This requirement means all filler metals used in B31.1 work must be listed in, and conform to, the relevant SFA specification in Section II Part C. The SFA specifications are the ASME adoption of AWS filler metal classifications — for example, SFA-5.1 (carbon steel covered electrodes), SFA-5.9 (stainless steel bare wires), SFA-5.28 (low-alloy bare wires for GMAW/GTAW). Each SFA specification defines the required chemical composition, mechanical properties, packaging, and marking requirements that the manufacturer must satisfy.

Where a project requires a filler metal not covered by Section II Part C — such as a proprietary superalloy, an experimental high-strength filler, or a non-standard combination — the code permits its use only if the WPS specifically requires that filler and qualified welders have demonstrated acceptable results with it under ASME Section IX testing. This provision requires documented procedure qualification testing (PQR), not just a manufacturer’s data sheet.

This is the primary strength requirement. The weld metal must be at least as strong as the base material. For a joint in A106 Gr. B carbon steel pipe (minimum UTS = 415 MPa / 60 ksi), the filler metal must deposit weld metal with a nominal tensile strength of at least 415 MPa / 60 ksi. Using a lower-strength filler would make the weld the weakest element in the joint, violating the fundamental principle that the weld should not be the limiting factor in the piping system’s pressure-carrying capacity.

The use of filler metals with substantially higher strength than the base metal is generally permissible under this clause but must be considered in the context of requirements (c) and the overall stress analysis, particularly where PWHT may be required.

This clause applies when two base metals with different strength grades are joined at a transition weld — for example, a P-No. 1 Group 1 carbon steel pipe (60 ksi) butting to a P-No. 5A Cr-Mo pipe (70 ksi). The weld metal need only match the weaker material (60 ksi) rather than the stronger. This is a critical provision that prevents over-engineering the weld: using a 70 ksi filler when only 60 ksi is required adds no benefit and may introduce unnecessary hardness, hydrogen cracking susceptibility, or PWHT complications.

Chemical composition matching is required for several interconnected reasons: corrosion resistance (an alloy steel weld in a stainless body would corrode preferentially), mechanical property compatibility after PWHT (the tempering response of the weld metal must match the base metal to achieve uniform hardness), high-temperature creep resistance, and thermal expansion compatibility. The code specifically calls out major alloying elements (e.g., Cr, Mo) and essential minor elements (e.g., V, Nb, W in advanced creep-resistant steels).

When two alloys of different chemical composition are welded together, the filler metal may match either parent material or have an intermediate composition — but it is not required to exactly match either one. The choice of approach depends on the specific alloys involved, the service environment, and the applicable PWHT. Using an intermediate composition ensures that both fusion boundaries experience a gradual alloy gradient, reducing the risk of carbon migration (decarburisation/carburisation) across the bond line, which is the primary long-term degradation mechanism at dissimilar ferritic-to-ferritic transition welds.

This is an absolute requirement with no exceptions under (d). When an austenitic stainless steel (e.g., 316L, 321) is welded to a ferritic or low-alloy steel (e.g., P22, P91, carbon steel), the weld metal must be austenitic. This requirement exists because austenitic weld metal provides the ductility and toughness necessary to accommodate the significant difference in thermal expansion coefficient between austenitic stainless (approximately 17 × 10⁻⁶/°C) and ferritic steel (approximately 12 × 10⁻⁶/°C). A ferritic or martensitic weld metal at this interface would experience high cyclic thermal stresses and would likely fail by thermal fatigue cracking over time.

Nonferrous metals — copper alloys, aluminium, nickel alloys, titanium, and cobalt alloys — have highly specific filler metal requirements that cannot be addressed by the simple strength-and-chemistry rules of sub-clauses (a) through (e). For copper alloys, the weld metal must match the deoxidation state and alloying of the base metal. For aluminium, the filler selection is governed by the specific alloy combination and the required post-weld properties. For nickel alloys, the manufacturer’s documentation and organisations such as the Nickel Institute, CDA (Copper Development Association), and AWS specifications provide the definitive guidance.

In power piping, the most common nonferrous applications involve copper nickel (90/10 CuNi or 70/30 CuNi) in feedwater heater tubing and seawater cooling systems, and nickel alloy cladding or overlay in corrosive service. Always confirm the recommended filler directly against the base metal heat certificate and the applicable SFA specification.

This provision recognises that real-world power piping projects can present metallurgical challenges that the standard rules of (a) through (f) do not resolve cleanly. The code lists specific examples of conditions where (g) may apply, but explicitly states the list is not exhaustive:

• Unusual materials or combinations: Novel alloys, proprietary materials, or combinations for which no standard filler is commercially available
• Highly corrosive service environments: Cases where the standard matching filler would be susceptible to a specific corrosion mechanism (e.g., intergranular attack, pitting) and a more electrochemically noble or resistant weld metal is required
• Dissimilar material welds: Cases beyond those covered by (d) and (e), such as ferritic-to-nickel alloy transitions
• Different mechanical properties desired: Design intent to create a deliberately softer or harder weld zone — for example, a soft interlayer in a crack-arrester design, or a hard-surfaced overlay on a valve seat