Study on Growing Mechanism of Defects on Tin Alloy Layer During its Quenching

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1. Relation between surface defects and crystal size(1) Tinplate is quenched into cooling water of 90ﾟ C and its surface is observed. All of the surface defects are located at the center of each crystal grain. This fact shows that the surface defects are growing due to the local cooling on the surface.(2) With the decrease of cooling water temperature, the size of -the crystal grains decreases while the number of the crystal grains without the surface defects increases. The configurations of the crystal grains without the surface defects are similar to those which are cooled in the air. It is also found that the duration time of boiling becomes shorter with the decrease of cooling water temperature. These facts show that the surface area being cooled without the local cooling increase when the boiling is not last for a long time, and thereby that the surface defects do not appear on such an area.II. Cause of local coolingBased on the experiments, a hypothesis is deduced that the local cooling are caused due to the contact between solid surface and water during the transition boiling. The hypothesis can explain the results that the number of the surface defects has a maximum value at the water temperature of 70ﾟC.III. Thermal stress on tinplate surfaceTensile stress is developed when the melted tin layer is solidified on the surface due to the local cooling. The tensile stress is actually the cause of growing the surface defects.