I'd also note that contrary to claims made on the thread:
ASTM A108 1018 is not brittle (actually it's really ductile and easy to successfully weld vs ASTM A513 1020 or 1026).
Both MIG and TIG can have excellent penetration. The issue is careful control of the weld parameters (that's what I employ my welding engineer to determine). The carbon equivalent of the steel is much more of a concern. High carbon equivalents make the tolerance for out of parameter welds smaller. At a carbon equivalent of over 0.5% you are basically assured that your welds will fail.
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