This blog post dives deep into a seemingly simple topic: minimum fillet weld sizes. You'll discover why the common belief that these sizes are just about strength is actually incorrect. Learn the real metallurgical reason behind these design rules (found in codes like AWS D1.1 and AISC 360-16): preventing embrittlement and costly hydrogen-induced cracking from rapid cooling, even in low carbon steels. We'll also cover crucial details like welding dissimilar thicknesses and the requirement for single-pass welds for these minimums. Finally, understand how this overlooked aspect connects to the widespread and expensive problem of overwelding in our industry.