The sudden emergence of duplex stainless steel pipe has become one of the major breakthroughs in the stainless steel pipe industry in the past few decades. Duplex stainless steel has good corrosion resistance and high strength, and is the best substitute for traditional stainless steel and structural steel. As the prices of important alloys such as nickel and molybdenum continue to rise, the price-stable duplex stainless steel is gradually favored by users. However, from a technical point of view, a stable price can be considered as an additional incidental advantage given by duplex stainless steel. Low nickel duplex stainless steel LDX2101® (Outokumpu commercial grade, EN1.4162, UNS S32101) has become a rookie in the duplex stainless steel family. LDX2101®’s technical characteristics such as corrosion resistance, weldability and strength are unique, saving resources and low nickel content making it more economically competitive.
Duplex stainless steel was born in the late 1920s. The test records kept by the Outokumpu Avista Research Center date back to December 23, 1930. The first generation of duplex stainless steel developed by Avista Iron Works in 1930 is a typical 25Cr-5Ni, without molybdenum or 1.5% Mo. Among them, 453S (25%Cr-5%Ni-1.5%Mo) is the pioneer of AISI 329. The early duplex stainless steel has a high content of sulphur, which is about 60% to 70% in the solid solution state. And the carbon content is as high as 0.1% at the time. Compared with austenitic stainless steel, the main advantage of duplex stainless steel is its high strength and better resistance to intergranular corrosion. Duplex stainless steel can be used in environments where normal austenitic stainless steels are inevitably subjected to stress corrosion cracking. After the invention of the AOD process in the 1970s, the decarburization rate of stainless steel increased and the addition of nitrogen was more precise. This resulted in a modern low-carbon nitrogen-rich duplex duplex stainless steel. Compared to their predecessors, modern duplex stainless steels have improved pitting resistance and weldability.
The first steel grade of modern duplex stainless steel is 2205, and its mechanical strength and corrosion resistance are significantly higher than 316L and 317L. In many application fields, it fully reflects the better cost performance of 2205, and has built many battles for a long time. Known as Super Duplex Stainless Steel, the 2507 was introduced in the 1980s for the more demanding environments that the 2205 could not meet. Also available in the same period is the 2304 with lower alloy content, but its use is limited. In 2007, the rapid rise in nickel prices caused interest in 2304, and the 2304 with high strength characteristics could replace 316L to meet the requirements of most application environments.
Duplex stainless steel is suitable for pressure vessels, scouring and abrading equipment for the paper industry, cargo tanks for chemical tankers, industrial tanks and bridges. At present, the apparent consumption of global duplex stainless steel has doubled in the level of 70,000 tons in 1997. This paper introduces the basic characteristics and research cases of modern duplex stainless steel to illustrate their characteristics in welded pipes, pipe fittings and Application in accessories.
Basic characteristics of modern duplex stainless steel
1.1 Mechanical properties
High strength is the commonality of duplex steels, which is twice as high as that of ordinary austenitic stainless steels and is significantly higher than that of ferritic stainless steels. The high strength is advantageous for thinning the thickness, thereby reducing the structural weight. The mechanical properties of modern duplex stainless steels are shown in Table 2 in the solution annealed state. The hardness of duplex steels exceeds that of austenitic and ferritic stainless steels. This is what most scouring and wear applications require. In terms of stretch forming. Duplex steel is found between high-form austenitic and ferritic steels. Due to poor plasticity, the geometry of ordinary austenitic steels is generally not achieved, and its high yield strength requires an increase in the strength of the forming.
1.2 Corrosion characteristics
Existing dual phase steels are resistant to chloride pitting and stress corrosion between 304 and 6 Mo austenitic stainless steels. The PRE value is a simple measure of the local corrosion resistance of steel grades. However, this is basically the same as the alignment of the laboratory CPT (Critical Pitting Temperature) test according to ASTM G150. Duplex stainless steel is much better resistant to chloride stress corrosion than austenitic stainless steel 304 and 316.
Laboratory erosion test:
The erosion test is carried out in three different liquids, divided into immersion and no immersion, and finally subjected to wear tests. The samples used were identical. In the 24-hour erosion test (test 1) of the double sample, the test materials were all in passivated and the weight defects were completely polished. However, if the test material is first immersed in a high chloride solution (1000 mg/l) for one week and then polished for 5 hours (Test 2 Environment 2), the passivation film may be partially broken, causing pitting and abrasion. In the 1NH2SO4 solution, the test materials were all melted and corroded, and the wear rate was extremely fast during the 5-hour grinding period (Test 3 Environment 3), and the erosion rate was about 1 mm/year (see Figure 2). In a mildly corrosive environment with a chloride content of 200 mg/l (environment 1), the erosion rate of 316L is higher than that of LDX2101® by 24% to 30%. In the environment with a chloride content of 1000 mg/l (environment 2), the erosion rates of the two steel grades differ by 16% to 18%. At the most corrosive (environment 3), the 316L erosion rate is only 6% higher than the LDX 2101®. Laboratory erosion tests have shown that the mechanical strength of the material has a large impact on overall performance because LDX 2101® has the least weight loss in all test environments.
