Correction of Internal Defects in Titanium Alloy Pipe Weld Joints

The internal defect of incomplete penetration in titanium alloy pipe welds refers to a situation where the joint and the weld metal or the weld layer do not fuse together. This defect weakens the cross-section of the weld operation and causes severe stress concentration, significantly reducing the joint strength. It often becomes the root cause of weld cracking. A slagged weld contains non-metallic slag, which is called slag. The slag reduces the cross-section of the weld operation, causes stress concentration, and lowers the strength and impact toughness of the weld.

Gas porosity in weld metal absorbs excessive gas (such as H2) or gas produced due to metallurgical reactions inside the molten pool (such as CO) when the molten pool cools and solidifies. It cannot be expelled in time and forms cavities inside or on the surface of the weld, which is called gas porosity. The presence of gas porosity reduces the effective cross-section of the weld and lowers the mechanical strength of the joint. If there are penetrating or continuous gas porosity, it will seriously affect the sealing performance of the weldment. Cracks occur in the welding joint area during or after the welding process and are called cracks. Cracks can occur on the weld itself or in the heat affected zone on both sides. Sometimes they occur on the metal surface, and sometimes they occur inside the metal.

Generally, according to the different mechanisms of crack formation, they can be classified into two types: hot cracks and cold cracks. Hot cracks occur during the crystallization process from liquid to solid in the weld metal, mostly in the weld metal. The main reason for their occurrence is the presence of low-melting-point substances (such as FeS, with a melting point of 1193°C), which weaken the connection between crystal grains. When subjected to large welding stress effects, it is easy to cause fractures between the crystal grains. When the weldment and welding rods contain a lot of impurities such as S and Cu, hot cracks are prone to occur. Hot cracks have the characteristic of spreading along the grain boundaries. When the crack penetrates the surface and communicates with the outside, it has a significant tendency to hydrogenation. Cold cracks occur during the cooling process after welding, mostly on the fusion line between the base metal and the weld. The main reason for their occurrence is the formation of quenched structures in the heat affected zone or the weld itself. Under high stress effects, it causes fractures within the crystal grains, and titanium alloys with high carbon content or more alloy elements are most prone to cold cracks. If too much hydrogen is melted into the weld, cold cracks can also occur. Cracks are the most dangerous defect. Besides reducing the cross-section of the bearing area, they also cause severe stress concentration, and the cracks will gradually expand during use, eventually leading to the damage of the component. Therefore, such defects are usually not allowed in the welding layout. Once found, they must be removed and re-welded.