The microstructure and mechanical properties of a modified 2.25Cr-lMo steel
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INTRODUCTION
I S H I G U R O et al. sought an improvement in the strength of 2.25Cr-lMo steel by microalloying additions.~ The objective was a pressure-vessel steel with better creep strength for thick sections for use in coal dissolver vessels, desulfurization vessels, and hydrocrackers. These investigators found that a low-silicon 2.25 pet Cr-I pet Mo-0.1 pet C-0.25 pet V-0.02 pet Ti-0.002 pet B steel* *All compositions are in weight percent.
gave improved creep strength and impact toughness over those for commercial 2.25Cr-lMo steel in either the annealed or normalized-and-tempered condition. It was also claimed that the steel had superior resistance to temper embrittlement and improved weld heat-affected zone properties. Low silicon (0.02 pet vs 0.2 to 0.4 pet for most commercial 2.25Cr-lMo steels) was chosen to increase resistance to temper embrittlement, and carbon content was restricted to 0.1 pet so as to avoid possible welding problems. The addition of 0.002 pet B was credited with increasing the hardenability of the alloy, allowing for the formation of "martensite or bainite or a mixture of the two at a cooling rate as slow as 0.17 ~ (10 ~ ''l (The 0.02 pet Ti ties up nitrogen, and boron is not removed from solution as boron nitride.) The start of the proeutectoid ferrite transformation at 700 ~ was moved from about 100 to about 2000 seconds. Precipitate stability is of prime importance for elevatedtemperature strength. Ishiguro et al. stated that the steel contains vanadium and titanium carbides which are more stable than the chromium and molybdenum carbides formed in 2.25Cr-lMo steel. 2 No proof of such a precipitate identification was given. When the constitution diagrams for CrMo-V steels are consulted, 3 it appears that at equilibrium a 2.25Cr-lMo steel containing 0.25 pet V would contain M 6 C (a molybdenum-rich carbide), M7C3 ( a chromium-rich carbide), and possibly MC which could contain vanadium and titanium. R.L. KLUEH and R.W. SWINDEMAN are Research Metallurgists, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37831. Manuscript submitted June 11, 1985.
METALI,URGICAL TRANSACTIONS A
Ishiguro et al. compared the creep properties of the modified and standard 2 . 2 5 C r - l M o steel taken from the 88 (88 T) position of a 200-mm plate. ~ The problem with such a comparison is that it was for different microstructures: the modified steel was entirely bainite, and the standard material contained large amounts of polygonal ferrite. The question, then, is whether the mechanical property differences are due to the titanium and vanadium additions to the modified steel or to the different microstructure caused by the increased hardenability of the modified steel. If the difference is due only to the difference in hardenability, boron (or manganese, or nickel) could be added to the standard 2.25Cr-lMo steel to increase its hardenability. 4,5 We determined tensile and creep-rupture properties on the modified 2.25Cr-lMo steel. The results were compared with properties that had previously be
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