Modelling phase transformation and tensile properties of micro-alloyed structural steels for fire resistance

Abstract

Fire is one of the most extreme situations in which structural steels can be damaged, culminating in catastrophic failure. Common structural steels incur subsequent changes in physical characteristics, stiffness, and mechanical properties because of high temperature treatment, which is irreversible after cooling. As a result, many alloy steels have been created to address these issues. Steel produces ferrite to austenite transformation temperatures of 700–750 °C and relative instability of carbide precipitates over 650 °C since it is not fire resistant. Few steel grades have been developed to prevent this, which has led to an increase in the use of certain steel grades at higher temperatures. The demand for fire-resistant high-temperature construction steels has increased as construction technology has improved, resulting in shorter construction periods and more efficient use of space. The aim of the research was to establish a common structural steel by adding a few specific alloying elements like Mo, Cr, Al, and Ti that will assist the steel to maintain its strength at high temperatures. Using thermodynamic data for phases and mobility, CALPHAD-based computational techniques provided insights of microstructure and its impact on different properties.The addition of Mo and Cr retarded the cementite growth and widened the melting range. Consequently, solidus line shifted at higher temperature. Addition of Mo passively raised the cooling rate by increasing time of formation of ferrite, pearlite, austenite and martensite. Mo and Cr enhanced the precipitation of intermetallic carbides of high melting temperatures and controlled and retarded the growth of such carbides. Thus, yield strength of fire-resistant steel was greater than that of conventional steels at elevated temperatures. Owing to low yield ratio, the alloys had a higher ductility. High temperature steels had the same workability and weldability as standard steels. Mo addition significantly modified the elevated temperature strength. To consider the economic perspective, an alternative to Mo can be Cr which is quantitatively found in the prediction. Addition of Cr over Mo might not give better high temperature mechanical properties than solely added Mo but provided superior properties than the base alloy. Combined Mo and Cr addition retarded the formation of cementite which is brittle in nature. Mo addition is exclusively responsible to increase the phase transformation time that will definitely reduce the residual stress of the sample while cooling. The predictions derived from statistical data will undoubtedly aid in the development of experiments for future research.