On Phase Distribution and Phase Transformations in Phosphoric Irons Studied by Metallography
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THE Gupta-period Delhi Iron Pillar is a living testimony to the high level of skill achieved by ancient Indian ironsmiths in the extraction and processing of iron. This Pillar has attracted the attention of engineers, especially corrosion technologists, because it has withstood corrosion for more than 1600 years. The presence of relatively high phosphorus (0.25 wt pct) in the forgewelded Pillar plays a major role in its excellent corrosion resistance.[1,2] Phosphorus facilitates the formation of a protective passive film on the surface, which provides the Pillar its exceptional corrosion resistance properties. Recently, the remarkable corrosion resistance of phosphoric irons (containing up to 0.5 wt pct phosphorus) has been demonstrated in aggressive concrete environments.[3,4] Recognizing the annual economic loss due to corrosion in both developed and developing nations, it is important to study the possible use of phosphoruscontaining steels in modern applications. Phosphorus in steel leads to ‘‘cold shortness,’’ due to the phosphorus segregation at the grain boundaries. However, carbon can replace phosphorus from the grain boundaries by the site competition effect, when Fe-P alloys contain a low amount of carbon.[5,6] Fe-P alloys containing 0.01 to 0.05 wt pct carbon have exhibited good ductility.[7,8] Moreover, improved mechanical properties have been reported for vacuum-melted pure Fe-P alloys, after suitable control of the microstructure.[8] GADADHAR SAHOO, formerly Doctoral Student, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, is Assistant Manager, R&D Center for Iron and Steel, Steel Authority of India Limited, Ranchi 834002, India. R. BALASUBRAMANIAM, Professor, is with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India. Contact e-mail: [email protected] Manuscript submitted August 10, 2006. Article published online July 6, 2007. 1692—VOLUME 38A, AUGUST 2007
The control of the microstructure is important in realizing ductile phosphoric irons. The phase diagram of Fe-P provides some ideas for heat-treating Fe-P alloys of low P content to render them ductile. The Fe-P alloy with P content in the range of 0.0 through 0.68 wt pct (Figure 1(a), reported by Kubaschewski[9] for pure Fe-P alloys) and 0.0 through 0.57 wt pct (Figure 1(b), reported by Vogel[10] for samples containing impurities) reveal austenite of lower phosphorus content at the grain boundaries of ferrite after heat-treating in the twophase (a + c) region. This inhomogeneity of P created at a high temperature persists even at room temperature. Therefore, the heat treatment of phosphoric irons of low P content will open several possibilities regarding microstructures and phase distribution; one of the aims of this article is to address these issues in novel phosphoric irons. The other aspect of the study was to realize good mechanical properties after heat treatment in the twophase (a + c) region, to highlight the fact that ductile phosphoric iro
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