Quantification of the Phase Transformation Kinetics in High Chromium Cast Irons Using Dilatometry and Metallographic Techniques

Abstract

Further development of high chromium cast irons (HCCI) is based on tailoring the microstructure, necessitating an accurate control over the phase transformation and carbide precipitation temperatures and can be achieved by thermal treatments (TT). To understand the underlying mechanisms controlling the transformation kinetics during the different stages of the TT, it is imperative to adjust the TT parameters to have information of the transformations occurring during non-thermal and isothermal heating cycles, since proper selection of the TT parameters ensures the optimum use of the alloying elements. In this work, the boundaries of the phase transformations for a HCCI containing 26 wt pct Cr for different cooling rates (continuous cooling transformation, CCT, diagram) were established by applying dilatometric measurements. Based on the CCT diagram, a temperature-time-transformation (TTT) diagram was constructed by isothermally holding the samples until complete phase transformation. For determining the initiation and finishing of the transformation, the lever rule assisted by derivatives was applied. The phases present after transformation were determined by combining X-ray diffraction (XRD) and metallographic characterization using optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the data obtained from the dilatometer was experimentally verified by isothermally heat treating some samples using laboratory furnaces. The transformed phase fraction from OM and SEM images was then correlated to the fraction obtained from the TTT diagram.

Inhibition of Carbide Growth by Sr in High-Alloyed White Cast Iron

“…The microstructure of the samples consists of eutectic M 7 C 3 carbides in an austenitic matrix. At the carbide-matrix boundaries, the martensite formed, which, according to other researchers [29,30], may be a result of C and Cr depletion in this area.…”

Analysis of the carbide precipitation and microstructural evolution in HCCI as a function of the heating rate and destabilization temperature

“…Once the target temperature was reached, the samples were taken from the oven and immediately water quenched to retain the microstructure. The as-cast material consisted of M 7 C 3 eutectic carbides (EC), an austenitic matrix and a martensitic layer at the matrix/EC interface, as previously described 4 , 7 , 15 .The treatment temperatures were selected based on previously published and unpublished results obtained from three different methods namely: high-temperature X-ray diffraction, dilatometry, and MatCalc kinetic simulations 4 , and the temperatures typically employed for the destabilization of HCCI 4 , 16 , where the temperature range 950–1000 °C corresponds to the highest precipitation rate 13 .…”