Formation of an ultrafine grained layer due to high temperature treatment of the surface of a cast iron cylinder bore

In order to meet environmental standards, a finishing laser treatment on the honed cylinder bores of V-engine blocks was performed. Samples of laser-treated cast-iron cylinder bores with lamellar graphite were taken. Scanning electron micrographs were taken on cross-sectioned specimens with a scanning electron microscope/focused ion beam dual beam instrument in order to evaluate the microstructure and grain size of the laser-treated layer. Specimens were found to be ultrafine grained. To simulate the thermal distribution during laser treatment, finite element method simulations were made for different laser energy densities. During laser treatment oil reserving holes were formed due to the inhomogenity of the pearlitic matrix and graphite lamellae, and the high temperature of the laser treatment. To quantify the number of oil reserving holes, a new image analysis method has been developed. It was found that the amount of oil reserving holes increased with higher laser energy densities.