焊接裂纹是比较危险的焊接缺陷，严重影响焊接结构的使用性能和安全性，焊接结构的许多损伤事故都是由焊接裂纹引起的，裂纹除了会降低焊接接头的强度外，还会造成严重的应力集中，促使裂纹的发展和焊接结构损坏，在焊接过程中，焊缝和热影响区金属冷却到固相线附近高温区产生焊接裂纹。
Welding cracks are relatively dangerous welding defects, which seriously affect the performance and safety of welded structures. Many damage accidents of welded structures are caused by welding cracks. In addition to reducing the strength of welded joints, cracks will also cause serious stress concentration, promote the development of cracks and damage of welded structures, The weld metal and heat affected zone metal are cooled to the high temperature zone near the solidus to produce welding cracks.
热裂纹的形成
Formation of hot cracks
焊缝金属中的热裂纹也称为凝固裂纹，焊接材料多为合金，合金的凝固自始至终都是在合适的温度范围内进行的，这是热裂纹产生的基本原因。焊缝金属中的许多杂质的凝固温度都低于焊缝金属的凝固温度，所以先凝固的焊缝金属将低熔点杂质推向凝固结晶的晶粒边界，形成一层液体薄膜。由于焊接时熔池的冷却速度很快，焊缝金属在冷却过程中收缩，导致焊缝金属产生拉应力，当拉应力将凝固的焊缝金属沿着晶粒边界拉开，并且没有足够的液态金属来补充它时，就会形成微小的裂纹。随着温度继续下降，拉应力增大，裂纹继续扩展，这就是凝固裂纹。
The hot crack in weld metal is also called solidification crack. Most of the welding materials are alloy. The solidification of alloy is carried out in a suitable temperature range from beginning to end, which is the basic reason of hot crack. The solidification temperature of many impurities in the weld metal is lower than that of the weld metal, so the first solidified weld metal pushes the impurities with low melting point to the grain boundary of solidification crystallization, forming a layer of liquid film. Because the cooling rate of weld pool is very fast during welding, the weld metal shrinks during the cooling process, resulting in tensile stress. When the tensile stress pulls the solidified weld metal apart along the grain boundary, and there is not enough liquid metal to supplement it, tiny cracks will form. As the temperature continues to decrease, the tensile stress increases and the crack continues to expand, which is called solidification crack.
当焊缝金属中含有大量低熔点杂质时，焊缝金属容易产生凝固裂纹，母材和焊接材料中含有有害杂质，尤其是硫，硫与钢中的铁化合形成硫化亚铁（FeS），硫化亚铁与铁反应形成一种共晶物质，凝固温度为988°，远低于丁钢铁的凝固温度，所以硫是引起钢材焊缝金属中发生凝固裂纹的主要的元素。另外，钢中碳含量高时，有利于硫在晶界处富集，这也是促使凝固裂纹形成的原因，因此，使用低碳含量的焊接材料有利于预防凝固裂纹的形成。
When the weld metal contains a lot of low melting point impurities, the weld metal is easy to produce solidification cracks. The base metal and welding materials contain harmful impurities, especially sulfur. Sulfur combines with iron in steel to form ferrous sulfide (FES), and ferrous sulfide reacts with iron to form a eutectic material. The solidification temperature is 988 ℃ °， So sulfur is the main element that causes solidification cracks in weld metal. In addition, when the carbon content in steel is high, it is conducive to sulfur enrichment at the grain boundary, which is also the reason for the formation of solidification cracks. Therefore, the use of welding materials with low carbon content is conducive to preventing the formation of solidification cracks.
预防热裂纹的措施
Measures to prevent hot crack
1.锰具有脱硫作用，如果母材和焊接材料含碳量高，而含锰量不足时，则容易产生热裂纹，一般要求母材、焊条、焊丝的硫含量不超过0.04％，低碳钢和低合金网用焊条和焊丝的含碳量一般不应超过0.12％，焊条电弧焊时，正确选择焊条类型，使用合格优质的焊条是预防热裂纹的重要措施。
2.对铡性大的焊件，焊接时立生的变形小，导致焊接应力增加，促使热裂纹的产生，焊接时，选择合适的焊接规范，要采取预热和缓冷措施，合埋地安排焊接方向和焊接顺序，以减小焊接应力。
3.调整焊缝金属的合金成分，如焊接铬镍不锈钢，适当增加焊缝金属的铬含量，可显著增加焊缝金属的抗热裂纹性能，在焊缝金属中加入能细化晶粒的元素，有利于去除集中分布的液态薄膜，预防热裂纹的产生。
4.热裂纹板易在孤坑产生，即弧坑裂纹，焊条电弧焊时需要注意填满弧坑，在不填充焊丝的钨极氩弧焊中，当电弧闭合时，焊接电流应逐渐减小，然后当焊接体积减小到很小时，焊接电流应切断，焊接难以去除弧坑裂纹的材料时，应使用引出板把弧坑引出。