The internal defects of titanium alloy pipe welds are not fully penetrated, which refers to a shortcoming that the workpiece and the weld metal or the weld layer are not partially fused. Incomplete penetration weakens the working section of the weld, constitutes a severe stress concentration, and greatly reduces the strength of the joint. It often becomes the source of weld cracking. Non-metallic slag is clamped in the slag-containing weld, which is called slag-containing. The inclusion of slag reduces the working cross section of the welding seam, constitutes stress concentration, and reduces the strength and impact toughness of the welding seam.
Generally, according to the mechanism of crack occurrence, it can be divided into two types: hot crack and cold crack. Thermal cracking occurs during the crystallization process from liquid to solid in the weld metal, mostly in the weld metal. The main reason for its occurrence is the existence of low-melting substances (such as FeS, melting point 1193 ℃) in the weld, which weakens the connection between the grains. When subjected to a large welding stress effect, it simply causes a break between the grains . When the weldment and welding rod contain many impurities such as S and Cu, thermal cracking will easily occur.
Hot cracks have the characteristic of spreading along grain boundaries. When the crack penetrates the exterior and communicates with the outside world, it has a significant hydrogenation tendency. Cold cracks occur during the cooling process after welding, and most of them occur on the fusion line of the base metal or the base metal and the weld. The main reason for its occurrence is that the quenching arrangement is formed in the heat affected zone or the weld seam. Under the effect of high stress, it causes cracking inside the grains and when welding titanium alloy materials that are easy to quench and have a high carbon content or many alloying elements. , Cold cracks are most likely to occur. Melting too much hydrogen into the weld can also cause cold cracks.
Cracks are the most risky shortcomings. In addition to reducing the load-bearing cross-section, severe stress concentration also occurs. During operation, the cracks will gradually expand and eventually cause damage to the component. Therefore, this kind of shortcoming is usually not allowed in the welding layout. Once it is found, it must be shoveled to re-weld.
At high temperatures, the stomata weld metal absorbs too much gas (such as H2) or gas (such as CO) generated due to the metallurgical reaction inside the molten pool, which is too late to be discharged when the molten pool is cooled and condensed, and is formed inside or outside the weld. A hole is a stomata. The existence of air holes reduces the useful working cross section of the weld and reduces the mechanical strength of the joint. If there are penetrating or continuous pores, it will seriously affect the sealing of the weldment. In the process of crack welding or in the future, the cracking of the metal part in the area of the welded joint is called a crack. Cracks can occur in the weld or in the heat affected zone on both sides of the weld. Sometimes it happens on the outside of the metal, sometimes it happens inside the metal.
