When welding titanium tubes and titanium alloy tubes, when the weld contains high oxygen and nitrogen, the weld or heat-affected zone becomes brittle. Titanium begins to absorb hydrogen at about 250 degrees, oxygen at 400 degrees, and nitrogen at 600 degrees. The air contains a lot of nitrogen and oxygen. Under the action of large welding stress, cold cracks will appear. Its characteristic is that the cracks occur several hours or even longer after welding, which is called delayed cracks. Studies have shown that the diffusion of hydrogen during welding titanium tubes is the main cause of this crack. During welding, hydrogen diffuses from the high-temperature deep pool to the heat-affected zone with a lower temperature. The increase in hydrogen content increases the amount of TiH2 precipitated in this zone, increasing the brittleness of the heat-affected zone. In addition, due to the volume expansion of hydride precipitation, a large organizational stress is caused, and hydrogen atoms diffuse and aggregate to the high-stress parts of the zone, resulting in cracks. The way to prevent this delayed crack is mainly to reduce the source of hydrogen in the weld joint.
When welding titanium tubes, pores are a major problem that is often encountered. The fundamental reason for the formation of pores is the result of the influence of hydrogen. The main process measures to prevent the formation of pores are:
1) Welding is carried out under the protection of high-purity argon gas, and the purity of argon gas should not be less than 99.99%
2) Thoroughly remove organic matter such as scale and oil on the surface of titanium tubes, titanium plates, and titanium plate tube eyes. Chemical and mechanical methods can be used for cleaning.
3) Apply good gas protection to the molten pool, control the flow and flow rate of argon gas, prevent turbulence, and affect the protection effect.
4) Select appropriate welding process parameters and welding specifications, increase the deep pool residence time to facilitate the escape of bubbles, and effectively reduce pores.
When welding titanium alloys, the argon gas protective layer formed by the argon arc welding gun can only protect the welding molten pool from the harmful effects of air, but has no protective effect on the weld and its surrounding areas that have been solidified and are in a high-temperature state. The welding titanium tubes and their surrounding areas in this state still have a strong ability to absorb nitrogen and oxygen in the air. As the degree of oxidation gradually increases, the color of the titanium alloy weld changes, and the plasticity of the weld decreases. Silvery white (no oxidation), golden yellow (TiO, slightly oxidized), blue (Ti2O3, slightly oxidized), and gray (TiO2, severely oxidized). When titanium alloys are welded, the possibility of hot cracks in the weld joint is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is very small, and the low melting point of eutectic formed by S and P is rarely generated at the grain boundary. In addition, the effective crystallization temperature range is narrow, and the shrinkage of titanium and titanium alloys during solidification is small, so the weld metal will not produce hot cracks.
When welding titanium tubes and titanium alloy tubes, when the weld contains high oxygen and nitrogen, the performance of the weld or heat-affected zone becomes brittle. Titanium begins to absorb hydrogen at about 250 degrees, oxygen at 400°C, and nitrogen at 600 degrees. The air contains a lot of nitrogen and oxygen. Under the action of large welding stress, cold cracks will appear. Its characteristic is that the cracks occur several hours or even longer after welding, which is called delayed cracks. Studies have shown that the diffusion of hydrogen during welding is the main cause of this crack. During welding, hydrogen diffuses from the high-temperature deep pool to the heat-affected zone at a lower temperature. The increase in hydrogen content increases the amount of TiH2 precipitated in this zone, increasing the brittleness of the heat-affected zone. In addition, the volume expansion of hydrides during precipitation causes greater organizational stress, and hydrogen atoms diffuse and aggregate to the high-stress parts of the zone, resulting in cracks. The way to prevent this delayed crack is mainly to reduce the source of hydrogen in the welded joint.
When welding titanium tubes, pores are a major problem that is often encountered. The fundamental reason for the formation of pores is the result of the influence of hydrogen. The main process measures to prevent the formation of pores are:
1) Welding is carried out under the protection of high-purity argon gas, and the purity of argon gas should not be less than 99.99%
2) Thoroughly remove organic matter such as scale and oil on the surface of titanium tubes, titanium plates, and titanium plate tube eyes. Chemical and mechanical methods can be used for cleaning.
3) Apply good gas protection to the molten pool, control the flow and flow rate of argon gas, and prevent turbulence from occurring, which affects the protection effect.
4) Select appropriate welding titanium tubes process parameters and welding specifications, increase the deep pool residence time to facilitate the escape of bubbles, and effectively reduce pores.
When welding titanium alloys, the argon gas protective layer formed by the argon arc welding gun can only protect the welding pool from the harmful effects of the air, but has no protective effect on the weld and its surrounding areas that have solidified and are in a high-temperature state. The weld and its surrounding areas of the titanium tube in this state still have a strong ability to absorb nitrogen and oxygen in the air. As the degree of oxidation gradually increases, the color of the titanium alloy weld changes, and the plasticity of the weld decreases. Silvery white (no oxidation), golden yellow (TiO, slightly oxidized), blue (Ti2O3, slightly oxidized), and gray (TiO2, severely oxidized). When welding titanium tubes, the possibility of thermal cracks in the weld joint is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is very small, and the low melting point of eutectic formed by S and P is rarely generated at the grain boundary. In addition, the effective crystallization temperature range is narrow, and the shrinkage of titanium and titanium alloys is small when solidifying, so the weld metal will not produce thermal cracks.
