The properties of Ni-Al bronze sand cast ship propellers in relation to section thickness

Abstract

From more than one hundred Nickel-Aluminium (Ni-Al) bronze sand cast ship propellers, the effect of thickness on the properties of the material has been investigated. For this purpose the mechanical and corrosion fatigue properties have been measured on pieces of material from the thickest part of the castings with varying thicknesses from 25 mm to 450 mm. For each range of cast section thickness the average of the results is presented. It is observed that the properties of the casting material deteriorate with increasing section thickness. Linear regression analysis has been applied to establish a relation between the properties of the casting material and section thickness. To provide a basis for the calculation of the corrosion fatigue strength of the casting material at a given section thickness, the measured corrosion fatigue life is related to the ultimate tensile strength. By comparing corrosion fatigue curves of small and large castings, corrosion fatigue strength conditions can be set up for each range of cast section thicknesses. It is shown that the deterioration of properties of Ni-Al bronze castings with increasing section thickness is associated with microstructural changes such as coarsening of grain size and phase structure attended with segregations. Relations can be given of tensile strength and corrosion fatigue properties to grain size.

From these relations, conditions for the structure can be drawn up to obtain the optimum properties of the casting material, even for thick sections of large ship propellers. These conditions can only be fulfilled by extensive testing and control of the manufacturing process.