PSI - Issue 41

Chouaib Zeghida et al. / Procedia Structural Integrity 41 (2022) 384–393

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Zeghida Chouaib et al. / Structural Integrity Procedia 00 (2022) 000–000

3.1. Concept of IHSI Before considering ways of removing weld-induced residual tensile stresses, it is worthwhile to review the factors that cause residual stresses. Briefly, welding produces high temperature gradients in a pipe. These gradients cause thermal stresses that exceed the temperature dependent yield stress of the material and result in residual stresses upon cooling. Certain temperature histories result in inside surface residual tensile stresses, while other thermal histories can produce compressive residual stresses. In the case of girth welded pipes, temperature gradients, which cause compressive residual stresses on the inner surface, arise from heating the pipe exterior while the inside is cooled. IHSI uses this concept for improving stresses. That is, as illustrated in Figure 2, the welded pipe is inductively heated from the outside while water flows through the pipe to remove heat from the interior surface.

Fig. 2. Concept of the IHSI Method.

3.2. Residual stress distribution after IHSI The stress distribution, deformation, and the temperature distribution at the time IHSI is executed are shown in Figure 3. In the IHSI method, a large temperature difference throughout the pipe wall emerges due to simultaneous cooling through applying water on the inside surface of the welded joints and the heating of the outside surface to a specified temperature set by the high-frequency induction heating method. At this time, the compressive yield and tensile yield is generated on the outside and the inside surface, respectively (Figure 3-a). When heating is stopped (cooling on the inside is continued), the temperature difference becomes smaller and stress on the outside surface that was generated in the heating process changes to tensile stress. Stress on the inside surface turns into the compression stress and remains as residual stress (Figure 3-b). Through IHSI, residual tensile stress on the inside of welded pipes can be relieved or changed to compressed sides. The process results in axial residual stresses that are compressive on the Inside Diameter (ID) and, thus, are favorable for impeding stress corrosion crack initiation and growth. The resulting residual stresses are also axisymmetric and self-equilibrating through the thickness. References (Harris, 1992) and (Failure Analysis Associates, 1990) provide experimental and analytical post-IHSI residual stress distributions through the wall, in the context of present study , only the axial stresses are of interest here. These references contain results for small, intermediate and large lines. When the stresses are linearly varying through the wall , axisymmetric and self equilibration , the stresses at the ID and outside diameter (OD) are related to one another by the expression:

3R 2 h 3R h  

   

(1)

ID

OD

where R is the inside radius of the pipe and h the wall thickness of the pipe.