Microstructure and mechanical properties of welded joints of L415/316L bimetal composite pipe using post internal-welding process

In order to reduce the welding cost and improve welding quality, L415/316L bimetal mechanical lined pipes were welded using the post internal-welding process in which the outer carbon layer is welded prior to the inner stainless layer. The mechanical property tests, such as tensile, bending and hardness tests, of the welded joints were carried out. Microstructure and the chemical compositions of the welded joints were analyzed by optical microscope (OM), scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and electron probe microscope analysis (EPMA). In addition, the pitting corrosion resistance of the welded joints was evaluated. The results show that the mechanical properties of the welded joints do meet the standard requirements. The cracks on the bending specimens are closely related to the martensite formed at ER309 L/ER70S-6 or ER309 L/L415 interface. In addition, pitting holes are found in the ferric chloride solution showing that ER309MoL welding wire is not suitable for welding 316L layer. From the perspective of mechanical properties, it is advisable to use the post internal-welding process for the welding of bimetal composite pipes.