Straight-gauge stainless tubing?

General Discussion
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I’ve been interested in building more with stainless steel tubes as it’s slightly cheaper than Ti for 3DP parts, but I’m not entirely sure how to go about sourcing straight-gauge tubing for custom bent chainstays. It looks like KVA sells a few straight-gauge variants, but at a premium. Ideally I’d find some OD = Ø3/4" (Ø19.1mm). Would 304 or 316 be reasonable for this application?

For comparison:

  • SS 304 has a UTS of 505MPa, yield of 215MPa, and elastic modulus of 200GPa
  • SS 316 has a UTS of 580MPa, yield of 290MPa, and elastic modulus of 193GPa
  • 4130 has a UTS of 670MPa, yield of 435MPa, and elastic modulus of 205GPa

My gut says that the lower yield strength would mean it’s feasible but perhaps using a thicker wall like 0.049-0.065" (1.24-1.65mm)?

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The legend Rob: 316 tube for a seatstay - bad idea? - #8 by IvanV

IMO, I have never seen an SS or CS buckle. Has anyone? Based on that, I think its pretty safe. The front triangle is a different story.

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I am going to build a tandem to ride with my wife, so I have put a lot of time into finding the best solution for straight gauge SS. I live in China, and can not get thin wall Cromo; heavy wall, no problem. I built close to 100 frames in the 8-90s, and have a decent college education in metallurgy.

I believe 304 is a better choice than 316. However, I will be using 201. The mechanical properties are very close to Cromo.

I can get, very cheaply, seamed tubing in wall thicknesses of 0.3, 0.5, 0.8, and 1.0 mm, and thicker. They are sold for “decorative” and structural use. I don’t care that there is a seam. Flat sheet is rolled and welded to make a tube. The huge advantage of this is that 201 work hardens very quickly, so the manufacturing process creates a tube with very good tensile strength.

The tubes I have bought ring like a bell, which indicate high hardness, but I wanted confirmation, so I bought some Rockwell C test coupons to do scratch tests to test hardness. Steel hardness and tensile strength are very linear. Scraps of other steel can also be used in the same way.

Here are some specs on 201 stainless:

ASME SA213 TP201 Tube Mechanical Properties

                              Yield (KSI)          Tensile    % Elongation      Rockwell Hardness

SS 201 Annealed 38 min. 75 min. 40% min. HRB 95 max.
SS 201 ¼ Hard 75 min. 125 min. 25.0 min. 25 – 32 HRC (typical)
SS 201 ½ Hard 110 min. 150 min. 18.0 min. 32 – 37 HRC (typical)
SS 201 ¾ Hard 135 min. 175 min. 12.0 min. 37 – 41 HRC (typical)
SS 201 Full Hard 145 min. 185 min. 9.0 min. 41 – 46 HRC (typical)

It is from this website:

I don’t know what your sources are where you live, but this is my solution.

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