A 2007 GMC Sierra 1500 presented with a repeating knocking sound while being driven. The rate of knocking increased as the vehicle accelerated, and fuel economy decreased. Disassembly of the engine revealed a cam lifter collapsed and was no longer in constant contact with its respective cam lobe. Each time the cam shaft rotated; the lobe forcefully contacted the lifter creating the knocking sound. Damage was noted on both the cam lobe and the surface of its partner lifter and the components needed replacing. Explico investigated the failure to determine the specific mechanism that resulted in the failure of the casing on the cam lobe.
Explico’s Eugene Lopez-Ona performed an investigation to determine the mechanism that resulted in the failure of the casing on the cam lobe. A metallurgical analysis was conducted using technical services provided by Titan Metallurgy. The analysis consisted of visual examination, dimensional documentation, magnetic particle non-destructive examination, scanning electron microscopy, microhardness testing, and metallographic preparation and examination. Metallographic specimens were prepared to view the microstructure in both longitudinal and transverse planes.
Vickers microhardness indentation tests were performed on a traverse of the transverse metallographic specimen. The hardness readings indicate a case depth of approximately 0.11”. The average hardness in the case is 58.1 HRC. The average hardness in the core of the cam lobe is 21.7 HRC. The image below presents a stitched view of the transverse metallographic mount with the case and core microstructures visible. The case microstructure consists of tempered martensite. The core microstructure consists of ferrite and pearlite.
The damaged surface was examined using a scanning electron microscope (SEM) and was mounted and prepared for metallographic examination. The SEM image (left) shows the leading edge of the damage to the cam lobe surface with a crack extending away from the leading edge. The metallographic image (right) shows a transverse view of the case hardened with the damaged surface of the cam lobe. At the surface, fresh martensite can be seen with cracks at the interface with the rest of the cam lobe.
The damage to the cam lobe resulted from sudden impacts with the stuck lifter. These repeated interactions were sufficient to cause the microstructure at the surface of the lobe to change from tempered martensite to fresh martensite. Fresh martensite is very brittle in comparison to tempered martensite, this property difference drives delamination at the interface with the bulk of the cam lobe material.