This article mainly describes the effect of preheating and preheating temperature on the surface forming and cracks of the cobalt-based alloy Stellite 6 of 2205 dual-phase steel by ShanghaiHYIndustryCo.,Ltd through hot-wire TIG automatic welding. Influence. Through experimental research, it is found that when there is no preheating, the weld formation is poor due to the poor fluidity of the deposited metal of the overlay welding, and the PT shows obvious micro cracks during the second layer welding; when the preheating is performed at 350℃ , The fluidity of the deposited metal is obviously improved and the weld formation is very beautiful. However, during the second layer welding process, the temperature gradient line of the temperature controlled by the thermocouple on the pipe end surface found obvious cracking when the temperature is below 200 ℃, but the temperature is 200 ℃ At the above, the cracks disappeared until all the pipes were welded, no cracks were found, and the macroscopically showed that the fusion was good, and through the hardness test, it was found that the HRC of the first layer and the second layer were both above 42, which satisfies the wear-resistant surfacing cladding material Minimum HRC35 requirement.
Cobalt-based alloy is a kind of wear-resistant material resistant to high temperature corrosion. In crude oil extraction, the pipeline medium is mainly a three-phase mixture of solid, liquid and gas, and the three-phase separation of solid, liquid and gas is carried out through the valve, especially in deep sea mining. Dual-phase steel and nickel-based alloys are widely used. Therefore, for the valve body and valve core, its corrosion resistance and wear resistance fully affect the service life of the pipeline and its components. Therefore, for the dual-phase steel 2205 cobalt-based TIG stack The welding test research has significant significance.
Stellite 6 contains a higher proportion of alloying elements, among which higher carbon content and tungsten content, which can produce tungsten carbide with higher hardness and quality during welding metallurgy, so as to achieve higher wear resistance, and thus can be used in crude oil extraction. To ensure that the valve body and the valve core are not worn, so as to ensure a long service life, and it contains up to 28% of chromium, which ensures the corrosion resistance of the deposited metal.
Ⅱ, common cobalt-based alloy surfacing methods
Due to the high hardness of cobalt-based alloys, it is difficult to form a disk-shaped solid wire by drawing. At present, as far as welding materials are concerned, there are mainly cobalt-based alloy welding rods, cobalt-based alloy welding rods, cobalt-based alloy metallurgical powders, and cobalt-based alloys. Flux-cored welding wire, etc. Common welding methods are mainly manual electrode arc welding, MIG welding, plasma cobalt-based alloy bar welding, plasma metallurgy powder spray welding, laser metallurgy powder spray welding, etc.
Ⅲ, automatic hot wire cobalt-based TIG surfacing method and its advantages and disadvantages
Compared with other welding methods, the automatic hot-wire cobalt-based TIG surfacing is achieved by using the cobalt-based flux cored wire to complete the TIG automatic surfacing by using the wire preheating method. Its main advantages are:
Welding full-position coverage: It can use the torch rotation or the workpiece rotation to realize all-position welding, which is not limited by the flat welding position during powder metallurgy spraying;
Wide product coverage: Because the welding gun can extend into the workpiece and realize automatic welding during TIG welding, it is not affected by the limitation of the welding gun working space such as laser welding and plasma arc welding. Therefore, for the inside of the valve and other channels Welding has a high advantage;
Low sensitivity to thermal cracking: Due to the low heat input during TIG welding, a higher degree of undercooling is achieved when the weld pool is cooled, thereby ensuring a shorter liquid film residence time for the low melting point eutectic during metal crystallization, which is effective Avoid crystal cracks;
Low welding stress: Due to the preheating effect of the welding wire and the pulse effect of TIG automatic welding during welding, low welding stress is shown under low heat input, so it is more effective in avoiding delayed low plastic cracks in cobalt-based alloys. High meaning
In addition, compared to fusion electrode welding, plasma arc welding, and laser welding, fully automatic hot wire cobalt-based TIG surfacing also exhibits certain shortcomings:
Tungsten electrode burns seriously: because the cobalt-based alloy is only flux-cored welding wire, the spatter generated during the welding process is easily sprayed on the surface of the tungsten electrode or even the end, thereby increasing the rapid burning of the tungsten electrode caused by arc discharge ；
Poor welding seam formation: Although lower welding heat input during TIG welding has a higher advantage in avoiding hot cracks, it is difficult to open the weld pool during TIG welding of cobalt-based alloys, resulting in poor welding seam formation;
High welding cost: Compared with plasma and laser welding of cobalt-based alloy metallurgical powder spraying, the efficiency of cobalt-based TIG surfacing is lower, and the production cost is significantly increased.
