If the generation of the nucleation rate faster, and crystal nucleus formation, growth speed is slow, it generated more cell number, grain is fine.

如果晶核的生成速度较快，而晶核生成后的成长速度较慢，则生成的晶核数目较多，晶粒较细。

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But in the backing welding, columnar grain-growth will be appeared in the direction of crystal direction, it effects the welding quality seriously.

但是单面焊打底层焊缝中出现的沿焊缝柱状晶结晶方向的柱状气孔，严重地影响着船舶焊接质量。

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