Abstract
Present paper reports the microstructural characterization of the failed screws. The analyzed chemical compositions of all the three screws indicate that these are fabricated from Fe-based alloy with Cr as alloying element. The analyzed chemical composition is close to AISI-51B37 standard. The microstructures of the screws in un-etched condition exhibit the presence of micro cracks and thin layer at the outer surface of threads. The micro cracks have originated from the root of the threads and propagated towards the centre. The thin layer with ∼12 μm thickness reflects the presence of mainly cadmium (Cd). This indicates that the Cd coating is applied on the outer surfaces of the threads for protection against corrosion and lubricity. The etched microstructure of the screws is very fine tempered martensite. The hydrogen content of the screws is ∼13 ppm which is significantly higher than that allowed in high strength steels (1 ppm). The fracture surfaces of the screws portray the presence of intergranular fracture features, ductile tearing and ductile dimples. The presence of high hydrogen content, integranular fracture features and ductile tearing indicate that the screws have failed due to the hydrogen embrittlement.
Published Version
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