方形钢管焊接性是指金属在特定结构和工艺条件下通过常用焊接方法获得预期质量要求的焊接接头的性能，焊接性一般根据焊接时产生的裂纹敏感性和焊缝区力学性能的变化来判断。
Square steel tube welding metal refers to the performance expected quality requirements of welded joints commonly used welding method is adopted in the specific structure and process conditions, welding general changes according to the crack sensitivity of weld zone and mechanical properties of welding the judge.
方形钢管焊接性能的好坏主要取决于它的化学组成。而其中影响最大的是碳元素，也就是说金属含碳量的多少决定了它的可焊性。钢中的其他合金元素大部分也不利于焊接，但其影响程度一般都比碳小得多。钢中含碳量增加，淬硬倾向就增大，塑性则下降，容易产生焊接裂纹。
The quality of welded steel pipe is mainly determined by its chemical composition. The most influential factor is carbon, which means that the amount of carbon in the metal determines its weldability. Other alloying elements in steel are also not conducive to welding, but their effects are generally much smaller than carbon. With the increase of carbon content in steel, the hardening tendency increases, and the plasticity decreases.
通常，把方形钢管材料在焊接时产生裂纹的敏感性及焊接接头区力学性能的变化作为评价材料可焊性的主要指标。所以含碳量越高，可焊性越差。

In general, the sensitivity of the cracks in the welded steel pipe and the mechanical properties of welded joints are the main indexes to evaluate the weldability. So the higher the carbon content, the worse the solderability.
所以，常把方形钢管中含碳量的多少作为判别钢材焊接性的主要标志。含碳量小于0.25%的低碳钢和低合金钢，塑性和冲击韧性优良，焊后的焊接接头塑性和冲击韧性也很好。焊接时不需要预热和焊后热处理，焊接过程普通简便，因此具有良好的焊接性。随着含碳量增加，大大增加焊接的裂纹倾向，所以，含碳量大于0.25%的方形钢管不应用于制造锅炉、压力容器的承压元件。
Therefore, the steel pipe in the square of the number of carbon steel as the main mark of welding. Low carbon steel and low alloy steel with carbon content less than 0.25% have good plasticity and impact toughness. It does not need preheating and heat treatment after welding. The welding process is simple and convenient. With the increase of carbon content, greatly increasing the tendency of welding cracks, therefore, the carbon content of more than 0.25% square steel pipe is not used in the manufacture of boilers, pressure vessels pressure components.
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