Influence of Structure on the Magnitude and Type of Residual Macrostresses in a Steel Tubing Coupling

The influence the structure of medium-carbon steel on the magnitude and distribution of residual macro-stresses in different sections of a tubing coupling is investigated. Heat treatment (quenching and tempering at different temperatures) of cylindrical samples cut from the coupling is carried out. Four types of structure are obtained, i.e., quenched martensite, tempered martensite, bainite and ferrite-pearlite. It is shown that according to the degrees of reduction of the longitudinal and tangential residual macrostresses the structures formed in the steel can be arranged in the following sequence: quenched martensite → tempered martensite → bainite ferrite + pearlite. At the same time, the different types of structure do not lead to significant changes in the radial macrostresses in the coupling. It is established that the magnitude of the longitudinal and tangential residual stresses in the coupling depends on the type of the microstructure formed in the steel during the heat treatment and on its defects. It is shown that formation of a bainitic structure in a tubing coupling is the most expedient, because it ensures a relatively high strength and elevated plasticity at comparatively low longitudinal and tangential residual stresses.