Polytherms of the physical properties of metallic melts

Abstract

The temperature dependence of the kinematic viscosity, electrical resistivity, surface tension, and density of liquid steels and alloys on heating and subsequent cooling is analyzed. On that basis, the polytherms of the physical properties of steels and alloys are systematized. On heating to certain critical temperatures, changes occur in the structure of the melt. Consequently, the cooling polytherms take a form more closely resembling the equilibrium classical laws and do not match the heating polytherms. Branching or hysteresis of the temperature dependences is only irreversible on heating to critical temperatures. Otherwise, partial or complete return to the primary melt structure is possible. That affects the degree of hysteresis of the polytherms. The degree of hysteresis and the data regarding the properties provide qualitative information regarding the transition of the melt structure from the equilibrium to the microhomogeneous state. The uniformity of the distribution of atoms of the alloying elements in microgroupings or clusters indicates the equilibrium of the structure, while the uniformity of the distribution of clusters that differ in structure over the melt volume reflects structural microhomogeneity. Data on the properties of multicomponent metals indicate that, after melting, the variation in melt properties on isothermal holding takes the form of familiar damping oscillations. With increase in temperature, the damping becomes aperiodic, and the relaxation time declines. The processes responsible for the isothermal variation in melt properties occur at the microscopic level. Nonequilibrium industrial metal usually contains inclusions inherited from the initial materials, such as insoluble graphite particles in the cast iron or associations and aggregations of carbide and nitride type. The melt takes a long time to reach equilibrium—usually longer than the time for diffusional mixing of the atoms within the nonequilibrium regions. With more complex chemistry and structure of the solid metal, the distance of the melt from equilibrium will be greater. In this system, new correlations are formed and broken more intensely. Cooperative interactions of the new spatial and time structures with those inherited from the initial materials occur here, as indicated by oscillating behavior of the properties of the metallic melts. Information regarding the state of the melt before solidification permits scientific analysis of the melting points and melting times of the steels and alloys. Such preparation of the melt affects its supercooling, its solidification rate, the formation of hardening phases and eutectics, the segregation of the elements, the dendrite and zonal; structure of the castings, and the overall product quality and production efficiency.