The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M 7 C 3 carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M 2 C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.

An HSS roll and a Ni-grain roll, commercially manufactured in a centrifugal casting method, were used in this study. CommunicationsTheir chemical compositions (as per input constituents) are listed in Table I. The specimens were cut from the shell part of commercially cast rolls. The HSS roll specimens Phase Analysis of Two Steel Work experienced a series of heat treatment, i.e., austenitization, Rolls Using Mössbauer Spectroscopy air cooling, and double tempering, and were taken at intermediate stages, such as (1) after austenitization at 1050 ЊC for SEI JIN OH, SOON-JU KWON, HONGSUG OH, 1 hour followed by air cooling, (2) after first tempering at SUNGHAK LEE, and KEUN CHUL HWANG 540 ЊC for 1 hour, and (3) after second tempering at the same condition. By the first tempering, a large amount of Responding to an increasing demand lately in the hot retained austenite is removed. The second tempering rolling process to produce rolled steel plates with homogeincreases fracture toughness by transforming martensite to neous thickness and even surface and to improve the productempered martensite. [6] The Ni-grain roll specimens were tivity, continuous studies on developing rolls with enhanced subjected to the stress-relief treatment only at 450 ЊC for 15 properties in wear resistance, strength, fracture toughness, to 30 minutes after casting without austenitization. The heatand thermal fatigue have been made. [1][2][3][4] Because rolls are treatment time of the specimens was decided according to cast in large sizes and cannot go through a subsequent hot that of large commercial rolls. deformation process, the cast structure composed of coarse After polishing and etching the roll specimens, their carbides is retained, and the matrix alone is transformed microstructures were examined by an optical microscope into tempered martensite by austenitization and tempering and SEM. The fractions of various carbides and graphites treatments. [5] Heat-treated rolls are affected by various facpresent in the roll specimens were measured using an image tors such as chemical composition, casting condition, and analyzer. Hardness of the matrix was measured by a Vickers heat-treatment condition, and thus have a complex structure hardness tester under a 25-g load, and hardness of the overall with many phases such as carbides, tempered martensite, bulk was measured by a Rockwell hardness tester (C scale). austenite, pearlite, and graphite. [6] Although most coarse X-ray diffractograms were taken in the 2 range of 30 carbides are not influenced by heat treatment, retained austo 100 deg using a monochromatic Cu K ␣ radiation. The tenite formed after austenitization seriously deteriorates diffractometer (Rigaku, D/MAX 3-C) was equipped with a hardness and wear resistance. Therefore, it is required to graphite monochromator and operated at a voltage of 40 kV carefully analyze the microstructural changes varying with and at a current of 40 mA. After calculating the exact angle the casting and heat-treatment conditions, together with parof peaks ...