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How to weld 90-degree outside corners

Proper welding of a corner joint starts with perfectly cut straight edges

Clamped firmly in place, the two pieces of steel forming this 90 degree outside joint are positioned properly, allowing for excellent weld penetration.

One day many years ago, another welding student and I were taking a break outside the door of the college welding shop. “I just love burnin’ stick,” he said, as we discussed what we were doing that day. (Burning stick is slang for arc welding, in case you didn’t get that.) I had to agree that being able to manipulate steel with welders and torches was pretty cool — or hot, or whatever.

But if you don’t take time to cut the steel properly, create good joint fitment and understand how to avoid the problems that applying a lot of heat to a piece of steel can create, the finished product will have a weak weld joint or won’t fit because it’s warped from heat distortion. That can turn love of burnin’ stick into extreme frustration and wasted effort.

Recently, it was necessary to fabricate a replacement integrated bumper and front frame crossmember on a vehicle being restored in the Grainews garage. That provided the perfect opportunity to take a close look at the proper procedure for fitting up and welding 90-degree outside corners. Here’s a look at the whole fabrication process from start to finish.

Getting the job done

To match the original part, which was impractical to repair, the new bumper had to be built out of 10 gauge steel. The original part was pressed into a “U” shape at the factory. Clearly, that wasn’t an option for us, so the replacement will need to be made up of three pieces cut from flat stock and welded together. Creating perfectly straight edges when cutting them, therefore, is critical.

We want the replacement to exactly match the original with precise dimensions, so the cut lines were marked on the steel using a steel scribe rather than a marking pen. The scribe scratches a precise, fine line into the base steel. It’s easier to make exact cut marks with this tool as well as precisely position the cutting guide than when using a thicker line created by a felt pen.

A plasma cutter was used to cut the bumper pieces out of flat stock. Clamping a framing square onto the base metal to use as a guide for the plasma torch created a very straight edge that isn’t possible on a freehanded cut. That was important for this job. Ideally, we’d use a straight edge that is as long as the entire cut, which the framing square wasn’t. But it was the best we had, and it worked.

Once the pieces were cut, their edges were dressed with an angle grinder to remove any slag created by the plasma cutter and smoothen them out a bit. All the surface rust on the new steel was removed as well, using a 60-grit flap disc on an angle grinder. That will allow for good electrical contact and help reduce the risk of contaminating the weld bead, and it well get the part ready for paint.

Because the bumper also acts as the front frame crossmember, it must be perfectly straight and not have any twist or distortion, so the welding process will have to be done carefully to minimize the amount of heat put into the part.

The pieces were clamped onto a welding table with perfectly square edges to hold them in place. Even though 10-gauge steel is more than 1/8 inch thick it can still warp from heat distortion. The clamps were left in place until the joint was fully welded to prevent the arc heat from pulling any piece out of position. Rather than overlapping them, aligning the inside edges of the pieces leaves a gap at the corner, which ensures good weld penetration. The weld bead creates new steel to fill in the gap.

We started the welding process in the usual way, by tacking the pieces together in a few places along the length of the joint. To finish welding it and avoid concentrating heat in one area, very short beads were systematically burned onto the end of each of those tacks, constantly moving from one to another throughout the process. That distributes the heat evenly along the entire piece to avoid warping. And don’t be in a rush. Working slowly also helps keep heat build up to a minimum.

Grinding the weld bead smooth makes for a seamless, finished appearance. To do that, we started with a regular disc on an angle grinder and removed the bulk of the excess material, creating an overall curved shape for the corner in the process. Moving the grinder back and forth along a wide area of the joint rather than just working a small section at a time helps level out the bead evenly. Grinding can create almost as much heat as welding, so this technique also helps minimize and evenly distribute any heat generated. To finish it off we switched back to the 60 grit flap disc to get a completely smooth surface and make the weld bead look like part of the base steel.

A test fit on the frame showed the bumper remained straight through the welding process and fit the frame exactly as planned. Running a constant weld bead along the joint from one end to the other would have certainly distorted and ruined the part.

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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