EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF TRIBOMECHANICAL BEHAVIOUR OF SiMoCr DUCTILE CAST IRON

Abstract

The versatility of ductile cast iron in the mechanical properties obtained through compositions and microstructure control in as-cast components offers reduced energy and material requirements by eliminating the cost of heat treatments. In the present study, three different melts Melt-1, Melt-2 and Melt-3 of SiMoCr ductile cast iron are prepared to study the effect of silicon, molybdenum and chromium on microstructure with reference to the variation in the matrix of ratio of pearlite and ferrite, nodularity, nodule size and nodule count and their resulting effects on tensile, hardness, wear and fracture properties of the material. For a given chemical composition and nodule size, an increase in the nodule count decreases the pearlite-ferrite ratio which further results in a reduction in tensile strength but an increase in percentage elongation. Elongated, sharp, wavy, or spiky edges of the nodules result in stress concentration and promote crack growth in the matrix. The spherical nodule plays a significant role in arresting the crack and reorients the crack path around the nodule. Nodule count also affects graphite shape, size and distribution. An increase in the nodule count decreases the nodule size in the matrix of ferrite and pearlite results in a finer grain structure which further increases the tensile strength. The uniaxial tensile tests are conducted from the specimens taken out from the as-cast sample bars at different strain rates and temperatures to analyze the combined effect of the amount of deformation, strain rate and temperature on the plastic behaviour of the material under different working conditions both in experiments and simulations. The various parameters of the Johnson-Cook material model are determined by following the standard procedure. Microstructural examination depicts that the percentage of various alloying elements affects the variation in the matrix of the ratio of pearlite and ferrite, v nodularity, nodule size and nodule count and thereby affecting the tensile properties and J-C parameters of the material. A common three-point bend test is performed as per the standard test procedure to determine the fracture toughness of the developed materials. The value of static fracture toughness obtained from the experimental data for the fatigue pre-crack specimen prepared from the test bars of Melt-2 is higher than the rest of the samples prepared from the test bars of the other two melts which may be attributed to the homogeneous distribution of the nodules in the matrix of ferrite and pearlite. According to the results obtained from FE simulations with fatigue pre-crack, the results for fracture toughness are found to be in good agreement with the results obtained from experimental data. A series of pin-on-disc wear tests is conducted to study the dry sliding behaviour of Melt-3, by using the Taguchi L16 array for the optimum combination of load, sliding velocity and temperature, on a rotary tribometer setup, at controlled temperatures. The analysis of variance, first-order regression analysis and confirmation tests are done to validate the results obtained for the material’s wear properties. Scanning electron microscope (SEM) and energy dispersive x-ray (EdX) analysis are also done for microstructural transformations. It is found that the sliding velocity has a higher influence on the specific wear rate. In contrast, the applied load is the key influencing parameter for a higher value of the coefficient of friction. In last, for testing the developed material in real-world situations, the wear and temperature response of brake discs made from Melt-3 of SiMoCr ductile cast iron against the brake pad under intermittent braking is also studied. A separate disc brake test rig setup is developed, for such intermittent baking tests, using a 15 hp induction motor, chain drive set for power transmission, brake shaft, brake caliper and SiMoCr brake disc. From this study, it is observed that the developed material showed good tensile and hardness properties. The highest values of the ultimate tensile strength (606MPa) and vi percentage elongation (13.98%) are observed in Melt-2. The average hardness value observed in the specimens of Melt-3 is the highest i.e., 298 ± 10.67 HV0.5 among the three melts. Melt-2 specimens showed the highest static fracture toughness as 54.49 MPa√m followed by fracture toughness observed in the samples of Melt-1 and Melt-3. The temperature evolved during intermittent braking at locking brake load is recorded experimentally as 198.7 °C, which agrees with the results obtained from numerical analysis and simulation. Overall, the material developed, SiMoCr DCI, in the present research will be a good contender for engineering applications where good tensile strength coupled with toughness, crack resistance and good wear properties are the key factors.