An Introduction to 9Cr-1Mo Alloys

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WINTER 2001 VOLUME 13, NO. 1 M A G A Z I N E An Introduction to the 9Cr-1Mo-V Alloys How to Comply with OSHA’s Ergonomics Program Regulation Foundry Quality System for Pressure Vessels in the 21st Century The “R” Word: The State of the U.S. Economy Making Traffic Flow II: Implementing the ‘Soft’ Side of Lean WINTER 2001 1 The 9Cr-1Mo-V materials are becoming more common as power plant temperatures and pressures are increased to improve efficiencies. The excellent high-temperature pr
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  WINTER 2001 1   M A G A Z I N E WINTER 2001VOLUME 13, NO. 1 An Introductionto the 9Cr-1Mo-V AlloysHow to Comply with OSHA’sErgonomics Program RegulationFoundry Quality System forPressure Vessels in the 21st CenturyThe “R” Word:The State of the U.S. Economy Making Traffic Flow II:Implementing the ‘Soft’ Side of LeanMaking Traffic Flow II:Implementing the ‘Soft’ Side of Lean  2 VALVE MAGAZINE   ALLOYS DONALD R. BUSH An Introduction to the 9Cr-1Mo-V The 9Cr-1Mo-V materials are becom-ing more common as power planttemperatures and pressures are in-creased to improve efficiencies. Theexcellent high-temperature propertiesof the 9Cr-1Mo-V alloys are predi-cated upon the formation of a particu-lar microstructure containing submi-croscopic carbides. The formation ofthis microstructure is affected by thedeoxidation practices used when thealloy is produced, by the heat treat-ment it receives, and by welding andpost-weld heat treatments. If theseprocesses are not properly executed,acceptable creep properties will notbe realized even though the compo-sition and room-temperature me-chanical properties will appear ac-ceptable.  WINTER 2001 3 he pursuit of improvedefficiencyand reducedemissions in powerplants has resulted in thedesign of systems utilizinghigher and higher steamtemperatures and pressures.This shift to higher tempera-tures requires the use of materials with appropriatehigh-temperature strength,creep properties, physicalproperties, and metallurgicalstability at the operatingtemperature of the unit toprevent catastrophic failuresby several potential long-termmechanisms, includingleakage due to creep-induceddimensional changes, burstingdue to stress rupture, orthermal fatigue due tothermally-induced cyclicstresses.In the 1970s, Oak RidgeNational Laboratory devel-oped a new alloy system,commonly referred to as 9Cr-1Mo-V, that provides excel-lent elevated-temperatureproperties. 1 The use of thisalloy in various forms hasaccelerated significantly inthe past few years as newpower plants are being built.This article discusses thegeneral characteristics of thealloy, the applicable specifications and grade names, and someof the technical issues involved in its use. THE ALLOY SYSTEM Austenitic stainless steels have commonly been used forhigh-temperature steam applications due to their excellentstrength retention at high temperatures. However, there aredrawbacks to their use. In addition to their relatively highcost, the austenitic stainless steels exhibit high thermalexpansion rates and low thermal conductivity. This results inthe development of high thermally-induced stresses duringheating and cooling, which can cause thermal fatigue.The modified alloy system is similar to the conventional9Cr-1Mo (9% chromium, 1% molybdenum) grades. Modifi-cations include additions of vanadium, niobium (sometimescalled columbium), and nitrogen, as well as a lower carboncontent. When produced and heat treated to form the propermicrostructure (See “The Importance of Microstructure” on T  4 VALVE MAGAZINE page 8), this alloy providesextremely good mechanicalproperties at elevated tempera-tures. In addition, because of itslower thermal expansioncoefficient and higher thermalconductivity, the alloy is muchmore resistant to thermal fatiguethan the austenitic stainlesssteels. SPECIFICATIONS ANDGRADES 9Cr-1Mo-V materials havebeen adopted in various forms ina number of ASTM/ASMEstandards, and many are listed inthe ASME Boiler and PressureVessel Code Section II Part Dallowable stress tables with amaximum temperature rating of 649 ° C (1200 ° F). The standardsand grade designations forvarious forms are shown in Table1 . Compositions, mechanicalproperties, and ASME allowablestresses are listed in Tables 2, 3, and 4. Note that these materi-als do not provide any majorbenefit over more conventionalmaterials such as F22 or WC9 for use below about 900 ° F(482 ° C), but that they provide much higher allowable stressesin the range from 900-1100 ° F (482-593 ° C). PROCESSING ISSUES Because it is critical that a specific microstructure bepresent in order to realize the high-temperature properties 1 ,there are a number of metallurgical issues that must beaddressed with respect to deoxidation practices, heat treat-ment, and welding processes used in the production of thematerial. DEOXIDATION During the production of raw material or castings, consider-ation must be given to the deoxidation practice used to tie updissolved oxygen in the molten metal prior to pouring ingotsor castings. Deoxidation is generally accomplished by addingsmall quantities of elements which have a high affinity foroxygen, a practice commonly referred to as “killing” inconjunction with wrought products. These elements combinewith the oxygen to form solid oxide inclusion particles,preventing the formation of gas porosity in the ingot orcasting. The most common deoxidizers are manganese,silicon and aluminum, although many other elements withhigh oxygen affinity, including titanium, zirconium, niobium,magnesium, and calcium might be utilized.The deoxidation practice is critical in the 9Cr-1Mo-Vmaterials because of the presence of nitrogen and the purposeit serves in creating a proper microstructure (See “TheImportance of Microstructure” on page 8). Many of theelements used for deoxidation also have a high affinity fornitrogen. If certain deoxidizing elements are utilized, or aresimply present in the melt as residuals in high enoughquantities, they can effectively tie up the nitrogen, and itwon’t be available during heat treatment to form the fineniobium carbonitrides. When this happens, large, blockycarbide phases will form instead. Without the fine niobium  a Allowable Stresses from ASME Code Cases 2192-2. Values should be acceptable for ASTM A217 C12A. Note: Section II Part D indicates values in italics are based upon time-dependent properties.
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