Influence of matrix and alloying on the high cycle fatigue properties of compacted graphite iron for cylinder heads

Sammanfattning: Continued improvements in combustion processes, introduction of biofuels, decreasing fuel consumption and increasing specific power will inevitably lead to an increase of the combustion pressure, which can be accompanied with a temperature increase. This will increase the loading on many components and drives the development of materials more resistant to the harsh conditions close to the engine. The cylinder head is one of the components that will be affected by the increased loads. Therefore, thermomechancial loads (TMF) originating from the cyclic temperature variations from the heating up and cooling down cycles of the engine and the high cycle fatigue load (HCF) emanating from the repeated combustion pressure pulses may cause failure of the component. Today, most cylinder heads for heavy trucks are cast in either lamellar graphite iron (LGI) or compacted graphite iron (CGI). In both cases, the graphite particles are embedded in a pearlitic matrix. A possible improvement of the endurance life of the component is to add silicon to the composition. Silicon promotes the formation of ferrite and the hypotheis is that the more ductile ferritic matrix enclosing the graphite particles will improve the resistance to macro crack initiation and propagation and thus the life of the component. The purpose of this master thesis is to investigate the effect of a ferritic matrix as well as other added alloying elements such as molybdenum and nickel on the high cycle fatigue properties of CGI. The HCF properties of three ferritic alloys with different amounts of molybdenum and four pearlitic alloys with additions of molybden or nickel were compared at a fatigue life of 2 miljon cycles and a stress ratio of R=0.1. The results show that the silicon alloyed ferritic base material, without molybdenum, had a fatigue strength that on average was about 10 % higher than the corresponding pearlitic base alloy without alloying elements. Adding 0.25 wt % molybdenum improved the fatigue strength for both the pearlitic and the ferritic alloy with about 5-10 %. Further, for the pearlitic materials, molybdenum also improved the static mechanical properties while no such effect was seen for the alloys with a ferritic matrix. It was also found that nickel did not alter the HCF or the static properties significantly on the materials with pearlitic matrix.