Abstract Scope |
Introduction.
Internally clad API 5L Grade X65 pipes are currently joined using Alloy 625 filler metal. Alloy 625 girth welds, however, are unable to meet the 100 MPa overmatching strength requirement established by DNV-OS-F101 for reel lay applications. As a result, industry is now considering the use of stronger filler materials, such as low alloy steel, to manufacture such girth welds. Careful consideration is required for the development of a LAS girth welding procedure. Dilution from the Alloy 625 internal cladding into the LAS weld metal causes sharp gradients in composition and microstructure that affect both hardness and strength properties. Testing and analysis is required in order to determine the relationship between microstructure and mechanical properties.
Technical Approach.
Newly developed single U-groove welds on API 5L Grade X65 steel were subjected to microstructural characterization, hardness testing, and tensile testing utilizing digital image correlation (DIC). Welds were manufactured utilizing an ER80S-G filler metal and an alloy 686 root pass. EDS traverses were performed in order to determine compositional gradients. EBSD phase balance analysis was conducted to determine the volume fraction of BCC structured phases and retained austenite in ER80S-G fill passes containing high dilutions from the alloy 686 substrate. Customized tensile testing was performed with DIC to acquire stress-strain behavior of highly diluted regions in the LAS weld metal.
Results.
Results indicate that the volume fraction of martensite and retained austenite directly influence hardness and strength. DIC results display an increase in strength in the weld metal as the composition transitions from LAS diluted with alloy 686 to undiluted LAS. EBSD analysis reveals volume fractions between 8 to 96 pct. of retained austenite in the first layer of ER80S-G deposited above the alloy 686 root pass. The high volume fractions of retained austenite results with hardness as low as 171 HV0.3 in the weld metal. DIC tensile testing also reveals an undermatching strength in the weld metal when high volume fractions of retained austenite are present. The weld metal containing the alloy 686 root pass and layer 1 of ER80S-G displays yielding at 566 MPa while, the adjacent HAZ and base material yields at higher stresses of 576 MPa. Layers 2 and 3 of ER80S-G contain less than 12 pct. dilution from the alloy 686 root pass and lower volume fractions of retained austenite. Layer 2 of ER80S-G contains less than 9 pct. of retained austenite and instead, contains a high volume fraction of untempered martensite. The formation of untempered martensite increases hardness to approximately 400 HV0.3, which in turn, also increases strength. The HAZ and base material display yielding prior to the adjacent weld metal containing layers 2 and 3 of ER80S-G. The strength of the weld metal continues to rise as the composition transitions to undiluted ER80S-G near the cap of the weld. The undiluted ER80S-G shows an overmatching strength than the neighboring HAZ and base material. Stresses in the weld metal increase to a maximum with no signs of yielding until necking and failure occurs in the base material.
Conclusions.
DIC tensile testing experiments display an overmatching strength in the ER80S-G weld metal. DIC results display a weld metal with a higher unknown yield strength than the ultimate tensile strength of the base material. All-weld metal tensile testing in accordance with DNV-OS-F101 reveals a minimum yield strength of 624 MPa. The 624 MPa weld metal yield strength exceeds the 100 MPa overmatching strength requirement established by DNV-OS-F101 for girth welds on internally clad API 5L Grade X65 pipes intended for reel lay applications. The ability to manufacture LAS girth welds enables further analysis of such a procedure to one day replace alloy 625 girth welds and reduce pipeline manufacturing expenses. |