Formation of structure and properties of hot-deformed powder steels microalloyed with sodium and calcium during thermal and thermomechanical treatment

One of the main problems limiting further growth in the production of parts by the hot forging of porous performs (HFPP) is that the obtained materials are prone to brittle fracture due to the poor quality of interparticle jointing formed during hot deformation, as well as the presence of impurities in the composition of initial powders. The paper studies the possibility of increasing the mechanical properties and endurance performance of hot-deformed powder steels by doping them with sodium or calcium microadditives and using thermomechanical treatment. Sodium bicarbonate and calcium carbonate were used for microalloying. Carbon was added as pencil graphite powder. The temperature of heating porous preforms before hot forging and the carbon content in steels were varied; the content of microalloying additives was, wt.%: 0.2 for sodium, and 0.3 for calcium. Mechanical properties as well as contact and low-cycle fatigue life were tested on 5 × 10 × 55 mm and 10 × 10 × 55 mm prismatic specimens, as well as ∅ 26 × 6 mm cylindrical specimens. In comparison with carburizing and thermal treatment, thermomechanical treatment improves the impact strength and endurance performance of hot-deformed powder steels with Na or Ca microadditives under the contact and low-cycle fatigue loading, and the hot repressing temperature of porous preforms is reduced without compromising the mechanical properties of powder steels obtained. It may be associated with the formation of a more fine-grained structure and higher microstresses of the crystal lattice. The cooling down of preform surface layers during hot forging process operations creates conditions for ausforming in them.