Study on formability of duplex steel expansion pipe and its threaded joints

Abstract

The threaded joint of equal well-diameter-expansion pipe is prone to fracture and leakage after expansion forming. The structural integrity and material selection of the expansion pipe threaded joint are the main difficulties. Taking dual-phase steel and other well-diameter expandable casings as the research object, a method combining finite element simulation and experimental verification is used to construct an enhanced constitutive model. Firstly, three kinds of reinforced constitutive models are constructed, and then finite element software is used for simulation, combined with experiments to analyze the axial length of the pipe, pipe wall thickness, thread clearance, material hardness, plastic toughness and microstructure. The results showed that after expansion the top and root of the threaded area of the tube after expansion have greatest change and the residual stress is relatively high. Microhardness and Charpy impact tests were carried out on the pipes before and after expansion to study the changes in hardness and toughness. It can be found that the toughness of the expansion pipe has decreased, and although the wall thickness will be reduced, the strength of the pipe will be significantly increased, which is the effect of work hardening. The topography of the crack surface is nearly identical over the entire range of expansion ratios considered in this study. In addition, the microstructure of the unexpanded and expanded tubes is similar, consisting of ferrite and austenite. This paper provides a theoretical reference for engineering practice.