Weldability and Common Problems of Q390 Steel

Since Q390 steel is a low-alloy high-strength steel, when welding high-strength steel, generally the material with the same strength as the base metal should be selected, and the toughness, plasticity and strength of the weld metal must be considered comprehensively. The actual strength of the joint is not lower than the product requirements.

For Q390 steel, when the thickness is less than 40mm, there is generally no need to preheat in advance, no need to control the interlayer temperature and thickness heat, and no need to use heat treatment to improve the structure after welding. However, in order to obtain the excellent strength and toughness of the weld structure and prevent cold cracks, it is necessary to preheat the Q390 steel in advance to control the cooling rate of the welding.

Q390 steel will encounter the following problems during the welding process:
1. Cold cracking (delayed cracking), which is one of the prominent problems in welding Q390 steel, as the carbon equivalent increases, the tendency of delayed cracking also increases. Delayed cracks mainly occur in the coarse-grained zone of the heat-affected zone, and sometimes in the weld.

2. Embrittlement in the overheating zone. During the welding and cooling process of Q390 steel, the quenched structure-martensite is easily formed in the heat-affected zone, which increases the hardness of the near-seam area and reduces the plasticity, resulting in cracks after welding.

3. For hot cracks, the use of welding rods that do not meet the requirements will cause excessive carbon and sulfur content in the weld metal, which will lead to cracks.

4. Certain welding methods will reduce the quality of welded joints of Q390 steel, such as electroslag welding. Due to the large line energy, the grains in the coarse-grained zone of the welding heat-affected zone will grow very thick, causing serious impact toughness of Q390 steel. After welding, normalizing treatment for grain refinement must be carried out to improve the impact toughness of Q390 steel.