Hadi Hedayati

Grade:  Master

Thesis Title:

Effect of Welding Sequence and Hydrotest Process on Welding Residual Stresses in SUS304 Stainless Steel Pipes

Year: Sept. 2007- Feb. 2010.

Abstract:

Weld joints often contain various types of flaws such as slag inclusions, gas pores or stick spots. From such flaws during the service, cracks can initiate, grow slowly and finally lead to failure. Fatigue behavior of a welded structure is complicated by many factors intrinsic to the nature of welded joints. Among these effects, the residual stresses as consequence of thermal multi-pass welding cycles can be a dominant effect on fatigue crack propagation. Tensile residual stresses are generally detrimental, increasing the susceptibility of a weld to fatigue damage, stress corrosion cracking and fracture. When assessing the risk for growth of defects such as surface flaws in piping systems, the welding residual stress may give a large contribution to the total stress field than stress caused by design loads. Moreover, in order to prevent inter-granular stress corrosion cracking in the root area of welds in stainless steel pipes, it is necessary to satisfy certain requirements concerning process condition, material properties and welding residual stress. Therefore, a good estimation of the welding residual stress fields is then needed.

In the present study, temperature fields and residual stress states in the weld joints of SUS304 stainless steel pipes are computed numerically using a thermo-elasto-plastic analysis. This weld joint is a circumferential butt-weld joint and is done in two passes using gas tungsten arc welding. Welding procedure is simulated in two and three dimensions using a subroutine developed in finite element ABAQUS software for applying heat flux. The applied heat flux is represented by a double ellipsoid model. Futhermore, the element birth and death technique is used to simulate weld passes and filler metal deposition into the weld pool. When the simulation is done, the effect of welding sequence on welding residual stresses are considered using four different welding sequences proposed. Finally, the effect of hydrotese process in decreasing the residual stresses is evaluated.

To verify the validity of simulation results, they are compared with the results of the experimental measurements of residual stresses attained by hole drilling method. As the results of the simulation show, selecting a suitable welding sequence can substantially decrease the amount of tensile residual stresses and can change them to compressive stresses in certain situations. Also the results of this research show that applying a hydrotest pressure after welding process can reduce welding residual stresses up to 70%.

  Keywords: multi-pass welding process, welding residual stresses, element birth and death, welding sequence, hydrotest process.