Effect of the Cooling Rate on the Microstructure and Properties of C92900 Bronze

Abstract—In mechanical engineering, antifriction tin bronzes are used for manufacturing friction parts, for example, C92900 bronze, which is used on aircraft braking systems. One way to improve the properties of leaded tin bronzes is to increase the cooling rate during solidification. In this work, the effect of the cooling rate and the changes in the content of alloying elements within the limits established by industry standards on the properties of C92900 bronze are investigated. In order to provide different cooling rates, the alloys are casted into molds made of resin-bonded sand, steel, and graphite, for which the cooling rates were 0.4, 5, and 14.6°C/s. The influence of the cooling rate and the composition of bronze on the freezing range, macrostructure, microstructure, thermal conductivity, hardness, tensile properties, and wear rate are investigated. By means of differential thermal analysis, it is shown that upper limit alloying of C92900 bronze leads to a decrease in the solidus temperature by 40°C, which should be considered during deformation processing and heat treatment. An increase in the cooling rate during the solidification of C92900 bronze ingots provides a significant grain refinement and change in the amount, size, and morphology of the phases. For example, in the case of metallic and graphite mold casting, the size of the lead particles decreases and its circularity increases. The change in the Sn content within the range established by industrial standard has a significant effect on the intermetallic γ-(Cu, Ni)₃Sn phase fraction. The increase in the cooling rate has no significant effect on the thermal conductivity of C92900 bronze, but it leads to an increase in hardness by 30 HB. It also improves the yield strength and ultimate tensile strength of the bronze. The wear test, carried out in accordance with the shaft–partial insert scheme in a kerosene medium using a steel counterbody, shows that an increase in the cooling rate during solidification leads to an increase in the wear rate of bronze from ~0.4 × 10^–8 to ~1.2 × 10^–8. The change in the bronze composition within the industrial standard range has practically no effect on the wear rate, but leads to a slight increase in the coefficient of friction.