Development of Predictive Formulae for the A1 Temperature in Creep Strength Enhanced Ferritic Steels

Download or read book Development of Predictive Formulae for the A1 Temperature in Creep Strength Enhanced Ferritic Steels written by Lun Wang and published by . This book was released on 2010 with total page 111 pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: The creep strength enhanced ferritic (CSEF) steels P91 and P92 are extensively used in fossil supercritical power plants due to their improved creep strength at elevated temperatures. Loss of creep strength and/or toughness may occur in CSEF steel welds due to formation of fresh martensite, ferrite, or retained austenite during welding and post weld heat treatment (PWHT). Predicting the critical phase transformation temperatures is of practical importance for the development of appropriate welding and PWHT procedures. The available empirical formula proposed by Andrew for the determination of the A1 temperature does not cover the composition range of CSEF steels and the effect of carbon and nitrogen has not been taken into account. Thermodynamic simulation software such as Thermo-Calc and JMat-Pro could predict equilibrium transformation temperatures (i.e. equilibrium formation of austenite, ferrite in steels). However, the microstructure in CSEF steels which is of practical importance for determination of the A1 temperature is tempered martensite (in as delivered condition). Santella developed two formulae for P91 and P92 steels based on thermodynamic simulation data recently. Predictions by Santella's formulae tend to be conservative estimates of the A1 temperature in P91 and P92 steels. Thermodynamic simulations and experimental measurement based Design of Experiment (DOE) approaches and fractional data analysis were applied to develop formulae for predicting the A1 temperature in P91 and P92 steels. The alloying elements with a significant effect on the A1 temperature in P91 steels screened out by the thermodynamic simulation based DOE approach are: Ni, Mn, Si, N, Cr, Mo, C, V. No interactions were found between these significant alloying elements. A measurement based DOE was developed based on the elements with significant effect on the A1 temperature determined by the thermodynamic simulation based DOE. The test samples were melted using a button melting system in argon environment. These samples were subjected to homogenizing, rolling, normalizing, and tempering in order to reproduce to the manufacturing process of commercial CSEF steels. The A1 temperatures in these samples were measured by Single Sensor Differential Thermal Analysis (SS-DTA). The results of the measurement based DOE were processed using fractional data analysis to develop predictive formulae for the A1 temperature in P91 and P92 steels. It was found that C, N, Si and Cr could influence the A1 temperature largely; Ni and Mn were not the only determining factor of the A1 temperature. The relations between the concentrations of most alloying elements and the A1 temperature of P91 and P92 steels were quadratic function. The predictions of these formulae were validated by comparison to measured values of the A1 temperature in P91 and P92 steels by dilatometry.