Donald R. Lesueur

Stress-Strain Rate Relations in Ultrahigh Carbon Steels Deformed in the Ferrite Range of Temperature

The stress-strain rate relations in ultrahigh carbon steels (UHCSs) have been analyzed at high temperatures in the ferrite range where dislocation slip is the principal deformation mechanism. Specifically, the present investigation centers on the factors influencing the strength of UHCS in the ferrite range of 500 to 700 C and in the strain rate range of 10{sup -3} to 10{sup 3} s{sup -1}. These steels contain iron carbide as a second phase either in the form of spheroidite or pearlite and vary in the grain size according to the processing history. The new variables that need to be taken into account are the effect of ferromagnetism and its change with temperature on the creep strength of the ferritic UHCSs. Recent studies have shown that ferromagnetism strongly influences the lattice and dislocation pipe diffusion coefficient and the elastic modulus of iron in the ferrite range of temperature. These variables are shown to influence the creep strength of ferrite-base steels and explain the high activation energies that are observed in many of the ferritic UHCSs.

High Strain Rate-High Strain Response of an Ultrahigh Carbon Steel Containing 1.3% C and 3% SI

The mechanical response of a UHCS-1.3C material deformed at approximately 3000 s{sup -1} to large strains (60%) has been studied. The influence of three different heat treatments, which resulted in pearlitic, martensitic and tempered martensitic microstructures, on the stress-strain response has also been examined. Failure, at both the macroscopic and the microscopic levels, and the ability of the material to absorb energy in compression have been evaluated. Failure for all heat treatments occurred due to shear localization. However, in the pearlitic condition, extensive buckling of the carbide plates was observed and the UHCS-1.3C material exhibited significant potential for compressive ductility (>60%) and energy absorption due to the distributed buckling of these plates. In the pearlitic condition, localization occurred due to adiabatic shear bands, in which austenite formed. Subsequent cooling produced a divorced-eutectoid transformation with associated deformation, which resulted in a microstructure consisting of 50 to 100 nm sized grains. The results show the large potential for use of UHCS in applications involving dynamic loading.