Investigation of structure and properties of welded joints of pipe steels

Abstract

The work is devoted to the study of microstructure and crack resistance of welded joints of pipe materials of main gas and oil pipelines for the Far North regions. Reliable operation of pipelines in harsh climatic conditions of such regions can be provided by the use of materials that have a combination of high values of strength, toughness, cold resistance and weld ability. The results of studies of samples from large diameter pipes with an operating pressure of up to 7.4 MPa at test temperatures of -10 and -20 °C. are presented. These pipes are made of hot-rolled low-carbon low-alloy steel. Various welding methods were used to produce welded joints: high-frequency induction welding, multi-arc automatic welding, laser and hybrid welding. This allowed us to evaluate the influence of the heating and cooling rate during welding on the characteristics of the structural States of the weld, the fusion line, and the overheating area in the zone of thermal influence. It was revealed that a rough needle structure of Vidmanshtett is formed in the zone of overheating of the longitudinal seam at a high cooling rate. Using a low cooling rate allows you to get the Vidmanshtett structure within 2-3 points. It is shown that the crack resistance of the base metal of the pipes practically does not change with a decrease in the test temperature. The values of crack resistance for the metal of the zone of thermal influence have a large spread, which may be due to the influence of technological features of cooling the pipe billet after welding.