Correction of titanium alloy pipe welds and application points of titanium pipe elbows

Internal defects in titanium alloy pipe welds are incomplete penetration, which refers to defects in the lack of fusion between the workpiece and the weld metal or between the weld layers. Insufficient penetration will weaken the working cross-section of the weld, causing severe stress concentration and greatly reducing the strength of the joint. It often becomes the source of welding cracks. Slag inclusion: Non-metallic slag inclusions are caught in the weld and are called slag inclusions. Slag inclusion reduces the working cross-section of the weld, causing stress concentration, which will reduce the strength and impact toughness of the weld.

The weld metal with pores absorbs excessive gases (such as H2) or gases (such as CO) produced by metallurgical reactions inside the molten pool at high temperatures. When the molten pool cools and condenses, it is too late to be discharged and forms inside or on the surface of the weld. The pores are stomata. The existence of pores reduces the effective working cross-section of the weld and reduces the mechanical strength of the joint. If there are penetrating or continuous pores, the sealing performance of the weldment will be seriously affected. Crack: During or after welding, partial rupture of the metal in the weld area is called a crack. Cracks may appear in the weld or in the heat-affected zone on either side of the weld. Sometimes it happens on the outside of the metal, sometimes on the inside.
Generally speaking, according to the different crack occurrence mechanisms, they can be divided into two categories: hot cracks and cold cracks. Hot cracks are generated during the crystallization process of the weld metal from liquid to solid state, and mostly occur in the weld metal. The main reason for its occurrence is the presence of low melting point substances (such as FeS, melting point 1193°C) in the weld, which weakens the connection between grains. When subjected to large welding stress, intergranular cracks are easily caused. . When welding parts and electrodes contain a large amount of S, Cu and other impurities, hot cracks are likely to occur. Hot cracks have the characteristic of propagating along grain boundaries. When cracks penetrate the surface and communicate with the outside world, they have a significant tendency to hydrogenate. Cold cracks occur during the post-weld cooling process, and often occur at the base metal or the fusion line where the base metal and the weld intersect. The main reason for its occurrence is the quenching mechanism formed in the heat-affected zone or weld, which causes cracks inside the grains under the action of high stress. Cold cracks are most likely to occur when welding easily quenchable titanium alloy materials with higher carbon content or more alloying elements. Too much hydrogen melted into the weld can also cause cold cracking. Cracks are one of the most dangerous defects. In addition to reducing the load-bearing cross-section, they also produce severe stress concentrations. Cracks will gradually expand during use and may eventually lead to component damage. Therefore, this shortcoming is usually not allowed in welding layouts. Once discovered, it must be removed and re-welded.

1. When processing titanium alloy pipe elbows, pickling is generally not used.

2. Titanium alloy pipe elbows play an important role and value in the use of equipment, and produce good operation in certain environments and media.
According to the requirements and surface conditions of the workpiece, titanium standard parts are processed by alkali washing, water-soluble detergent, chlorine solvent sandblasting, shot blasting and other methods.
Welding rods should be connected and dry when used. The calcium titanium type should be dried at 150℃ for 1 hour, and the low hydrogen type should be dried at 200-250℃ for 1 hour (drying cannot be repeated, otherwise the coating will easily crack and fall off). Pay attention to the adhesion of the electrode coating. Oil and other dirt will increase the carbon content of the weld and affect the quality of the weldment. During elbow welding, due to repeated heating, carbides precipitate, reducing corrosion resistance and mechanical properties. The hardenability after welding is large and cracks are prone to occur. If the same type of electrode is used for welding, it must be preheated to above 300°C and slowly cooled to about 700°C after welding. If the weldment cannot be subjected to post-weld heat treatment, chromium-nickel-titanium alloy pipe electrodes should be used.
Proper handling of titanium alloy pipe elbows can improve their safety performance and extend their service life.
3. When installing titanium pipe elbows, sealing performance must be ensured to avoid leakage and affect the normal operation of the pipeline.

4. When installing the titanium alloy pipe elbow, the titanium alloy pipe elbow can be installed directly on the pipe according to the connection steps and installed according to the direction of use. Generally, it can be installed at any position on the pipeline, but it needs to be easy to operate and inspect. Pay attention to prevent the flow direction of the medium in the titanium alloy pipe elbow from vertical to upward flow under the valve disc. Titanium alloy elbows can only be installed horizontally.

1. Titanium alloy pipe elbows should be inspected regularly. The processing surface must be kept clean at all times, remove dirt, and be stored neatly in a ventilated and dry place indoors. Stacking or open storage is prohibited.
2. When using ball valves, globe valves, and gate valves for titanium alloy pipe elbows, they can only be fully opened or fully closed. They are not allowed to be used to regulate flow, as this will prevent erosion of the sealing surface and accelerate wear.
3. The gate valve and the upper thread stop valve are equipped with inverted sealing devices. When the hand wheel is turned to the highest position, the surface of the head will never get rusty or damaged. Always keep titanium alloy pipe elbows dry and ventilated, keep containers clean and tidy, and store them according to correct storage procedures.