Whether GB/T 1591 Q345D weldable steel plates need preheating

Whether GB/T 1591 Q345D weldable steel plates need preheating? First, we need to know the purpose of preheating.

Preheating of weldable steel is to prevent cold cracks, which is directly related to the carbon equivalent. The carbon equivalent of GB/T 1591 Q345D weldable steel plates is less than 0.3%, so there is no problem for weldability of Q345D steel plates.  Generally,  the cold crack sensitivity is increased when the carbon equivalent exceeds 0.4%. So usually Q345D steel plates not need preheating.

There is also exceptions. If the thickness of GB/T 1591 Q345D weldable steel plates   is large, the preheating is needed because the cooling is very fast. If the thickness of Q345D steel plate is above 70mm, the risk of non preheating is very large.