SS Bar Bending

[GTFiji]

I have an application where I am specifying a SS for a 1" bar ~5' long that will be subject to a bending load (i.e. supported on the ends and loaded in the middle). I am looking at 303 (drawn to 100,000psi), 416 H900 temper & 17-4 H900 temper. These materials are being evaluated based on corrosion, machinability, appearance, tensile strength & hardness for my application. What about bending? When I compare Modulus of elasticity's for each of these they are about the same in MPa.

Does this mean each bar will bend the same amount given identical loading below the yield strength?

 

[mcguire]

To a first approximation that's correct. Modulus is dependent on composition, not structure. The differences in composition are minor from a modulus point of view. The cold drawn bar, however,should be stress relieved to give a clear yield point. Without stress relieveing you'll get a little creep at room temp at less than the 0.2% offset yield.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[MikeHalloran]

Yes, E is very nearly the same for all the alloys you mention, and for all steels as well.

If deflection is important to you, as it seems to be, fatter bar (or tube) will work better, even in less expensive alloys.

If you could describe your application in more detail, you could get a _lot_ more help with it here.

Same charge as always.





Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

Just wanted to mention that there is no such thing as 416 H900. The H900 is an aging heat treatment and it is only for Precipitation hardening alloys such as 17-4PH, 15-5PH, etc. 416 is heat treat by a heating and then quenched and tempered just as low alloy steels such as 4340, 4230 etc.

The impact strength of 17-4PH, especially large size bar in the H900 and H925 conditions, may be very
low at subzero temperatures; consequently, the use of 17-4PH for critical applications at low temperatures
should be avoided.

 

[GTFiji]

Thanks much for the terrific feedback, it is very helpful. I have a high volume consumer product that I have only prototyped to date and I am trying to hone in on a material and manufacturing process. One of my selection criteria was resistance to bending as I described. Now that I understand that for a given cross-section most Stainless Steels will bend the same amout I can rule out "bending" as a selection criteria.

I am going to start a new thread stating all my requirments for this product as I need help with selecting a Stainless Steel, manufacturing process, and finishing process.

 

[EdStainless]

Since defelction is based on E and section modulus you will also find that heavy walled tubing is almost as stiff as bar. Roughly speaking a 1" x 0.200" tube will have about 87% of the stiffness of a 1" solid, but it only has 64% of the weight.

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

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

 

[GTFiji]

My cross section is a an octagon shape 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".

Is E a function of cross-sectional area? If the tube is that stiff there must be some other effect from the geometry. What could I do to my "shape" to make it stiffer? I thought increasing my tensile strength by going to different grades of stainless steel was the answer but I now know that is not the case!

 

[EdStainless]

pull your strength of materials book and look up calcualting the section modulus.
the section mod is based on geometry. Knowing that and the materials modulus you can figure out the actual stiffness.

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

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

 

[IFRs]

For this shape and length, have you looked at an aluminum extrusion? You will get the final shape from the mill, (no milling, grinding, etc) aluminum is not as stiff as steel but you can put the metal where you need it and remove it from where it is less useful. Do you have a maximum load and deflection to work towards?

 

[EdStainless]

The Al will be too soft and flex too much. He has built around an iron based material, It sounds like he is stuck.

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

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

 

[GTFiji]

Ed is correct. I would like a tensile strength of 150,000 psi if possible (see my "Material Selection Challenge" thread). I may be able to accept slightly less. I would love to use aluminum from a forming standpoint but I need the weight, hardness & tensile.

 

[MikeHalloran]

If the face channels are 0.9" deep, they have to meet each other. I hope you dropped a zero there.

If you need 150ksi tensile, you're pretty well screwed out of stainless, too.




Mike Halloran
Pembroke Pines, FL, USA

 

[GTFiji]

Yes, the channels are actually 0.09" deep, thanks.

I found spec. sheets for 416 & 17-4 PH on

http://www.alleghenyludlum.com

that both show 150 ksi plus for these tempered at 900 deg.

 

[unclesyd]

I don't know the exact end use but 17/4 PH SS @ H900 isn't a good choice, especially if people are involved. To use 17/4 PH SS I would not use it for an application like you propose unless it was heat treated at H1125.

 

[EdStainless]

I still like the idea of cold finished 201.

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

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

 

[MikeHalloran]

Must be a really high end consumer product, to afford a long exotic bar like that.



Mike Halloran
Pembroke Pines, FL, USA

 

[GTFiji]

unlesyd - Why not use 17/4 PH SS "if people are involved"? The bar will generally not be grabbed by peolple but occasional hand contact could occurr. Why H1125 instead?

The bar will be used for a high quality fitness product.

 

[unclesyd]

The reason for my recommendation is based on our experience with 17/4 Cond A and some recommendations by Armco after some catastrophic failures of same. Our failures were in shafts and special fasteners that were fabricated from 17/4 Cond A despite recommendations to the contrary. People were involved in one of the shaft failures and the recommendations from Armco, incorporated into our standards, were that 17/4 Cond A was not to be used in open machinery, such as ours, where personnel were in proximity.

By analysis we found that the published physical values for 17/4 Cond A were quite variable. We found early on that if your design required ductility and toughness you had to heat treat. The recommendation that H1125 heat treatment was put forward by Armco early on to maximize the physical properties for shafts and fasteners.

17/4 doesn’t machine or cold work very well in Cond. A

Personally I’ve never seen or heard of a recommendation to use 17/4 in Cond A in a dynamic component.

 

[EdStainless]

If you really need that strength level use 17-7 in the RH950 condition, but the TH1050 should be fine.
I just don't seet he reason to go to that expense with this product.
Either 201 or a lean duplex in the cold finished condition should be enough.

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

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

 

[GTFiji]

Thanks again for all the great input.

SS is chosen for it's high quality appearance and resistance to corrosion. The 3 original reasons I was looking to increase tensile of the bar above our SS 303 prototypes were as follows:

1) Less bending under load (I now know Modulus is the driving factor)

2) Increased surface hardness. I have 1/4" wide bearings riding over the flat portions of the bar. Bearing is SS 440c @ 58-65Rc. Each bearing is loaded up to 50 lbs ontothe bar. The 303 began lifting (small chips) quite severely well before 40,000 cycles.

3) This bar will be lifted during exercise. I must guard against fracture if awkwardly dropped onto the floor. It will take abuse. A large safety factor is prudent.

For these reasons I am targeting a 150,000psi tensile. I may be able to go under this but must run cycle testing and analysis to validate.

My thread has wondered a bit but I need help defining a cost effective material and process to produce bars in volume. (see Material Selection Challenge thread)

Thanks to all!!!!