Failure Analysis of Hexhead Capscrew made of 4140 Steel in a Moist Environment

The 4140 steel screws have been used in ships. The customer had a problem in frequent failure of such screws being used in certain valves in the propulsion unit. The broken head of the capscrew was photographed. There was a brownish pit in one corner of the broken capscrew head. It was decided that further investigation using Rockwell "C" Hardness, SEM /EDAX and TEM would be necessary. Rockwell "C" Hardness was carried out at five different spots at the bottom of the capscrew and the average hardness was HRC 34, which is within the specification of HRC 28-35. Next, Secondary Electron Imaging (SEI) of the broken head revealed brittle fracture and there was no sign of the striation lines which are apparent in a fatigue fracture. So it was not a fatigue failure. Semi Quantitative Chemical Analysis was carried out using EDAX in the brown fracture area. There was a strong peak of oxygen which indicated that there was corrosion in the pitted area. As the pitted zone was divided into several sections and analyzed using EDAX, it was found that the strength of oxygen varied from one end to the other indicating that the amount of oxides was higher in the outer zone of the pit and less in the inner side of the pit. High magnification (25,000x) SEM SEI photography and later TEM of a carbon film replica of small area having magnification (90,000x) revealed that there was some dislocation pile-up indicating strain fields in the corrosion pits. Therefore, it was confirmed that the failure was due to stress corrosion produced due to torquing under service conditions. In the future, the right amount of torque will be used to fasten the screws and /or a modified heat treatment could produce capscrew having hardness around 30 HRC.

To see the resume of the expert associated with this case study, see the link below.