Techniques for avoiding cracking when making concave fillet and groove welds.
Some of the lessons we learn best are taught in the School of Hard Knocks. I'd like to recount one such lesson that I remember vividly.
While I was welding superintendent at Globe Shipbuilding during World War II, we built 29 ocean-going vessels. Included in this total were 11 cargo vessels: 338 foot ships with 1,750-hp diesel engines. They were built for long-range merchant marine service, and had a top speed of 11 knots.
The masts of these cargo vessels had a masthead. It contained the block or sheaves for the hoisting cables. It was quite a weldment, all held together with 0.25-in. fillet welds. Normally, we used E6012 electrode for these welds, and we had never had any problems when welding with this filler metal.
During WWII, we fought shortages of everything: steel, acetylene, oxygen, welding consumables — you name it, we probably needed it and couldn't get it when we needed it. The ongoing shortages had prompted an informal bartering system. When we weren't trading, we were borrowing.
One day, we ran out of E6012 electrode. We contacted a nearby shipyard; they were out of E6012, too, but they had some E6020 electrode they could loan us. This so-called “hot rod” would get us up-and-running again, so we took it, even though we'd never used it before. Beggars, after all, can't be choosers.
The night shift went to work, welding three masthead assemblies for the cargo vessels. The next morning, we discovered that many of the welds had cracked — right down the center of the bead. The cracked welds were directly traceable to the borrowed electrode.
The E6012 electrode we were familiar with deposited a bead with a slightly convex face; the E6020 gave us a concave bead.
We initially “blamed” the E6020 electrode, but the actual cause of the cracking was the bead shape we obtained with that electrode. Since concave welds can be produced with a variety of products, it is important to understand the mechanism behind this cracking.
In the shipyard, we were making fillet welds, so we will start by discussing fillets. As the molten weld metal begins to solidify, grains grow from the base metal to which the weld is being applied. The grains continue to grow as thermal energy is conducted away from the weld, with the general direction of growth being toward the center of the bead. The last portion of the weld to solidify is the center of the bead, near the surface.
While the weld is cooling, it is also volumetrically shrinking. The resultant contraction pulls on the surface of the weld. For convex beads, the shrinkage puts the surface into compression. For concave beads, however, the shrinkage puts the surface into tension. These contraction forces caused the welds made with the E6020 electrodes to crack.
While the problem we experienced in the shipyard involved fillet welds, concave weld passes in groove welds can also crack. Root passes are more prone to such cracking, as are single pass weld layers. Split layer techniques encourage convex beads, which in turn are more crack resistant.
Since the cracking is solidification-related, the cracks either will be present when the weld cools or they won't form at all. There is nothing inherently wrong with an uncracked, concave weld bead, providing that the required throat dimension is achieved. However, the size of concave fillets can be deceiving: what looks like a huge weld may, in fact, have a very small throat. That is why it is wise to check fillet weld sizes for both the leg dimension and the throat size.
Part of the reason I recall my shipyard story so vividly is that the welds made with E6020 electrode had been made on the third shift. We had to chip those welds out with pneumatic chipping hammers — back in WWII, arc air gouging hadn't yet been invented. It took all of the first shift just to remove the cracked welds. So, after having spent two shifts of work (the third and the first), we were back where we had started: needing to weld the mastheads. Somehow, somewhere, we found enough E6012 electrode to finish the project — successfully — making convex fillets.
No diplomas are issued from the School of Hard Knocks, but the lessons learned there are never forgotten.
Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the “Top 125 People of the Past 125 Years” by Engineering News Record. Blodgett may be reached at (216) 383-2225.