .19 to .185 - probably the reason for the minor increase in weight
the spine of the new knife is at a 90 degree angle and you can strike a ferro rod with it to generate sparks. The old spine was rounded and could not be used to strike sparks. (I like the old spine better, I use of 2 hand techniques when I carve - tough on my thumbs)
the steel has a new and better heat treat with the addition of being a little easier to sharpen - as stated above
the blade edge below the secondary bevel is longer to make it a better carver
no longer charge for ferro notch or the bow divot (I believe)
the one off custom new 5.1 has more of a spear point the old 5.1 had more of a drop point - the new and the original 5.1 have the same drop point
they offer a new handle treatment, "micro texturing", which makes their handles more tacky. In the case of the canvas micarta scales they are very gripy even when they are wet
In addition there are significant changes to the new 4.1 and 7/7 as well.
Not sure if this helps but to make a long story even longer, old spec = original heat treat, first wave of the new 5.1 and 4.7 3v knives received an improved heat treat or second generation heat treat, I believe the "delta" gets the 3rd iteration heat treat. Just to make matters even more complicated I believe they've also changed the heat treat for their new 20cv knives as well. My new 5.1 in 20cv holds an edge much better than my first generation 5.1 in 20cv.
