Relative anisotropy of structures and ultrasound attenuation response between laboratory casting in permanent mould (vacuum induction melted) and casting processed through electroslag refining

Abstract

In the present study, ingots of 'modified 15CDV6' (0·3C–CrMoV) steel were laboratory melted and cast by the vacuum induction melting–electroslag refining (VIM–ESR) and VIM (water cooled steel mould) routes. The ingots have been evaluated for their relative anisotropy differences by ultrasound attenuation (UA) studies along respective radial and axial directions. The expected sources of UA are crystallographic texture and the presence of defects. These defects include inclusions, grain boundaries, segregation and microporosity. Apart from microporosity, the presence of defects has been characterised along the respective casting directions. Although both castings/ingots had gone through successive phase transformations after solidification (liquid→δferrite→austenite→ferrite), the electroslag refined (ESR) material had significantly more anisotropy in crystallographic texture, but insignificant anisotropy in inclusion distribution, as compared with the vacuum induction melted (VIM) material. The grain boundary character distributions and long range compositional fluctuations were similar and nearly isotropic between the two ingots. The ESR material, in general, was denser, which may indicate a lower presence of microporosity. Taking grain size differences into account, the UA estimated anisotropy was more for ESR material, a response clearly dominated by the anisotropy of the crystallographic texture. At a higher frequency, and correspondingly finer resolution for defects, the anisotropy owing to defects played a more dominant role, but even then the highest frequency of 2 MHz was not adequate to highlight a dominance of defect anisotropy.