Fatigue damage of the surface layers of metal is a characteristic cause of failure of rolling bearings, gears and a number of other machine parts operating under cyclically repeated contact loads. Resistivity to the development of contact damage of steels obtained by hot forging porous blanks is determined by the presence of cohesive bonds between the particles of the base powder, as well as by the presence of non-metallic inclusions and grain size. The possibility of increasing the contact endurance of hot-deformed powder steels due to micro-doping with calcium has been studied. Iron powders with various content of impurities, as well as atomized powder of low-alloyed chromium-molybdenum steel were used as the basis for preparation of the blends. Calcium was doped as calcium carbonate. Mixing was performed in a planetary centrifugal mill. Samples for mechanical testing were obtained by hot forging porous blanks. After hot forging the samples were carburized to compensate for the loss of carbon in the surface layer. It has been established that doping calcium microadditives is favourable for increasing the energy content of damage under the conditions of exposure to contact-fatigue and bending loads. This is due to a decrease in the size of austenite grains at the expense of inhibition of their growth during the adsorption of calcium at the grain boundaries. Microalloying with calcium changes the localization of seats of contact fatigue damage. In samples-witnesses without microadditives of calcium cracks originate near non-metallic inclusions of sharp-angled shape in the near-surface zone. In microalloyed specimens the cracks are located in the subsurface layer in the area of Hertz maximum shear stresses.

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The main problem in the production of bimetals (BMs) is the need to ensure adhesive interaction at the contact boundary of layers to prevent their peeling during operation. Hot forging of porous preforms (HFPP) provides the possibility of obtaining high-density powder BMs with a minimum amount of pores both in the volume of the layer material and at the layer interface to increase adhesion strength. Production of hot-forged powder BMs may involve mixing of working layer and substrate charge materials, which can lead to uncontrolled interface «blurring». This study uses the previously proposed method for pre-pressing of hard-to-deform material powder to produce «structural steel – high-speed steel» porous BM preforms. Two-layer cylindrical ∅20×30 mm samples were obtained in order to determine mechanical properties and conduct structural analysis. The BM base material was PK40 steel, and the working layer was atomized powder of M2 high-speed steel featuring satisfactory compressibility properties. The porous preforms of BM samples were pressed in a specially designed mold at a hydraulic press enabling two-sided pressing of two-layer powder moldings with predetermined distribution of layer densities and strengths. Cold-pressed BM preforms were sintered in protective environment, and then subjected to hot repressing using a laboratory drop hammer. Some preforms were examined as sintered. In addition, hot repressing of cold-pressed green preforms was performed. Satisfactory process strength of the working layer material is observed at its porosity (Pwl) in the range from 34 to 45 %. When Pwl> 45 %, powder is not molded, and at Pwl< 34 % the working layer delaminates. The maximum layer bonding strength and thermal shock resistance of BM provides the use of a flow route that involves preliminary sintering of cold-pressed preforms and subsequent hot forging. The optimum pressure of working layer pre-pressing is 145 MPa.

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