Girth welded replica of API X70M material have been produced on NG-GMAW welding technique. The particular area of interest is to develop suitable girth welding process parameter using NGGMAW. The major aim of the work was to replicate welds having tensile strength between 650 and 680 MPa. Design of Experiment (DoE) method by Taguchi design, using some selected welding processes was adopted. Two process parameters (factors) – arc voltage and wire feed rate, (the variables), and three levels were used. The resultant joint property on tensile strength of X70M pipeline was examined. The targeted mechanical property was achieved by selecting the best process parameters. Their effects on ultimate tensile strength – UTS was analysed using statistical technique – analysis of variance - ANOVA and Signal to Noise - S/N ratio with ‘thebigger-the–better’ value. Validation was done using MIDAS NFX (an FEA) mechanical engineering software. In conclusion, process parameters that affects or influences the girth welded properties of API X70M under field conditions were identified. Guidance for the specifications and selection of processes that could be used in field-welding for optimum performance has been recommended. Keywords: Optimization, Girth-Weld, Process Parameters, Tensile Strength, NG-GMAW

Morphology and optimisation of impact energy of weldment of low alloy-high strength steel were investigated in this work.  The study involved the use of design of experiment via Taguchi method based on three variables, each having three levels. Minitab 16 software was used for the analysis of variance (ANOVA) and signal to noise (S/N) ratio of the impact energy. Direct current electrode positive (DCEP) was used in the electric manual metal arc (MMA) welding. The input parameters at the three levels were current (60 A, 70 A, and 80 A), metal thickness (5 mm, 7 mm, 9 mm) and root gap (2.0 mm, 2.5 mm, 3.0 mm).  It was found that the input variables significantly affected the microstructure of the weldment in all the nine experiments conducted.  Optimization of welding parameters was carried out  to obtain the optimal input values required to give the optimal impact energy of weldment.  The optimal values for the input values were  80 A for current, 9 mm thickness and 2.5 mm root gap and these were substituted in the regression equation to obtain the optimal impact energy. It was equally observed that the input variables affected the microstructure of each of the nine samples.