Hastelloy C-276 is a Nickel alloy containing element W. It is widely used in various chemically corrosive media due to its superior corrosion resistance to moderately oxidizing and strongly reducing acids. middle. Heat exchanger pipes in petrochemical equipment mostly use C-276 seamless pipes. Due to the high price of seamless pipes, most manufacturers use welded C-276 sheets to obtain welded pipes of different specifications, and strictly control the manufacture of welded pipes. process to make it have excellent corrosion resistance and avoid premature failure in use, resulting in economic losses.
Alloy C-276 welded pipe is not subjected to cold rolling and solution annealing treatment after welding, and the dendrite structure produced by the weld will greatly reduce the corrosion resistance of Hastelloy C-276 welded pipe, resulting in easy corrosion at the weld of the welded pipe. The comparison shows that cold rolling and solution annealing treatment can significantly improve the corrosion resistance of C-276 welded steel pipe.
At present, ERNiCrMo-4 welding wire is usually used as the filler material for welding, and its chemical element content is basically the same as that of the base metal. After welding, both the base metal and the weld can achieve good mechanical properties. However, in the environment of strong corrosive medium, it is not enough for welded pipe fittings to only have excellent mechanical properties, but also to have good corrosion resistance. A chemical company used C-267 welded pipe in the acetic acid production unit with an annual output of 300,000 tons. After 4 years of service, a condenser of an acetic acid water separation tower was found to leak, and the failure parts were mainly concentrated in the weld of the C-267 welded pipe. There are local corrosion holes.
There are 5 grades of C-276 welded pipe fittings mentioned in the ASTM B626 standard. For heat exchange tubes used in strong corrosive media, the highest-level manufacturing process should be adopted, that is, welding, cold rolling, solution annealing, and non-destructive testing should be carried out in sequence, and they can be put into use only after they are qualified. The standard stipulates that the welding of C-267 welded pipe should be automatic welding without filler material. Generally, people generally pay attention to the welding process of C-276 welded pipe, and believe that the welding process determines the quality of welding, which in turn determines the corrosion resistance of welded pipe fittings. The effect of cold rolling and solution annealing on the corrosion resistance of C-276 is rarely noticed.
2.1 Cold deformation treatment
After the welded pipe fittings are cold-spun, the pipe wall will be thinned. When the wall thickness is reduced by 80% (cold rolling amount), the surface structure of the weld is observed. The dendritic structure and impurities are decomposed into many small pieces, and the grains are arranged linearly. The strength of the welded pipe that has only been cold-rolled has been improved, but the corrosion resistance has not been improved, and the corrosion resistance of the welded joint without post-weld cold rolling treatment is even worse, because the cold deformation makes the surface of the weld large. The residual stress, under the combined action of stress and corrosion, the corrosion resistance is even worse. The corrosion test was carried out on the welded pipe after 80% cold rolling treatment, and it was found that there were serious corrosion grooves at the weld. Therefore, solution annealing treatment should be carried out in time after cold rolling to eliminate residual stress generated during cold rolling.
2.2 Solution annealing treatment
Experiments were carried out on the Hastelloy C-276 welded pipe after solution annealing, and the metallographic structure was observed. The strength and plasticity of the weld have been significantly improved. After 80% cold rolling and solution annealing treatment, the corrosion test was carried out on the weld structure. It was found that the weld, fusion line and heat affected zone were basically not corroded, and no serious corrosion groove appeared. Therefore, if the solution annealing treatment is carried out in time after the cold deformation treatment, the corrosion resistance of the welded joint of the welded pipe can be improved.
Welds and heat-affected zones are weak areas of welded pipes. The corrosion resistance of welds and heat affected zones can be enhanced by cold spinning deformation followed by solution annealing. Cold rolling treatment can form a small amount of fiber structure, the fiber structure is deformed to form a recrystallized core, and a carbide core can be formed inside the deformed grain, and the carbides precipitated along the grain boundary during sensitization treatment are minimized. Cold deformation treatment + solution annealing can produce a good recrystallization structure, the structure of the weld area is relatively uniform, and the structure and properties of the seamless tube similar to forging can be obtained.
(1) C-276 welded pipe should be cold rolled and solution annealed after welding. Cold rolling treatment is the basis of solution annealing treatment. Cold rolling accumulates structural distortion energy and provides power for recrystallization.
(2) The solution annealing treatment provides the recrystallization temperature, eliminates the residual stress caused by cold rolling, disperses the segregation elements to eliminate the dendrite structure, and finally recrystallizes and refines the weld structure and enhances the corrosion resistance of the welded pipe.
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