Duplex stainless steel is gradually replacing 304 and 316L in the paper industry. The alloy additive nitrogen enhances the pitting resistance while increasing the strength of the duplex stainless steel. In addition, higher temperature components made of duplex stainless steel, such as steam boxes, are also resistant to stress corrosion.
1.3 Physical properties
The physical properties of duplex stainless steel and austenitic stainless steel are basically the same, the important difference is the coefficient of thermal expansion (linear expansion), and the duplex stainless steel is lower. In general, a low coefficient of linear expansion is an advantage, especially in a mixture of stainless steel and carbon steel, which reduces the risk of thermal fatigue.
2.1 Process tube design
Some chemical plants have high requirements for pipes that transport chemicals and fluids. This is because corrosive substances go from one process to the next. Pipes and components are designed to use standard parts as much as possible. Specifications and inspection standards are based on the European standard (EN-standard, such as: 13480-3) or American standard (ASME/ANSI-standard).
The chemical plant design is mainly divided into the following parts:
- Factory layout
- Flow chart, including chemical, mechanical and physical processes
- Pipeline assembly drawing
The selection of steel pipes and fittings is often limited by the total purchase price, which is generally determined by the weight and length of the required pipe. Therefore, designers and purchasing managers are keen on new steel grades that are expected to reduce pipeline costs. To reduce the weight, it is necessary to use a thin steel pipe wall, and the benefits are as follows: transportation and installation costs are reduced, and the amount of welding on site is significantly reduced. Lightweight brackets can also save some money.
Whether the wall thickness of the pressure tube can be thinned depends on several factors. The ASME standard is more conservative than the EN standard in wall thickness design. However, in both standards, the economical duplex stainless steel LDX2101® has a wall thickness that is 40% thinner than 304L. The actual design thickness should refer to the ASME comparison table. Also consider the design pressure. For pipes with lower pressure, the reduction in wall thickness is smaller. If the wall thickness of the original design is already thin, it cannot be reduced.
There are other factors to consider when designing the pipeline. Internal pressure is not the only load to be considered in the piping design. The support spacing should also be appropriate. The support spacing depends on:
- Bracket type
- Pipe diameter and wall thickness
- Medium in the tube
- Whether insulation is required
- Whether to absorb shock
Taking into account the above factors, changes in the temperature of the surrounding or in-tube medium will create additional loads, which should be considered in material selection or piping design.
2.2 Hollow steel design
Stainless steel has been used as a decorative material for buildings. Hollow sections for construction, construction and transportation are often carbon steel. Carbon steel structural design is moving in the direction of high strength steel. This is because the design is more economical when the material strength is fully utilized. Carbon steel is prone to corrosion in many applications and requires thorough corrosion inspection and corrosion protection, including planned and unplanned shutdowns or power outages (water, gas) – for maintenance, maintenance and painting. The related costs required for the maintenance of painted carbon steel are expected to increase in the future. The application of duplex stainless steel in building components has attracted a lot of attention.
Duplex stainless steel is the most cost effective in corrosive environments. Compared with 300 series stainless steel, the cost has decreased to varying degrees. Duplex stainless steels have twice the yield strength than 300 series stainless steels. For the same load, thinner duplex stainless steels can be used. Duplex stainless steel is also cheaper to make than other materials because it is easy to machine and weld. The ferrite phase improves the weldability of duplex stainless steels compared to high nickel alloys. This alone saves a lot of soldering time. Using high-speed steel cutting tools, the economical duplex stainless steel LDX 2101® is machined similarly to 316 stainless steel or easier. Moreover, the labor required for duplex stainless steel machining is also less than that of high nickel alloys.
The structural parts can be designed according to the European standard EN1993-1-4:2006. The basis for structural design in EN 1993-1-4 is the limit state analysis. The limit values used are the maximum limit state and the usability limit state. The maximum limit state means that once this limit is exceeded, the component or the entire structure collapses. The usability limit state indicates that if the limit is exceeded, the specified use criteria cannot be achieved. The design specifications specified in the EN 1993-1-4 standard apply to solution annealed materials with yield strength fy = 480 N/mm2 and cold worked materials with strength classes C700 and CP350.
According to the Euro-InoxDesign Manual, after cold working, the austenitic stainless steel has improved yield strength and tensile strength and can be used as the basic material for the strength class CP500. According to the European-Stainless Steel Design Manual, the design strength of the members subjected to the axial force and the members subjected to the axial force + bending force is equal to 0.2% proof strength, and the material temperature is also considered. When the ignition temperature is reached, stainless steel performs differently than other metals, and it can maintain its mechanical properties (mainly elastic modulus and yield strength) at a temperature of 30 minutes in a standard fire. The stability of the yield strength of stainless steel depends on the alloy composition of the material, ie the stainless steel grade selected.
The structural design of stainless steel is not more complicated than carbon steel. The thermal expansion coefficient of duplex stainless steel is the same as that of carbon steel, which is much lower than that of austenitic stainless steel. The fatigue strength of welded joints is as good as that of carbon steel joints. Their structural strength is better than standard austenite, and the bearing capacity of the members is greater.
