Socket Weld Fatigue
By Bill Mohr on Thursday, December 16th, 2010
While moving the piles of paper on my desk, the pile of papers on socket weld fatigue showed up again. Socket weld fatigue failure has proved to be difficult to eliminate from in-service components, but is mercifully rare compared to all the socket welds used in pressurized piping systems. In socket welds the pipe is joined to a connector using an external fillet weld. The locations of likely failure boil down to two. A crack can grow from the external weld toe on the pipe or from the weld root into the weld metal of the fillet. Both of these can cause a leak. The second is hard to inspect for, so fillets are often enlarged to make this failure mode less likely. But the lifetime is heavily dependent on other parameters. The data in that pile of papers indicate that tension loading of the pipe can give failure with fewer and smaller cycles than bending of the pipe. Indeed there are distinct differences between lifetimes for different modes of bending. Rotating bending, where the peak load sweeps around the pipe, is more damaging than reversed bending in a single direction. Preventing failures in socket weld systems can involve making the welds big enough and smooth enough at the toe for the desired service. It can also demand that the pipe be sufficiently supported to prevent resonant vibrations that are restrained by the socket weld. Here’s hoping that your socket welds are safe from this rare, but repeated kind of failure by leakage.