Ni-base alloy 625 is used in the Oil & Gas industry for girth welding of internally clad high strength steel (HSS) pipes. Such welds however, cannot be used in pipe-reeling applications. DNV specifies that for pipe-reeling, the deposited girth weld metal shall overmatch the base metal yield strength by 100 MPa. As a result, industry is now considering the use of other filler materials. Low alloy steel (LAS) filler metals are a potential solution. Such a welding procedure however, requires using state of the art arc welding processes and evaluation techniques to overcome weldability and microstructural challenges.
API 5L Grade X65 plates were machined with a narrow-gap U-groove joint geometry to simulate the joint used by the Oil & Gas industry for joining internally clad HSS pipes. A combination of welding processes such as CMT, GMAW-SC, and GMAW-P were used to develop a welding procedure that could produce defect free welds. Welding parameters and bead placement were further optimized to improve deposition rates and help reduce hardness of previous weld passes. Lastly, welds were subjected to mechanical testing in accordance to the governing standard, DNV-OS-F101, to obtain bend, hardness, and tensile testing properties.
Defect free butt welds were developed and optimized for joining API 5L Grade X65 plates that simulate the joint used in industry for joining internally clad HSS pipes. The developed welding procedure consists of depositing an alloy 686 root pass and, ER80S-G fill and cap. Parameter optimization was conducted using the CMT welding process to minimize dilution of the alloy 686 root pass into the first layer of ER80S-G. The incorporation of the GMAW-P process helped increase deposition rates, improve bead penetration, and achieve defect free welds. Alterations to bead placement helped create a temperbead welding effect that reduced hardness of previous weld passes and minimized dilution. As a result, the formation of martensite was limited to only the first layer of ER80S-G. Mechanical testing was conducted in accordance to DNV-OS-F101 to obtain bend, tensile, and hardness results. Two of three bending specimens met the desired criteria when subjected to 20 % strain while the third specimen experienced a crack that initiated from a lack fusion defect. All-weld metal tensile tests exhibited an average of 683 MPa yield and 744 MPa UTS that exceed the 100 MPa overmatching strength requirement. Digital Image Correlation (DIC) was incorporated into tests to examine how dilution from the alloy 686 root pass affected the strain distribution in the initial passes of ER80S-G. In the as-welded condition, hardness mapping showed values above the 250 HV0.3 requirement but limited to the first layer of ER80S-G. Further testing is required to evaluate the localized mechanical properties of the diluted layers of ER80S-G and, examine if post weld heat treatment could suffice to reduce hardness below the specified requirement.
DNV specifies that for pipe reeling applications the girth weld metal shall overmatch the base metal yield strength by 100 MPa. ER80S-G butt welds were developed, optimized, and tested to produce defect free welds capable of meeting bend and tensile testing requirements. Although high hardness has been limited to only one layer of ER80S-G, hardness continues to be a challenge in the as-welded condition due to the formation of untempered martensite. Further investigation is needed to reduce hardness via PWHT. Results enable the development of new girth welding procedures of internally clad HSS pipes for pipe reeling applications.