With the increasing development of industrial technology. It is difficult for general stainless steels to meet stress corrosion, pitting corrosion. To meet the requirements for crevice tunnel corrosion. For this reason, metallurgists have conducted a lot of research. And developed austenitic-ferritic stainless steel. That is duplex stainless steel. Traditional austenitic stainless steel has insufficient resistance to local corrosion.
Like intergranular corrosion, stress corrosion, pitting corrosion, and crevice corrosion. Especially the fracture caused by stress corrosion, which is harmful. Duplex stainless steel is a new type of steel developed in the past two decades. By controlling the various alloying elements and the heat treatment process.
The ferrite and the austenite phase in the solid solution structure account for about 50% each. Combine the excellent toughness and weldability of austenitic stainless steel. With that of ferritic stainless steel. The combination of high strength and resistance. Chloride stress corrosion makes duplex stainless steel. It has the advantages of ferritic stainless steel and austenitic stainless steel.
Duplex 2205 stainless steel is about half of the ferrite phase and austenite phase. Generally, the content of the low-volume phase needs to reach 30%. In the case of low C content, the Cr content is 18%-28%, and the Ni content is 3%-10%. Some steels also contain Mo, Cu, Nb, Ti, N, and other alloying elements. This type of steel has the characteristics of austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness. No room temperature brittleness, and improved intergranular corrosion resistance and welding performance. It still maintaining iron 475℃ brittleness, high thermal conductivity. And superplasticity of the element stainless steel. Compared with austenitic stainless steel. It has a high strength. Resistance to intergranular side food and chloride stress corrosion resistance. Duplex 2205 stainless steel has excellent pitting corrosion resistance. Also nickel-saving stainless steel.
Through correct control of the chemical composition and heat-treatment process. The duplex 2205 stainless steel has the advantages of both ferritic and austenitic. It combines the excellent toughness and weldability of austenitic stainless steel. With iron the high strength and chloride stress corrosion resistance. The elements of stainless steel are combined. It is these superior properties. That makes the duplex stainless steel develop as a weldable structural material. Since the 1980s, it has become a combination of martensitic and austenitic. A steel type juxtaposed with ferritic and ferritic stainless steel.
Application of duplex 2205 stainless steel
Duplex 2205 stainless steel has the phenomenon of local pitting, stress and cavitation corrosion. Caused by strong acids and alkalis in petrochemical industry. And vessels, pipes and parts. Generally difficult for stainless steel to handle.
But, duplex 2205 stainless steel not only has strong corrosion resistance but also has good strength and toughness. For this reason, it has obtained wide applications in engineering and energy transportation. Such as bridges. , Airplanes, ships, automobiles, and decorative buildings in coastal cities and chemical zones.
- 1.1 Oil and gas industry
This is one of the main areas where duplex 2205 stainless steel. This field covers a wide range, complex working conditions, and diverse media. It is also an area where duplex stainless steel is used earlier and more.
Duplex 2205 stainless steel like DP3 steel as the seawater cooler tube. The seawater outlet temperature is 40℃. It has been used intermittently for 15 years and the effect is very good.
The coolers of the pulverized coal gasification unit of coal chemical plants are all made of imported 2205 steel pipes.
1.1.2. Fertilizer Industry
Because duplex 2205 stainless steel has good anti-corrosion fatigue performance in urea medium. It is very suitable for manufacturing the key equipment of urea production-A press pump body. The pump bodies of the A-press pumps of small and medium fertilizer plants are made of 18-5Mo steel. Dozens of companies also use high-chromium lead-containing duplex stainless steel. Also, the pump valve forgings of steel have passed the inspections of Japan JIS G0573. And sulfuric acid methods are exported to Japan in batches. The price is cheaper than that of local production in Japan.
Besides, the 2205 stainless steel taking advantage of its wear and corrosion resistance. Used for the internal parts of high-pressure shut-off valves in the urea plant. The effect is good.
The chemical tanker industry has been the largest user of 2205 stainless steel. Its consumption accounts for about 50% of hot rolled steel sheets. The liquid cargo carried by chemical tankers is diverse. Included chemical and petrochemical products, food, etc.. The cabin materials needs to be both corrosion-resistant and high-strength. Now 2205 duplex stainless steel has replaced 316L and 317L. It has become the standard material for offshore chemical tankers.
1.1.4. Paper and Salt Light Industry
2205 stainless steel has better resistance to crevice corrosion in neutral chloride solution. The industry has developed the application of this steel in vacuum salt production. Salt and salt co-production equipment by using this feature. The large-scale brine adopts duplex stainless steel lining and composite plates. It solves the problem of equipment salt scaling and corrosion. The longest use history is 10 years. Duplex stainless steel has mature experience in large-scale vacuum salt-making equipment.
