LOCAL SURFACE STRENGTHENING OF HIGH-DUCTILE CAST IRON

Piston rings work in extreme conditions of high temperatures, abrasive friction, high thermomechanical stress, oil starvation and aggressive environmental influences. The purpose of the work is to develop a method of increasing the wear resistance of piston rings made of high-strength cast iron due to the use of volumetric thermal annealing in combination with surface hardening by laser hardening with a low-power laser. The structure was studied using an optical microscope. Microhardness was measured on a PMT-3 microhardness tester. Wear is investigated using the roller-pad method on a friction machine. High-strength cast iron was subjected to normalization in order to ensure the required level of mechanical properties in cross-section. Laser technologies are used for surface strengthening of high-strength cast iron. A comparison of cast iron hardening regimes was carried out: volume hardening, laser hardening with melting, laser hardening with UV radiation. The study showed that cast iron has the highest hardness after quenching from the liquid state, less – after quenching with laser radiation in the UV range with a wavelength of λ = 0.355 μm, cast iron has the lowest hardness after bulk quenching. It was found that during laser hardening with UV radiation, the depth of the hardened layer depends on the initial microstructure, which is probably related to the different thermal conductivity of the microstructures. The pearlite-ferrite structure of cast iron after normalization has a higher thermal conductivity than the structure of martensite formed after volume hardening. For samples with a martensite structure, the energy of the laser beam provides heating to quenching temperatures at a greater depth. Hardening of high-strength cast iron with a UV laser ensures the formation of a finely dispersed martensite structure, increasing the hardness and wear resistance by 2 times while maintaining the quality of the surface layer.