Q345D steel is a commonly used low alloy plate in China. It is equivalent to steel
S355J2 in Germany standard and A529Gr50 in the ASTM standard, with slightly different chemical compositions and the same mechanical properties. Under normal delivery conditions, it is the same as the German S355J2G3, and even almost the same performance as the S355J2G4. They can be used interchangeably.
The welding of Q345D steel plate needs to be preheated first, because the carbon content is relatively high.
1. The purpose of heat is to prevent the occurrence of cold cracks. Cold cracking is directly related to carbon equivalent. Therefore, to distinguish the carbon equivalent of this material, the carbon equivalent of Q345A (formerly called 16Mn) and Q345D are the same. The carbon equivalent of Q345 is less than 0.3%, and it can be said that the weldability is okay (generally, the carbon equivalent of 0.4% increases the sensitivity to cold cracks). So Q345A is usually not warmed up. But when the board is very thick, it needs to be preheated because it cools quickly. Once 20 # steel was welded with J422 (E4303). If the board thickness is above 70mm, there is a great risk of not preheating.
2. By the way, the difference between this Q345D and the usual Q345A is that there are fewer impurities S and P, and the low temperature impact performance is better. From the welding point of view, when there are fewer impurity elements in the base material, it is beneficial to control the welding hot cracks.
3. Of course, since the base material chooses Q345D material (more expensive than Q345A, and less supply), generally speaking, it has special uses, so the welding material will not be used casually, it is best to use it according to the welding material specification, if the electrode specification says Preheat the base material, then preheat. Because the welding materials are not used according to the manufacturer's instructions, the weld performance will be greatly reduced.
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