1 Introduction
    904L is a solution of sulfuric acid corrosion and the development and application of the super austenitic stainless steel, its resistance to atmospheric pressure at any concentration, any temperature of acetic acid corrosion, has excellent resistance to intergranular corrosion and stress corrosion, its chemical composition range of common austenitic stainless steel and nickel-base alloy between, Mo, Cu and other high content of alloying elements. In order to master 904L welding performance, we conducted a number of such materials, welding process test to finalize a reasonable welding parameters, and maintenance of the device has been successfully applied.

   2 904L welding characteristics of
   2.1 has a good resistance to intergranular corrosion under the "intergranular chromium depleted" theory, when the carbon content of austenitic stainless steel is higher than 0.08% when the heating process, the over-saturated in the form of carbon can Cr23C6 precipitation along the grain boundaries, it is likely to cause intergranular chromium depleted, thereby reducing the austenitic stainless steel corrosion-resistant crystal Q; when the carbon content of austenitic stainless steel is below 0.02% 1 0.03%, then carbon all dissolved in austenite, even in the heated between 450-850 ℃, the grain boundary will not form a "poor chrome zone" caused by intergranular corrosion.  Super austenitic stainless steel 904L as the carbon content of less than 0.03%, used in welding small welding heat input, speed up the cooling rate, in contact with the corrosive medium the final weld seam, the basic phenomenon of intergranular corrosion can be eliminated.

    2.2 has a high sensitivity of thermal cracking as compared with other stainless steel, austenitic stainless steel welding hot cracking greater sensitivity, mainly related to the following features: First, the thermal conductivity of austenitic stainless steel small, linear expansion coefficient, local heating and cooling in the welding conditions, the welded joints of high-temperature residence time is longer, weld metal and near-seam zone at high temperatures to withstand high tensile stress and tensile strain, in the cooling the process can form a larger tensile stress in the weld metal solidification phase there is a big ask tensile stress is to produce a necessary condition for solidification crack; second, austenitic stainless steel easy to form a strong orientation of the weld metal columnar crystal, some Harmful Impurities easy to form a low melting point in the intergranular liquid film, resulting in weld solidification cracking. In order to prevent heat generated austenitic stainless steel weld cracks can be taken the following measures: ① strict control over materials, sulfur, phosphorus and other harmful impurities; ② used to form a weld wire bipolar organization, thereby disrupting austenite columnar crystal orientation, reducing the intergranular volume of low melting point eutectic segregation; ③ low-line power, narrow weld technology to improve the pool of cooling rate.

    2.3 has a certain tendency to stress corrosion cracking of austenitic stainless steel due to poor thermal conductivity and linear expansion coefficient, welding welding would have a greater tensile stress, while the tensile stress is the stress corrosion cracking of the existence of an important and indispensable conditions for , under the influence of the corrosive medium, 904L of the welded joints have a certain tendency to stress corrosion cracking. The tests showed that welded joints overheated areas on the most sensitive to stress corrosion cracking.

    3 904L Welding Process
    3.1 Welding
    904L usually Tungsten Inert Gas welding, arc energy of large, heat concentration; and have the cooling effect of argon gas flow, heat affected zone is small, will help improve welded joints intergranular corrosion resistance, while the welding quality and stability, weld shape good, and no spatter and slag.

    3.2 clean-up and welding welding welding should pay attention to the following points: ① the base metal surface and the wire surface must be clean to prevent pollutants from melting one seam; ② high temperature and high temperature weld pool must be effective protection of argon, weld metal to prevent oxidation. Welding ago, we were right on both sides of the specimen groove 2O ~ 30 mm are available within the grinder to clean up, and used acetone to the groove and the wire to scrub to remove impurities and oxidation of the surface film.In order to ensure adequate protection of the back seam, can be used 62 aluminum produced a dedicated back protector.

    3.3 Attention should be using a small welding current, low voltage, narrow weld and fast welding; bottoming when soldering to ensure good fusion with the root of the weld can not be too thick, when closed arc-shaped into a gentle slope; extinction should be filled crater, such as the contraction hole or crater cracks must be worn grinder heavy welding; 904L hot cracking due to the sensitivity of a strong, multi-layer welding must be asked to control its temperature not greater than 6O ℃ . To speed up the weld cooling rate, after welding, forced cooling water can be used.

    3.4 weld seam inspection is finished, we based JB4708 (steel pressure vessel welding procedure qualification "requirement of the specimen was examined. Weld surface, without holes, welding tumor, dents, defects such as undercut, weld-wave uniform , weld metal and base metal transition smooth for welding test specimens, 100% RT test results consistent with JB4730 (Pressure Vessel NDT "standard grade Ⅱ requirements, not found is not fusion, cracks and other defects. For specimens la stretch test and side bending, back bending test results meet the performance requirements.

    4 Application results
    In October 2005, at a factory chemical plant solvent dehydration tower BT403 overhaul process, the workshop found that the device is made of 904L of the population distribution pipe weld serious corrosion in need of repair. We are right to take over based on the above parameters for the welding repair, using good effect, the device is running corrosion has not occurred.