From Carburization to Repair: Degradation-Controlled Creep and Tensile Behavior of GTAW-Welded 35Cr–45NiNb Tubes

Weld repair has emerged as a critical strategy for restoring and extending the service life of centrifugally cast 35Cr–45NiNb tubes employed in ethane pyrolysis furnaces; however, its metallurgical implications remain insufficiently understood. These components operate under prolonged exposure to carburizing atmospheres, which promote severe microstructural degradation and progressive loss of mechanical integrity. This study presents an assessment of Gas Tungsten Arc Welding (GTAW) repairs applied to tube segments in as-cast (AC) and carburized (C) conditions, establishing the microstructural aspects governing their tensile and creep performance. Welds joining as-cast–as-cast (AC–AC), as-cast–carburized (AC–C), and carburized–carburized (C–C) segments were characterized. The alloy microstructure consisted of an austenitic γ-(Fe,Ni) matrix reinforced by Cr 7 C 3 , Cr 23 C 6 , and NbC carbides, with carburized regions additionally containing coarse precipitates, probably the G-phase (Ni 16 Nb 6 Si 7 ). Weld metal refinement and continuous fusion boundaries demonstrated that GTAW can successfully restore metallurgical integrity. However, heat-affected zones adjacent to carburized segments exhibited substantial carbide coarsening and morphological instability. Room-temperature tensile strength decreased from ∼586 MPa (AC–AC) to ∼459 MPa (AC–C) and ∼416 MPa (C–C), while total elongation dropped from ∼5% to ∼0.5% with increasing incorporation of carburized material. Creep rupture life at 1100 °C under 16 MPa declined from ∼107 h (AC base metal) to ∼62 h (AC–AC), ∼34 h (AC–C), and ∼23 h (C–C). Fractography revealed a transition from mixed ductile–brittle fracture in AC specimens to predominantly faceted brittle fracture in carburized and C–C weldments. Despite these degradations, all welded joints retained mechanical performance compatible with industrial operating conditions. Overall, the results demonstrate that GTAW is a technically viable route for repairing 35Cr–45NiNb tubes, while highlighting the microstructural instabilities that limit long-term performance and must be considered in predictive service life assessment or repair qualification procedures. • GTAW enables effective repair of 35Cr–45NiNb tubes with pre-carburized segments. • Welding induces carbide coarsening in HAZ, especially near carburized material. • All welded joints preserve the austenitic γ-matrix and metallurgical continuity. • Tensile strength, ductility, and creep life decrease with increasing carburization. • GTAW is reliable for industrial repair while informing alloy and process design.