Duplex stainless steels have excellent corrosion resistance, but its insufficient wear resistance limits its application scope. Therefore, alloying of duplex stainless steels to form hard phases, such as carbides, becomes one of the important research directions to enhance their wear resistance keeping their good corrosion resistance. Specifically, carbides are considered as an ideal hard phase to strengthen Fe-Cr-C alloys, where their types, hardness, volume fraction, morphology, size, spacing and interconnecting feature are the important factors affecting the wear resistance of the alloys. It has been shown that, Si as an alloying addition can modify the carbide precipitates in type, phase morphology and distribution in Fe-Cr-C alloys, having an obvious influence on their wear and corrosion resistance as well as their mechanical properties. In this work, the microstructure and properties of two types of ultra-high chromium (40%, mass fraction) and high carbon (1.5%) duplex stainless steels with different concentrations of Si (0.46% and 1.36%) are investigated. The composition, as-cast microstructure and solidification process as well as the changes in the microstructure, phase structure after solution treatment were studied chemical analysis, OM, SEM, EPMA and XRD. The mechanical properties including hardness, tensile strength, fracture toughness, corrosion resistance and wear resistance have been tested correspondingly. It is revealed that, both the two kinds of duplex stainless steels have a constitution of three phases in as-cast state, i.e. gamma phase, sigma phase and M23C6. During the solidification process of the steel with 0.46%Si, 6 ferrite dendrite forms at the beginning, followed by eutectic (delta+M23C6), peritectic gamma, and finally eutectic (gamma+ M23C6), in which the delta phase transformed into eutectoid (gamma(2)+sigma) in the subsequent cooling process. For the duplex stainless steel with 1.36%Si, the increase of delta ferrite amount is observed leading to obviously increased content of sigma phase, the morphology of peritectic gamma becomes intermittent and irregular shape, and no eutectic (gamma+M23C6) forms. After solution treatment, the two kinds of steels are composed of ferrite, austenite and M23C6 type carbide. Note that, the volume fraction and continuity of alpha ferrite are promoted obviously by increasing Si content from 0.46% to 1.36%. The Si addition slightly improves the hardness, tensile strength and fracture toughness of the duplex stainless steel, while has little effect on the corrosion resistance and slightly reduces the wear resistance.
