Type IV Cracking Susceptibility in Weld Joints of Different Grades of Cr-Mo Ferritic Steel
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INTRODUCTION
THE Cr-Mo ferritic steels are widely used in fabrication of power plants and petrochemical industries. Different grades of Cr-Mo ferritic steel with chromium in the range 0.5 to 12.0 wt pct and Mo in the range 0.25 to 2.0 wt pct are in use, and the selection of a specific grade is based on their creep-rupture strength and oxidation resistance. The Cr-Mo ferritic steels derive their creep strength from the solid-solution strengthening, precipitation strengthening by the complex interand intragranular metal carbonitride particles and from the phase-transformation induced dislocation substructures. The strengthening mechanisms are vulnerable to change on thermal and creep exposures, and degradation in creep strength of the steels occurs on service exposure. Large-scale plants are usually fabricated employing fusion-welding techniques. The deposited weld metal and the heat affected zone (HAZ) of the base metal of the joint have different microstructures possessing appreciably different mechanical properties than the base metal.[1,2] These result in creep-rupture strength of Cr-Mo ferritic steel weld joint lower than the base metal due to the different types of cracking developed during creep exposure.[3] Type-IV cracking at the outer range of HAZ is considered life-limiting factor of high-temperature welded components fabricated out of Cr-Mo ferritic steels.[4] K. LAHA, Scientific Officer ‘‘G’’, K.S. CHANDRAVATHI, Scientific Officer ‘‘E’’, P. PARAMESWARAN, Scientific Officer ‘‘F’’, and K. BHANU SANKARA RAO, Associate Director, Materials Development and Characterization, are with the Metallurgy and Materials Group, Indira Gandhi Center for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] Manuscript submitted on July 30, 2007. Article published online December 20, 2008 386—VOLUME 40A, FEBRUARY 2009
To decrease environmental pollution by CO2 emission, the traditionally used 2.25Cr-1Mo steel is now increasingly replaced by higher creep-resistant 9Cr-1MoVNb (modified 9Cr-1Mo) steel in an effort to increase the efficiency of power plants by operating them at higher temperatures and pressures. However, many concerns have been reported regarding the premature failure of welded components fabricated out of the more advanced higher creep-resistant ferritic steels.[5,6] This study is concerned with the relative susceptibility to type-IV cracking under creep condition in the weld joint of different grades of Cr-Mo steel. The steels investigated are low-chromium 2.25Cr-1Mo (grade 22) and high-chromium 9Cr-1Mo (grade 9), and 9Cr-1MoVNb (grade 91).
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EXPERIMENTAL DETAILS
The chemical compositions of the 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels are shown in Table I. The steels were received in normalized and tempered conditions. The normalization was carried out at 1223 K for 17 minutes, 1223 K for 15 minutes, and 1333 K for 6 hours; whereas, tempering was carried out at 1003 K for 1 hours, 1053 K for 2 hours, and 1043 K for 4 hours, respectively, for the 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels. Similar
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