Experimental determination of the residual stresses in a Kraft recovery boiler tube

Neutron diffraction was used to determine the residual stresses in a spiral weld overlay tube used in Kraft recovery boilers by the pulp and paper industry. The specimen was a 2.5 inches OD carbon steel tube covered with a layer of Inconel 625 weld overlay. Residual strains in the carbon steel and weld overlay layers were determined using the ferritic (211) and austenitic (311) reflections, respectively. Residual stresses in each material were derived from the measured strains using Hooke`s law and appropriate elastic constants. Tensile stress regions were found not only in the weld metal but also in the heat affected zone in the carbon steel. The maximum tensile stress was located in the weld overlay layer and was found to be 360 MPa, or about 75% of the yield strength of the weld metal. The experimental data were compared with a finite element analysis based on an uncoupled thermal-mechanical formulation. Overall, the modeling results were in satisfactory agreement with the experimental data, although the hoop strain (stress) appears to have been overestimated by the finite element model. Additional neutron diffraction measurements on an annealed tube confirmed that these welding residual stresses were eliminated after annealing at 900{degrees}C for 20 minutes. 18 refs., 7 figs