Ⅳ, the idea of automatic hot wire cobalt-based TIG surfacing test
In view of the advantages and disadvantages of the above hot wire TIG cobalt-based alloy surfacing welding, combined with the lower thermal conductivity and higher expansion coefficient of 2205 dual-phase steel, while the linear expansion coefficient of cobalt-based alloy is small, especially when the temperature is low. Due to its large restraint degree, it is difficult to realize the larger elastic-plastic strain of the deposited metal under stress load. This test is divided into two types: no preheating method and temperature increasing preheating method.
1. Test materials
This test uses 2205 duplex steel and Stellite 6 cobalt-based flux cored wire.
2, test method
Test is divided into two types, A and B. The welding parameters are shown in Table 2. The base material of the A test uses a 200×150×20 2205 duplex steel plate, and the B test uses a DN200×150 2205 duplex steel pipe.
Ⅴ . Fully automatic hot wire cobalt-based TIG surfacing test results
1, A test result
After the welding of the first layer, it was found that no cracks appeared in the PT test of the two tests, as shown in Figure 1. However, after the welding of the second layer, there were obvious microcracks on the surface of the deposited metal in the A test, as shown in Figure 2, and the surface of the entire weld was poorly formed.
2, B test results
2.1 Weld forming
The B test adopts the overall preheating method in the furnace, and during the welding process, the outer side of the 2205 duplex steel pipe is insulated with insulation cotton, and the temperature of the upper pipe port is recorded every 4 minutes with a thermocouple, and the first layer is welded After the completion of the heat treatment in the furnace at 350℃ for 1h, the test found that the surface formation of the weld seam during the first and second layer welding was significantly improved compared to the test without preheating in item A.
2.2 Port temperature changes
Port temperature changes with welding time and welding height: The furnace preheating at 350℃ was continued before the second layer welding, but after the furnace was assembled in the welding station for about 10 minutes, it was found that the temperature dropped significantly. When the actual welding starts, the surface port temperature is only 180℃, and with the increase of welding height and continuous welding heat input in the subsequent welding process, the surface port temperature slowly decreases first, and then gradually rises until the welding is completed. When, the port temperature reaches 420℃.
2.3 Welding crack
During the second layer welding process, when the length of the pipeline was welded for nearly 30mm, an obvious sound of brittleness of the weld was heard. At this time, the measured port temperature was 167℃, and the PT test showed that the crack was a half-circle transverse cracking. At the moment of instant transverse cracking in the circumferential direction of the weld, a longitudinal crack source is also formed, which will extend upward with the release of stress in the subsequent welding process. When the port temperature continues to rise to about 180°C, the crack propagation stops until the welding is completed. And it did not produce any cracks.
2.4 Macro inspection
Weld section samples were cut and polished with sandpaper to show their low-magnification morphology by the chemical reagent etching method recommended in the ASME-IXQW-470 standard. Visual observation, and the test results were observed and evaluated according to the ASMEIXQW-183 standard. The sample inspection results show that the weld metal and the cross-section of the heat-affected zone are completely fused without cracks.
Ⅵ . Conclusion
The test proved that when dual-phase steel 2205 is welded with one layer of Stellite 6, no preheating and 350℃ preheating can ensure that no cracks occur in the deposited metal, but when there is no preheating, the fluidity of the weld pool is poor. This makes the weld surface poorly formed, and when preheating at 350°C, the fluidity of the weld pool is improved, and the final weld bead is more beautiful in shape;
When there is no preheating, the surface microcracks of the weld are obvious, and it is difficult to achieve the quality of the multi-layer weld bead;
Carry out preheating in the furnace at 350°C. After the furnace is released, the temperature will drop significantly during the welding and assembly process of the workpiece, and there is a higher risk of low plastic stress cracking below 200°C;
When the temperature is above 200℃, no welding cracks are seen, that is, for the duplex steel 2205 material, the preheating temperature of hot wire TIG cobalt-based alloy automatic surfacing should be above 200℃, and electric heating should be continued during the welding process Ways of heating and heat preservation to achieve temperature control, that is, a qualified surfacing cladding can be obtained in the end;
When hot wire TIG is used for Stellite 6 cobalt-based TIG surfacing welding of duplex steel 2205, a better fusion of the surfacing layer and the base metal can be achieved;
When hot wire TIG is used for Stellite 6 cobalt-based TIG surfacing welding of duplex steel 2205, the minimum HRC35 requirements for the second and first layers can be guaranteed, and the effective thickness of the wear-resistant composite layer is relatively high.
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