Microstructure and creep behavior of a niobium alloyed cast heat-resistant 26 pct Cr steel
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I.
AMONGthe
INTRODUCTION
heat-resistant alloy castings described in
Metals Handbook, 1,2 "straight-chromium" alloys are the
simplest ones. They are ferritic at all temperatures but the low chromium grades are slightly hardenable. The chief advantages of these alloys are resistance to oxidation and to sulfur-rich atmospheres and a low nickel content for applications where that element may have an unwanted catalytic action. On the other hand, the "straight-chromium" alloys lack ductility/impact resistance at low temperatures as well as creep resistance. Therefore, these castings are mainly used in applications where loads are moderate such as boiler baffles, furnace grate bars, tu3~eres, lute rings, etc. Nickel up to 7 pct and nitrogen up to 0.15 pct are added to the higher chromium grades to improve creep strength. The microstructure of these higher-chromium alloys is comprised of coarse dendrites and an interdendritic constituent which freezes with a eutectic-like morphology (ferrite plus M23C6 carbides, where M = iron + chromium). Secondary precipitation also takes place in the ferritic matrix. Therefore, the creep resistance of these alloys appears to depend on carbides to prevent grain boundary sliding and to restrict dislocations motion. Clearly the "eutectic-like" carbides are critical for the former and the secondary carbides for the latter process. In particular, by increasing the amount and distribution of these "eutectic-like", interdendritic carbides, an improvement in creep resistance should be expected. The purpose of this work is to investigate the effect of niobium additions on microstructure and creep behavior of cast heat-resistant 26 pct Cr steel. II.
EXPERIMENTAL METHODS
The experimental alloys (Table I) were melted in a 30 kg induction furnace and cast into U molds as prescribed in ASTM A-370. The niobium, as ferroniobium, was added to a standard melt (alloy I), the composition of which complies with specified composition of a HC steel (ASTM-A 297). This procedure yielded alloys with similar impurities and EDUARDO A. A.G. RIBEIRO, formerly with the Metallurgy Department, Polytechnic School, University of S~o Paulo, SP, is now with Acos Villares S.A., Av. Dr. Ramos de Azevedo 133, S~o Caetano do Sul (09500) SP, Brazil. R. PAPALEO is with the Metallurgy Department, Polytechnic School, University of S~o Paulo and Companhia Brasileira de Metalurgia e Minerac~o (CBMM) - Av. Juscelino Kubitschek 1703, S~o Paulo (04543) SP, Brazil. J. R. C. GUIMARAES is with Companhia Brasileira de Metalurgia e Mineraqao (CBMM) Avenida Juscelino Kubitschek 1703, S~o Paulo (04543) SP, Brazil. Manuscript submitted May 30, 1985. METALLURGICALTRANSACTIONS A
compositions (except for the niobium content). Elimination of Mo, Ni, and N, as well as the low Si content of the experimental alloys with regard to the typical HC chemistry, aimed at working with a simpler material to make the niobium effect more obvious. Alloy I may be classified as a lean HC type steel. Standard methods were used to investigate the microstruct
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