Effects of microstructure and carbide spheroidization on fatigue behaviour in high V–Cr–Ni cast irons
Introduction
A new cast iron with vanadium carbides (VCs) dispersed within austenitic matrix microstructure, high V–Cr–Ni cast iron, has been developed, which has excellent corrosion and wear resistance and high strength [1]. Therefore, this cast iron is expected for extensive engineering applications as a structural material that can be used as a replacement of traditional cast irons such as nodular cast irons. However, the mechanical properties, in particular fatigue properties that are critical for structural materials have not been studied.
In a previous work [2], the authors have investigated the fundamental fatigue behaviour of a high V–Cr–Ni cast iron with a specific microstructure, and indicated that VC had a contradictory role in fatigue behaviour where cracks frequently initiated at VC cluster, while it acted as a barrier to early small crack growth. When considered these findings and the complicated constituent microstructure of high V–Cr–Ni cast irons, it is very significant to understand the role of microstructural variables in fatigue behaviour for establishing an optimum microstructure in practical application and contributing further material modification.
In the present study, rotary bending fatigue tests were performed using smooth specimens of high V–Cr–Ni cast irons with different C and V contents, i.e. microstructures and VC morphologies, and the effects of microstructure and VC spheroidization on fatigue behaviour were discussed on the basis of crack initiation, small crack growth and fracture surface analysis.
Section snippets
Material
The materials used are six high V–Cr–Ni cast irons with different microstructures and VC morphologies. Their chemical compositions are listed in Table 1. As can be seen in the table, the materials posses the Cr and Ni content comparable to austenitic stainless steels and considerable C and V content.
In this study, C and V content has been widely changed in order to produce different microstructures, in particular the amount of VC that would exert significant influence on fatigue behaviour. Each
Microstructure characterization
Fig. 2 shows an example of EDS analysis of the microstructure in 2C6.5V, where (a) is SEM image, (b), (c) and (d) are the colour maps of V, C and Cr, respectively. As can be seen in Fig. 2(a), the microstructure consists of three distinct regions: granular precipitates, flat or featureless region and lamellar region. In the granular precipitates, V and C are strongly detected, thus they are VCs. The flat or featureless region is believed to be the austenitic matrix microstructure, because of
Relationship between fatigue strength and tensile strength
Fig. 13 illustrates the fatigue strength characterized in terms of fatigue ratio, σ/σB. Also included are the results for three types of cast irons, FCD450, FCD700 and ADI, for comparison [2]. It should be noted that all high V–Cr–Ni cast irons exhibit higher relative fatigue strength compared with those in FCD700 and ADI which have nearly equal tensile strength, and such a trend is particularly remarkable in 3C10V(1) with a very high fatigue ratio of 0.68 at fatigue limit. Such excellent
Conclusions
Rotary bending fatigue tests were performed using smooth specimens of materials with different C and V contents, i.e. microstructures and morphologies of vanadium carbides (VCs) in high V–Cr–Ni cast irons, and the effects of microstructure and VC spheroidization on fatigue behaviour were discussed on the basis of crack initiation, small crack growth and fracture surface analysis. The conclusions obtained are summarized as follows:
- 1.
In the materials with non-spheroidal VC, fatigue strength roughly
Acknowledgements
The authors wish to thank Mr Y.Yamakage for experimental assistance.
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