Titanium is a chemically very active metal, which has a great affinity for gases such as oxygen, hydrogen and nitrogen at high temperatures, especially in the titanium welding process, this ability is stronger with the increase in welding temperature. Practice has proved that if the absorption and dissolution of titanium and oxygen, hydrogen, nitrogen and other gases are not controlled during welding, it will undoubtedly bring great difficulties to the welding process of titanium welded joints.
ONE: Foreword
In recent years, with the development of economy, especially with the deepening of reform and opening up, my country's economic construction has made great progress. At the same time, my country has also made great progress in the welding of pipelines and other projects. Titanium welding is a relatively common type of welding. In the process of titanium welding, how to do a good job in the quality control of its welding has a very important influence on the color of the welding seam of titanium welding. Due to the intuitiveness of titanium weld color, it is of great significance to study the relationship between titanium welding seam color and welding quality. In this paper, the author discusses the relationship between the welding quality of titanium welding and the color of titanium welding seam based on his years of research on the quality control and process of titanium welding and practical work experience, hoping to have a certain amount of research in this field. effect.
TWO The influence of the characteristics of titanium on titanium welding
1. Effects of Oxygen and Nitrogen
Oxygen and nitrogen are interspersed in titanium, which distorts the titanium lattice, increases the deformation resistance, increases the strength and hardness, but reduces the plasticity and toughness. It is unfavorable to contain welding oxygen and nitrogen in the weld and should be avoided.
2. The effect of hydrogen
The increase of hydrogen will make the impact toughness of the titanium weld metal drop sharply, while the plasticity will drop a little, and the hydride will cause the brittleness of the joint.
3. carbon impact
At room temperature, carbon is dissolved in titanium in the form of gaps, which increases the strength and reduces the plasticity, but it is not as obvious as oxygen and nitrogen. When the carbon content exceeds the solubility, hard and brittle TiC is formed, which is distributed in a network and prone to cracks. The national standard stipulates The carbon content of titanium and its titanium alloy should not exceed 0.1%. During welding, the oil pollution of the workpiece and welding wire can increase the carbon content, so it needs to be cleaned up during welding.
Three Weldability Analysis of Titanium
Titanium has good weldability. Because of its small thermal conductivity (0.041Cal/℃·cm·s), titanium metal only melts in the arc combustion range and has good fluidity; moreover, it has Small thermal expansion coefficient (8.6×10-6/℃, much smaller than carbon steel), which greatly improves the weldability of titanium metal.
Four The relationship between the weld color and welding quality of titanium welding
1. The color change of titanium and titanium alloy titanium tube weld seam and its defect generation mechanism
The welding seam defects and generation mechanism of titanium and titanium alloy titanium tubes are as follows. When welding titanium tubes, the argon gas protection layer formed by the argon arc welding torch can only protect the welding pool from the harmful effects of air, and it will not affect the solidified The welding seam and its surrounding areas in the high temperature state have no protective effect, while the titanium tube welding seam and its surrounding areas in this state still have 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 species 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 ° C. Slightly oxidized) blue (slightly oxidized by Ti2O3) gray (severely oxidized by TiO2).
2. The quality of titanium welding can be judged by the color of the titanium weld surface
The test for different colors and hardness of titanium welds is as follows
(1) It has been proved by experiments that as the color of the weld deepens, that is, the degree of oxidation of the weld increases, the hardness of the weld increases. Through the test of the peer experiment, the hardness of the titanium metal increases, such as oxygen and nitrogen in the weld. And other harmful substances increase, and greatly reduce the quality of welding.
(2) The weldability of titanium has a very important relationship with its chemical and physical properties, but the key point is that in the case of high temperature, the high activity of titanium is easily polluted by air, and its grains expand when heated. When the welded joint cools, it forms a brittle phase. The melting point of titanium is very high, which can reach 1668 ± 10 °C, which is more energy than welding steel. At the same time, the chemical properties of titanium are relatively active, and it is much easier to interact with O and H than steel, and it will be sharp when it is above 600 °C. compound. At 100 °C, a large amount of H and O are absorbed, and the ability to dissolve H is tens of thousands of times greater than that of steel, which in turn generates titanium hydride, which reduces the toughness sharply. Gas impurities increase the propensity for cold cracking and delayed cracking, increasing notch susceptibility. Therefore, the purity of argon gas for welding should not be lower than 99.99%, the humidity should not be higher than 0.039%, and the hydrogen content of the welding wire should be below 0.002%. The heat transfer coefficient of titanium is 1/2 of that of steel, and the transition from α to β occurs at 882 ° C. No matter how high the temperature is, the β grains grow rapidly and the performance deteriorates significantly. Therefore, the temperature should be strictly controlled, especially the welding. High temperature residence time in thermal cycling. There is no problem of hot cracks and intergranular cracks when welding titanium, but there are porosity problems, especially when welding α+β alloys.
Five Titanium Welding Considerations
Based on the above research, titanium metal should pay attention to the following issues when welding:
1. During the titanium welding process, the welding area and the high temperature area after welding should be strictly protected to avoid the air entering the welding and high temperature area from seriously affecting the quality of the weld. necessary.
2. Weld grooves are required to be mechanically processed (no grinding is allowed);
3. Spot welding should be avoided and high frequency arc starting should be used.
4. Avoid post-weld heat treatment; if it is necessary to carry out post-weld heat treatment, the heat treatment temperature should be less than 650 ℃.
Six concluding remarks
The quality control of titanium welding has a very important influence on the weld color of titanium welding. At the same time, the quality of titanium welding can also be judged according to the weld color of titanium welding. There is a very important relationship between the two.