Microstructural changes caused by deep cold rolling and their effects on fatigue life of ductile iron
Date of Issue2015
School of Mechanical and Aerospace Engineering
Ductile iron is a commonly used material in the automotive industry. Ductile iron has better fatigue life than other cast iron due to the presence of spheroidal graphite nodules in the microstructure. High nodularity of the graphite nodules has a positive effect on the fatigue life of ductile iron. The fatigue life of ductile iron can be improved by mechanical surface treatment. An example of which is deep cold rolling. Deep cold rolling improves the fatigue life of ductile iron by inducing strain hardening and improving the surface roughness. Compressive residual stresses are also introduced into the sub-surface region which helps to close up any cracks that have formed. In the experiments, the ductile iron used had a pure ferritic matrix. By varying the rolling ball size and pressure in the experiments, the microstructural changes of ductile iron and effects of these changes are analysed. While rolling increased the matrix hardness, it was found that increasing the rolling ball size from 6mm to 13mm reduced the increment in hardness. Increasing the rolling ball size also had the effect of decreasing the surface roughness and preserving the graphite nodularity on the rolled surface and cross sectional area. For a given rolling ball size, increasing the rolling pressure increased the hardness of the ferrite phase and the surface roughness. It was also found that the effect of rolling pressure on graphite nodularity was only present on the rolled surface, in which increasing the rolling pressure for a given ball size was detrimental for graphite nodularity. From the results gathered, it can be said that rolling ball size had a more extensive effect than rolling pressure on the microstructure of ductile iron.
Final Year Project (FYP)
Nanyang Technological University