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.
Highlights
GCr15SiMn bearing steel is one of the most important special steels and is utilized to produce bearing rings and rollers that are applied widely in large equipment like tunnel boring machines, wind power equipment, precision machine tools, and so on
With the increasing demand for high-quality and large-sized bearing steel, the electroslag remelting (ESR) process has been widely used as an important technology to produce bearing steel due to its superiority in the production of large-sized ingots and the improvement of solidification structure, cleanliness, and mechanical properties
It is obvious that freckles are harmful solidification defects, and those steel ingots with freckles have to be discarded because the freckles are practically impossible to eliminate thoroughly in the subsequent forging or heat treatment process
Summary
GCr15SiMn bearing steel is one of the most important special steels and is utilized to produce bearing rings and rollers that are applied widely in large equipment like tunnel boring machines, wind power equipment, precision machine tools, and so on. Freckles are a kind of unexpected but common channel- or stringer-like macro segregation defect They are a typical reflection of solute inhomogeneity initiated by convective instabilities in the mushy zone during solidification [1] and appear as long black linear trails with a diameter of up to several millimeters. It is obvious that freckles are harmful solidification defects, and those steel ingots with freckles have to be discarded because the freckles are practically impossible to eliminate thoroughly in the subsequent forging or heat treatment process This can be attributed to these factors: (i) Large freckle channels and slow diffusion of solutes in the solid phase require an extremely long homogenizing treatment time (several years or even decades) and high treatment temperature (intensify liquation), (ii) it is difficult to eliminate micro pores and eutectic or primary carbide, (iii) freckles consist of equiaxed
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