Numerical And Experimental Investigation Of Residual Stresses In 9Cr-1Mo Steel Welded Joint

9Cr-1Mo steel is a kind of high-alloy steel that has been specifically designed for operating at high temperatures and that has found wide use in the power generation industry in recent years.For the successful installation and use of this advanced steel,all aspects of their behavior,in terms of solid-state phase transformation(SSPT)in welds and heat-affected zone(HAZ)of welded structures,must be investigated.The purpose of this thesis is to investigate,and seek a greater understanding of,the influence of SSPT behavior of welds in 9Cr-1Mo steel on the residual stresses formation in their multi-layer and multi-pass joints of gas tungsten/metal arc welding structures.This understanding has been achieved through a combination of numerical simulation and experimental measurement.First,a series of experimental procedures have been designed and manufactured including plate and pipe weldments with thickness 6mm~20mm,and the surface welding residual stresses of these weldments have been measured using deep hole drilling method.Then,based on the SYSWELD software,the thermal and mechanical properties of P91 and P92 steel considering the SSPT have been modelled,meanwhile,the 3D finite element mesh and moving heat power source model have been carried out to simulate the welding processes of single-layer and multi-pass remelted welds in 6mm-thickness plate,multi-layer and single-pass butt-welds in 10mm-thickness plate,multi-layer and multi-pass butt-welds in 20mm-thickness plate and 21.4mm-thickness pipe.Comparison to the experimental data,the simulated results of welding residual stresses have a good agreement with that measured on discrete points and give us a complete distribution contour of the weldments.Some conclusions are drawn as follows through numerical simulation results:(1)The longitudinal residual stress in single-layer and multi-pass weldment of 6mm thickness plate shows a tension-compression interphase distribution due to SSPT,and a compression stress with-150 MPa is formed in the last-pass weld and HAZ while a tension stress almost exceeding the yield strength of parent metal is accumulated in the parent metal outside of HAZ.In addition,the longitudinal residual stresses in former welds depend on the peak temperature of the nodes in thermal cycle,and the SSPT has a significant influence on the formation of welding residual stresses.(2)The longitudinal residual stress in multi-layer and sing-pass weldment of 10 mm thickness plate shows that the last-weld and HAZ forms a compressive stress zone with-150 MPa while the welds near the bottom surface forms a large tension stress.Although the tension transverse residual stress forms in the middle zone of the plate,the peak stress of entire model is lower than 400 MPa.Repair welding has a great influence on both the longitudinal and transverse residual stresses middle zone of the plate.(3)The longitudinal residual stress of upper surface of the 20 mm thickness butt-weld plate joint shows the same distribution as that of the single-layer and multi-pass welding plate,which forms a tension-compression interphase distribution.The transverse welding residual stress shows a lower tensile stress forms on the upper surface but a higher tensile stress on the bottom surface.In the internal of the plate,a lower tensile stress zone is formed longitudinally while a wider compressive zone is formed in transverse direction.(4)The circumferential stress of the outer surface of 21.4mm steel pipe is formed a tension-compression interphase distribution and that of the inner surface and internal of pipe is formed tensile stress.In axial direction,the outer and inner surface of the pipe forms compressive stress zones while internal of pipe shows a higher tensile stress zone.The results of this thesis facilitate a better understanding of formation process of welding residual stresses in 9Cr-1Mo steel parent metal and weld of plate and pipe welded joints and provide results that are immediately applicable and useful to engineers and designers of the power generation industry.