Photo – “Sparks” generated during arc welding are each a small droplet of molten metal that solidifies before hitting the ground. This image, taken using a scanning electron microscope, shows solidified spatter magnified several hundred times.
There are times when you just can’t get out of the way of Murphy’s Law. A manufacturer of industrial transmissions contacted us to determine the cause of a batch of failures their normally bulletproof products experienced. These failures were unprecedented in this mature designs history.
The gears, shafts, and bearings were well and truly destroyed, and a great deal of debris from that destruction had collected in the sump of the transmission cases. Careful examination of the debris using one of our scanning electron microscopes (SEM) revealed something both interesting and unusual. A high percentage of the debris was consistent is size and was perfectly spherical.
We have a great deal of experience in examining particles and identifying their source. They cause a lot of failures. A raindrop is perfectly spherical until it hits the ground. So is the spatter that is produced in arc welding. But being a rapidly solidifying liquid, molten arc spatter cools sufficiently to retain its spherical shape by the time it reaches the floor.
Enough weld spatter particles had collected in the cases to destroy the transmission in service, but not enough to be discovered during assembly. It doesn’t take much.
There was another problem however. No welding was done at this manufacturing facility and all parts were scrupulously cleaned prior to assembly.
Discussions between the manufacturer and its customers regarding the source of the weld spatter started to get a little heated. Then someone remembered that an area of the shop had been expanded to accommodate additional machinery. And that some structural welding had been done during that expansion. Near a segment of the assembly line.
The manufacturing dates of the affected transmissions were quickly corroborated with the time during which the expansion was underway. Our analysis of the weld spatter matched the composition of the weld filler metal in the structural welds, and the source of the weld spatter was finally identified.