Professional Welder’s Tip: Perfectly Joining a Round Pipe to a Square Tube at 90°. In this video, I share real-world professional welding techniques to help you join a round pipe to a square steel tube at a precise 90-degree angle with a tight fit and clean weld finish. You’ll learn how to mark accurately, cut a proper fish mouth using an angle grinder, tack weld to prevent distortion, and complete the weld for strong, durable, and professional-looking results. Perfect for DIY projects, steel frames, metal furniture fabrication, and general metalworking.