Tensile fracture of coarse-Grained cast austenitic manganese steels
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I.
INTRODUCTION
HADFIELD (cast austenitic manganese) steels have long been known for their high toughness and hardening properties. I~-61 The tensile deformation of coarse-grained cast Hadfield steels has been characterized in a previous work, and the main macroscopic features observed were orange-peel formation, coarse slip bands, and lack of tensile necking. [7] The present work is a characterization of tensile fracture mechanisms, in an attempt to understand both the high toughness of the material and the relationship between inhomogeneous deformation and subsequent fracture processes.
II.
EXPERIMENTAL
The composition and microstructure of five heats of cast Hadfield steel employed in this study have been reported previously, t7'81 and will consequently be briefly described here. The steel was cast into keel blocks in order to optimize soundness. Carbides were dissolved by heating to 1050 ~ followed by quenching into water at room temperature. The resulting microstructure is austenitic. The typical grain size is widely distributed, and ranges from 0.25 to 1 millimeter. Some microporosity and residual interdendritic carbides (undissolved or reprecipitated during cooling from solution treatment) are discernible along with dendritic segregation (visible after repeated etching). Round (8 mm diameter) and flat square cross-section (6 mm side) tensile specimens were machined from the bottom of the keel. Final dimensions were obtained by fine grinding, in order to minimize the formation of a surface-hardened layer. Most of the tensile testing was performed on a closed loop servo-hydraulic machine, under load-control (104 N/min) at ambient temperature in air, employing an average of five specimens for each heat and geometry.
D. RITTEL, Doctoral Candidate, and I. ROMAN, Associate Professor and Head, are with Materials Science Division, Graduate School of Applied Science and Technology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. Manuscript submitted September 18, 1987.
METALLURGICAL TRANSACTIONS A
III.
RESULTS
A striking difference is observed when the mechanical properties of round and square specimens are compared, as illustrated in Table I.* Both types of specimens possess *The results shown in Table I are for heat MS4 in Reference 8, with 0.25 wt pct mckel added. These results illustrate the general trend which was observed in all investigated heats.
nearly identical yield strength, whereas tensile elongation, reduction of area, and consequently true fracture strength are markedly higher for round specimens. Examination of the samples, during loading and after rupture, disclosed numerous cracks on the surface of both types of specimens (Figure 1). These cracks developed on planes almost perpendicular to the loading direction. However, the average size of the cracks varied for each type of specimen. As can be seen from Figure 1, the cracks are visually detected on the surface of the square specimen (after 0.25 plastic strain), whereas they are hardly noticeable on the surface of the fractur
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