Hot ductility and sulphur segregation in 1 %C—1 %Cr steels

Abstract

Hot torsion tests have been carried out at 800-1300 °C on vacuum melted 1 %C-1 % Cr steels containing ca . 0.005 % S with Mn:S ratios of 72, 11 and 1 by mass. These steels (A, B and C) had similar hot strengths but the ductility depended markedly on Mn:S ratio. In the single phase austenite range above about 910 °C, steel A had high ductility, which decreased systematically with increasing temperature to about 1270 °C, after which it fell precipitously to nearly zero at 1300 °C. In contrast, steel C had low ductility, which increased with increasing temperature to attain a similar value to steel A at 1270 °G. Steel B showed an intermediate behaviour. Also, the ductility of steel A was independent of grain size whereas increasing the grain sizes of steels B and C by pretreatment at 1250 °C resulted in a significant decrease in ductility. Thermodynamic data for (Mn, Cr, Fe) S inclusions indicate greatly increased solubility of sulphur with decreasing Mn: S ratio. Thus in steel A the sulphur is not entirely in solution even at 1300 °C, whereas complete solution is expected at 1110 °C in steel B and at 1040 °C in steel C.