Titanium and titanium alloys have very strict protection requirements when welding. When the carbon content of the weld is 0.55%, the plasticity of the weld almost disappears and becomes a very brittle material, and this brittleness cannot be eliminated by post-weld heat treatment.
There are many elements in titanium alloys, which have an impact on the physical properties of titanium. Among them, carbon is a common impurity in titanium and titanium alloys. When the carbon content is below 0.13%, carbon is in alpha titanium, and the limit of weld strength is somewhat limited. Increase, the plasticity is somewhat decreased, but not as strong as the effect of oxygen and nitrogen. However, when the carbon content of the weld is further increased, the number of reticulated TiC in the weld increases with the increase of the carbon content, so that the plasticity of the weld decreases sharply, and cracks are prone to occur under the action of welding stress.
The welding seam defect of the titanium tube is due to the fact that the argon gas protection layer formed by the argon arc welding torch can only protect the welding pool from the harmful effects of the air when the titanium tube is welded.
However, it has no protective effect on the welded seam and its surrounding area that has been solidified and is in a high temperature state, and the titanium tube weld in this state and its surrounding area still has a strong ability to absorb nitrogen and oxygen in the air. .
Oxygen is absorbed from 400°C, nitrogen is absorbed from 600°C, and air contains a large amount of nitrogen and oxygen. With the gradual increase of the oxidation level, the color of the titanium tube weld changes and the plasticity of the weld decreases.
Silvery white (no oxidation) golden yellow (TiO, titanium begins to absorb hydrogen at about 250 ℃. Slightly oxidized) blue (slightly oxidized by Ti2O3) gray (severely oxidized by TiO2).
1. The effect of carbon
Titanium and titanium alloys are relatively stable during welding, but at room temperature. Liquid droplets and molten pool metals have strong absorption of hydrogen, oxygen, and nitrogen, and in the solid state, these gases have interacted with them.
As the temperature increases, the ability of titanium and titanium alloys to absorb hydrogen, oxygen and nitrogen also increases significantly. Titanium begins to absorb hydrogen at about 250 °C, oxygen from 400 °C, and nitrogen from 600 °C. After the gas is absorbed, it will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the welding quality.
2. Hydrogen is the effect
The main reason is that with the increase of hydrogen content, hydrogen is the most serious factor affecting the mechanical properties of titanium in the gas impurities. The change of the hydrogen content of the weld has the most significant effect on the impact properties of the weld.
The flaky or needle-like TiH2 precipitated in the weld increases. The strength of TiH2 is very low, so the flaky or needle-shaped HiH2 acts as a notch, and the combined impact performance is significantly reduced; the change of the hydrogen content of the weld has an effect on the strength.
