Failure of Tension Springs During Installation
Two large tension springs fractured during installation. The springs were manufactured from a grade 9254 chromium-silicon steel spring wire. The associated material specification allows wire in the cold-drawn or oil-tempered (quenched-and-tempered) condition. The specified wire tensile strength range was 1689 to 1793 MPa (245 to 260 ksi). The finished springs were to be shot peened for greater fatigue resistance. Investigation (visual inspection, 3x images, 2% nital etched 148x SEM images, chemical analysis, hardness testing, and EDS analysis) supported the conclusion that the springs failed during installation due to the presence of preexisting defects. Crack surfaces were found to be corroded and phosphate coated, indicating that the cracks occurred during manufacture. Installation, which presumably entailed some axial extension, resulted in ductile overload failure at the crack sites. Recommendations included evaluating the manufacturing steps to identify the process(es) wherein the cracking was likely occurring. It was further recommended that a suitable nondestructive method such as magnetic particle inspection be implemented.