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Welding of Ferritic Austenitic duplex stainless steel

TIME:2016-12-10   AUTHOR:   SOURCE:   VIEWS: 409

Ferritic Austenitic stainless steel is a kind of duplex stainless steel, which is developed on the basis of ultra low carbon ferritic stainless steel. Compared with ordinary stainless steel, the mass fraction of Ni is low, but the mass fraction of Cr and N is high, and it has a good ability of pitting corrosion and stress corrosion resistance. In addition, the mass fraction of Fe in the crystal structure is high, so it has higher yield strength than other stainless steel.

The ferrite austenite duplex stainless steel welding body characteristics of Ferritic Austenitic duplex stainless steel with good weldability, Ferritic Austenitic duplex stainless steel is heated to a temperature sufficient, appeared austenite to ferrite transformation. With the increase of temperature, ferrite increases and austenite decreases. The temperature rose to 1250<1300>, can be transformed into ferrite organization. At room temperature for cooling, can obtain the ferrite microstructure. At the beginning of the cooling from the liquid phase, the ferrite was formed at 1 450>. When the temperature is lower than 1300>, the crystal nucleus is formed at the ferrite grain boundaries, and the austenite is formed gradually. The slower the cooling rate, the more the austenite, the less the austenite. Compared with ferritic stainless steel, the stainless steel has a lower tendency to crack, and the tendency of embrittlement is lower than that of austenitic stainless steel.

However, if the welding process is not well controlled, the Bi phase structure of the material will cause embrittlement of the weld and heat affected zone. Since the duplex stainless steel is mostly used for large components, it is impossible to carry out post weld heat treatment, and the weld and heat affected zone should have all the chemical composition and mechanical properties after welding. The results show that when the content of ferrite in the weld and heat affected zone is larger than 80%, the toughness will be decreased and the crack will be increased. Therefore, it is necessary to control the chemical composition of the weld, especially the mass fraction of Ni and the cooling rate, to prevent the tendency of single phase ferrite and grain coarsening and the formation of cracks.

First, the cooling rate should be kept in a suitable range, so that the molten metal has enough time to generate enough austenite. The cooling speed is determined by heat input, preheating temperature, interpass temperature and thickness of base material, especially in the welding of large workpiece, welding heat affected zone of the cooling rate is low, will prolong the residence time caused by temperature range embrittlement, the residence time of weld and near weld zone of the base material in high temperature area and also extend this is not conducive to welding, weld pool and root control. Therefore, it is necessary to pay attention to the use of a small amount of welding heat energy, the temperature between the channel is not more than 150>, so that the weld metal has a reasonable ferrite austenite phase ratio. Secondly, properly increasing the mass fraction of Ni, Mn and N in the welding rod or wire can promote the formation of austenite. This is a simple and easy method for the production site where the cooling rate is not easy to control. In addition, the electrical conductivity of the electrode with different temperature, so the use of smaller diameter welding welding auxiliary device features the device itself, can meet the requirements of long pipe welding. For small pieces of welding, simply remove the active cone, the use of three jaw self centering chuck clamping workpiece.

Both complement each other, improve the flexibility, expand the scope of application of the equipment.

Convenient operation. A pair of rollers is equivalent to a V shaped iron, the workpiece can be automatically positioned to adjust the heart positioning, without locking and other operations.

The optical axis is provided with four of the roller (according to the need to increase or decrease), does not need additional on the positioning fixture, can conveniently realize the butt welding with the diameter of the workpiece, and can ensure enough accuracy of.

Automatic conversion function with angular speed. The welding process parameters are given as the line speed of the weld formation. The device has no relative sliding between the workpiece and the roller. When the diameter of the workpiece is changed, the angular velocity of the workpiece can be changed automatically without changing the rotation speed of the roller. The use of three jaw self centering chuck every time according to the fixture speed (i.e. the angular velocity of the workpiece and the workpiece diameter), different line speed conversion method. If the relationship between line speed and wheel speed regulating handle to make calibration, is used in the future will no longer be the diameter of the workpiece, directly according to the calibration value can be adjusted to the line speed, which will bring great convenience to use.

4 conclusion the device has been applied in practice, and achieved good results. The welding quality has reached the design requirements, thus solving the problems in production, with little investment, in exchange for the improvement of existing equipment processing capacity. For other similar structures, but the size of the workpiece is relatively large, can be used in the basic structure of the device, only the optical axis and the size of the wheel to make appropriate adjustments, the workpiece can be welded.