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

María Agustina Guitar,U Pranav Nayak,Frank Mücklich,Anna Scheid,Leandro Nakamura,Dominik Britz,Dirk Ponge

doi:10.1007/s11661-020-05808-y

María Agustina Guitar, U Pranav Nayak + Show 5 more

Open Access

https://doi.org/10.1007/s11661-020-05808-y

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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.

Highlights

HIGH-CHROMIUM cast irons (HCCI) containing12-30 wt pct Cr and 2-3.5 wt pct C, are extensively used for high abrasion-resistant applications such as components that manipulate and mechanically process aggregates and raw materials.[1,2,3] Their wear resistance and mechanical properties mainly depend on the type, morphology and distribution of carbides, and on the nature of the supporting matrix structure which, in turn, depends on the chemical composition and on any subsequent thermal treatments (TT).[4,5,6] The currentManuscript submitted January 21, 2020
The martensite finishing temperature (Mf) was not detected during the measurement, which might indicated that Mf finds at lower temperatures
It was possible to establish the boundaries of phase transformations for different cooling rates in a Continuous cooling transformation (CCT) diagram for a HCCI containing 26 wt pct Cr

Summary

Introduction

HIGH-CHROMIUM cast irons (HCCI) containing12-30 wt pct Cr and 2-3.5 wt pct C, are extensively used for high abrasion-resistant applications such as components that manipulate and mechanically process aggregates and raw materials.[1,2,3] Their wear resistance and mechanical properties mainly depend on the type, morphology and distribution of carbides, and on the nature of the supporting matrix structure which, in turn, depends on the chemical composition and on any subsequent thermal treatments (TT).[4,5,6] The currentManuscript submitted January 21, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS A trend is to expand the uses and duty life of HCCI by exploring alternative TT that would affect the type and nature of precipitates. With this in mind, microstructure tailoring must be the base for the further development of HCCI with the aim to obtain the desired performance of the material for a specific application. Microstructure tailoring must be the base for the further development of HCCI with the aim to obtain the desired performance of the material for a specific application The goal of this approach is to find the best matrix/precipitates combination based on the nature, size and distribution of secondary carbides, which can be controlled by adjusting the temperature and annealing time at the destabilization and/or sub-critical diffusion (SCD) steps.[2,4]

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