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
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
This gives formability data, as does
Guide To Copper Beryllium and
Cumulative Stress and the Bauschinger Effect
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
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).