Evolution of mechanical properties of ferrite and pearlite phases during spheroidization process and their relationship to the overall properties of low alloy steel

Abstract

Mechanical properties of ferrite-pearlite low alloy steel are strictly determined by the properties of ferrite and pearlite phases. In this present paper, the evolution of mechanical properties of ferrite and pearlite phases during spheroidization process is investigated by using nanoindentation tests. Meanwhile, the overall mechanical properties of the material were measured using uniaxial tensile tests. Test results indicate that with the intensification of spheroidization of layered cementite in pearlite, the hardness and yield strength of pearlite phase decrease from 2.72 GPa to 1.73 GPa and 411.5 GPa–256.1 GPa, respectively. However, the mechanical properties of ferrite phase exhibit an opposite trend, with hardness and yield strength increasing from 1.6 GPa to 1.67 GPa and 188.1 GPa–208.4 GPa, respectively. Uniaxial tensile tests results indicate that overall yield strength of steel decreases significantly as the degree of spheroidization increases, from the original 276 MPa–229 MPa. Although the strength of pearlite decreased by 38%, the overall strength of alloy steel decreased by only 15% due to an 11% increase in ferrite strength. During spheroidization process, the relationship between pearlite hardness, yield strength, and interlayer spacing of cementite in pearlite can be expressed in Hall-Petch type relationships as: H=0.482×S−12+1.34 and σy=91.44×S−12+149; and the relationship between the strengths of ferrite-pearlite aggregate and ferrite and pearlite phases is σy=Vf1.95σf+(1−Vf1.95)σp.