There are variances between the titanium weld and the titanium base material used in the titanium pressure vessels. When the impurity content of the titanium welding wire matches that of the base material, the plasticity and toughness of the titanium weld must be significantly lower than that of the titanium base material. Titanium pressure vessels demand that the titanium weld's post-weld strength and plasticity do not fall below that of the base material in the annealed state. To accomplish this, the only practical measure under typical welding process circumstances is to employ titanium welding wire with significantly lower impurity content than the base material.
Compared with the titanium base material, the mechanical properties of the titanium weld have the following differences
When welding titanium, inert gas is utilized wherever possible. The goal of protection is to reduce contamination of the titanium weld with iron, carbon, nitrogen, oxygen, hydrogen, and other pollutants from the environment. Good protection can only help to reduce pollution. The welding process is not about eliminating contaminants in the weld; it is simply about how much pollution there is. As a result, the contaminants in the weld are always higher than those in the welding wire. If the impurity composition of the welding wire and the base material are identical, the impurity composition of the weld is always higher than that of the base material, resulting in a poorer plasticity and toughness of the weld.
Titanium welds are cast structures that are looser than the titanium parent material structure after pressure processing, and will necessarily have flaws such as dendritic structure, porosity, shrinkage, internal stress, and so on (within the acceptable range of nondestructive testing). The plastic toughness of titanium welds is less than that of titanium parent materials.
The solubility of impurities such as nitrogen, oxygen, and carbon in the liquid phase, R phase, and metallographic phase of titanium is different. Phase change will cause the redistribution of impurity elements. The cooling speed of the weld after welding is relatively fast. The impurity elements have no time to diffuse evenly in various parts, resulting in dendrite segregation of impurity elements. There are local high impurity content areas, causing local lattice distortion. When the phase transition from p phase to a phase occurs during the cooling process, the R phase transition to the 'martensite phase occurs partially. Phase transition, producing a metastable phase. The element content distribution of the titanium parent material is relatively uniform, which also makes the plastic toughness of titanium welds lower than that of the titanium parent material.
Titanium pressure vessels are generally not annealed after welding. The weld is in the post-weld state, while the titanium base material is in the annealed state, which also makes the plastic toughness of the weld lower than that of the base material.
