What elements affect titanium welding?
When the carbon content of the weld is 0.55%, the plasticity of the weld almost disappears and becomes very brittle material, which can not be eliminated by post weld heat treatment.
There are many elements in titanium alloy, which have influence on the physical properties of titanium. Carbon is a common impurity in titanium and titanium alloy. When the carbon content is less than 0.13%, the weld strength limit of carbon in α titanium increases and the plasticity decreases, but the effect of carbon is not as strong as that of oxygen and nitrogen. but
When the carbon content of the weld is further increased, the amount of reticulated tic in the weld increases with the increase of carbon content, which makes the plasticity of the weld decrease sharply, and cracks are easy to appear under the action of welding stress.
The weld defect of titanium welded pipe is due to the fact that during the welding of titanium welded pipe, the argon gas maintenance layer formed by argon arc welding gun can only maintain the weld pool from the harmful effect of air.
However, it has no protective effect on the weld and its surrounding areas which have solidified and are in high temperature state, while the weld and its surrounding areas in this state still have strong ability to absorb nitrogen and oxygen in the air.
Oxygen is absorbed from 400 ℃, nitrogen is absorbed from 600 ℃, and air contains a lot of nitrogen and oxygen. With the increase of oxidation level, the weld color of titanium welded pipe changes and the weld plasticity decreases.
Silver white (no oxidation) golden yellow (TiO, about 250 ℃) titanium begins to absorb hydrogen. Slightly oxidized) blue (Ti2O3 slightly oxidized) gray (TiO2 heavily oxidized).
Titanium and titanium alloy are relatively stable but in the process of welding, at room temperature. The liquid droplet and molten pool metal have strong absorption of hydrogen, oxygen and nitrogen, and in the solid state, these gases have taken effect with them.
With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen, oxygen and nitrogen also increases obviously. Titanium begins to absorb hydrogen at about 250 ℃, oxygen at 400 ℃ and nitrogen at 600 ℃. After these gases are absorbed, the embrittlement of welding joint will be directly caused, which is a very important factor affecting the welding quality.
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. The change of hydrogen content in the weld has the most significant effect on the impact performance of the weld.
Sheet or needle like TiH2 precipitated in the weld increases. The strength of TiH2 is very low, so for example, the notched and impact properties of sheet or needle like hih2 are significantly reduced; the change of hydrogen content in the weld has no obvious effect on the increase of strength and the decrease of plasticity.
The hardness and tensile strength of the weld increase obviously, and the oxygen content of the weld increases linearly with the oxygen content in argon.
However, the plasticity decreased significantly. In order to ensure the performance of the welding joint, the oxidation of the weld and the welding heat effect should be strictly prevented during the welding process.
Effect of nitrogen
Nitrogen and titanium plate react violently at high temperature above 700 ℃. The lattice deflection caused by the formation of brittle titanium nitride (tin and the formation of interstitial solid solution between nitrogen and titanium) is more serious than that caused by the same amount of oxygen.
Therefore, nitrogen is more significant than oxygen in improving the tensile strength and hardness of industrial pure titanium weld and reducing the plasticity of weld. When the nitrogen content of the weld is more than 0.13%, the weld cracks due to over brittleness.
Source: China Titanium Welded Pipe Manufacturer: www.titaniuminfogroup.com