Views: 67 Author: Site Editor Publish Time: 2024-12-10 Origin: Site
Duplex stainless steel is composed of two phases, austenite and ferrite. When the ratio of the two phases is about 50%, duplex stainless steel combines the excellent toughness and weldability of austenitic stainless steel with the higher strength and chloride stress corrosion resistance of ferritic stainless steel, making it have the advantages of both austenitic and ferritic stainless steel.
Duplex stainless steel 2205 is produced by AvestaPolarit of Sweden. The commercial grade is 2205CodePlusTow. It has been included in A240 and A480 of ASTM and ASME. The UNS number is S32205. It belongs to the second generation of duplex stainless steel. 2205CodePlusTow is different from the same duplex stainless steel 2205 with UNS number S31803. It increases the lower limit of nitrogen content and passes the test of harmful metal phase precipitation. 2205CodePlusTow has higher strength, corrosion resistance and metallurgical stability after welding. It is easy to obtain a balanced two-phase structure in the welded joint. The high nitrogen content more effectively inhibits the precipitation of harmful metal phases, which is very beneficial for welding.
Duplex stainless steel welding control and post-weld inspection are key steps to ensure welding quality and performance. In terms of welding control, welding parameters such as welding speed, current and voltage need to be strictly controlled to ensure the stability of the welding process and the quality of the weld. At the same time, choosing the right welding method and welding materials is also crucial, as they directly affect the strength and corrosion resistance of the welded joint.
Duplex stainless steel 2205 has good weldability, and the sensitivity to cold cracks and hot cracks in welding is relatively low. Usually, no preheating is done before welding, and no heat treatment is done after welding. Due to the high nitrogen content, the single-phase ferritization tendency of the heat-affected zone is relatively small. When the welding material is selected reasonably and the welding line energy is properly controlled, the welded joint has good comprehensive performance.
The sensitivity to hot cracks is much smaller than that of austenitic stainless steel. This is because the nickel content is not high, and there are very few impurities that are easy to form low-melting eutectics, and it is not easy to produce low-melting liquid films. In addition, there is no danger of rapid growth of grains at high temperatures.
The main problem of duplex stainless steel welding is not the weld, but the heat-affected zone. Because under the action of the welding heat cycle, the heat-affected zone is in a fast-cooling non-equilibrium state, and more ferrite is always retained after cooling, thereby increasing the corrosion tendency and the sensitivity to hydrogen-induced cracking (embrittlement).
Duplex stainless steel contains about 50% ferrite, and also has 475℃ brittleness, but it is not as sensitive as ferritic stainless steel.
During the welding process of duplex stainless steel, under the action of thermal cycle, the organization of weld metal and heat affected zone undergoes a series of changes. At high temperature, the metallographic structure of all duplex stainless steels is composed of ferrite, and austenite is precipitated during the cooling process. The amount of austenite precipitation is affected by many factors.
The mechanical properties and corrosion resistance of duplex stainless steel welded joints depend on whether the welded joints can maintain an appropriate phase ratio. Therefore, welding is carried out around how to ensure its duplex organization. When the amount of ferrite and austenite is close to 50% each, the performance is better and close to the performance of the base material. Changing this ratio will reduce the corrosion resistance and mechanical properties of duplex stainless steel welded joints. The optimal value of ferrite content of duplex stainless steel 2205 is 45%. Too low ferrite content (<25%) will lead to a decrease in strength and stress corrosion cracking resistance; too high ferrite content (>75%) will also damage corrosion resistance and reduce impact toughness.
The balance between ferrite and austenite in the welded joint is affected by the content of alloying elements in the steel, as well as the filler metal, welding thermal cycle, and shielding gas.
According to research and a large number of tests, nitrogen content in the base metal is very important. Nitrogen plays an important role in ensuring that sufficient austenite is formed in the weld metal and the heat affected zone after welding. Nitrogen, like nickel, is an element that forms and expands austenite, but nitrogen's ability is far greater than nickel. At high temperatures, nitrogen's ability to stabilize austenite is also greater than nickel, which can prevent the appearance of single-phase ferrite after welding and prevent the precipitation of harmful metal phases.
Due to the effect of welding thermal cycle, when the composition of autogenous welding or filler metal is the same as that of the base metal, the amount of ferrite in the weld metal increases sharply, and even pure ferrite structure appears. In order to suppress the excessive increase of ferrite in the weld, the welding trend of duplex stainless steel is to use a weld metal dominated by austenite. Generally, two approaches are adopted: increasing nickel or adding nitrogen in the welding material. Usually the nickel content is 2% to 4% higher than that of the parent material. For example, the nickel content of 2205 filler metal is as high as 8% to 10%. Using nitrogen-containing filler materials is better than filling materials that only increase nickel. Both elements can increase the proportion of austenite phase and stabilize it, but adding nitrogen can not only delay the precipitation of intermetallic phases, but also improve the strength and corrosion resistance of weld metal.
At present, filler materials are generally based on increasing nickel, and then adding nitrogen equivalent to the parent material content.
For duplex stainless steel 2205, Sandvik22.8.3.L (ER2209) welding wire is used for tungsten inert gas arc welding, and Avesta2205AC/DC welding rod is used for arc welding to meet the requirements of welding materials. These characteristics of duplex stainless steel 2205 and welding materials in alloying elements provide a certain range for the selection of welding process parameters, namely welding line energy, which is very beneficial for welding.
The biggest feature of duplex stainless steel welding is that the welding thermal cycle has an impact on the structure in the welded joint. Phase change will occur in both the weld and the heat-affected zone, which has a great impact on the performance of the welded joint. Therefore, multi-layer and multi-pass welding is beneficial. The subsequent welds have a heat treatment effect on the previous welds. The ferrite in the weld metal is further transformed into austenite, becoming a two-phase structure dominated by austenite; the austenite phase in the heat-affected zone adjacent to the weld also increases accordingly, and can refine the ferrite grains, reduce the precipitation of carbides and nitrides from the grains and grain boundaries, thereby significantly improving the structure and performance of the entire welded joint.
Welding process parameters, namely welding line energy, also play a key role in the balance of duplex structure. Since duplex stainless steel is 100% ferrite at high temperature, if the line energy is too small, the heat affected zone cools quickly, austenite does not have time to precipitate, and the excess ferrite will be kept undercooled at room temperature. If the line energy is too large and the cooling rate is too slow, although sufficient austenite can be obtained, it will also cause the ferrite grain growth in the heat affected zone and the precipitation of harmful metal phases such as σ phase, causing joint embrittlement.
In order to avoid the above situation, the best measure is to control the welding line energy and interlayer temperature, and use filler metal.
When tungsten inert gas welding, 2% nitrogen can be added to the argon gas to prevent the loss of nitrogen on the weld surface due to diffusion, which helps to balance ferrite and austenite.
The welding process should usually specify the welding line energy range and the highest interlayer temperature. It is usually recommended that the line energy is 110-215kJ/mm, and the interlayer temperature is controlled below 150℃.
Preheating is usually not required. Before welding, if the workpiece is wet or the workpiece temperature is below 5℃, it should be properly preheated. There are also cases with large constraints and especially thick-walled structures. Preheating to 100℃ is beneficial. Further details about preheating should be consulted. The oxyacetylene flame should not directly contact the material surface, and no hot spots should appear.
