Study Of Welded Joints' Crack Resistance Of Marine Structural Steel

With the development of China's offshore oil and gas resources' exploitation, the demand for marine engineering for offshore platforms is increasing. In order to improve the efficiency of the construction, SAW and FCAW under CO₂ is used in offshore platform jacket constructing,resulting in more and more cracking problems in recent years. Therefore, the study of marine structural steel welded joints' crack resistance is necessary, which can guarantee the safety of marine engineering construction under the premise of improving the efficiency of the construction.In this paper, DH36 is chosen as base material, and G-BOP cracking test and the y-groove Tekken cracking test are respectively used to study welded joints' crack resistance of single-wire SAW and FCAW under CO₂. The results show that:Welded cracks of SAW joints of DH36 in the G-BOP test are initiated near the fusion zone root. Small dimples appeared at the crack initiation, and the crack propagating area has quasi-cleavage morphology. Pro-eutectoid ferrite appears in the weld area, probably causing the tissue uncoordinated, and under the longitudinal restraint conditions and the role of diffusible hydrogen, it is easy to produce transverse cracks. The order of four kinds of consumables crack resistance is A#>B#>C#>D#, under the heat input of 2.35 kJ/mm and 3.45kJ/mm. Without preheating, increasing heat input from 2.35kJ/mm to 3.45kJ/mm, the effects to different welding consumables for crack resistance are not the same. The cracking rate of welding consumable A# is not affected, and that of B# and C# increase, while that of D# decreases. This is probably because of the diffusible hydrogen content is rather different between the related consumables. Increasing heat input from 2.35kJ/m to 3.45kJ/mm, can promote pro-eutectoid ferrite in the weld zone of the four consumables, reducing submerged arc welded joint stiffness in the region. Under two heat input 2.35kJ/mm and 3.45kJ/mm, there are no obvious correspondence between hardness values and the cracking rate of welded joints.Crack resistance of the wire 1# and 2# is poor, while 3# and 4# is good. For 1# and 2#, preheating 90 degrees or appropriately increasing the heat input can significantly improve crack resistance for 3# and 4#, their crack resistance is scarcely affected. Cracking rate and hardness near the fusion zone have good relationship in the y-groove Tekken cracking test of single channel, that is the greater the hardness, the higher the cracking rate. In the dual-channel y-groove Tekken cracking test, hardness of welded joints decrease significantly. Crack resistance of 4# is also better than that of 2#. The impact of preheating, interlaminar temperature and heat input are not the same to crack resistance of the two wires. Interlaminar temperature has the greatest impact on crack resistance of 4#. Appropriately increasing interlaminar temperature can help reduce its cracking rate. For 2#, a small heat input, and increasing preheat temperature and interlaminar temperature can significantly reduce the rate of crack, increasing its crack resistance. Under the dual-channel y-groove Tekken cracking test, hardness of welded joints and cracking rate is not clearly related.Using a specimen, in single pass welding diffusible hydrogen content of deposited metal after welding is decreasing as the cooling time prolongs, and the same to the escape velocity. Increasing the heat input from 1.01kJ/mm to 1.57kJ/mm, the law of the diffusion of hydrogen to escape does not change, but at the same cooling time after welding, weld metal diffusible hydrogen content was obviously reduced. The results of diffusible hydrogen content of different interlaminar conditions in two pass and three pass welding show in multi-pass welding controlling inter-layer temperature, and ensuring the residence time of high temperature, are rather conducive to reducing hydrogen content,which is good to welded joints' crack resistance.