Prediction of Bainite Start Temperature in Alloy Steels with Different Grain Sizes

Singon Kang, Suchul Yoon, Seok-Jae Lee
2014 ISIJ International  
KEY WORDS: bainite start (Bs) temperature; alloying element; prior austenite grain size (PAGS); alloy steels. Bainite microstructure in alloy steels yields materials with an excellent combination of mechanical properties such as high strength and toughness, resistance to creep and fatigue, and hydrogen embrittlement. 1,2) During cooling, bainite transformation is initiated at the bainite start (Bs) temperature, which is the highest isothermal temperature at which upper bainite is observed, and
more » ... eases when the temperature reaches the martensite start (Ms) temperature. Addition of alloying elements mostly lowers the Bs temperature. Especially, carbon and manganese known as strong austenite stabilizing elements effectively decrease the Bs temperature. Several quantitative investigations have been performed to determine the relationship between the addition of alloying elements and Bs temperature variations in alloy steels. Some empirical equations have been proposed to predict the effect of adding alloying elements to carbon and alloy steels on Bs temperature. 3-12) Prior austenite grain size (PAGS) can also affect the Bs temperature. Lee et al. 13) reported that Bs temperature decreased with decreasing PAGS. Bs temperature variations due to the decrease in PAGS simultaneously influenced bainite transformation kinetics. [13] [14] [15] However, few researchers have attempted to develop the equations for Bs temperature by considering both chemical composition and PAGS effects. Therefore, in the present study, we developed a simple empirical equation for Bs temperature prediction that includes both the alloying element effect and the PAGS effect based on experimental data obtained from the literature. We compared the accuracy of our empirical equation with existing equations and verified the performance of our equation using experimental data. Experimental Bs temperature data for carbon and alloy steels were extracted from published time-temperaturetransformation (TTT) diagrams. 16) Both chemical composition and PAGS were used to evaluate existing equations and to derive a new equation. We selected the following alloying elements in the present work: C, Mn, Si, Ni, Cr, and Mo. Data for other alloying elements were excluded. The chemical composition range of the selected steels was limited to that of low alloy steels, thus high alloyed steels such as a stainless steel was excluded. Ranges of chemical composition, PAGS, and B s temperature used for equation derivation are summarized in Table 1 . The total number of B s data points used in the present work was 97. The effects of alloying elements on B s temperature prediction are generally expressed by a linear type relationship as follows: [3] [4] [5] [6] 8, 10)
doi:10.2355/isijinternational.54.997 fatcat:h7472skusffonnvvpxoziaty5y