Precipitation and its effect on the design of cast heat resistant alloys

Heat resistant cast alloy (H-Series) steels such as HP modified and micro-alloyed HP, are used extensively in the petrochemical industry for reformer furnace tubing and ethylene pyrolisis coils. The HP alloys have a microstructure consisting of an austenitic matrix with finely dispersed carbides. The H-Series steels have evolved over many years of alloy development by trial-and-error additions of alloying elements. It is possible to further improve the performance of these steels through the use of computational thermodynamics based on a detailed understanding of the composition-microstructure-property relationships. This paper highlights some of the progress made in understanding the relationship between precipitation and creep properties of these alloys through using a combination of thermodynamic modeling, microstructural characterization, and mechanical property measurements. It also provides information on the creep properties of a new alloy designed based upon the improved understanding of the structure-processing-property relationship in these alloys.