manufacture of beryl. copper spring for max life 5

[BobM2]

I'm bending a strip of berylium copper to be used as a contact arm in a switch. Bending it to get the 30 deg. bend would leave a residual tensile stress on the side that would see a tensile fatigue stress. How do people bend (manufacturing steps) strips to get maximum fatigue life?

 

[kenvlach]

What is the alloy and the temper?
Are you starting with finished (already age hardened) spring strip?

Ordinarily, wrought Be copper material is supplied in either the solutionized or solutionized plus cold worked condition. Then, after the customer forms to final shape, the material is age hardened at 315 to as high as 480^oC, depending upon alloy.

"C17200 and C17300 are used in parts that are subject to severe forming conditions but require high strength, anelasticity, and fatigue and creep resistance..."
---Metals Handbook, 9th Edn.,vol. 2, ...Nonferrous..., p. 303.
Table 31, p. 304, gives Hardness, conductivity and fatigues strengths for C17200 & C17300 in various tempers. Highest hardness, conductivity and fatigue strength (285-315 MPa)are all found for the TH04 temper: solutionized, cold worked to full hard, and age hardened to HRC 40-45.

 

[BobM2]

Right now I've purchased some 17410 half hard, mostly because it was readily available in small quantities. Would further heat treating after forming help relieve the residual tensile stesses?

 

[TVP]

Yes, stress relieving after forming is a good idea for springs. Unfortunately, I do not frequently work with copper alloys, so I don't know the stress relieving temperature to use. It must be lower than the aging temperature.

 

[israelkk]

First, since the reat treatment temperature is higher than the stress relieve temperature it will do both: stress releive and heat treat. But, you may end up with a different bending angle because of the stress relieve.

However, there may be another way. If you can stay with the half hard strength you can do the following: first bend the spring for a higher bending angle, let say 50 degrees, then reverse the bend in such amount (let say ~-30 degrees) such that after both actions the spring will end uo with the 30 degrees you intended to. This way you will have the compressive stresses in the side that see the tensile fatigue stress. This is a tricky process and you may have to experiment to find the correct two angles (the 50 degrees and the -30 degrees are just for the sake of the process explanation).

There is a way to calculate how much maximum residual compressive stress can be achieved in the final product but it is beyond this forum scope.

 

[kenvlach]

The reverse bending method of residual stress reduction described by israelkk is described and illustrated in Brush Wellman documents; see links below.

Further heat treatment will reduce residual stress, but it has drawbacks and may not be necessary. As additional heat treatment has some cost and risks (surface and internal oxidation w/o sufficiently protective furnace atmosphere, possible warping, reduced properties due to overaging), some further consideration may be useful. Also, the surface of mill-hardened strip is thoroughly cleaned and inhibited before delivery, so this surface treatment should be repeated after any additional heat treatment.

The need for stress relief depends upon 3 basic factors:
1) Initial material condition,
2) Nature and extent of deformation, and
3) Application load & cycles.

Initial material condition. The initial condition of the C17410 strip is TH02 temper (˜ Brush Wellman ½ HM), i.e., cold worked to 50% hard + age-hardened. This is the only ½ hard temper per ASTM B768, and all alloy 174 produced by Brush Wellman – apparently the sole supplier – is mill hardened. Properties are given in ASTM B768 and the MatWeb & BrushWellman websites. Since the material was already age-hardened (unlike Be copper alloys normally supplied per ASTM B194), this eliminates the concomitant stress relief occurring during subsequent age-hardening.
Mechanical properties of Brush Wellman Beryllium Copper Alloy 174 Strip; 1/2HT (TH02) Temper (UNS C17410) from

http://www.matweb.com:

Mechanical Properties Metric English Comments
Hardness, Rockwell B 89-98 89-98 (Rockwell 30T: 76-81)
Hardness, Vickers 180-230 180-230
Tensile Strength, Ultimate 655-790 MPa 95000-115000 psi
Tensile Strength, Yield 550-690 MPa 79800-100000 psi
Elongation at Break 10-20 %
Modulus of Elasticity 138 GPa 20000 ksi
Fatigue Strength 280-310 MPa 40600-45000 psi
(Reverse Bending (R=1); 10^8 cycles)

Nature and extent of deformation. Depends a lot on the radius of the bend and whether it was in the longitudinal grain direction (not transverse).

In general, with mill-hardened Be copper, “further age hardening is not required. However, it may be desirable to stress relieve the parts to remove residual stresses induced during fabrication. This treatment is particularly desirable for highly formed cantilever-type springs and intricate machined shapes that require maximum resistance to relaxation at moderately elevated temperatures.” – Metals Handbook, 9th Edn., Vol.2…Nonferrous…, p. 256.

