There are three main factors causing cold crack in welding:

(1) hardening tendency of steel;

(2) welding tensile stress;

(3) Hydrogen content and distribution of welded joints.

In view of the above three aspects of the influencing factors, the main measures can be taken:

(1) Selection of alkaline low hydrogen electrode, flux. Alkaline low hydrogen electrode and flux welding joint have low hydrogen content, good desulfurization and dephosphorization performance, and high impact toughness. Before use, it is necessary to dry at 350 ° C ~ 420 ° C for 1 ~ 2 h, in order to effectively remove the water in it, thereby reducing the oxygen content of the welded joint and reducing the cold crack tendency of the joint.

(2) Welding spare parts must be inspected before they can be paired. Do not force the pair to avoid excessive assembly stress.

(3) Clean the welds before welding. The steel parts to be welded and within 20 mm on both sides should be carefully cleaned to remove moisture, rust, oil and other debris.

(4) Reasonable arrangement of welding sequence. The principle is to try to make most welds can be welded under the condition of small stiffness, and further reduce welding stress.

(5) Preheating before welding, slow cooling or heat treatment after welding. Preheating before welding is usually an important technological measure to prevent cold cracks in welding of high strength steel. Slow cooling or heat treatment after welding can make the diffused hydrogen fully escape, reduce the welding residual stress, improve the structure, reduce the hardening, and thus reduce the welding cold crack tendency. High strength weathering steel welding generally do not need to take preheating and slow cooling after welding process measures.

The following conditions need to be preheated before welding: when the welding ambient temperature is less than 5 ° C, the welding parts are partially preheated to 75 ° C ~ 125 ° C; When welding cracks and other defects on the high-strength weathering steel base material with a thickness of not less than 8 mm, the welding parts must be partially preheated to 100 ° C ~ 150 ° C before welding. Where preheating is performed before welding, the welding layer temperature shall not be lower than the local preheating temperature of the weldment.

(6) Select the appropriate welding line energy. In manual arc welding and mixed gas shielded welding, the welding line energy is generally small, and the appropriate increase of reasonable welding line energy can extend the cooling time of the welded joint, reduce or avoid the quenching structure of the welding heat affected zone, and at the same time, it is conducive to the escape of hydrogen and reduce the tendency of cold cracking.

(7) Select the appropriate welding method and welding operation specifications. In several common welding methods of medium and thick plate, the low temperature impact toughness of welded joint is best by double-sided multi-layer gas shielded welding, followed by double-sided multi-layer manual welding, and then automatic welding of double-sided single-layer submerged arc with unclear roots. Mixed gas shielded welding head has low hydrogen content and good cold cracking resistance, so it should be preferred. In multi-layer welding, the previous pass preheats the next pass. The second layer of welding pass plays a role in the previous layer of welding pass, so the multi-layer welding head has higher crack resistance than the single layer welding head. When the weld section is large, multi-layer and multi-pass welding should be used. Manual electrode welding must maintain short arc operation. Manual welding rod welding and mixed gas protection semi-automatic welding should be 20 ~ 30 mm away from the weld end, and then pull to the weld end for normal welding after the arc is stable. The end of the weld must adopt the back-welding finishing method, and the back-welding length should be 25 ~ 40 mm; Weld pit must be fully welded.