Materials and Filler Metal Selection
Materials and Filler Metal Selection
Selecting the correct filler metal is critical for weld integrity. Factors include base metal composition, strength, toughness, corrosion resistance, and service conditions. In Canada, common standards are CSA W48 for filler metals and CSA W59 for steel structures. Filler metals are classified by strength (e.g., E4918 for 480 MPa tensile strength), position, coating type, and current type. For dissimilar metals, filler metal selection must consider dilution and thermal expansion. For low-alloy steels, matching filler metal composition may be necessary to avoid hydrogen cracking. For stainless steels, use low-carbon (L) grades to prevent sensitization. For aluminum, use 4043 or 5356 depending on base alloy.
A common trap is selecting a filler metal based solely on strength without considering toughness or hydrogen content. For example, E7018 is low-hydrogen and suitable for high-restraint joints, while E6010 is cellulosic and produces deeper penetration but higher hydrogen. Another trap is using a filler metal with a different classification than specified on the WPS; this is a nonconformance. Also, storing filler metals properly (e.g., keeping electrodes dry) is essential to prevent porosity.
Worked example: Welding ASTM A36 steel to 304 stainless steel using GTAW. The filler metal should be 309L (ER309L) to accommodate dilution and prevent cracking. Using 308L might lead to excessive martensite formation in the dilution zone. The WPS must specify ER309L, and the welder must use it.
Rule of thumb: For dissimilar metal joints, choose a filler metal that is at least as alloy-rich as the more alloyed base metal, and consider dilution.