The electronic microscope analysis used to study the martensite morphology in high strength low alloy steels

The class of HSLA steels have a great development due to the spread of quenched and tempered steels. The present paper has in view the research by means of electronic microscope analysis of structural characteristics of a HSLA steel after quenching and tempering. The martensite-bainite structure is investigated by means of secondary electrons images, transmitted electrons images or by diffraction of electrons on selected areas. Martensite has an aspect in slats, it is made up of packets with a high dislocation density. The martensite packets are made up of subgrains with slight differences of orientation among them, having the form of a thin blade with depth of 0.1 μm. At the tempering over 450 C begin the loss of carbide coherence and the advanced decomposition of the quenching structure. Between 450 C and 600 C in the bainite crystal the ferrite cells become more evident. There appear subgrains with limits at the short angles. Gradually as the subgrains grow in the ferrite poliangled matrix appears limits for the big angles. There starts the matrix recrystallisation. In this stage these can be noticed a powerful growth of the alloy elements precipitates. The electronic microscope analysis allowed the study of the evolution of morphologycal aspects of the martensite-bainite duplex structure leading to an efficient use of this type of steel. (orig.)