Triratna Shrestha

In Situ Tensile Deformation and Residual Stress Measurement by Neutron Diffraction in Modified 9Cr-1Mo Steel

AbstractThe deformation behavior of monolithic modified 9Cr-1Mo (Grade 91) steel during uniaxial tensile loading was studied using the in situ neutron diffraction technique. The residual stress distribution across gas tungsten arc welds in the Grade 91 steel was measured by the time-of-flight neutron diffraction method using the SMARTS diffractometer at Lujan Neutron Scattering Center, Los Alamos National Laboratory. Grade 91 plates were welded using the gas tungsten arc welding (GTAW) technique. The load sharing by different grain orientations was observed during the tensile loading. The residual stresses along three orthogonal directions were determined at the mid-thickness, 4.35 and 2.35 mm below the surface of both the as-welded and post-weld heat-treated plates. The residual stresses of the as-welded plates were compared with those of the post-weld heat-treated plates. The post-weld heat treatment significantly reduced the residual stress level in the base metal, the heat-affected zone, and the weld zone. Vickers microhardness across the weld zone of the as-welded and post-weld heat-treated specimens was evaluated and correlated with the observed residual stress profile and microstructure.

Studies of the Effect of Heat Treatment on Hardness and Creep Behavior of a Modified 9Cr-1Mo Steel

In this study, heat treatment was carried out on modified 9Cr-1Mo steel specimens to determine the microstructural evolution for various normalizing temperature/time and tempering temperature/time combinations. Normalization was carried out in the temperature range of 1020–1100°C and time range of 2–8 hours, while tempering was performed in the temperature range of 690–790°C and time range of 2–20 hours. Optical microscopy was then used to visualize and chronicle the microstructural characteristics at varying levels of heat treatment. Vickers micro-hardness measurements were performed on each sample to obtain hardness values as a function of normalizing and tempering temperature/time. Creep tests have also been performed on as-received and welded specimens of modified 9Cr-1Mo steel, in the temperature range of 500–700°C and stress range of 50–200 MPa. Microstructural analysis was carried out on the specimens before and after creep deformation using both scanning and transmission electron microscopy. The microstructural evolution during the heat treatment and creep tests provided useful information to understand and characterize the creep deformation mechanisms of modified 9Cr-1Mo steel.Copyright