Microstructural Dependence of Tensile and Fatigue Properties of Compacted Graphite Iron in Diesel Engine Components

Engine components experience high temperatures during operation, which deteriorates the mechanical properties and induce an acceleration of fatigue damage. Compacted Graphite Iron (CGI) provides a combination of thermal and mechanical properties satisfying the requirements of diesel engine components of cylinder heads. In order to explore the effect of the microstructural state of CGI on the tensile properties and fatigue lifetime, an annealing treatment of 720 h at 420 degrees C is carried out to generate two additional initial conditions of the CGI microstructure: annealed in open atmosphere and annealed in vacuum, whereby these results are compared with the as-cast condition. It is observed that the annealing treatments cause an increase in yield strength, and concurrently an increase in ductility during static tensile loading at room temperature. During cyclic loading at room temperature an increase of lifetime is observed. These variations are far more pronounced after vacuum annealing. As the stronger bond of the ferrite/graphite interface compared to the pearlite/graphite interface, provides the better resistance to delamination, it will strengthen the material both in static and dynamic loading. The change of the local microstructure at the interface of the graphite and the metal matrix explains the concurrent increase in strength and ductility.