Material Selection Challenge
Material Selection Challenge
(OP)
My product application is in the fitness industry. We have working prototypes but need to define our material selection for production quantities. The product requires an octagon shape bar that is 1.0" flat to flat. (4) of the (8) sides have a 0.9" deep by 0.35" wide channel machined into them. The overall bar length is 62.0". The flats will have roller bearings riding on them so surface hardness is an issue. Some small slots and holes are machined into each end of the bar as well.
Product requirements are:
Corrosion - Indoor use in non-industrial setting (ie gym). Product must look high quality and not rust. Surface to be electropolished (or other?) for cosmetic purposes.
Tolerence - Flat to flat dimension must hold +0.0/-0.005", overall bar flatness not to exceed 1/16" over 5ft.
Tensile - 150,000psi desired, slightly less may be acceptable. The stronger the better for safety factor.
Hardness - Bearing wear is an issue. SS 303 on prototypes not acceptable. Desire increased surface hardness.
Cost - Must be minimized, as always.
Our prototypes have been fully machined out of SS303. We need to upgrade the material for tensile strength and harness purposes. I have found one supplier that can cold draw my finished cross-section to my requirements in 303 (work hardened to 100,000psi). We need a higher tensile though.
What SS materials would be most cost effective to meet my requirements? What tempering would I need, if any? What process would be the most desirable?
- A square bar fully machined to shape, then harden, then electropolish.
- Draw an octagon cross section bar, then finish machine, then harden, then electropolish.
- Fully drawn cross section to finished shape with machined end slots only, then electropolish.
All input & ideas greatly appreciated!
Product requirements are:
Corrosion - Indoor use in non-industrial setting (ie gym). Product must look high quality and not rust. Surface to be electropolished (or other?) for cosmetic purposes.
Tolerence - Flat to flat dimension must hold +0.0/-0.005", overall bar flatness not to exceed 1/16" over 5ft.
Tensile - 150,000psi desired, slightly less may be acceptable. The stronger the better for safety factor.
Hardness - Bearing wear is an issue. SS 303 on prototypes not acceptable. Desire increased surface hardness.
Cost - Must be minimized, as always.
Our prototypes have been fully machined out of SS303. We need to upgrade the material for tensile strength and harness purposes. I have found one supplier that can cold draw my finished cross-section to my requirements in 303 (work hardened to 100,000psi). We need a higher tensile though.
What SS materials would be most cost effective to meet my requirements? What tempering would I need, if any? What process would be the most desirable?
- A square bar fully machined to shape, then harden, then electropolish.
- Draw an octagon cross section bar, then finish machine, then harden, then electropolish.
- Fully drawn cross section to finished shape with machined end slots only, then electropolish.
All input & ideas greatly appreciated!





RE: Material Selection Challenge
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Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm
RE: Material Selection Challenge
RE: Material Selection Challenge
RE: Material Selection Challenge
Arunmrao - what is "CI"?
Any thoughts on material and process (ie temper) if I went to a square bar fully machined to my shape? This is how I made prototypes but it was very expensive.
RE: Material Selection Challenge
RE: Material Selection Challenge
RE: Material Selection Challenge
RE: Material Selection Challenge
RE: Material Selection Challenge
RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM
RE: Material Selection Challenge
What do the the wizards think?
Good to have the good old Rock Island Arsenal on board! I like 201, too.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Material Selection Challenge
RBA
RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM
RE: Material Selection Challenge
"The beauty of the 17-4 is that it is age hardenable which means we can slowly heat it to 900F, hold it at heat for an hour and then slow cool it back to room temperature which keeps it nice and straight and doesn't mess up your dimensions. Unfortunately, all the plain carbon and alloy steels don't get hardened that way. Those materials require us to heat them up to at least 1500F, hold at heat to soak out the material, and then rapidly quench them in a cold liquid like oil, water, or salt. The higher temperatures along with the rapid quenching will make your bars warp and distort far more than the 17-4. The flatness that you call out would be very tough to hold and we'd probably have to manually straighten (bend) the parts back to form using a hydraulic press. Not to mention, they would come back to you very dark and sooty looking. It's also probably more expensive from a heat treat standpoint too. I'm not sure if that outweighs the base material cost or not. You also lose all the corrosion resistance with those steels and would have to consider plating or painting them."
I like the idea of cold drawing to my shape to ensure a consistent part dimensionally but I have yet to define a material that gets me to 150ksi and 38Rc or above. I have also been able to only find one domestic supplier that will even talk to me about drawing this shape and he has been slow to respond. Most mills like the standard stuff!
All ideas/leads welcomed.......
RE: Material Selection Challenge
17-4 PH is fundamentally an air hardening martensitic stainless steel, as are all the PH grades except A-286($$).It just gets additional strengthening from copper precipitation. The hardness doesn't do much for wear resistance when acquired via precipitation.
410, 416,420, 431, etc. are also air hardening martensitic stainless steels. No fundamental reason for one to distort more than the other. The tempering of martensitics is below 900F.
The difference in corrosion resistance between 17% Cr and 12% or 16% martensitics in pretty small for indoor use. Nearly all cutlery is 410 or 420 and does fine.
You may be over-engineering the part with 17-4 PH. See if the price you can get 416, 420, or 431, or even 410 for, is enough savings to go up that learning curve.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Material Selection Challenge
RE: Material Selection Challenge
Distortion comes from non-uniform cooling. If one area transforms to martensite (in doing so it expands 4% by volume) while another area is still soft austenite, the austenite deforms. Slow uniform cooling helps prevent distortion and the high chromium level of 416 permits slow cooling while keeping full martensitic end product.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Material Selection Challenge