Characterisation of mesosegregations in large steel ingots

Abstract

The chemical composition mapping of low-alloyed steel ingots used for the nuclear industry is crucial in the manufacturing of forgings and their final quality mastering. Mechanical properties of forged and hot-rolled steels may be affected by chemically segregated bands. These bands arise from segregations that appear at the scale of a few grains in the as-cast structure: the so-called mesosegregations. While segregation at the scale of dendrite arms (microsegregation) and the scale of the casting (macrosegregation) is well understood and can be readily characterized, only little is known about the formation of mesosegregation. The first step towards understanding the cause behind mesosegregation formation can be brought through comprehensive chemical characterisation at the scale of several grains (mesoscopic scale), which requires using different characterisation techniques compared to micro- or macrosegregation characterisation. We developed a sampling and characterisation methodology that allows segregations to be mapped at the mesoscopic scale using micro X-ray fluorescence (μXRF). Characterisation technique, sampling methodology, and sample size must be adapted to consider the different solidification structures; both at smaller (dendrite arms, grains) and larger (macrostructure) scales. Segregations were characterised on a 113 x 98 mm2 steel plate extracted from a low-alloyed steel large ingot.