In the field of pulp and paper industry, there is almost a blank in China. The kraft cooking method still uses low-carbon steel cooking pots. The use of bleaching equipment has been popularized abroad. At present, there are also domestic introductions, but the number Very rarely. In summary, it can be seen that the use of duplex stainless steel in China has certain limitations.
For example, the large-scale use of duplex stainless steel in foreign countries. Such as the pulp and paper industry, oil and gas industry, transportation industry. Even the construction industry. According to national conditions, using the performance advantages of duplex stainless steel. Also to continuing to expand its application in the chemical and petrochemical fields. The technical transformation of the pulp and paper industry needs to develop. As for the oil and gas pipelines, it is currently difficult to promote.
The price of 2205 stainless steel is too high. But it is still possible to use duplex stainless steel. Even super duplex stainless steel to manufacture the chlorine ion by oil and gas fields. such as gas gathering pipelines and heat exchange equipment. The development of the marine transportation industry are in the ascendant. Requiring a large number of large sheets of steel plates. This gap needs to be filled. As for the application in the construction industry, it has not been involved yet. In fact, the urban sculpture landscape of the coastal. 2304 steel can completely replace 304 and 316 austenitic stainless steel.
Nowadays more and more people require higher stainless steel than 316L in the stainless steel handrail field. As when it use for a house nearby the sea, 316L still not strong enough to corrosion-resistant. Thus, people tend to use 2205 stainless steel instead of 316L.
Advantages and applications of duplex 2205 stainless steel
1.2.1 Compared with austenitic stainless steel. The advantages of duplex stainless steel are as follows:
1) The yield strength is more than twice. Ordinary austenitic stainless steel, and it hasenought plastic toughness for forming. The wall thickness of storage tanks made duplex stainless steel 30-50% less than austenite. It is beneficial to reduce costs.
Table 1-2 Part of the grades and chemical composition of duplex stainless steel ( %)
2) It has excellent resistance to stress corrosion cracking. Even the duplex stainless steel with the lowest alloy content has higher resistance. Stress corrosion cracking than austenitic stainless steel. Especially in an environment containing chloride ions. Stress corrosion is a prominent problem. That is difficult to solve for ordinary austenitic stainless steel.
3) The corrosion resistance of 2205 duplex stainless steel. It is most used in many media, is better than ordinary 316L austenitic stainless steel. While super duplex stainless steel has high corrosion resistance. In some media, such as acetic acid and formic acid. It can even replace high-alloy austenitic stainless steel or even corrosion-resistant alloy.
4) It has good local corrosion resistance. Compared with austenitic stainless steel with the same alloy content. Its wear corrosion resistance are better than austenitic stainless steel.
5) The coefficient of linear expansion is lower than austenitic stainless steel. Close to that of carbon steel. It is suitable for connection with carbon steel. And has important engineering significance. Such as the production of composite plates or linings.
6) Regardless of dynamic load or static load. It has higher energy absorption capacity than austenitic stainless steel. This has obvious advantages for structural parts to cope with sudden accidents. Such as collisions and explosions, and has practical application value.
1.2.2 Compared with ferritic stainless steel. The advantages of duplex stainless steel are as follows:
1 ) Comprehensive mechanical properties. Especially plastic toughness, are not as sensitive to brittleness as ferritic stainless steel.
2) Except for stress corrosion resistance. Other local corrosion resistance is better than ferritic stainless steel.
3 ) Cold working process performance. And cold forming performance are far better than ferritic stainless steel.
4 ) The welding performance is also much better than that of ferritic stainless steel. Generally, preheating is not required before welding. And no heat treatment is required after welding.
5 ) The application range is wider than ferritic stainless steel.
Basic requirements of dual-phase steel joints and measures to meet the requirements
2.1 basic requirements
The welded joint does not have defects. That exceed the quality standards and the mechanical properties. It meet the expectations of the welded structure. The performance requirements of the use. No welding hot cracks and cold cracks, stress corrosion, pitting corrosion. The appearance of δ phase embrittlement.
2.2 Preventive measures
1 ) Duplex stainless steel has good weldability. Generally, welding materials with similar composition as the base metal. Since the carbon content has a great influence on corrosion resistance. Tthe carbon content of the deposited metal does not need to be higher than the base material. Equipment that is corrosive or only to avoid rust pollution. Choose to contain Ti or other ultra-low carbon welding material. For workpieces with high acid corrosion resistance, often use Mo Welding materials. The welding material will not have welding hot cracks and cold cracks.
