Characteristics and Formation Tendency of Freckle Segregation in Electroslag Remelted Bearing Steel

Abstract

Undesirable macro segregation defects, freckles, restrict the commercial production of large-sized electroslag remelting (ESR) bearing steel ingots through degradation of the mechanical properties and service lifetime. In order to clarify the freckle characteristics and formation tendency as well as the formation mechanism, freckles from an industrial large-sized GCr15SiMn ESR ingot were investigated through structural and compositional analysis, along with simulation calculation. The results show that freckles consist of (Si, Mn, Cr)-enriched equiaxed grains and occur in about the 1/2 radius region at the middle-upper part of the ESR ingot, where the secondary dendritic arm spacing (SDAS) and solidification front angle are large but cooling rate is small. The absolute value of relative Rayleigh number, Ra, also reaches its maximum in the 1/2 radius region, with a liquid fraction of 0.3–0.5, corresponding to the region where freckles form. Based on the experimental and simulation results, to evaluate the freckle formation in industrial-scale GCr15SiMn ESR ingots, the threshold value of relative Ra, a freckle criterion considering the compositional and thermal effects, was determined to be about −0.023.

Advances in Metal Casting Technology: A Review of State of the Art, Challenges and Trends—Part II: Technologies New and Revived

“…The contributions by Wan et al and Yan et al do not center on shape casting but rather on the processing of semi-finished casting products, such as billets or ingots [378,379]. The paper by Wan et al is concerned with the avoidance of carbon segregation typically occurring during the casting of high-carbon steel billets.…”

“…Experimental investigations addressing location, compositions, structure and geometrical features of segregates in sample ingots were accompanied by theoretical considerations (e.g., CALPHAD approaches realized using the Thermo-Calc software package), finally leading to a model of freckle segregation formation which integrates, among others, geometric, thermodynamic, fluid dynamic and compositional aspects as well as composition-dependent viscosity levels. As a result, a threshold for freckle formation was suggested in terms of a relative Rayleigh number limit for the material in question [379].…”