The fuel cam on an injector pump for a marine diesel engine failed after 1392 running hours. Failure occurred at the contact point between the roller and the load bearing surface. The specified alloy is an EN36B steel, case hardened to a depth of 1.5mm and to hardness value of 60 Rockwell C.
Inspection of the wear surface shows cracks and steep sided pits. The microstructure of the bulk fuel cam material is typical of the specified alloy (Figure 1), with cracks running parallel to the failure surface (Figure 2)
The loading on the worn surface of the cam is rolling contact fatigue, characterised by a polished surface with interconnected cracks, pits and spalling.
A section through a surface not subject to wear shows a case hardness of greater than 60 Rockwell C. On a section through the worn surface, the surface hardness does not meet specification. The case depth was found to be approximately 0.8mm. It is likely that the case hardened layer has been removed in the worn area.
The microstructure and hardness measurements are typical of a quench and tempered EN36B steel. However, spectrographic analysis shows some inconsistencies in Cr, Mn, Ni and Mo levels. The success of case hardening depends on the method and the alloy. Case depth is determined by the temperature during case hardening, the carbon concentration at the surface of the steel and the alloy composition.
It is likely that the load bearing surface of the cam was overloaded, either by misalignment or excessive pressure. The shallow case depth means that the area subjected to load during rolling contact could not be sufficiently strong to withstand the design load. This is illustrated in Figure 3.
During rolling contact fatigue the shear stress peaks below the surface during each cycle. The schematic on the left hand side shows that the stress field may exist outside of the case hardened area since the case depth is below specification for this particular component. Conversely, if the case depth were within specification, the stress field would be contained within the case hardened region. Case hardening increases the yield strength of a material in a localised area. If the load is transferred to a region with lower yield strength, the material will deform, initiating rolling contact fatigue.