Fracture Analysis on 10.9 Grade High Strength Screws

10.9 grade high strength screws made of 20 steel fractured after being used for about one month. Macro-examination, chemical compositions, hydrogen content, micro-hardness, microstructure, EDS and SEM analysis were carried out on the cracked screws. The results show that the screws existed micro cracks before using. The primary cracks made stress concentration, so the hydrogen in material moved to the crack top, enriched and caused of tension stress. The hydrogen content continuous to increase, while it come to the up-limited of the fracture hydrogen content, the screws occurred hydrogen ductile-brittle fracture.