What are the precautions for welding of Gr9 titanium welded pipe?

1. Gr9 titanium weldability analysis

Titanium is an active metal, can be formed at room temperature oxygen-containing dense oxide film, maintain high stability and corrosion resistance. 540 ℃ above the generated oxide film is not dense. Titanium and oxygen, nitrogen, hydrogen at high temperatures react faster, titanium at 300 ℃ above the rapid absorption of hydrogen, 400 ℃ above the beginning of oxygen and nitrogen absorption, 600 ℃ above the rapid absorption of oxygen, 700 ℃ above the rapid absorption of nitrogen. Oxygen, nitrogen dissolution of titanium to reduce the plasticity and toughness of titanium, hydrogen dissolution of titanium to enhance the brittleness of titanium, the toughness of the sharp decline. Carbon is solidly dissolved in titanium through the interstitial space to improve the strength of titanium and reduce plasticity, and when the carbon content exceeds the solubility, hard and brittle TiC is produced, which is distributed in the form of a network and is prone to cracking. In addition, the melting point of titanium is high, the specific heat and thermal conductivity is small, the cooling rate is slow, the welding heat-affected zone at high temperatures for a long time, the high temperature is very easy to overheat coarsening, resulting in a decrease in the plasticity of the joint. The weldability of titanium can be analyzed from the above characteristics of titanium. In order to ensure the performance of welded joints, the intrusion of carbon, nitrogen, oxygen and hydrogen should be controlled. Therefore, how to keep titanium clean and protected at high temperature is the key issue. At the same time, the temperature of the welded joint should be controlled to avoid overheating affecting the performance of the welded joint. Argon is an inert gas, neither with titanium to form a solid solution, nor react with other elements, so titanium welding is suitable for argon arc welding, but the purity of argon gas should be very high, general purity ≥ 99.99%.

2. Welding preparation

(1) welding site and environment

Welding environment should be clean, smoke-free, dust-free, independent. Welding environment of the natural conditions: wind speed ≥ 1.5m / s, relative humidity > 80%, temperature <5 ℃, rain and snow without defense shall not be welded. For example, when welding in the steel structure workshop, the titanium welding area should be separated from the steel structure area to prevent contamination of iron powder, gas and other harmful impurities; welding should be away from the vents and open doors and windows, and the site should be paved with rubber mats.

(2) the production of protective cover, in order to make titanium in the high temperature zone (above 200 ℃) completely protected by argon, must make a good protective effect, welding operation is convenient protective cover. Because the structure of the shield directly affects the quality of welding and welding efficiency, so after many structural design and welding tests, the structure of the shield form was finally determined. In addition, the specific gravity of argon is higher than air, so the protective cover can be considered when making the protective cover above the protective cover to leave an operating hole.

(3) Bevel processing bevels should be processed, for example, bevels should be processed using tools such as beveling machines, pneumatic internal grinders, alumina or silicon carbide grinding and polishing discs. When machining the bevel, please process slowly (or use cooling water to cool), so as not to process the temperature is too high. After cutting the bevel surface should be flat and smooth, there can be no cracks, laminations, inclusions, burrs, flying edges and oxidation color, etc. The surface of the bevel and within 30mm on both sides of the metal luster is silver white.

(4) cleaning before welding wire, bevel surface and both sides within 20mm for surface cleaning. According to the degree of surface contamination can be selected degreasing, mechanical cleaning or chemical cleaning, mechanical cleaning allows the use of stainless steel wire brush. Welding before the application of acetone or anhydrous ethanol and other solvents to clean the groove and wire, do not touch the hands after cleaning, welding in a timely manner within 4 hours, expired welding should be cleaned again. Clean welding wire shall not be thrown to the ground, should be placed in a clean titanium tube to prevent re-contamination.

3.Welding

(1) Welding process evaluation

Welding process evaluation according to NB/T47014-2011, welding process evaluation of titanium pipe material used for TA2, titanium pipe specifications for φ45mm × 3mm. in preparing the pre-welding program, should be the principle of weld operability and good performance of welded joints. In order to ensure that the titanium coil sizing welded joints to meet the requirements of use, welding process evaluation test according to the pre-welding process regulations. Argon arc welding machine model WSM315, using high-frequency arc, argon using pure argon (99.99% Ar). Welding process evaluation test results can be seen welding process evaluation test results qualified.

(2) slice corrosion test

In order to test the corrosion resistance of welded joints, the use of argon arc welding welding a titanium tube specimen, sealing are used to protect the water, including the middle weld deliberately not to protect, the weld is yellow, part of the blue, the rest of the silver-white. Titanium tube specimen sliced samples, take 30% sulfuric acid (boiling) after 24h soak corrosion test, the results show that the silver-white weld has a slight corrosion, while the blue golden-yellow weld corrosion is serious, indicating that in the welding process, the weld by the through-water protection of corrosion-resistant weld as well.

(3) Titanium coil welding

Welding process evaluation test and corrosion test qualified before welding products. According to the welding process assessment of the preparation of welding process regulations, according to the welding process regulations of the parameters and requirements for the welding of the coiler. Because the coiler is longer, heavier when welding, it is more difficult to rotate manually, so the coiler can be placed on the rotatable support wheel, which can be rotated and welded. At the same time, the first seven straight titanium tube for a group of group welding after a welding, welding in two layers, bottoming welding tube through pure argon for protection (first argon 10 ~ 15min, the air inside the tube before welding), welding at the tube outside the protective cover for protection, to be welded weld mouth sealed with tape, the mouth of the tube sealed with tape and inserted into the argon tube through the argon, the last mouth of the tube sealed with tape, and hit a few small holes to exclude air and argon. All welded ports welded bottom welding after the cover welding, cover welding to protect the water inside the tube, pay attention to the water must be filled with water inside the tube to prevent the tube from retaining air, outside the tube with a protective cover through the argon gas. When welding, the temperature of the titanium tube must be controlled at all times below 200 ℃.