Cold working can increase the strength value of stainless steel hollow profiles. Cold working refers to the flattening of strips for the production of hollow sections, such as hardened cold rolling, which can be carried out in a pipe mill or in a steel mill. The mechanical properties of the entire batch of hollow tubes are to be recorded in the inspection report.
In the vertical load support, the use of high-strength hollow profiles serves as a load-bearing function and can beautify the building. It is an economical and practical solution.
2.3 Water heater
This is an aluminum heat exchanger with an external stainless steel tube. The principle is to let the heated gas flow through the aluminum heat exchanger, and the gas is added to the water around the heat exchanger. In order to prevent corrosion of the aluminum heat exchanger, the outer surface of the aluminum core is sealed with a cold drawn stainless steel tube.
Users have been using 316 titanium stainless steel early on, and the weight has been reduced by 30% with LDX2101®. Duplex stainless steel with low nickel and molybdenum content allows users to save a large amount of alloy and the material price is relatively stable. Duplex steel is generally better at stress corrosion cracking than 316 stainless steel.
The performance of stainless steel varies from series to series, and changes from austenitic or ferritic series to duplex steels place new demands on production equipment and processes. Duplex steel has a high yield strength and requires more force during forming. When the thickness of the pipe wall is thinned by the high strength characteristic, the influence of the increased forming force is not sufficiently taken care of. The resilience of duplex steel is significantly higher than that of other stainless steel series.
When the duplex stainless steel is welded by the automatic TIG method, the traveling speed of the torch is slower than that of the welded austenitic stainless steel, but the duplex stainless steel is easier to weld than the ferritic stainless steel. 2205 and other duplex steels are commonly used in heat exchanger tubes and enclosures. In heat exchanger applications, the coefficient of thermal expansion of duplex steels is generally lower than that of austenitic stainless steels.
An example of this application is the drain pipe at the Boliden mine in northern Sweden. The water in the mine is taken from the underground 450 meters. Pipe diameter DN600, pressure between 45 and 200 meters underground 45 bar, 200 meters to ground 32 bar. The pipeline is 1200 meters long. However, the applicable temperature range of duplex steel is -40 to 300 °C, which cannot completely replace austenitic stainless steel.
If the most commonly used 304L is compared with the LDX 2101®, the welding factor is 0.7 according to the EN standard. For a pressure of 45 bar, the wall thickness of the 304L tube is 13.4 mm, and the wall thickness of the LDX 2101® tube should be 7.2 mm. The section with a pressure of 32 bar has wall thicknesses of 9.6 mm and 5.1 mm, respectively. When the design pressure is 45 bar, the closest standard specifications are 14 mm and 10 mm, and the design pressure is 32 bar. The standard specifications are 8 mm and 6 mm, respectively. With 304L, the pipe weighs 196 tons and the LDX 2101® weighs 113 tons. This can reduce the use of 83 tons of stainless steel, saving 42.5%. In addition to the direct benefits of saving tubing, there are many aspects that are indirectly beneficial. The length of the weld of 230 ring seams is 437 meters, the welding of 304L steel pipes takes about 2100 hours, and the welding of LDX 2101® takes only 1325 hours because of its The tube wall is thin. Save 775 hours and shorten on-site work time. The weight reduction of the steel pipe can save energy required during the hoisting process, thereby reducing CO2 emissions. The freight rate from the production plant to the site is also reduced. The distance between the two places in the project is 1500km, electric train transportation, CO2 emission reduction 125kg. If transported by truck, CO2 emissions will be 75 times higher. Due to the low amount of LDX2101®, steelmaking emits less CO2 than 304L.
- Cut costs
- Pipe weight reduction
- Reduce welding labor costs
- Saving welding consumables
- Steel pipe lifting energy saving
- Transportation energy saving
- Reduce CO2 emissions (smelting, lifting, etc.)
Considering raw material costs and exchange rates, the total savings are between $300,000 and $500,000.
Duplex stainless steel welded pipes not only replace austenitic and ferritic stainless steels, but also other metals such as anti-corrosive carbon steel and aluminum, and even polymers. Application examples cover a wide range of areas: construction, mine drainage and domestic water heaters. Global economic development places higher demands on energy and transportation. More families need modern home facilities, such as home hot water centers, and this demand will continue to rise in the future. 2205 is one of the grades of duplex stainless steel series. The corrosion resistance of this series of steels is strong and weak. Duplex stainless steel is gradually being accepted. In applications where the 316L is unsatisfactory, more and more users are choosing duplex stainless steel because it is suitable for a wider range of corrosive environments.
Due to the light weight, high strength and good corrosion resistance of duplex stainless steel, the application items are increasing. It cannot be ignored that the high nickel price in 2006-2007 has increased the status of duplex stainless steel, and the increase in sales of duplex stainless steel includes the replacement of austenitic stainless steel for purely price reasons. The comprehensive performance of duplex stainless steel is perfectly suited for many existing and newly developed applications. In terms of corrosion resistance, the grade of duplex stainless steel is more complete than a decade ago. It is certain that duplex stainless steel will continue to grow in the past decade.
Source: China Duplex Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)