Correlation of microstructure and thermal-fatigue properties of centrifugally cast high-speed steel rolls
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1/8/04
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Correlation of Microstructure and Thermal-Fatigue Properties of Centrifugally Cast High-Speed Steel Rolls CHANG KYU KIM, JONG IL PARK, JAE HWA RYU, and SUNGHAK LEE This is a study of thermal-fatigue properties in centrifugally cast high-speed steel (HSS) rolls. The thermal-fatigue mechanism was investigated, with special focus on the roll microstructure and the increase in tensile stress which led the specimens to fracture when it reached the tensile strength. The thermal-fatigue test results indicated that the thermal-fatigue life decreased with increasing maximum temperature of the thermal-fatigue cycle. The results were then interpreted based on the amount of carbides and the cyclic-softening phenomenon associated with the exposed time to elevated temperatures. The coarse intercellular carbides on the specimen surface acted as fatigue-crack initiation sites as they cleaved at a low stress level to form cracks. The roll having the lowest matrix hardness and the smallest amount of intercellular carbides, thus, showed better thermal fatigue properties than the other rolls. For the improvement of the thermal-fatigue properties of the rolls, this study suggests a homogeneous distribution of carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing the roll compositions.
I. INTRODUCTION
AS rolled products, having a more uniform and lower thickness, smoother surface, and higher strength, have been widely demanded, hot-rolling conditions have become severe.[1,2] In order to match these demands, rolls having improved mechanical properties such as wear resistance, hardness, fracture toughness, and thermal fatigue have been required.[3,4] Among such properties, thermal-fatigue properties play an important role in quality improvement of the rolled products and the roll durability, because surface roughening and cracking are closely related to thermal fatigue.[5,6] Thermal fatigue of the rolls is caused by the sudden increase and decrease of temperature as they contact with rolled plates at high temperatures, followed by water cooling. The surface region (less than 1.0 mm from the surface) which experiences the thermal fatigue is placed under a complete constraint by the roll interior, which is not affected by the heat.[7,8] Thus, the roll surface can be consistently roughened, and cracks can be initiated at the surface because of the continuous expansion and contraction caused by the contact with hot-rolled plates and subsequent water cooling.[9,10] In order to establish appropriate rolling conditions, the thermal-stress variation at the complete constraint state, as well as the role of microstructural factors on thermal-fatigue properties, should be understood. In other words, to analyze the thermal fatigue phenomenon and to examine factors related to the thermal fatigue during hot rolling, a systematic understanding CHANG KYU KIM, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technolo
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