Non-metallic inclusions and quality of pipe joints obtained by high-frequency electric resistance welding

Abstract

An incisive analysis of studies of the most frequent defects of pipe joints obtained by high-frequency electric resistance welding (HFW) is included herein with particular focus on nonmetallic inclusions (NMIs), which are the cause of defects in such welded joints. Their composition, origin and methods of elimination under industrial conditions are discussed. In order to validate the nature of defects the interpretation procedure of the composition of NMIs detected in the discontinuities of welded joints has been developed. Examples of its application on the low-carbon line pipe steels of different grades X52–X56 are presented. The compositions of NMIs found in 64 defects of welded joints of A 516-55 (09G2S) steel pipes are summarized. These compositions significantly differ from each other and correspond to the different nature of NMIs: indigenous deoxidation products based on spinel MgO∙Al2O3 (8 %), products of steel modification by calcium (62 %), exo-indigenous NMIs with residuals of mould flux (4 %) or MgO (8 %) and indi-exogenous NMIs based on MgO (18 %). The unique variation ranges of the main elements included in these groups of NMIs are used for automated determination of their origin when carrying out metallographic examination of defects by means of the Thixomet image analyzer in the conditions of factory practice. The detailed origin of NMIs makes it possible to indicate the exact location in the technology of ladle treatment and steel casting for their improvement and to enhance on this basis the quality of line pipe joints obtained by HFW.