The welding technology of 2205 duplex stainless steel
In the stainless steel processing technology, welding processing of 2205 duplex stainless steel is a very test of welding skills, this article is concerned about such stainless steel welding processing considerations.
The second generation of duplex stainless steel is generally called standard duplex stainless steel, its composition is characterized by ultra-low carbon, nitrogen, steel in the typical composition is 22% chromium, 5% nickel, 0.17% nitrogen, and the first generation of double Compared with stainless steel, 2205 duplex stainless steel increased the nitrogen content and enhanced the resistance to stress corrosion and pitting resistance in acidic media with high chloride ion concentration. Nitrogen element is a strong austenite forming element, added to the duplex stainless steel, not only can enhance the strength and will not significantly damage the toughness of stainless steel, but also inhibit the precipitation of carbides.
Duplex stainless steel
2205 duplex stainless steel is very significant organizational characteristics. In the greenhouse under the steel body solid solution of austenite and ferrite about half of each, both the two-phase structure characteristics. It has the characteristics of small thermal conductivity, resistance to pitting, crevice and chloride stress corrosion, and also has austenitic stainless steel with good toughness, low brittleness transition temperature, resistance to intergranular corrosion, good mechanical properties and good weldability advantage.
In addition performance is also very prominent. Yield strength and stress corrosion resistance, duplex stainless steel than Austenitic stainless steel yield strength is nearly 1 times the same pressure level conditions, can effectively save the material. The austenitic stainless steel has a low linear thermal expansion coefficient close to low carbon steel. So duplex stainless steel and carbon steel is suitable for connection, its engineering significance. But forging and cold forming is not as good as austenitic stainless steel.
The duplex stainless steel 2205 has good weldability, and the sensitivity of welding cold cracks and thermal cracking is small. Generally no preheating before welding, and after welding is not heat treatment. Because the nitrogen content is high, the single-phase ferrite tendency of the heat-affected zone tends to be small, and when the welding material is selected reasonably, the welding line energy control has good comprehensive performance.
2205 duplex stainless steel hot cracking sensitivity is much lower than the austenitic stainless steel. This is because the nickel content is not high, easy to form low melting point eutectic impurities very little, easy to produce low melting point liquid film. In addition, the grain is not at risk of rapid growth at high temperatures.
In addition, the main problem of duplex stainless steel welding is not in the weld, but in the heat affected zone. Due to the thermal cycling in the heat, the heat affected zone is in the fast-cold non-equilibrium state and always retains more ferrite after cooling, which increases the corrosion tendency and hydrogen-induced crack (brittleness) sensitivity.
In the processing of duplex stainless steel, under the action of thermal cycling, the weld metal and heat affected zone of the organization occurred a series of changes. At high temperatures, all the microstructure of all duplex stainless steels is composed of ferrite, and the austenite is precipitated during the cooling process. The amount of austenite precipitated is affected by many factors.
The mechanical properties and corrosion resistance of the duplex stainless steel welded joints are affected by the appropriate coupling of the welded joints, so the welding is done around how to ensure its biphase organization. When the amount of ferrite and austenite close to 50%, the performance is better, close to the performance of the base metal. If this relationship is changed, cope with reduced corrosion resistance and mechanical properties of duplex stainless steel welded joints. Two-phase stainless steel 2205 ferrite content of the best 45%, too low ferrite content of less than 25% will lead to strength and resistance to stress corrosion cracking capacity decreased; too high ferrite content of more than 75% will be detrimental to Corrosion resistance and reduced impact toughness.
The influence factors of the proportion of stainless steel welded head are mainly determined by the equilibrium relationship between ferrite and austenite in welded joint, which is affected by the content of alloying elements in steel and the influence of filler metal, welding heat cycle and shielding gas.
The biggest feature of duplex stainless steel welding is that the thermal cycling has an effect on the organization within the welded joint, and both the weld and the heat affected zone undergo phase change, which greatly affects the performance of the welded joint. So the multi-channel multi-channel welding is useful, follow-up weld on the front welds have heat treatment, weld metal ferrite further into austenite, austenite dominated by the two-phase structure; The austenite phase in the heat affected zone of the weld is also increased and the ferrite grains can be refined to reduce the precipitation of carbides and nitrides from the crystal and the grain boundaries, thereby making the properties of the entire welded joint significantly improve.
The number of welding processes is the welding line energy on the balance of the two-phase organization also plays a key role. Because the duplex stainless steel is 100% at high temperature ferrite if the line energy is too small, heat affected zone cooling speed, austenite too late to precipitate, excessive ferrite will be kept in the greenhouse under the cold. If the line energy is too large, the cooling rate is too slow, although you can get enough austenite, but also cause the heat affected zone ferrite grain growth and σ phase of harmful metal phase precipitation, resulting in the formation of brittle The In order to prevent this from happening, the optimal measure is to control the welding line energy and the interlayer temperature, and to use the filler metal.
Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.ugsteelmill.com)