FEATURES OF THE STRUCTURAL STATE FORMATION IN A CYLINDRICAL FORGED BILLET OF LARGE-SIZED CRANKSHAFTS

Abstract

The article presents a comprehensive analysis of the chemical composition, macrostructure, gas saturation, mechanical properties, and the uniformity of the hardness distribution, microstructure, and non-metallic inclusion contamination in the full cross-section of a large cylindrical forging made of 38ХН3МА steel. It is demonstrated that a high forging grade of 6 minimizes central porosity and the occurrence of liquation in the cross-section of a forged blank with a diameter of 600 mm. Point heterogeneity caused by the presence of non-metallic inclusions corresponds to a score of 2 and manifests itself at a depth of more than 100 mm from the forging surface. The study confirms that modern open-metal steelmaking technology, combined with secondary metallurgy and vacuum treatment, ensures a sulfur and phosphorus content of 0.008% and a hydrogen content of 1.6 ppm. The forging surface hardness is uniform across the cross-section and ranges from 192 to 212 HB. The microstructure is also uniform across the cross-section: the predominant structure is granular pearlite. However, despite the high degree of uniformity in hardness and microstructure, the cross-section of a large forging exhibits pronounced structural banding combined with an uneven distribution of non-metallic inclusions, the main types of non-metallic inclusions being sulfides and brittle silicates. The metal of the forging exhibits a high degree of cleanliness with respect to non‑metallic inclusions up to a depth of 150 mm from the surface, whereas contamination increases in the axial zone. Structural heterogeneity must be taken into account in the manufacturing technology of large crankshafts when the journal surface falls within the axial zone of the workpiece.