Analysis of yield strength drop occurring during spiral piping of two API-X70 steels

摘要

An investigation was conducted into the effects ofmicrostructural parameters on the yield strength drop occurringduring spiral piping of two high-strength API-X70 steels processedwith titanium and molybdenum additions. Compression-tensiontests were conducted as well as tensile tests in order to investigatethe Bauschinger effect during piping, and then the test data werecompared via microstructures, tensile properties, textures, andyield point phenomenon. The tensile results indicated that the yieldstrength of the Ti-containing steel dropped, together with yieldpoint phenomenon, after piping, whereas this drop did not occur inthe Mo-containing steel dropped, together with yield pointphenomenon, after piping, whereas this drop did not occur in theMo-containing steel. It appeared that competing mechanismsexisted in the steels studied herein; the yield strength decrease dueto the Bauschinger effect accompanied by the yield strengthincrease due to the strain hardening effect. In the case of the Ti-containing steel, the yield strength drop was associated with thesmall strain hardening overrode by a large amount of theBauschinger effect. For the Mo-containing steel, the large strainhardening, commonly associated with continuous yieldingbehavior, overrode the Bauschinger effect, and finally caused theincrease in yield strength after piping.