Solidification structure and abrasion resistance of high chromium white irons
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INTRODUCTION
HIGH chromium white cast irons (WCIs) are used in a variety of applications where stability in an aggressive environment is a principal requirement, including the mining and mineral processing, cement production, and pulp and paper manufacturing industries. Superior abrasive wear resistance, combined with relatively low production costs, makes these alloys particularly attractive for applications where grinding, milling, and pumping apparatuses are used to process hard materials such as ore, coal, gravel, and cement. Their exceptional abrasive and erosive wear resistance results primarily from their high volume fraction of hard carbides, although the toughness of the matrix also contributes to the wear resistance. High Cr WCIs can be described as in situ composites with large, hard proeutectic and/or eutectic M7C3 carbides in a softer iron matrix (i.e., austenite, martensite, ferrite, pearlite, or bainite). The matrix structure observed most often in the as-cast high Cr white irons is austenite, but this can be changed after relatively simple heat treatments to one that is primarily, or completely, martensite. Although dependent on the specific composition of the high Cr white iron, the carbides are typically of the M7C3 type, where M includes Fe, Cr, and other carbide forming elements. The M7C3 carbides grow as rods and blades with their long axes parallel to the heat flow direction in the mold. The cast macrostructure of these alloys must also be well characterized, because wear-resistant alloys are typically used in castings with thick section sizes.[1,2,3] Several researchers have reported on the effects of solidification on the microstructure of a 15 wt pct Cr WCI,[4–11] ¨ .N. DOG ˇ AN, National Research Council Research Associate, and J.A. O HAWK, Supervisory Materials Engineer, are with the Albany Research Center, United States Department of Energy, Albany, OR 97321. G. LAIRD II, Mechanical Engineer, formerly with the Albany Research Center, United States Bureau of Mines, is with Predictive Engineering, Inc., Corvallis, OR 97333. Manuscript submitted August 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
and have performed detailed experiments concerning the relationship between its macrostructure and fracture toughness.[12] Other studies have examined the macrostructure of 30 wt pct Cr ferritic, hypoeutectic irons and its correlation to tensile strength and hardness.[13] Nonetheless, questions still remain about the influence of cast macrostructure and microstructure on the mechanical properties of the range of commercially important high Cr alloys containing between 15 and 26 wt pct Cr. Moreover, before further improvements can be made in these eutectic alloys,[14] their solidification behavior and mechanical properties must be well documented in order to provide a foundation for the subsequent development of new, or improved, grades of wearresistant alloys. A survey of the literature on the wear behavior of high Cr WCI makes it clear that wear resistance is a sensitive function of co
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