Effects of aluminum content on the mechanical properties of a 9Cr-0.5Mo-1.8W steel
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I.
INTRODUCTION
IT has been known since the early 1950s that aluminum was responsible for the decreased creep resistance of carbon steel,[1] and a few article have since been published about the influence of aluminum on creep properties of carbon and low alloy steels.[2,3,4,5] According to these works, aluminum addition impaired creep resistance by the removal of ‘‘active nitrogen’’ and by the refinement of prior austenite grains. Experimental evidence[3] also showed that in low alloy steels of the Cr-Mo type, nitrogen was almost completely fixed by chromium by forming the CrN-type chromium nitride, and thus no active nitrogen existed. In this case, the decreased creep resistance was supposedly due mainly to the grain refinement effects of aluminum. As for the highly alloyed 9 to 12 pct chromium ferritic steels, however, the effects of aluminum on creep properties are not yet clear. It was suggested[6] that the decreased longterm creep rupture strength and ductility of some 12 pct chromium steels were due to the precipitation of aluminum nitrides precipitated on prior austenite grain boundaries which promoted intergranular failure. On the other hand, aluminum in a small amount was found not to affect the elevated temperature strength of 9 pct Cr steels.[7,8] Considering the importance of aluminum content in the quality control and the possibility of aluminum as a deoxidizer for the high chromium heat resistant steels, the current study was carried out to clarify the effects of aluminum content on the mechanical properties of a 9Cr-0.5Mo-1.8W steel. The steel was invented in Japan[9,10,11] to improve the steam parameter in the advanced fossil power plants and approved[12] in 1994 by ASME code committee as P92 and T92.
H. NAOI, Chief Researcher, M. OHGAMI, Senior Researcher, and X. LIU, Old Guest Researcher, are with the Steel Research Laboratories, Nippon Steel Corporation, Chiba-Ken, 293, Japan. T. FUJITA, Emeritus Professor, is with the University of Tokyo, Tokyo, 113 Japan. Manuscript submitted June 10, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURES
The aluminum content in a Fe-9Cr-0.5Mo-1.8W steel was varied in four levels covering the range of 0.003 to 0.094 pct. The chemical analyses of the four experimental heats are given in Table I. A 20-kgf ingot for each heat was cast after melting in a vacuum-induction-melting furnace, and then hot-rolled at 1180 7C to 900 7C to 15-mmthick plate. After normalizing at 1050 7C for 60 minutes, the 15-mm-thick plates were air cooled to room temperature, and then tempered at 780 7C for 1 hour. Creep rupture tests were carried out at 600 7C, 650 7C, and 700 7C under constant loads, and tensile tests were conducted at 20 7C to 700 7C using specimens of 6 mm in diameter and 30 mm in gauge length. Impact tests were performed for the asnormalized and tempered specimens and the specimens aged for up to 3000 hours at 600 7C by using standard Charpy V-notched specimens. Microstructures and precipitation characteristics were investigated by mean of
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