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inside of the bent portion when it is subjected to internal pressure and also the relation between
reduction ratio and fracture elongation. Because reduction ratio (namely, the compressive strain
occurring at the inner surface) can be determined from R/r through eqn (I), both reduction ratio
and R/f (corresponding to the magnitude of cold working) are plotted on the horizontal axis. The
figure shows that a dangerous situation develops when R/t is less than about 2 because the strain
on the inside of the bent portion due to the load imposed at water proof testing exceeds the rupture
elongation. Hence, when carrying out bending work, it is important, from the mechanical standpoint,
to perform the work so that R/t exceeds approximately 2 in order to prevent hydrogen embrittlement
cracking for similar applications.

5. CONCLUSION
The damage occurring at the cold-bent corner portions of thermal storage tanks made of ferritic
stainless steel (SUS436L) is believed to be due to hydrogen embrittlement cracking brought on by
tensile strain which developed during water proof tests performed after the stainless steel had been
charged with hydrogen by galvanic corrosion during acid washing at the fabrication stage. The
corner portions were analyzed by the finite element method and the dependency, on bending radius,
of the strain developing at the corner portion during water proof tests was clarified. Additionally,
the dependency of magnitudes of cold working on fracture elongation of tensile test specimens
hydrogen-precharged was ascertained. Based on these results, it was clarified that, from the mech-
anical standpoint, inner radius/thickness (Rlt) must exceed approximately 2.

REFERENCES

1. Grubb, J. F., in Mew Developments in Stainless Steel Technology, ed. R. A. Lula, ASM, Metals Park, OH, 1985, p. 175.
2. Metals Hundbook, “Failure Analysis and Prevention”, Vol. IO, 8th edn, ASM, Metals Park, OH, p. 182.
3. Sailo, K., Japan SOC. Mech. Engrs. (in Japanese), 1972, 16(53). 133.