Not sure about ASME, so take that for what it's worth, but in D1.1 the bend test jig has specific dimns for the radius of the plunger/dies depending on the material yeilds. I'll look it up and furnish the exact figure number in Section 4, if you are interested.
John Wright
edit:
Figure 4.15 for guided bend
Figure 4.16 for wrap around guided bend
Figure 4.17 an alternative roller equipped guided bend