Welding of Ferritic Austenitic Dual Phase Stainless Steel

Ferritic austenitic duplex stainless steel is a type of duplex stainless steel developed on the basis of ultra-low carbon ferrite stainless steel, with a duplex structure at room temperature. Compared with ordinary stainless steel, its Ni mass fraction is low, but the Cr and N mass fractions are high, indicating good resistance to pitting corrosion and stress corrosion. In addition, the high mass fraction of Fe in its crystalline structure results in higher yield strength than other stainless steels.

The welding characteristics of ferritic austenitic duplex stainless steel. Ferritic austenitic duplex stainless steel has good weldability. When heated to a sufficient temperature, the transformation from austenite to ferrite occurs. As the temperature increases, ferrite increases and austenite decreases. When the temperature rises to 1250<1300>, it can transform into pure ferrite structure. At this point, further cooling can be carried out to obtain pure ferrite structure at room temperature. When cooling from the liquid phase, ferrite is formed by solidification at 1 450>. When the temperature is below 1300>, crystal nuclei are first formed at the ferrite grain boundaries, gradually forming austenite. The slower the cooling rate, the more austenite is generated. Conversely, the less austenite is generated. Compared with ferritic stainless steel, this duplex stainless steel has a lower tendency for welding cracks; Compared with austenitic stainless steel, there is a lower tendency for embrittlement after welding.

However, if the welding process is not well mastered, the dual phase structure of this material will cause embrittlement of the weld and Heat-affected zone. Since this 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 directly have all required chemical composition and mechanical properties after welding. The test shows that when the ferrite content in the weld and the metal structure in the Heat-affected zone is more than 80%, the toughness will be reduced and the generation of cracks will be increased. Therefore, it is necessary to control the chemical composition of the weld seam, especially the mass fraction and cooling rate of Ni, to prevent the tendency of single-phase ferrite and coarse grain size, as well as the generation of cracks.

Firstly, the cooling rate should be maintained within an appropriate range, allowing sufficient time for the molten metal to generate sufficient austenite. The cooling rate is determined by the input heat, preheating temperature, interpass temperature and base metal thickness. Especially when welding large workpieces, the low cooling rate of the base metal Heat-affected zone will extend the residence time of the temperature range that causes embrittlement, and the residence time of the base metal in the weld and near weld zone in the high temperature zone will also be extended, which is not conducive to the control of the welding pool and the welding of the root. Therefore, attention should be paid to using small welding heat energy, and the interpass temperature should not exceed 150>, so that the weld metal has a reasonable ferrite austenite dual phase ratio. Secondly, increasing the mass fraction of Ni, Mn, and N in the welding rod or wire appropriately can promote the formation of austenite. This is a simple and feasible method for production sites where the cooling rate is difficult to control. In addition, the conductivity of welding rods varies with temperature, so it is necessary to choose a welding auxiliary device with a smaller diameter. The device itself is independent and can meet the requirements for welding long pipe fittings. For small weldments, simply remove the active tower wheel and use a three jaw self centering chuck to grip the workpiece.

The two complement each other, improving flexibility and expanding the application range of the equipment.

Easy to operate and use. A pair of rollers is equivalent to a V-shaped iron, and the workpiece can be automatically aligned and positioned without the need for locking or other operations.

The optical axis is equipped with four pairs of rollers (which can be increased or decreased as needed), and it is convenient to achieve butt welding between workpieces of the same diameter without the need for additional alignment positioning fixtures, while ensuring sufficient alignment accuracy.

Equipped with automatic conversion function of angular velocity. The usual welding process parameters are given as the linear velocity of the weld formation. In this device, due to the lack of relative sliding between the workpiece and the roller, the linear velocity of the workpiece is the linear velocity of the outer diameter of the roller. When the diameter of the workpiece changes, as long as the rotational speed of the roller is not changed, the angular velocity of the workpiece can be automatically changed to keep the linear velocity unchanged. This is very different from the method of converting the line speed based on the fixture speed (i.e. workpiece angular velocity) and workpiece diameter every time using a three jaw self centering chuck. If the corresponding relationship between the linear speed of the roller and the speed adjustment handle is calibrated first, it will no longer be limited by the workpiece diameter in future use. The linear speed can be adjusted directly according to the calibration value, which will bring great convenience to use.

Conclusion: After practical application, the device has achieved good results, and the welding quality has met the design requirements, thus solving the problems in production. With minimal investment, it has improved the processing capacity of the existing equipment. For other workpieces with similar structures but significant differences in size, the basic structural form of this device can be used, and these workpieces can be welded by adjusting the dimensions of their optical axis and rollers appropriately.