1、热的目的是为了防止冷裂纹的产生。冷裂纹与碳当量有直接关系。因而要分清该材料的碳当量，Q345A（以前的牌号叫16Mn）、Q345D的碳当量是一样的。

1, the purpose of the heat is to prevent the generation of cold cracks. Cold crack is directly related to the carbon equivalent. So to distinguish the carbon equivalent of the material, Q345A (formerly known as 16Mn), Q345D carbon equivalent is the same.

Q345的碳当量0.3%不到，可以说焊接性还可以（一般说碳当量0.4%上以冷裂纹敏感性增大）。 所以Q345A通常情况下是不预热的。但是当板很厚时，由于冷却很快，还是要预热。曾经20#钢用J422（E4303）焊裂过。如果板厚在70mm以上，不预热风险很大。

Q345 carbon equivalent of less than 0.3%, it can be said that the welding can also be (generally said that the carbon equivalent of 0.4% on the cold cracking sensitivity increased). So Q345A is usually not warm up. But when the plate is very thick, because the cooling is very fast, or to preheat. Once 20# steel with J422 (E4303) welding crack. If the thickness of the plate in the 70mm above, do not preheat the risk of a large.

2、顺便说一下，这个Q345D与平时我们常用的Q345A区别就在于杂质S、P更少，低温冲击性能更好。从焊接角度说，当母材杂质元素少了，对于控制焊接热裂纹是有好处的。

2, by the way, the Q345D and the usual Q345A we used to distinguish between the impurity S, P less, low temperature impact performance is better. From the welding point of view, when the parent material impurity elements less, for the control of welding hot cracks are good.

3、当然既然母材选择Q345D材料（比Q345A贵，且供货少），一般说都是有特殊用途，所以焊接材料不会随便选用，最好是按焊接材料说明书使用，如果焊条说明书说要对母材预热，那就预热。因为没有按厂家说明书使用焊接材料，焊缝性能会大打折扣的。

3 of course, since the parent material Q345D materials (and less expensive than Q345A, delivery), generally there are special purpose, so the welding material will not casually use, it is best to use manual welding materials, welding rodinstructions to say if the parent material preheating, the preheating. Because of the use of welding materials, the welding performance will be greatly reduced.