Formation of Structure and Properties of Hot-Deformed Powder Steels Microalloyed with Sodium and Calcium in the Process of Thermal and Thermomechanical Treatment

One of the main problems limiting further growth of the production of parts by the hot forging of porous preforms (HFPP) is that the materials are prone to brittle fracture due to the poor quality of interparticle jointing formed during hot deformation and the presence of impurities in the initial powders. We study an approach to enhance the mechanical properties and endurance performance of hot-deformed powder steels by adding sodium or calcium microadditives and using thermomechanical treatment. Sodium bicarbonate and calcium carbonate are used for microalloying. Carbon is added in the form of a pencil graphite powder. The temperature of heating porous preforms prior to hot forging and the carbon content in steels vary; the content of microalloying additives is, wt %, 0.2 for sodium and 0.3 for calcium. Mechanical properties and the contact and low-cycle fatigue life are tested on 5 × 10 × 55 mm and 10 × 10 × 55 mm prism-shaped specimens and (diameter) 26 × 6 mm cylindrical specimens. Thermomechanical treatment is shown to enhance (in comparison with carburizing and thermal treatment) the impact strength and endurance performance of hot-deformed powder steels with Na or Ca microadditives under contact and low-cycle fatigue loading and reduce the hot repressing temperature of porous preforms without compromising the mechanical properties of the powder steels. This may be associated with the formation of a more fine-grained structure and the emergence of high microstresses in the crystal lattice. Cooling down of preform surface layers in the process of hot forging creates conditions for ausforming in them.