Characterization of Grain-boundary Precipitates after Hot-ductility Tests of Microalloyed Steels

The hot ductility of microalloyed steels was investigated by interrupted tensile tests at the temperatures of 850 and 950 degrees C. Analyses of microstructural damage during plastic straining of the steels were performed using an experimental setup that allowed rapidly quenching the tensile specimens after straining to a predefined level. Microstructural investigations on the materials were carried out on longitudinally sectioned samples. Further analyses on crack surfaces were performed by fracturing the strained specimens in liquid nitrogen and by analyzing the surfaces formed by high-temperature decohesion through conventional and field emission SEM. It was demonstrated that AIN and Nb(C,N) precipitates, in isolated or combined form, affected the prior-austenite grain boundaries. Differences in hot cracking sensitivity among the steels was accounted for by modifications of the precipitate size and volume fraction.