Evaluation of the fatigue behavior of ductile irons with various matrix microstructures
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I. INTRODUCTION
THERE is an increasing trend to add small amounts of Ti to steels to increase their strength and deep drawability, with the aim of making lighter and thinner components, particularly in the automobile industry. As a result, there is an increasing amount of Ti-containing steel entering the scrap metal cycle and, therefore, becoming available for use by the iron foundry industry, including use for the manufacture of ductile iron castings. Extensive research[1–5] demonstrated that the solubility of Ti in iron matrices of ferrite, pearlite, and ausferrite is quite small. Nearly all titanium in cast iron will form inclusions, which are mainly TiC, TiN, and Ti(C,N), usually of a few microns in size and having a cuboidal/triangular shape. Several researchers[6–10] have concluded that in ductile irons, Ti decreases nodularity (i.e., the fraction of graphite particles having a well-formed nodular shape). However, Mg and rare-earth elements can neutralize the deleterious effect of Ti. As for the effect of Ti on the graphite nodule count, Riposan et al.[9] indicated that the nodule count stayed roughly constant until 0.1 wt pct Ti and then increased strongly with the Ti content. Again, it was also found that the addition of rare-earth elements could neutralize the effect of Ti content on nodule count. When ductile iron castings are used for engineering applications, it is often important to consider their fatigue properties. One of the aims of this present study is to assess whether the fatigue properties of ductile cast irons with various matrices are affected by the presence of up to 0.11 wt pct Ti, resulting from the use of Ti-containing steel scrap. The effects of Ti content on fatigue properties could conceivably J. LUO, Research Fellow, and P. BOWEN, Head, Department of Metallurgy and Materials, and R.A. HARDING, Senior Research Fellow, IRC in Materials Processing, are with the University of Birmingham, B15 2TT, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
derive from the effects of Ti content on the graphite nodule size, count, and morphology and on the nature and size distribution of inclusions. The general factors affecting fatigue properties of ductile irons are reviewed as follows. A. Inclusions and Graphite Nodules Inclusions can start cracking by either interface debonding or internal inclusion cracking[11] at the very beginning of the fatigue life. While fatigue crack formation by interface debonding is mostly dependent on the bonding strength of the interface, the possibility of crack formation by inclusion cracking increases with an increase in the difference of elastic properties between inclusions and the surrounding matrix (this will increase the stress in inclusions). Inclusion cracking is also enhanced with a decrease in the ductility and increase in the brittleness of inclusions. However, the exact situation is often unclear. In the literature, inclusions are often treated as stress raisers, and the incl
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