Abstract
It is advantageous for industrial pipes to have small bending radii because of several advantages such as smaller space and less costs. However, the mechanical properties of such bent pipes often deteriorate, especially at the outer sections, due to the smaller thickness; in general, the properties worsen as the pipe bending radii decrease. Therefore, it is important to investigate the mechanical properties of bent pipes having smaller radii than is conventional. In this study, to verify the applicability of such bent pipes with small bending radii, the fatigue properties for a small radius of curvature from 1.5–2.0 diameter ratio (DR) have been investigated. Our pipe material was low carbon steel (SA106Gr-B) having the chemical compositions of 0.3C, 0.29Mn, 0.4Cr, and 0.4Cu (mass%). The high cycle fatigue, microstructure, hardness, and residual stress of two bent sections — extrados and intrados regions — were compared with those of the raw material section. Low carbon steel pipes have better fatigue characteristics at the bent section than at the raw material section. The improvements in the fatigue strength of specimen resulted from grain refinement and phase transformation, while compressive residual stress at surface of bent pipe remained by thermo-mechanical processing can give better fatigue properties in fields.
