Fatigue Fracture of a 4337 Steel Articulated Rod Originating at an Electroetched Numeral
An articulated rod (made from 4337 steel (AMS 6412) forging, quenched and tempered to 36 to 40 HRC) used in an overhauled aircraft engine was fractured after being in operation for 138 h. Visual examination revealed that the rod was broken into two pieces 6.4 cm from the center of the piston-pin-bushing bore. The fracture was nucleated at an electroetched numeral 5 on one of the flange surfaces. A notch, caused by arc erosion during electroetching, was revealed by metallographic examination of a polished-and-etched section through the fracture origin. A remelted zone and a layer of untempered martensite constituted the microstructure of the metal at the origin. Small cracks, caused by the high temperatures developed during electro-etching, were observed in the remelted area. It was concluded that fatigue fracture of the rod was caused by the notch resulting from electroetching and thus electroetched marking of the articulated rods was discontinued as a corrective measure.