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

1.表面缺陷与晶粒度的关系(1)马口铁在90ﾟC的冷却水中淬火，观察其表面。所有的表面缺陷都位于每个晶粒的中心。这说明表面缺陷是由于表面局部冷却而长大的。(2)随着冷却水温度的降低，晶粒度减小，无表面缺陷的晶粒度增加。无表面缺陷的晶型与在空气中冷却时的晶型相似。随着冷却水温度的降低，沸腾持续时间变短。这些事实表明，当沸腾时间不长时，未局部冷却的表面积增大，因此表面缺陷不会出现在这样的区域上。II.局部冷却的原因在实验的基础上，推测局部冷却是由于过渡沸腾过程中固体表面与水的接触引起的。这一假设可以解释在水温为70ﾟC.时表面缺陷数有一个最大值的结果。拉应力实际上是导致表面缺陷长大的原因。

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.