| An investigation was conducted into the effects of microstructural parameters on the yield strength drop occurring during spiral piping of two high-strength API-X70 steels processed with titanium and molybdenum additions. Compression-tension tests were conducted as well as tensile tests in order to investigate the Bauschinger effect during piping, and then the test data were compared via microstructures, tensile properties, textures, and yield point phenomenon. The tensile results indicated that the yield strength of the Ti-containing steel dropped, together with yield point phenomenon, after piping, whereas this drop did not occur in the Mo-containing steel. It appeared that competing mechanisms existed in the steels studied herein; the yield strength decrease due to the Bauschinger effect accompanied by the yield strength increase due to the strain hardening effect. In the case of the Ti-containing steel, the yield strength drop was associated with the small strain hardening overrode by a large amount of the Bauschinger effect. For the Mocontaining steel. the large strain hardening, commonly associated with continuous yielding behavior, overrode the Bauschinger effect, and finally caused the increase in yield strength after piping. |
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