Hey Robert,
Date: Thu, 21 Nov 2013 21:29:19 -0600 From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx> Subject: [AR] Re: Project update - propellant tank hydro tests I was going to use a stainless steel snap ring inside the LOX tank. But I hadn't thought about the screws on the outside of the tank becoming brittle. I assumed they would just get stronger as they're the normal alloy cap screws (180 ksi typ). Anybody have data on or practical use of alloy steel fasteners at cryo temps? I could certainly use stainless since the load on the washer that holds in the seal isn't very much. The main tensile force imparted to those screws isn't due to the tank pressure but instead due to the tank deformation. I selected the thickness of the seal retaining washer specifically to be thin enough to allow it to deform and not rip out the #4 screws and yet be thick enough to hold in the seal under pressure.
I've done some reading up on the steel-at-low-temperature-problem recently (Canadian/Russian winter, not cryo).
TLDR: I'd only trust austenitic stainless (304 or 316) and possibly Aluminium fasteners at Lox temperatures.
Roughly speaking, Carbon steel will have a brittle-transition between 0 and -20 deg C. Alloy steels can do better, with brittle transitions below -80 deg C.
The main factors seem to be: - Alloyed elements: Nickel good, Carbon bad. - Phase: Austenitic good, Martensitic ok, Pearlite bad. - Grainsize: Small is good, big is bad.A good resource for your case would be "Low temperature metals" by A. Hurlich, a copy can be found here:
<http://www.bnl.gov/magnets/staff/gupta/Summer1968/0311.pdf> Cheers, Michael