Originally Posted by FEENIX
I agree! To note that the material is harder to machine is OK but to make such a bold statement/recommendation not to use it is another story.
Obviously, other gunsmith/rifle builders are not having the same problem, checkout this thread from another forum >>> Lothar Walther barrels
, esp. #14.
I've worked with many forms of stainless steel in the past, plus about every form of 4xxx steel they sell. Add to that much of the air hardening catalog. With the exception of Ferritic and Graphmo, Hestalloy, and Waspalloy, these are about as tough to machine as you normally encounter. I think MAR-10 machines slightly better than 17ph-4 because the chips come off the metal easier, and all the heat seems to end up in the chip. None of these steels like light cuts. The insert you use is the trick, and the metal dosn't like you to push the metal off the surface. It wants you to cut it off. Cutting processes are closely guarded, and even the coolants used are secretive. Now I can't remember billing figures for 17ph-4 or 18-5, but MAR-10 is very expensive steel. Last time I bought two rounds of it (5" x 72") I paid $6500! The advantages of 17ph-4 and 18-5 (have not worked enough with 15-5 to base an opinion) are that it's about 99.9% corrosion resistent. Seems to handle heat better than the typical 410 / 416 / 420 / 440c SS steels. Plus the are far less prone to stress cracking. They do not rust in de ionized water, while the other four will be coated with rust in less than a week.
To machine these steels, the spindle group needs to be over built. They will ruin a set of Bridgeport bearings in less than a week. In a lathe, they are about all the average Okuma can handle (the larger models do much better). A Monarch hydraulic will cut them very well if you keep the chips from stringing. Machine operators hate the stuff, because they are always getting burnt. (you,ve had no fun till a large chip lands in your lap!!!!) They are a real pain to grind, but seem to hone well with diamond stones. Dosn't like to be gun drilled! I never did quite figure out a good grind for the drills, and you need much more coolant pressure that with regular steels. EDM processes are easy with the stuff.
But as negative as I posted about these exotic stainless steels, they are easy when compared to Hestalloy!!!! That stuff is a constant learning curve. The scrap rate is always high.
with the exception of MAR-10 (the samething as 17ph-4 but in pre treat form), the real issue with them is that they are so tough to machine. Probably due to a much higher nickel content. Mar-10 comes it at about 30RC, and don't remember the others being that hard.