The excellent weldability of A572 Grades 50 and 60 is one of their defining advantages over older high-strength steels. This is a direct result of their low carbon content and carbon equivalent (CE). Weldability refers to the ease with which a metal can be welded without introducing defects like cracks, while still achieving required mechanical properties in the weld and Heat-Affected Zone (HAZ).
Carbon Equivalent (CE): This is a calculated value that predicts the hardenability and crack susceptibility of the HAZ. The most common formula (IIW) is: CE = C + (Mn/6) + ((Cr+Mo+V)/5) + ((Ni+Cu)/15). For A572 Grade 50/60, typical CE values range from 0.38 to 0.45. According to AWS D1.1 Structural Welding Code, steels with a CE ≤ 0.45 generally do not require preheat for thicknesses up to about 1 inch (25 mm) when using common low-hydrogen processes. This is a major practical benefit.
Welding Processes: All standard arc welding processes are suitable:
Shielded Metal Arc Welding (SMAW): Use low-hydrogen electrodes (E7018, E8018-C3).
Flux-Cored Arc Welding (FCAW): Very common in fabrication shops (E71T-1, E81T1-K2). Gas-shielded (FCAW-G) or self-shielded (FCAW-S).
Gas Metal Arc Welding (GMAW): With solid wire (ER70S-6) and argon/CO2 shielding gas.
Submerged Arc Welding (SAW): For automated welding of splices in girders or long seams.
Potential Challenges and Best Practices:
Preheat for Thick Sections: While not needed for thin material, AWS D1.1 mandates preheat for thicker sections to slow the cooling rate and prevent hydrogen-induced cold cracking. For example, a 2-inch thick A572 Gr60 joint may require a preheat of 125°F (50°C). The specific preheat temperature is always determined by the CE, thickness, and hydrogen level of the consumable, per the governing code.
Heat Input Control: Excessive heat input can over-temper or grain-coarsen the HAZ, potentially reducing toughness. Procedures specify a maximum heat input (in kJ/inch).
Filler Metal Selection: Filler metal strength should match or slightly overmatch the base metal. For A572 Gr50, use 70-ksi filler (e.g., E70XX, ER70S-X). For A572 Gr60, 70-ksi filler is often acceptable and provides good ductility, but 80-ksi filler (E80XX) may be specified for full-strength matching, especially in critical, highly constrained connections.
Joint Preparation and Cleanliness: Removing mill scale, rust, moisture, and contaminants (oil, paint) from the weld zone is essential to prevent porosity and cracking.
Post-Weld Heat Treatment (PWHT): Generally not required for A572. However, stress-relief annealing may be specified for exceptionally thick, highly restrained welds in critical applications to reduce residual stresses.
Welding Procedure Specification (WPS): All welding must be performed according to a qualified WPS. This document, qualified by testing, specifies all variables: process, base/filler metals, preheat/interpass temp, amperage, voltage, travel speed, etc. The fabricator is responsible for developing and proving their WPSes.
In summary, welding A572 is considered straightforward for experienced fabricators. The key is to follow qualified procedures and the applicable code (AWS D1.1 or D1.5 for bridges), paying particular attention to preheat requirements for thicker material and using low-hydrogen practices consistently.