Repair welding of service-exposed, aged Heat Resistant Nickel-Base alloys is notorious for being extremely difficult. There is a high propensity for cracking in the heat affected zone of the weld which manifests during or immediately after the welding process. Industry experience and current literatures converge upon one conjecture-that the success of the weld is based largely on optimistic 'trial and error'.Prior Failure Analysis of characteristic weld cracks observed upon welding revealed the root cause to be the synergistic action of high Heat Input of the welding process and the residual stress which lead to a localized Creep Embrittled HAZ which eventually failed due to brittle intergranular fracture. Three weld parameters were identified for the reduction of heat input and residual stress; Groove design, Welding Pattern and Interpass Temperature. The joint was upgraded from a Double-angle V-groove to a U-groove, use of a Stringer Pattern instead of Weaving and a higher Interpass Temperature.Mechanical testing revealed that the modified procedure was superior. Void Counts of the HAZ showed a reduction in void percentage from 5.31% (Original Procedure) to 0.74% (New Procedure) near the tube's ID and a reduction from 13.3% (Original Procedure) to 3.40% (New Procedure) at OD areas. Comparative Hardness Testing of the Original and New Weld Procedures' weld joints concurred with the creep void analysis. Hardness profiles of the New Procedure showed higher values compared to the Original Procedure suggesting superior tensile strength of and less fissuring.The success of the new weld procedure was further authenticated by tensile testing. The 0.2% Yield Strength of the New Procedure was found to be 383.18N/mm~2 versus 339.70N/mm2 (Original Procedure) reflecting an improvement of 43.48N/mm2. UTS results also reflected an increase from an average of 475.10N/mm~2 (Original Procedure) to a superior 492.55N/mm~2 (New Procedure).
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