OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 4 2024 where k is a coeffi cient depending on the particle shape and is close to 1; λ is the radiation wavelength; β is the half-width of diff raction refl ection; θ is the diff raction angle. Dislocation density ρD [24] was calculated from eff ective crystallite sizes according to the formula 2 3 . D D − ρ = (2) The size of the coherent scattering regions D was estimated from the most intense diff raction refl ection 110 lying in the region of small angles 2θ. The broadening of refl ections caused by the Kα1 – Kα2 doublet, which is signifi cant at large diff raction angles, can be neglected for it. The contribution to the broadening of diff raction lines due to microstrain is also present; the relative lattice strain d d Δ [25] was determined by the formula 4tg d d Δ β = θ. (3) The separation of the contributions of microstresses and crystallite refi nement to the broadening of diff raction refl ections showed that microstrains have the main infl uence on the broadening of refl ections. Results and discussion The microstructure of undeformed rings (Fig. 2 a) consists of graphite inclusions and pearlitic matrix. In addition, the microstructure shows a small amount of ferrite grains, but its amount is not high, about 5 %. Photographs of the microstructure (Figs. 2 b, 3) of the cross-sections of the compression rings, which have served its service life, indicate that the cast iron has a fi ne plate-like pearlitic base with insignifi cant (not more than 5 %) inclusion of ferrite grains [6, 9, 11]. This corresponds to international standards for compression rings. Schemes of structures permissible for the material of compression rings were selected according to GOST 3443-87 “Castings from cast iron with diff erent graphite shape. Methods of structure determination” [6]. a b Fig. 2. Photo of the microstructure of the upper compression ring: a – before deformation during operation; b – after deformation during operation The fi rst photo represents the microstructure of the upper compression ring and shows the presence of lamellar cementite: the base is fi ne-plate pearlite with insignifi cant inclusion of ferrite grains; the presence of some cementite elements indicates increased brittleness of this material. The measured Brinell hardness of the ring material is HB135 (regulated hardness for compression rings of marine engines of this size is 92–102). The increase of hardness is accompanied by higher brittleness of the material. The second photo shows the microstructure of the lower compression ring: the primary base is pearlite with inclusions of
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