Effect of B₂O₃ on ESR Slag Viscosity for Remelting 9CrMoCoB Steel

Abstract

The viscosity of the slag constituted by 55%CaF₂–20%CaO–3%MgO–22%Al₂O₃–x%B₂O₃ (mass fraction: x ≤ 3) was measured by rotating cylinder method during the electroslag remelting (ESR) of 9CrMoCoB steel to analyze the effects of the B₂O₃ content on slag viscosity, break point temperature, and activation energy for viscous flow. Meanwhile, the slag’s structural characteristics were studied by Raman spectrometry. Results indicated that when T ≥ 1663 K, the viscosity changed gently with temperature, showing the value of about 0.055 Pa·s, which was rarely affected by w(B₂O₃). However, with the increase of w(B₂O₃) in the slag, the Ca₃B₂O₆ with a low melting point was formed, reducing the slag’s break point temperature from 1628 K to 1563 K. According to the NBO/T value and Raman spectra, B₂O₃, as a network generating oxide, improved the slag’s degree of polymerization, thus strengthening and complicating its network structure at high temperatures. In addition, as the w(B₂O₃) increased from 0 to 3%, the activation energy for viscous flow rose from 64 kJ·mol⁻¹ to 94 kJ·mol⁻¹ in the slag.