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Welding of stainless steel

Author:     Date: March 13, 2019    Source:    Focus on:

one Welding characteristics of stainless steel:

Because the resistance coefficient of stainless steel is much higher than that of low carbon steel, the welding rod and the base metal in the welding zone are easy to be melted by heating, and the matrix around the melting zone is overheated, resulting in uneven deformation and coarse grains in the welding zone.

l   Stainless steel has large linear expansion coefficient, small thermal conductivity, and is difficult to transfer heat. Welding heating makes the structure expand and produces large shrinkage deformation and tensile stress when cooling, which is easy to cause thermal cracks.

l   The intergranular corrosion is easy to occur in the HAZ of stainless steel after welding. The reason is that in the heat affected zone of welding, at the sensitized temperature( four hundred and fifty —850 The results show that the corrosion resistance of the steel decreases obviously due to the poor chromium content in the matrix, so the steel is preferentially corroded in the corresponding corrosion environment, and the grain boundary of the steel is widened due to the corrosion. At this time, the plasticity and strength of the corroded parts have been seriously lost, cracks and brittle fracture appear during cold bending, and there is no metal sound on the ground of corroded parts.

two Stainless steel welding preventive measures:

l   Control welding current: the welding specification of stainless steel is less than that of low carbon steel, and the current is about that of low carbon steel 80%

l   Accelerate welding speed: use the faster welding speed as far as possible, the purpose is to reduce the width of heat affected zone, shorten the residence time of weld in sensitized temperature range, make the weld in a stable state, and refine the weld microstructure.

l   Reasonable selection of welding materials: proper welding materials and protective atmosphere should be selected during welding. The chemical composition of the welding wire has an important influence on the corrosion resistance of the weld. The welding rod should have the similar chemical composition with the base metal, which can make the weld metal and the base metal have similar chemical composition, which is generally considered to achieve the best corrosion resistance.

l   Cleaning before and after welding: the surface of materials must be cleaned before welding, and welding slag shall be removed after welding.

three Welding of stainless steel water pipe

Stainless steel water pipe is welded by automatic pipe making machine after the stainless steel rolling plate is divided into strips. In order to reduce the adverse effect of welding on the corrosion resistance of stainless steel, modern automatic pipe making machine adopts argon protection, preheating, high welding speed, timely cooling and other measures, so that the stainless steel water pipe formed by welding still has good corrosion resistance and can be used in the atmosphere, Water and other weak corrosive medium are used for a long time.

According to the standard of Japan water pipe Association, stainless steel pipe used for general water supply and drainage can be directly used without heat treatment if it is produced by automatic arc welding.

four Stainless steel welded pipe fittings

In the installation and construction of large and medium-sized stainless steel water pipes, the traditional connection method is welding pipe connection. The welding method of stainless steel pipe is recommended TIG Welding method. However, due to the thin wall of stainless steel pipe and the simple construction site conditions, it is very difficult to weld without damaging its characteristics. The way to solve this problem by Japan water pipe association is to process the parts. All welding operations are carried out in the factory, and the connection can be completed as long as the parts are hoisted in the field.

For small diameter stainless steel water pipe, it is not suitable to use welded pipe fittings. There are reasons :1. It is difficult to carry out argon protection due to the poor site conditions ; two The technical level of workers is uneven , The same worker's welding work is also very difficult to complete from head to end ; three Due to the slow speed of manual welding ,   Welding heat affected zone Stay for a long time At sensitization temperature( four hundred and fifty —850 In the range of ℃), the substrate is poor in chromium and difficult to passivate, resulting in a significant decrease in corrosion resistance . From the previous engineering cases in China, the corrosion resistance of stainless steel was seriously damaged by field welding. Soon after the completion of the project, the welding parts began to rust, and even rusted through seriously, resulting in water leakage of the pipeline.

At present, the recommended connection method is: for the small diameter stainless steel water pipe, the clamping type connection is adopted; Large and medium-sized stainless steel pipes are connected by grooved pipe fittings. Avoid welding on site as much as possible.

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