Effects of Alloying Elements on Microstructure and High Temperature Tensile Properties of Cast Austenitic Stainless Steels

Abstract

In automotive turbo-chargers and exhaust systems, excellent high-temperature properties are required for retaining their structures at extremely high exhaust gas temperatures, and thus heat-resistant austenitic stainless steels have been actively developed. Heat-resistant austenitic stainless steels have been intensively used worldwide for turbo-chargers because they satisfy strict requirements and demands of high economy, environmental friendliness, and high performance. They also have advantages of excellent high-temperature hardness, strength, and thermal fatigue life over conventional stainless steels. Austenitic cast steels are highly alloyed steels having excellent high-temperature properties. In recent years, they have been applied to heat-resistant automotive turbo-chargers for several inherent advantages over conventional stainless steels, such as excellent hardness, strength, fracture toughness, thermal fatigue life, and fine and stable microstructure. Their cast parts retain their shape for a prolonged time offering a good opportunity for producing high-quality cast parts for high-temperature operation. They usually contain nickel for the stabilization of austenite matrix, chromium for reduced oxidation and corrosion and increased hardness, and tungsten and niobium for carbide formation and red hardness. Sometimes molybdenum is added for improving high-temperature hardness and for strengthening the austenite matrix. Firstly, in this study, attempts are made to identify the correlation between microstructures and mechanical properties of heat-resistant austenitic cast steels. It also intends to present essential conditions in alloy designing and manufacturing by investigating effects of alloying elements. Three cast steels were fabricated by varying contents of W, Mo, and Al, and their respective effects on room- and high-temperature tensile properties were investigated. The steels were designed in such a way that the formation of homogeneously distributed hard carbides was maximized by adjusting the content of carbide formers to enhance high-temperature tensile properties. Intensive investigation was conducted on various kinds of phases existing at high temperatures, i.e., austenite, ferrite, and carbides, to analyze the correlation between microstructural factors and mechanical properties. In addition to the experimental approach, fractions of phases were verified by thermodynamic calculations, and the quantitative data were compared with experimentally obtained data to effectively evaluate the high-temperature performance heat-resistant austenitic cast steels. Secondly, austenitic stainless steels were designed in a way how the formation of homogeneously distributed hard carbides was optimized by adjusting alloying elements to improve high-temperature tensile properties. This attempt is also divided into two parts. The one is, a part of W was replaced by Nb or Cr because W is a very expensive alloying element. Three austenitic stainless steels were fabricated by varying contents of W, Nb, and Cr, and their room- and high-temperature tensile properties were investigated. In addition to the experimental approach, fractions of equilibrium phases existing at high temperatures, i.e., austenite, ferrite, and carbides, were examined by thermodynamic calculations, and the quantitative fraction data were compared with experimentally obtained data to effectively explain the high-temperature performance of the heat-resistant austenitic stainless steels. The other is, five heat-resistant austenitic stainless steels were designed in a way how the formation of carbides was optimized by adjusting W content and by replacing a part of W by Mo in order to achieve excellent high-temperature tensile properties, and their room- and high-temperature tensile properties were evaluated. The correlation between high-temperature tensile properties and microstructural factors such as austenite, ferrite, and carbides was investigated by actually measured fractions of microstructural factors together with thermodynamically calculated fractions.