Characteristics of Microstructure and Mechanical Properties in Al-bearing Medium Carbon Martensitic Steels

As-quenched α'-martensitic structure and its mechanical properties of Fe-1.5%Mn-0.5%C-Al alloys containing various concentrations of Al were investigated. Microstructural observations of specimens heat-treated under various conditions confirmed that a austenite single-phase region can be obtained in the temperature range from 1373 K to 1473 K when Al is increased up to 3%. Dilatometry tests showed that the Ms temperature increases at a rate of 25 K/mass% with increasing Al content. The hardness of α' tended to decrease due to the decrease in the concentration of carbon in solid solution by auto-tempering and the formation of coarse blocks near the prior austenite grain boundaries. The coarse blocks had a butterfly shape, and there were no lath structures or transformation twins inside them, but rather high-density dislocations and carbides. Tensile tests on the 2%Al steel showed that the above coarse blocks can produce a large strain, which suppresses early rupture, resulting in a significant increase in ductility without any loss of strength.