However, “In applications not requiring severe forming, mill-hardened copper beryllium (Brush Alloys 165, 190, 290, 3, 10, 171 and 174) can be extremely cost effective. These materials are heat treated by Brush Wellman to deliver maximum formability at desired strength levels. Since millhardened strip requires no additional cleaning or heat treating after forming, manufacturing costs can be effectively reduced.” -- Formability of Copper Beryllium Strip

http://www.brushwellman.com/alloy/tech_lit/at008_0295.pdf

This gives formability data, as does Guide To Copper Beryllium

http://www.brushwellman.com/alloy/tech_lit/GuideToCopperBeryllium.pdf

and Cumulative Stress and the Bauschinger Effect

http://www.brushwellman.com/alloy/tech_lit/oct99.pdf

These documents show ‘good’ and ‘poor’ bends. A good bend is in the direction of rolling of the strip (not transverse), with at least the minimum specified radius, and with the design load in the same direction. A ‘poor’ bend is in the tranverse direction and/or with design load opposed to the bending stress.

A simple mechanical procedure is given to reduce residual stress (for ‘good’ geometries): Bend to a greater angle then desired, then bend back to the desired angle. This will reduce the residual tensile and compressive stresses on the 2 sides of the strip. I would recommend doing this with care to avoid kinking, as the bent material will have work hardened.

Application. What is the design load and the maximum number of switching cycles? If your load and cycles are well below the fatigue data given above, the need for stress relief is of course reduced. See also the Flexure Fatigue Strength plot (an S-N band) for high conductivity alloys given in Guide To Copper Beryllium.

Should you wish to conduct a stress relief, probably only a partial relief (as at aging temperatures) rather than a full relief at high (solutionizing) temperature is advisable. Heat treatment temperatures for C17410 seem to be proprietary to Brush Wellman. Limited data are given in Heat Treating Copper Beryllium

http://www.brushwellman.com/alloy/tech_lit/at0015_0295.pdf

This does not give age-hardening data for C17410 (alloy 174), as it is only supplied in the mill hardened condition. From data for the other low-Be, wrought high conductivity alloys, the standard age hardening treatment is 2 hours at 480^oC. Some ideas can also be obtained from Aging/overaging and Stress Relaxation Resistance Plots in Guide To Copper Beryllium.

Please give the strip thickness, inside radius of curvature of the 30^o bend, whether it conforms to Brush Wellman’s ‘good’ bend criteria, whether the mechanical residual stress reduction could be applied, and the design load (at maximum switch extension).

 

[BobM2]

All great advice, Thanks! I like the idea of reverse bending to get the compressive residual stress on the side that will see tensile fatigue loading. I'm trying to get as much deflection as I can so the loading is quite high about 42,000 psi (0 to 42,000 psi cyclic loading). However I only need about 100,000 cycles so I might be o.k.
If I do the reverse bend to get the favorable compressive stress then I would think I would want to start out with something fairly hard and strong (gives me higher compressive residual stress after forming) and not do any stress relief. Make sense?

 

[kenvlach]

Well, due to work hardening, it's rather quite difficult to manipulate residual stresses. Practice bending and straightening the handle of a stainless steel spoon or fork and you will learn quickly that the reverse bend location will be to one side or the other of the original bend. Of course, the proper forming equipment would help, but at a cost $$.

Regarding material, you would be better off starting with Be copper in the solutionized condition and with an amount of cold rolling inverse to the amount of cold forming that you wish to perform. After forming, age hardening would both increase strength and reduce residual stress.

As a practical matter, use as large a bend radius as the geometry of your part permits. This minimizes both forming and load stresses in the outside fiber at the bend (the stress concentration/failure point).
Ken

 

[israelkk]

you should remember that if you do the reverse bending you get compressive stresses in compressive stresses in the side that see the tensile fatigue stress. Therefore you have in actual higher yield/tensile strength than in the alloy spec. Theoretically you can go up to 1.33 of the yield/tensile strength of the alloy before the reverse bending. To know the correct and maximum reverse bending you need to experiment. There is a technique to do it but it is difficult to explain it in this forum.

You can do the reverse bending on the TH04 too. This way you will have the strongest spring. Do not do any stress relieve because it may cancel the favorable compresive stress that the reverse bending created. To stabilize the spring you can do multiple same reverse bendings (12 times is the best) or you can do one and hold it in the loaded position for 24 hours.

You need to do Goodman fatigue analysis because you load the spring from 0 to say 40000psi i.e R=0 instead of R=-1 (reverse bending).