Effect of annealing on properties of carbonaceous materials. Part III: macro and microstrengths

Carbonaceous materials including cokes, chars, and pyrolyzed coals were annealed at temperatures

ranging from 973 K to 1773 K (700 C to 1500 C) in an inert atmosphere. Macro and

microstrengths of original and annealed carbonaceous materials were characterized by the

tensile strength and fracture toughness. Fracture toughness was determined for inert maceralderived

component (IMDC) and reactive maceral-derived component (RMDC) using ultramicro

indentation. Experimental data obtained by tensile tests were processed using the Weibull

statistical method to find ‘‘inherent’’ strength. Tensile strength of chars and coals was significantly

increased by annealing at temperatures ranging from 973 K to 1373 K (700 C to

1100 C); further increase in annealing temperature to 1773 K (1500 C) increased their tensile

strength only slightly. Tensile strength of cokes decreased with the increasing annealing temperature;

the major effect was observed in the temperature range from 1573 K to

1773 K (1300 C to 1500 C). Fracture toughness of chars and coals was enhanced significantly

by heat treatment at temperatures ranging from 973 K to 1373 K (700 C to 1100 C) as a result

of pyrolysis, while that of cokes increased slightly by heat treatment. Fracture toughness of

IMDC was higher than RMDC.Macrostrength of carbonaceous materials was strongly affected

by their porosity and microstrength. The effect of pore geometry on macrostrength was marginal.

Decreasing the porosity was more effective compared with increasing the microstrength in

improving the macrostrength of carbonaceous materials.