Effect of Varying High Frequency Induction Bending on the Longitudinal SAW Weld of API X80 Steel Pipe

Abstract

Hot induction bending is a process applied to tubes produced for the oil and gas industry, where bending temperatures above Ac3 can be reached. The base metal is submitted to two thermal cycles, one during welding, generating the heat affected zone (HAZ), and another during bending when this HAZ is reheated. Also, after the bending process is completed, heat treatment is required, which represents a third thermal cycle. The objective of the present work was to compare the effect of two different induction bending process conditions on the longitudinal SAW weld of two API 5L X80 steel pipes of 20” diameter, 19mm thickness, Pcm of 0.17% and Σ NbTiV of 0.11%. The pipes, were produced by the UOE process from a steel plate obtained by thermomechanical processing without accelerating cooling. The bending parameters applied were 105 kW power at 2500 Hz frequency and 205 kW at 500Hz A major microstructural decomposition of the welded joint occurred for the 205 kW / 500Hz condition due to the higher heat input in function of the doubled power input during bending. These microstructural changes led to an inversion of the microhardness profile from the internal pass to the external pass as compared to the welded joint before bending. Tensile strength values obtained after bending were above the minimum limit (621MPa) established for grade X80 by the API 5L norm, showing that these microstructural changes promoted by the bending process do not compromise the structural integrity of the joint.