Correlation of microstructure and microfracture mechanism of five work rolls
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GWIHWAN BYUN, Section Manager, is with the Hot Rolling Technical Team, Hot Rolling Department, Pohang Iron and Steel Co. ,Ltd., Pohang, 790-360 Korea. SEUNGCHAN OH, Research Engineer, is with the Materials Research Department, Kia Technical Center, Seoul, 790-784 Korea. CHANG GIL LEE, Senior Researcher, is with the Materials Engineering Department, Korea Institute of Machinery and Materials, Changwon, 641-010 Korea, SUNGHAK LEE, Professor, is with the Center for Advanced Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. Manuscript submitted May 5, 1997. 234—VOLUME 30A, JANUARY 1999
Table I.
Chemical Compositions of the Five Rolls Used (Weight Percent)
Roll
C
Si
HSS Hi-Cr Ni-grain Adamite DCI
2.0 2.8 3.2 1.5 3.35
1.0 1.0 0.83 0.5 2.0
Mn
Cr
1.0 5.0 1.07 15.6 0.68 1.93 1.3 1.17 0.5 0.1
Ni
Mo
P
S
1.0 1.28 3.6 1.12 2.7
2.5 1.27 0.32 0.3 0.75
0.04 0.034 0.048 0.027 0.4
0.02 0.017 0.016 0.023 0.2
V Mg 4.0 — — — —
— — — — 0.1
ness of the five currently commercialized work rolls and by clarifying their respective fracture mechanisms. They are high speed steel roll (HSS roll), high chromium cast iron roll (Hi-Cr roll), Ni-grain cast iron roll (Ni-grain roll), Adamite roll, and ductile cast iron roll (DCI roll). The microfracture behavior was observed by using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber, while simultaneously measuring apparent fracture toughness. These experiments enable evaluation and interpretation of fracture toughness, and thus the results are expected to be used as the basic data for the selection of work rolls depending on the rolling stands. The rolls used in the present study are the HSS, Hi-Cr, and Ni-grain rolls fabricated by a centrifugal-casting method and the Adamite and DCI rolls fabricated by a static-casting method. Their chemical compositions are shown in Table I. The HSS and Hi-Cr rolls went through the heat-treatment process of austenitization at 950 7C to 1050 7C, air cooling, and double tempering at 450 7C to 600 7C. The Ni-grain roll was tempered at 450 7C to 600 7C without austenitization, while the Adamite roll was fabricated through austenitization, air cooling, and tempering. The DCI roll was fabricated through austenitization, air cooling, and austempering[9] treatment. Specimens were all obtained from the shell part of the rolls. After the roll specimens were polished, the HSS roll was etched by Murakami etchant (3 g K3Fe(CN)6 1 10 g NaOH 1 100 mL H2O),[10] in which M2C (black), M7C3 (light pink), and M6C (pink) carbides are selectively etched but not the matrix and MC carbides. The other rolls were etched by 3 pct nital etchant to observe the microstructures by an optical microscope. The chemical composition of carbides was analyzed by energy dispersive spectroscopy (EDS), and the volume fraction and size of carbides and graphites were quantitatively analyzed by an image analyzer. The microhardness of the matrix and carbides was measured by a vickers hardness tester under 25-g load, while t
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