FAILURE I1NVESTIGATION. OF SURFACE DEFECTS ING IN HOT-ROLLED IS- 2831 GRADE. BILLETS

Abstract

Steel Pipe manufacturing units generally face problems with the surface quality of the rolled coil, which is welded through Electrical Resistance Welding (ERW) process to produce seamed pipes. The rolled coil is produced by rolling the cast billets in a rolling mill. The billets manufactured by one such an industry contained many defects like piping, blow holes etc., Rolling of such defective billets into rolled flats resulted in surface defects like lamination, short longitudinal surface cracks, slivers. A detailed metallurgical investigation was carried out to understand the genesis of these defects and propose preventive solutions. The investigation carried out, was in two phases. In the first phase, billet samples were macroscopically analyzed by macro-scopic studies like Hot Acid Etching, Fluorescent Dye-penetrant Testing, Sulphur Printing and Oxide Printing. The blow holes found in the transverse sections of the billet sample were analyzed using EDS under FE-SEM to find that the surface of the blow holes is oxidized and also covered with slag inclusions like A1203 & Si02 . Sulphur prints and Oxide Prints revealed the presence of sulphide and oxide inclusions, respectively in the region adjoining the piping defect and also in blow holes (only oxides). In the next phase, the crack regions in the defective rolled strips from three different heats were observed under FE-SEM for crack morphology. Further, they were EDAXed and X-ray mapped to observe the presence and distribution, respectively of different elements in the form of inclusions, primarily slag inclusions in the form of oxides and other inclusions in the form of FeO and sulphides. The crack portion in the defective strip was found to contain very less amount of pearlite revealing the fact that the blow holes near the edges of the billet were pre-oxidised during hot soaking before rolling. The presence of slag inclusions, FeO in the blow holes and also in the cracked and laminated region of the rolled strip, make it clear that the blow holes couldn't get welded during hot rolling because of the presence of different inclusions in the form of their oxides, which were also responsible for the initiation and propagation of cracks and hence surface defects on the rolled flats were observed. The raw material melted in the furnace contained objects having deleterious tramp elements like Cr, Cu (from condenser tubes), S etc. So, the chemistry of the product varied from the specified standard namely IS :2830.