Date: Mon, 24 Aug 1998 09:15:19 -0400
From: Dave Hepner
More than you ever wanted to know about stainless steels:
301 Fe; Cr 16-18, Ni 6-8, C 0.08-.02, Mn 2.0 max, Si 1 maxand so on. Then there are the subtypes such as 304L and 316L, which stand for low carbon grades.
302 Fe; Cr 17-19, Ni 8-10, C 0.08-02, Mn 2.0 max, Si 1 max
303 Fe; Cr 17-19, Ni 8-10, C 0.08-0.02,Mn 2.0, Si 2-3,
304 Fe; Cr 18-20, Ni 8-10, C 0.08 max, Mn 2.0 max, Si 1 max
308 Fe; Cr 19-21, Ni 10-12, C 0.08 max, Mn 2 max, Si 1 max
309 Fe; Cr 22-24, Ni 12-15, C 0.20 max, Mn 2 max, Si 1 max
310 Fe; Cr 24-26, Ni 19-22, C 0.25 max, Mn 2.5 max, Si 1 max
316 Fe; Cr 16-18, Ni 10-14, C 0.10 max, Mo 1.75-2.5, Mn 2 max, Si 1 max403 Fe; Cr 11.5-13, C 0.15 max, Mn 1 max
405 Fe; Cr 11.5-13.5, C0.08, Al 0.10-0.30
416 Fe; Cr 12-14, C 0.15 max, P or S or Se 0.07 min, Zr or Mo 0.60 max
Generally speaking, the 300 series of stainless steels are non magnetic, while the 400 series are mildly magnetic. Notice that the 400 series contains no nickel. Commercial fasteners are generally made of either 316 or 18-8 grades of stainless. The 316 is non magnetic, The 18-8 is magnetic. Types 304 and 316 are the most common grades of stainless steel. 316 is also more brittle and is a pain to machine. Parts that require machining are usually made of 18-8 or 304. Sorry, I don't have the composition of 18-8 handy, but I am certain that Professor Falco has it tucked away somewhere.
David Hepner
Date: Mon, 24 Aug 1998 10:50:38 -0700
From: Charles Falco
I haven't got time right now for an extended discourse on stainless steels. However, a few relevant facts are that the general difference between the 300 and 400 series are the latter contains no nickel (431 is an exception, containing 2% Ni). Tensile strengths--the max stress before the steel finally snaps--vary greatly, from 127,000 psi (127 kpsi) for type 431, to 71 kpsi for 405. Type 316 is fairly common, with a tensile strength of 85 kpsi and a yield strength--the stress that causes irreversible stretching of the metal--of 43 kpsi. These are a bit lower than good quality non-stainless steels (but factors of >2x lower than ultra-high strength, heat-treated steels). However, there are stainless steels with low contents of expensive components (like Cr) that have yield strengths as low as 25 kpsi.
Let's say we want to make a brake stay out of a 1/2"-wide piece of 1/16"-thick metal. That stay will have a cross-sectional area of 0.0313 in^2 (although, see below for more on this point). If we used a 25 kpsi stainless to make the stay, it would irreversibly stretch when a stress of 25,000 pounds/in^2 x 0.0313 in^2 = 781 pounds were applied to it (and snap at 66 kpsi x 0.313 in^2 = ~2000 lbs). As can be imagined, slowing a ~600 lb. bike + rider puts one dangerously close to disaster with this stay. However, it's worse than this, since the ends of that stay will be pierced by 1/4" or 5/16" holes in order to bolt it to the backing plate and to the frame. Assuming the smaller of these holes, that still leaves only 1/4" of material in the cross-section of the stay, thus cutting the yield and breaking points in half--i.e., to ~390 lbs. and ~1000 lbs.
Charles Falco