US Navy all-stainless steel submarine program (AL-6X alloy)

Six percent molybdenum stainless steel proposed for hull alloy to defeat tropical-water pitting; molybdenum cost made it a "very pricey stainless steel." Linked back to the critical-pitting-temperature curve.

I told you the Navy was interested in making an all-stainless-steel submarine, and the alloy they were looking at was AL-6X, which is six percent molybdenum. Molybdenum is a pretty pricey alloy, so this was going to be a very pricey stainless steel. The reason: unless you stay in the tropics for a long time, you wouldn't have pitting problems with a very high molybdenum content, because you're going into a salt solution. So there is some corrosion on stainless steels that relates to the Navy.

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Mentioned alongside titanium hulls as Navy responses to magnetic-detection vulnerability.

I said they may be important for detecting magnetic fields, in what's called SQUID — superconducting quantum interference devices. The US Navy got very concerned in the late 80s that the Soviets would be able to put a SQUID in a satellite, and all of a sudden these big magnetic submarines would be magnetically visible in the whole ocean. They'd light up like a light bulb to a SQUID detector. At the time, you could fly an aircraft at 10,000 feet above the ocean with a SQUID detector and you could find the magnetic field signature of the steel submarines. The US Navy ever since has wanted to build a titanium submarine because it's non-ferromagnetic. They now want to build a stainless steel submarine because it's non-ferromagnetic. They want to get rid of this lousy ferromagnetic iron because superconductors can detect those submarines. The question is, could they do it from 100 miles in the air? With high-temperature superconductors it might have become possible. People were talking about magnetically levitated trains and all these wonderful things, and I said, you're not going to see magnetically levitated trains and big high-field magnets in our lifetimes or our grandchildren's lifetimes. There's the sound bite that someone will remember — we won't see it in our grandchildren's lifetimes.

Brief reference — AL-6 is named as the alloy the Navy wanted to use for an all-stainless submarine hull because of pitting resistance. Program abandoned ~2000–2005.

I'm going to catch up on a few things from the stainless steel lectures. This is molybdenum content. I told you to put molybdenum in stainless steel for corrosion resistance. This is the critical pitting temperature — at what temperature can you avoid pitting problems? Here's 316 stainless steel. 317 is kind of medical grade; the L versions help, because they form fewer carbides. AL-6 — this is what the Navy wants to make submarines out of. You can see why: the ocean can get pretty corrosive, and if you made it out of some other stainless you'd end up pitting your submarine. Most of them, they looked at it and they've given up. That was something of ten or fifteen years ago — the all stainless steel submarine. They've given up on the technology before that. They've been worrying about that for a while.

Brief aside — Navy ~10 years prior (so circa early 2000s) considered building super-austenitic stainless submarines for non-magnetic signature, then abandoned the program. Sets up the comparison to Chinese naval expansion.

Down here there's a little aside. If you go to 317, which has lots of molybdenum, you can add even more molybdenum, up to six percent. Now you're talking sixty-thousand-dollar-a-ton steel. Add more nickel, nitrogen, get some higher strength, and you get what they call the super austenitic stainless steels. The US Navy about 10 years ago was interested in building submarines out of this material. Why? Because they're not magnetic. The problem with magnetic submarines is, if you've got a superconducting quantum interference device up there in a satellite, you can see 100 feet down, and you can see this long magnetic hot dog in the ocean. But they finally gave up — our navy has more capital ships than all the rest of the world's navies combined. Except the Chinese are starting to try to catch up with us, sort of like what happened to Germany and France and Britain after World War One. So maybe we're heading for something else.