2 ) Design welding joints. Avoid the accumulation of corrosive media at the welded joints. Reduce or cut stress Concentrate and the residual stress of welded joints. Using common process measures, the heating temperature is 800-900 ℃. To get the ideal stress relief effect. The structure design should use butt joints to avoid ten-word cross weld. Single V Change to shape groove Y-shaped groove.
3 ) Use a small heat input, that is, a small current. A large welding speed, reduce lateral swing, wait for the previous welding. After the seam is cooled to the preheating temperature, the next welding seam is welded. 750-800 ℃ annealing treatment, after annealing Fast cooling to prevent appearance δ Harmony 475 ℃ embrittlement.
3 SAF2205 Weldability of dual-phase steel and possible problems in welding
3.1 SAF2205 Weldability of duplex steel
3.1.1 SAF2205 Performance characteristics of duplex steel
1) Molybdenum-containing duplex stainless steel. It has good chloride stress corrosion resistance under low stress. 18-8 type austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solution above 600℃. Heat exchangers made of this type of stainless steel are used in trace chloride. There is a tendency to produce stress corrosion cracking. But duplex stainless steel has good resistance.
2) Duplex stainless steel containing molybdenum has good pitting corrosion resistance. When having the same pitting corrosion resistance equal value (PREN=Cr%+3.3Mo%+16N%).The critical pitting potential of duplex stainless steel and austenitic are similar. The pitting corrosion resistance of duplex stainless steel is equal to AISI 316L. The pitting and crevice corrosion resistance of 25% Cr. Especially nitrogen-containing high-chromium duplex stainless steel exceeds AISI 316L.
3) It has good corrosion fatigue and wear corrosion resistance. Under certain corrosive media conditions. It is suitable for making pumps, valves and other power equipment.
4) Good comprehensive mechanical properties. It has higher strength and fatigue strength. And its yield strength is twice that of 18-8 austenitic stainless steel. The elongation in the solid solution state reaches 25%. The toughness value AK (V-notch) is above 100J.
5) Good weldability, low thermal cracking tendency. Preheating is not required before welding.No heat treatment is required after welding. It can be welded with 18-8 austenitic stainless steel or carbon steel.
6) The hot working temperature range of duplex stainless steel with low chromium (18%Cr). It is wider than that of 18-8 austenitic stainless steel, and its resistance is small. It can be rolled to produce steel plates without forging. The hot working of duplex stainless steel with high chromium (25%Cr). It is more difficult than austenitic stainless steel. And products such as plates, tubes, and wires can be produced.
7) The work hardening effect of cold working is greater than 18-8 type austenitic . In the initial stage of deformation of the tube and plate, greater stress is required to deform.
8) Compared with austenitic stainless steel. It has higher thermal conductivity and lower linear expansion coefficient. Making it suitable for equipment lining and production of composite plates. It is also suitable for making heat exchanger tube cores with higher heat transfer. Efficiency than austenitic stainless steel.
9) There are still various brittleness tendencies of high chromium ferritic stainless steel. And it is not suitable to be used in working conditions higher than 3000C. The lower the chromium content in duplex stainless steel. The less harmful the brittle phases such as σ.
3.1.2SAF2205 Microstructure characteristics of duplex stainless steel
At present, due to high smelting quality requirements and high prices. The output of duplex stainless steel is not high. Accounting for about 1% of the world’s stainless steel output. But it has increased after the 1990s.
The output in 1990 was about 100,000 tons, and it reached 110,000 tons, about 200,000 tons in 2000. China began to study duplex stainless steel in the 1960s. Including low chromium (Cr18), medium chromium (Cr22) and high chromium (Cr25). The main products are tubes, plates and composite plates. The output is not large. 2000t. The consumption of duplex stainless steel in 2001. It was about 4000t, 1/2 of which was imported with the project.
The structure of duplex stainless steel, according to W(Ni)eq, W(Cr)eq and Schaeffer diagrams. Generally the ratio of austenite (A) and ferrite (F) is about 60%: 40%. But the temperature deviation of chemical composition and solution treatment may occur. A or F≥70%, which will have certain influence on performance, thus, it is best to control each to 50%.
Table 3-2 Mechanical properties of different stainless steels
|Tyep||Steel||Heat treatment||Mechanical properties||Hardness|
Solid solution and fast cooling
Solid solution and fast cooling
Solid solution and fast cooling
Solid solution and fast cooling
Solid solution and fast cooling