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Inclusion morphology and fracture : toughness of pipeline steels Maiti, Ranen


The effect of increasing hot deformation on inclusion parameters such as area fraction (AF), the average inclusion length (Co), the inter-inclusion distance (D), the aspect ratio (FF) and the density of distributions (N);bas been quantitatively analysed by use of Quantimet 720, an image analysing computer and direct microstructural measurements. Two types of X-70 pipeline steel, one a semi-killed conventional (CON) steel, the other a fully killed and calcium treated for inclusion shape control (CAT) steel were examined in this research. Four stages of hot reduction of the CON steel viz. S2 (38% reduced), S3 (70% reduced), S4 (88% reduced) and S6 (97% reduced) and two stages of the CAT steel viz. CI (63% reduced) and C2 (95% reduced) were used in the investigation. The effect of inclusion parameters on the yield strength, total ductility and elastic and elastic-plastic fracture toughness of these materials were investigated to establish the role of inclusions in the ductile, ductile-brittle transition and brittle fracture processes. The elastic and elastic-plastic fracture toughness of the specimens were measured by KJC testing as per ASTM standard E-399-78a method and unloading compliance J-integral technique as per ASTM standard respectively. It was observed that the semi-killed CON steel containing elongated inclusions shows anisotropy in elastic-plastic fracture toughness; whereas the CAT steels with globular inclusions exhibited isotropic elastic-plastic fracture toughness. The inclusion parameters shape, size and distribution do not affect the yield strength of the X-70 steel. The inclusion parameters markedly affect the elastic-plastic fracture toughness properties of the X-70 CON steel, particularly in the ductile and transition temperature region where failure occurs by the mechanism of microvoid coalescence. The effect of inclusions on the elastic fracture toughness in the brittle temperature region, where fracture occurs by cleavage mechanism, is insignificant. The most significent inclusion parameters which affect the elastic-plastic fracture toughness in the ductile fracture regime are the area fraction AF and the inter-inclusion spacing (D). The elastic-plastic fracture toughness of the CON and CAT X-70 pipeline steel at RT has been correlated (i) with AF by the following relation: [figure 1]. (ii) with D by the following relation: [figure 2].

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