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1、熱的目的是為了防止冷裂紋的產生。冷裂紋與碳當量有直接關系。因而要分清該材料的碳當量，Q345A（以前的牌號叫16Mn）、Q345D的碳當量是一樣的。
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的碳當量0.3%不到，可以說焊接性還可以（一般說碳當量0.4%上以冷裂紋敏感性增大）。 所以Q345A通常情況下是不預熱的。但是當板很厚時，由于冷卻很快，還是要預熱。曾經20#鋼用J422（E4303）焊裂過。如果板厚在70mm以上，不預熱風險很大。
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、順便說一下，這個Q345D與平時我們常用的Q345A區別就在于雜質S、P更少，低溫沖擊性能更好。從焊接角度說，當母材雜質元素少了，對于控制焊接熱裂紋是有好處的。
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、當然既然母材選擇Q345D材料（比Q345A貴，且供貨少），一般說都是有特殊用途，所以焊接材料不會隨便選用，最好是按焊接材料說明書使用，如果焊條說明書說要對母材預熱，那就預熱。因為沒有按廠家說明書使用焊接材料，焊縫性能會大打折扣的。
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.