Non-metallic inclusions and hook cracks of high-frequency induction welded pipes

Abstract

Based on the analysis of foreign experience, it is shown that hook cracks are the most common defect of pipe joints obtained by high-frequency induction welding (HFIW). Particular attention is paid to the non-metallic inclusions (NMIs) that caused this defect. Their composition, origin and ways of elimination are considered. It was found that in the foreign literature the nature of certain oxides found in the vicinity of defects is often declared without objective evidence of their origin. The interpretation technique of NMIs composition, which caused the formation of defects of pipe joints obtained by HFIW, is presented. This technique is a part of the "Metal products quality system", implemented on the basis of Thixomet image analyzer, working in the laboratories of Vyksa Steel Works JSC. The compositions of NMIs found in the discontinuities of welded joint defects of 09G2S steel grade were interpreted using thermodynamic simulation of primary NMIs and generalized for two sets of 65 and 70 defects in each set. These compositions differ significantly from each other and correspond to the following indigenous inclusions: Al2O3, MgO·Al2O3, calcium aluminates, CaO and CaS. The same compounds can form exo-indigenous NMIs conglomerates after the interaction between liquid calcium aluminates and refractories based on MgO or ZrO2, and/or mould powder. The unique ranges of concentration changes of the main elements included in individual phases and their combinations in NMIs conglomerates are used for automated determination of their origin when conducting metallographic examination of defects using the Thixomet image analyzer. Examples of using the origin of NMIs to find the exact place in the secondary steelmaking and steel casting technologies for their improvement and on this basis to improve the quality of HFIW pipe joints are also presented.