GREY IRON Composition Microstructure

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The major alloying elements of cast irons are carbon (C) and silicon (Si), making grey iron a FeC-Si alloy.[6] The chemical composition of grey iron usually contain a carbon content in the range of 2.5 – 4.0 % and a silicon content around 1.0 – 3.0 %. The minor alloying elements e.g. phosphorus (P), sulfur (S) and manganese (Mn) are also added to the melt.[7] Addition of phosphorus has been observed to reduce both the mechanical properties i.e. tensile strength of the grey iron, and the eutectic temperature. The later mentioned effect has shown to increase the fluidity of the melt.[8] Sulfur has normally been considered to be an undesirable element in grey iron not only due to its promotion of e.g. intermetallic carbides and an increased chill tendency which exhibits negative effects, but also due to its reaction with iron. The reaction between S and Fe produce the phase FeS, which has a low melting point and can in elevated temperatures adopt a brittle behavior. To avoid the undesirable outcome of the FeS phase manganese is added to the melt to tie up the sulfur and produce MnS. Addition of sulfur has also exhibited positive effects e.g. promoting nucleation of graphite and increase the strength of the material up to a certain level.[9] The trace elements, which has concentrations lower than 0.01 %, can involve e.g. aluminium (Al), bismuth (Bi), calcium (Ca), lead (Pb), tellurium (Te), titanium (Ti), tin (Sn) or nitrogen (N), and can be present in the grey iron either intentionally or unintentionally. The trace elements can have an important influence on both the microstructure and the properties of the grey iron.[6]

Physical properties

Cast irons, including grey iron, cannot be seen as homogenous when looking at the different physical, or thermal, properties. The amount of graphite affects the density and the specific heat while the size, shape and distribution of the graphite flakes highly affects the thermal conductivity. The matrix structure however is having a significant influence of the thermal expansion of grey iron.[7] These four physical properties are described in this section. Density and specific heat are mostly of interest in order to be able to determine the thermal conductivity, which together with thermal expansion are important properties of a cylinder head and similar cast components that are subjected to heat during its service. E.g. thermal stresses are developed across the cylinder head due to non-uniform expansion caused by temperature gradients within the component during usage. Low thermal expansion limits the thermal stress and high thermal conductivity reduce the temperature gradient.

The component studied

The component studied in this thesis is a CH11 cylinder head from a Volvo truck diesel engine, weighing in at around 220 kg. It is cast in grey iron with the material composition as stated in Table 2. A cylinder head is a component which is bolted on top of the cylinder block of an engine and it is housing the valves for the inlet air and the exhaust gas, the fuel injector and cooling channels. The cylinder head is subjected to high temperatures during the combustions but also fluctuating temperatures and pressures.[24] A key aspect for the material in a cylinder head is that it must transport away the generated heat efficiently to limit thermal fatigue, i.e. have high thermal conductivity, while also maintaining the correct dimensions i.e. have low thermal expansion. The thermally induced stresses are also reduced by low thermal expansion, small thermal gradients and low stiffness of the material. The findings in this research is not limited to cylinder heads but could also be of interest in other components where the physical properties are of great interest, e.g. brake discs, cylinder blocks and furnaces.

Mechanical property testing

The mechanical property testing in this thesis consisted of tensile testing since it was used in test the two previous cylinder heads. Tensile testing The tensile test was performed at room temperature at JTH test lab using the Zwick/Roell Z100 tensile testing equipment which has a load capacity of 100 kN. The strain elongation on each tensile bar was measured with a laser extensometer. The tensile testing was executed to be able to collect the mechanical properties at various locations of the cylinder head. The proceeding steps were roughly as follows:  Measuring the tensile bars  Configure the software parameters  Execute the tensile tests  Collect the data given The software used when performing the tensile tests needed the input value of the minimum diameter of the necking section of each tensile bar. The measurements were taken in three different areas of the 50 mm necking section, at the bottom area, in the middle and at the top area as could be seen in Figure 28. This was done in order to investigate if the fracture occurred where the tensile bars had the minimum diameter.

Physical property testing

Four different tests, described in section 2.5, were performed to measure the material’s physical properties. Dilatometer testing The thermal expansion of the samples was measured by a Netzsch DIL 402C push-rod dilatometer, as seen in Figure 16. Due to technical problems with the machine, only six samples were measured, the three from slow solidification and the three from intermediate solidification. This is discussed further in section 5.1.2. The heating chamber was emptied of its air, creating a vacuum, and then filled with Helium gas. This process was done three times to ensure a high enough fraction of Helium in the atmosphere in the heating chamber. The testing was performed at two different occasions, each time doing a standard sample test, with a 12 mm long polycrystalline Al2O3 rod, before testing the material samples. At the first occasion samples 5B2F, 4B2F and 3B2F was tested and then the other three were tested. The testing was run from room temperature up to 500°C with a heating rate of 10 K/min. All samples were measured before testing and their lengths can be found in Appendix 1.

Contents :

  • 1 Introduction
    • 1.1 BACKGROUND
    • 1.4 OUTLINE
  • 2 Theoretical background
    • 2.2 GREY IRON Composition Microstructure
    • 2.3 MECHANICAL BEHAVIOR Modelling of tensile deformation curve
    • 2.4 PHYSICAL PROPERTIES Thermal expansion Density . Specific heat Thermal conductivity
    • 2.5 MATERIAL TESTING Tensile testing Dilatometer (DIL) Density determination Differential Scanning Calorimetry (DSC) Laser Flash Apparatus (LFA)
  • 3 Method and implementation
    • 3.2 SAMPLE EXTRACTION Mechanical property sample extraction Physical property sample extraction
    • 3.3 TESTING Mechanical property testing Physical property testing
    • 3.5 MICROSTRUCTURE ANALYSIS Optical microscope analysis Image analysis
    • 3.6 CONNECTING CASTING SIMULATION AND FE SOFTWARE Design of simplified geometry Setting up the FE-model Casting simulation
    • Connecting experimental results to casting simulation results
    • Mapping the casting simulation results to the FE-mesh FEA
  • 4 Results & Analysis
    • Comparing the UTS-values between the three cylinder heads
    • Microstructure analysis comparing six selected samples
    • Dilatometer – thermal expansion
    • Density
    • DSC – Specific heat
    • LFA – Thermal diffusivity
    • Thermal conductivity
    • Microstructure analysis
    • 4.4 FEA RESULTS
    • Experimental values
    • Extreme values
  • 5 Discussion and conclusions
  • 6 References
  • 7 Search terms
  • 8 Appendices
    • 8.1 APPENDIX
    • 8.2 APPENDIX
    • 8.3 APPENDIX

Connecting casting simulation and FE software including local variation of physical properties.

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