Fatigue failure analysis of heavily loaded steel chain elements

Introduction
Chain components are machine elements that suffered from extreme loading conditions, such as high tensile stresses, cycling stressing, friction, and sometimes aggressive service environment (such as humidity, seawater, chemicals). The presence of discontinuities (surface flaws, inclusions, etc.) and/or abnormal service conditions may lead to premature failure of chain elements.

Investigation
Failure analysis process was followed to a fractured steel chain component used for the power transmission driven by a high power motor. The chemical composition indicated that the fractured link was made of a low alloy carbon steel grade C40 (W.Nr. 1.0511). Optical evaluation indicated the occurrence of fatigue as dominant failure mechanism followed by a brittle overload mode (Fig. 1 and 2). Beach marks constitute cyclic crack-arrest marks depicting the slow crack growth area are shown in Fig. 1a. Final, fast fracture zone comprised mainly of radial marks pointing the origin, as shown in Fig. 1b.

 

a) Fatigue zone indicating the characteristic concentric progression marks (beach-marks)

b) overload zone constituting the instant fast fracture zone, containing chevron radial marks pointing the origin.

 

 

Fig. 1: Macrographs showing the fracture surface topography of the fractured components (approximate width ~ 50 mm); (a) Fatigue zone indicating the characteristic concentric progression marks (beach-marks) and (b) overload zone constituting the instant fast fracture zone, containing chevron radial marks pointing the origin.

Micro-fractographic analysis performed via secondary electron imaging in SEM highlighted the principal fatigue crack propagation and overload zone features. Fatigue surface manifested the presence of fine striations which demonstrate the actual crack advancement per load cycle (Fig. 2a). Final fast fracture is characterized by transgranular cleavage fracture mode (Fig. 2b). Counting of striations in the fatigue zone may lead to the determination of the total number of cycles to failure and trace back to service history when the crack initiation event was started.

Figure 2a & 2b: SEM micrographs depicting characteristic topographic feature of the fracture surface:

Fig. 2a:

Fig. 2b:

Fig. 2: SEM micrographs depicting characteristic topographic feature of the fracture surface: (a) fatigue striations showing the direction of the crack propagation, (b) cleavage facets outlining the transgranular mode of fast fracture area.

Further EDS elemental analysis identified iron oxide particles (FeO) which may constitute inherent weaknesses against fatigue loading, serving as stress raisers and offering an easy path for crack propagation.

Fig. 3a:

Fig. 3b:

Fig. 3: Optical micrographs showing the microstructure of the failed element: (a) core structure showing the presence of ferrite and pearlite (hardness ~ 250 HV), (b) surface structure of the hardened layer, showing the presence of bainite (hardness ~ 730 HV).

Optical microscopy revealed a layered structure: the core structure consisted of ferrite and lamellar pearlite while the hardened layer – reaching a thickness of approximately 2 mm –consists of bainite (Fig. 3). Non metallic inclusions, attributed probably to sulfides and oxides, were also identified.

 

Conclusion
The findings of the investigation suggest strongly that, the steel element, connected to a high power transmission motor failed in-service due to fatigue initiated from the inner surface and followed by brittle transgranular overload fracture. Chemical analysis and metallographic evaluation revealed that the failed component was manufactured from a low alloy carbon steel surface hardened while the core stayed at the soft annealed condition. In addition, a distribution of non metallic inclusions, such as FeO, that may have a serious impact in fatigue behaviour, was found in the microstructure. Further increase of general hardness level and material cleanliness is recommended since it is expected to have a beneficial effect, increasing the fatigue strength and lifetime of the component.

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