Effect of scrap using in charge on the microstructure and properties of ZhS6U nickel-based superalloy. Part 2. Structure analysis and mechanical properties of ZhS6U prepared with scrap

Abstract

The second part of the article considers influence of the scrap amount on structure and mechanical properties of ZhS6U-VI nickelbased superalloy. As the use of scrap is associated with the possibility of alloy contamination by nonmetallic inclusions and loss of alloying elements, the influence of scrap on alloy structure and mechanical properties is in great importance. The samples with diameter of 12 mm were melted in a vacuum arc furnace and were casted into a copper mold from the virgin ZhS6U-VI alloy without scrap and from alloys with 50 % and 100 % of scrap. The alloys structures were investigated using optical microscopy on etched metallographic sections that were cut from the samples’ cross-sections. The studies were carried out on the as-cast samples and the samples after solution heat treatment for 4 hours at 1210 °C. The slightly higher nonmetallic inclusions content were observed in the structure of the alloy melted with scrap in comparison with virgin alloy melted without scrap. For the sample that was melted from 100 % of scrap the inclusions secure level is 3 (ASTM E 45-97) with an average size of inclusions of 28.4 ± 0.2 μm. Herewith the presence of single large inclusions with a size of not more than 70 microns was noted. However, it has no effect on the alloy mechanical properties. Mechanical properties after heat treatment (ultimate strength (UTS) = 1090 – 1100 МPа and elongation (El) = 9 – 11 %) were obtained on the samples melted using 50 and 100 % of scrap and fully correspond to the TU1-92-177-91 standard for ZhS6U-VI nickel-base superalloy. During solidification most of the large nonmetallic inclusions are concentrated under casting surface, which makes their machining difficult. Because of that the use of 100 % scrap without its preliminary processing is not recommended. Acceptable results were achieved when the 50 % of scrap was used.