Magnetic Nondestructive Evaluation of Creep Behavior in Water-Quenched Modified 9Cr-1Mo Steel
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UCTION
TUBES of Cr-Mo steels are used in thermal, nuclear power plants and in the petrochemical industries.[1,2] These tubes are welded per industrial requirements. The region of the weld joint in such tubes primarily consists of weld metal, heat affected zone (HAZ), and base metal. Out of these, the HAZ of a weld joint is reported to be the most vulnerable region to failures.[3] In the HAZ of welded 9Cr-1Mo steel, the transition from a coarser to fine grained martensitic structure occurs from weld region to base metal. In addition to the coarse grain martensite, the fusion boundary also contains d-ferrite. There is an intercritical structure where transformed austenite and tempered ferrite appear toward the base metal.[4] Postweld heat treatments were usually carried out for relieving residual stress and microstructural modifications in order to enhance creep behavior.[5] Several investigations were carried out on evaluation of mechanical properties using various simulated microstructures.[6,7] However, only a small amount of literature is available to find the changes of physical properties on such simulated structures. Notable among them is the variation of magnetic properties of the steel with the microstructural changes and their correlation with the mechanical properties. Such reports open up the scope of using magnetic techniques such as magnetic Barkhausen emissions (MBE) as a nondestructive tool for evaluation of structural changes in service-exposed steel components. The creep behavior of modified 9Cr-1Mo steel having initial microstructure as tempered martensite was studied using MBE and J.N. MOHAPATRA, Senior Research Fellow, J. SWAMINATHAN and M.K. GHOSH, Scientists, and A. MITRA, Deputy Director (Scientist F), are with the National Metallurgical Laboratory, Council of Scientific and Industrial Research, Jamshedpur-831007, India. Contact e-mail: [email protected] Manuscript submitted November 14, 2008. Article published online February 12, 2010 900—VOLUME 41A, APRIL 2010
magnetic hysteresis loop techniques.[8] In this article, the creep behavior of water-quenched modified 9Cr-1Mo steel, after austenitization at 900 °C/30 minutes, has been investigated using the MBE technique.
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EXPERIMENTAL
As-received modified 9Cr-1Mo steel in the form of tube was cut to form a flat specimen for creep test. The chemical composition of the sample is shown in Table I. The specimens were austenized at 900 °C for 30 minutes and water quenched to have martensitic microstructure. The creep test was carried out at 600 °C and 125 MPa stress level. The material failed at 772 hours of time in such an aggressive environment. The 772 hours of exposure of the material is considered as the 100 pct expended creep life of the material in the present study. Magnetic MBE studies were carried out using the surface probe in the creep-tested sample, which was interrupted at different intervals for taking magnetic measurement. The magnetizing field and frequency for the MBE were 4 kA/m and 40 Hz, respectively, for MBE measurement. A 30 to 300
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