Precipitation and Grain Size Effects on the Tensile Strain-Hardening Exponents of an API X80 Steel Pipe after High-Frequency Hot-Induction Bending

This study discusses the use of the Morrison model to estimate the strain-hardening exponent (n) in the presence of precipitation hardening for an API X80 steel pipe. As the grain size becomes larger, high values of n are expected according to the Morrison equation. However, the grain size alone is not sufficient to explain the changes of the strain-hardening exponent (n) after hot-induction bending. The vanadium in the ferritic solid solution has an important influence on the decrease of the precipitation hardening, and consequently on the increase of the values of n, despite the refinement of the grain size and high dislocation densities. Therefore, the effects of grain boundaries on the capability to uniformly distribute deformations within the plastic regime become negligible, which limits the application of the Morrison model to estimate the values of n.