New approach development for complex dissimilar girth welding of high strength low alloy steel pipes cladded with Ni-based alloy

Abstract

In the present thesis, a systematic study to develop a special welding procedure for dissimilar girth welding of API 5L X65 steel pipe cladded with Alloy 625 is presented. The main objective was to join the pipes without solidification cracking and achieve the minimum yield strength overmatch required for subsequent installation of the pipeline by reel lay method. To achieve this goal, it was proposed to use two different fillers metals: a Ni-based alloy to offer a corrosion resistance, similar to the cladding, and a low alloy steel (LAS) to provide the YS overmatch. Thermodynamic simulation studies were performed to select the alloys for the dissimilar welding proposed. The solidification of Ni-based alloys 625, 686, C-276, 22, 59, 680 and 617 when mixed with API 5L X65 and AWS ER100S-G steels (LAS) were investigated using thermodynamic simulation, in order to investigate the metallurgical compatibility for dissimilar welding, in terms of solidification cracking. Among all alloys investigated, the alloys 22, 686 and 59 presented the lowest solidification crack susceptibility when mixed with the steels. An experimental study evaluating the mix effect of AWS ER100S-G steel with alloys 625, 22, 686 and 59 also was performed. The results shown that alloys 22, 686 and 59 presented few small discontinuities, while the Alloy 625 exhibited large cracks with presence of Nb-rich eutectic phases. Four dissimilar girth welds were produced with different alloys (625, 22, 686 and 59) for the first two weld passes, while the other weld passes used the AWS ER100S-G steel (LAS). The welded joint produced with Alloy 625 exhibited numerous cracks along of the first two LAS weld passes that detrimental mechanical properties. The three welded joints produced with alloys 22, 686 and 59 did not presented solidification cracks. Moreover, these welded joints achieved the minimum YS overmatch required by DNVGL-ST F101 for subsequent installation by reel lay method, not only for API 5L X65 steel, but also for API 5L X70 and X80 steels. The mixture between the LAS and Ni-based alloys created a complex microstructure, where the two Ni-based weld passes showed a -FCC matrix, while the first two LAS weld passes showed a martensitic matrix with retained austenite (Aret) and cubic carbonitrides. The third LAS weld pass exhibited a bainitic matrix with martensite, martensite-austenite (MA) microconstituent and carbides, and the others LAS weld passes showed an acicular ferrite (AF) matrix with MA particles. The toughness of the welded joints at -15°C was unsatisfactory due to brittle fracture.