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INTRODUCTION

FERRITIC spheroidal graphite cast iron (ferritic S. G. I.) is an important engineering material and is usually stable up to 873 K. However, intermediate temperature embrittlement occurs over a temperature range of 673 K. Recent papers by Yanagisawa e t a l . H'2'3] have provided some understanding of these characteristics. Those authors suggested that the minimum ductility of ferritic spheroidal graphite cast irons at 673 K was a direct result of intergranular fracture influenced by dynamic strain aging and a triaxial stress field. The triaxial stress field developed in the ferrite matrix between graphite nodules. Although Lui ~41 showed that adding Mo or Ce would render ferritic spheroidal graphite cast irons immune to intermediate temperature embrittlement, the mechanism is not well understood. Yanagisawa's investigation t~l showed that the fracture surface of the steel (3.08 pct Si, 0.67 pct C) changes from a dimple pattern in the smooth specimen to predominant intergranular fracture in the artificial necked specimen at 673 K. Tetelman e t a l . ' s I51 investigation showed that the cleavage stress of a notched bar of high nitrogen steel increases as the root radius of the notched decreases. Agogino t6j reported that the phenomenon of embrittlement of steel subject to high temperatures was recognized along with the potential of notches and high tensile stresses in accelerating the embrittlement. Bridgman [7] demonstrated that strains in a tensile test could be greatly increased if the test was carded out under pressure to reduce the value of the tensile mean stress in the neck. Since the graphite nodules can be considered as spherical v o i d s f '21 we changed the triaxial stress field by elongating the spheroidal graphite to study the influence of the triaxial stress field on intermediate temperature embrittlement of ferritic spheroidal graphite cast irons. C.G. CHAO, Graduate Student, T. S. LUI, Associate Professor, and M. H. HON, Professor, are with the Department of Materials Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. Manuscript submitted April 14, 1987.

METALLURGICAL TRANSACTIONS A

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EXPERIMENTAL PROCEDURE

The materials used were melted in a basic frequency induction furnace from a charge of raw pig irons, spheroidized by Fe-45 wt pct Si-4 wt pct Mg alloy, inoculated with Fe75 wt pct Si alloy and cast into a 65 m m • 90 m m • 120 mm CO: mold of Y block shape. The chemical composition identified by emission spectrometry is listed below: wt pct

C 3.52

Si 2.80

Mg 0.05

Mn 0.012

P 0.035

S 0.002

The casting was machined into various size round bars which were heated to 1193 K and rolled for an axisymmetric reduction of (rb = 1 -- ( d J d o ) : ) from 10 pct to 70 pct. dl and do are the final diameter and the original diameter of the round bars, respectively. The detailed rolling process is listed in Table I. Before each pass, the round bars were reheated to 1193 K and soaked for 20 minutes. After the rolling process, those round