Study of the Hardness and Microstructure of Quenched ABNT 1045 Steel Bars and Their Relationship to the Lumped Thermal Systems

Lumped thermal systems are characterized by uniform interior temperature during heat transfer processes, having as only important factor the time. Thus, the present research has proposed to investigate one of these systems in order to determine a maximum radius along which the microstructure and hardness are homogeneous after performing a quenching heat treatment. Cylindrical samples with diameters equal to 12.7, 14.0, 15.0, 16.0, 16.5, 17.0, 18.0 and 19.0 mm of ABNT 1045 steel were produced with ratios of 20:1 between the length and radius. Afterwards, they were heated in a kiln to 845°C, where they remained between 30 minutes (smaller diameter) and 45 minutes (larger diameter), for complete austenitization. Thereafter, they were rapidly cooled in water to room temperature. With the samples already sectioned, the hardness values were measured and the microstructures along the radius were verified by metallography. The results indicated that for the samples of 12.7, 14.0 and 15.0 mm, the hardness measurements were the same in the center, in the middle of the radius and near the surface, characterizing martensitic microstructures that were corroborated by the metallographies. For the samples of 16.0, 16.5, 17.0, 18.0 and 19.0 mm, the results were not homogeneous along the radius. In this way, it has been found that the maximum diameter at which ABNT 1045 steel cylindrical bars behave as lumped thermal systems is 15.0 mm.