PSI - Issue 14

Vijay Sai et al. / Procedia Structural Integrity 14 (2019) 491–498

496

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Ch. Vijay Sai/ Structural Integrity Procedia 00 (2018) 000–000

3.2. Mechanical analysis

Fig. 7. Variation of longitudinal residual stress in plate weld joints.

The magnitude of longitudinal stress in Plate joints and hoop stress in pipe joints, and their distribution was evaluated. These stresses are of similar type, and are of the highest magnitude, with respect to stress in other directions. Fig.7 shows the distribution of longitudinal residual stress in the three plate joints and Fig.8 shows distribution of hoop residual stress in SS304, P91 and dissimilar weld joints. In circumferential weld joints the residual stress varies across the thickness. The peak values of stress are tabulated in Tables 2 and 3. In both plate and circumferential weld joints, peak tensile residual stress is observed in the P91 joints, while the peak compressive residual stress is in the martensitic zone of the dissimilar weld joints. In plate dissimilar weld, the residual stress distributions are very similar to the stress distributions in their respective joints. However, in circumferential dissimilar joint, the residual stress distribution varies significantly, in comparison to respective joints. Peak tensile residual stress is observed in SS304, in this joint. Within dissimilar welds, compressive residual stress is higher in circumferential joints, whereas tensile residual stress is higher in plate joint. In all joints of P91, the difference between maximum and minimum residual stress is observed to be nearly same.

Table 2. Summary of longitudinal residual stress in plate butt weld joints. Weld joint

Longitudinal stress (MPa)

Dissimilar weld joint

SS304

330 676 700 322

P91

P91 weld joint SS304 weld joint

Residual stresses are the outcome of eigen strains in a material and its mechanical properties. The same strain produces a higher magnitude of stress in a higher yield material, in comparison to a lower yield material. Eigen strains, developed are proportional to the peak temperature attained, the coefficient of thermal expansion and the degree of restraint. Therefore, accounting the steeper temperature distribution, and higher yield, P91 material will have a higher residual stress. Without considering martensite formation in their simulation, Zuber et.al (2017) observed a residual stress of 550MPa in 6mm thick P91 plate joint. However, in ferro-martensitic steels like P91, the formation of martensite at low temperatures upends the stress distribution prior to it. Volumetric expansion accompanies the formation of martensite, resulting in compressive stress, at the location of its formation. Any lowering of this compressive stress in longitudinal direction, however results in the increase of tensile eigen strain in