Material Selection Challenge 4

[GTFiji]

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!

 

[EdStainless]

Ask your supplier if he can cold draw 201. He should be able to get to cold draw bar to 120ksi or so.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[TVP]

As Ed mentioned, an austenitic grade like 201 can be work hardened to higher strength levels. The bar would not need to be subsequently hardened. The Cu and Mn alloying additions in Type 201 keep the cost down relative to Type 301 or 302. A lot depends on what your cold drawn bar supplier can produce.

 

[arunmrao]

Fitness equipments in India are manufactured in CI. They are then machined,powder coated and finished. Handle bars and few other parts are made in 304 grade

 

[GTFiji]

Thanks for the 201 option. I have found one supplier that would work with me on this. I will ask him to quote 201 and predict it's tensile after cold draw.

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.

 

[arunmrao]

Cast Iron

 

[mcheema]

Age hardening grade like Type 630 (according to ASTM A 564) can go upto 38 HRC in annealed cond (cond A). It is machinable and can be subsequently age hardened (cond H900) upto 40 HRC.

 

[tripleZ]

If you're looking for the surface hardness, why not mask the bar off and have the slot hard chrome plated? You should be able to get the apparent hardness, but I'm unsure of what the cyclical rate would do to the surface over time. Although, if you're looking for the surface finish obtainable through electropolishing (definitely a higher class gym than the weight pit I bodybuild in), you might be able to go with CI or some other cheap grade, thinly plate the bar and buff the heck out of it for the shine. Then you save on material costs as well. Just random thoughts on my end.

 

[IFRs]

Too bad you can't use extruded aluminum - you would get final shape from the mill.

 

[rayband]

It is probably a bit late to add to this thread, but GTFiji, have you made a decision and run any trials? If you tried the suggested hard drawn Type 201, I expect you would have much trouble holding the 1/16 in flatness spec after machining. While I am a SS advocate, I suspect that a serious cost/performance analysis would show a hardenable steel like 4340 with electroplated finish would be near the top.

RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM

 

[mcguire]

I was waiting for someone to recommend 416. It is cheap. It has enough corrosion resistance for a gym. You can machine it as annealed pretty easily. Then you can harden and temper. Since it will air harden you needn't have distortion.
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/

 

[rayband]

I had not thought of T416, but it would seem a great choise for GTFigi if cost & availability are OK.

RBA

RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM

 

[GTFiji]

I am still searching for the lowest cost/highest quality solution. My latest run consisted of 17-4ph bars machined and heat treated to H900. The results have been very good. The parts look good after electropolishing to remove the slight discoloration. The tolerences I mention are also held after heat treat and no more machining was required. The issue is cost. I am exploring taking the machining off-shore which I hate to do. I like the idea of 4340 heat treated and then plated but I am afraid the heat treat process of heating and quenching will distort the bar and drive further machining. I would also like to explore the lower cost 416 but my concern is the same, distortion after the high temp. quench. Here is a response from a heat treater that fueled my concern:

"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.......

 

[mcguire]

GTFiji
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/

 

[GTFiji]

I was looking on the Allegheny Ludlum website at the Technical Blue Sheet for 416. It lists the mechanicals for the material that was oil quenched from 1800 f and then tempered at various charted temperatures. The 900 f temper gave 170ksi tensile and 42 Rc. I am targeting 150ksi and Rc in upper 30s or better. Can I get there with just a temper (low temp heat and air cool)? I thought the quench from high temp is what increased the distortion. Can the 416 just be tempered at lower temps and air cooled only (ie no quench)?

 

[mcguire]

In the section size you're dealing with, an 1800F soak, and air cooling and low temperature tempering, 400F to 500F, will work fine.
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/

 

[Mech151]

Check out austemper to reduce distortion (or carboaustemper for surface hardness). Also, depending on the material, you could consider a case harden in the area of interest.