| Abstract Scope |
The 2020 revision of AASHTO/AWS D1.5 introduced a provision allowing welding over protective coatings on structural bridge steel, which was subsequently removed in a later revision. The ability to weld over protective coatings is valuable, as temporary zinc coatings are commonly applied to protect structural steel during fabrication before the final protective coating system is applied. This study, conducted at The Ohio State University and sponsored by the Alaska Department of Transportation and Public Facilities, investigates the weldability of ASTM A709 Grade 50 steel across a range of surface conditions using drawn arc stud welding.
Two uncoated reference conditions are examined: steel with mill scale removed and steel retaining its mill scale. Three zinc coating types are investigated: mist coat applied at two thickness levels, arc spray metallized at two thickness ranges, and hot-dip galvanized at a single nominal thickness range that exhibits considerable thickness variation across the plate. Existing AWS standards are adapted as a benchmark for evaluating weld quality through visual examination, macroetch testing, ultrasonic testing, and mechanical testing. Porosity from zinc volatilization and liquid metal embrittlement are recognized concerns when welding over zinc coatings. Technique modifications including arc blow control, current path optimization, and ferrule geometry selection are explored to allow zinc to be displaced from the weld interface prior to solidification.
Completed testing on the uncoated conditions and mist coat demonstrate sound welds meeting all acceptance criteria. Thicker coating conditions present greater weld quality challenges, where overall coating thickness/variation is identified as a key factor contributing to reduced repeatability. Final results across all conditions will inform specific recommendations and limitations for drawn arc stud welding over zinc coatings, with findings intended for submission to the AWS Committee for consideration in future revisions to AASHTO/AWS D1.5. |