PSI - Issue 30

A.A. Antonov et al. / Procedia Structural Integrity 30 (2020) 6–10 A.A. Antonov et al. / Structural Integrity Procedia 00 (2020) 000–000

9

4

According to the data obtained, the typical change in the hardness values of the HAZ metal with each filling pass is connected with the specifics of the formation of multilayer welds during hot pass welding with preheating of fixed pipe joints and the peculiarities of thermal deformation processes in the HAZ metal during repeated local exposure to the welding source. The results of measurements of the RWS distribution reveal that the values of the circular and axial residual stresses decrease from the outer side of the pipe wall to the fusion line and tend to zero. At a distance of approximately 10 mm from the fusion line, the axial and circular stresses increase and reach ~ 600 and 300 MPa, respectively. A gradual decrease is observed with increasing distance from the fusion line. Herewith, the maximum values of axial residual stresses are recorded in the HAZ metal, where significant deformations and shrinkage occur. They slowly decrease with increasing distance from the fusion line. Therefore, such a distribution of RWS is qualitatively associated with the specifics of the softening process of the HAZ metal and shrinkage, which were detected on the outer and inner sides of the wall surfaces joined by welding of fixed pipe joints. The nature of the RWS distribution changes in the inner side of the pipe wall. The values of the axial and circular RWS begin to increase at approaching the fusion line and reach ~ 375 MPa and ~ -100 MPa, respectively, at a distance of 5 mm from the fusion line. When the distance from the fusion line of the root weld increases, the values of the axial RWS decrease and vary from 375 to 60. Then they increase to 167 MPa. Circular RWS increases from compressive stresses of -300 MPa to tensile stresses of 168 MPa. Such distribution of RWS on the inner surface of the pipe wall in the welded joint area is also qualitatively associated with the thermal deformation processes during multilayer welding with a hot pass of fixed joints with pre-heating, which contribute to longitudinal and transverse shrinkage in the metal of the welded pipe joints. Thus, as the work of Ammosov et al. (2017) showed that the distribution of RWS in the welded joints of siphon pipes has specific features and determines the operational reliability and durability of the pipe joints, taking into account the functional and nonfunctional effects on the underwater crossing of pipeline system across the Lena river. The tensile test results are presented in Fig. 2 and Table 2. According to the data obtained, there is an increase in ductility and thereby an increase in tensile strength indices relative to the base metal. Thus, the mechanical properties of the weld metal during welding of low-carbon steels undergo some changes in comparison with the base metal. Multilayer weld metal, especially during welding with coated electrodes, has a lower critical temperature of transition to a brittle state due to the structure refinement of the weld metal under the influence of heat released during the application of subsequent seams. The thermal effect of reheating is similar to that of normalization. In this case, it can be seen that the selected welding modes for a given thickness of the weld materials are meant to provide the necessary ductility.

Fig 2. Tensile diagram of the sample (20 °C).