1 Types of stainless steel
According to the chemical composition, stainless steel can be divided into Cr stainless steel, Cr-Ni stainless steel and Cr-Ni-Mo stainless steel. According to the application field, it can be divided into medical stainless steel, atmospheric corrosion resistant stainless steel, anti-oxidation stainless steel, and Cl-resistant stainless steel. However, the most commonly used classifications are classified according to the organization of steel. They are generally classified into ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, duplex stainless steel and precipitation hardened stainless steel. In petroleum and petrochemical applications, austenitic stainless steel, ferritic stainless steel and duplex stainless steel account for a large proportion.
Ferritic stainless steel generally has a Cr content of between 13% and 30%, and a C content of generally less than 0.25%. After annealing or aging, carbides are precipitated at the ferrite grain boundaries to achieve corrosion resistance. In general, ferritic stainless steels have lower corrosion resistance than austenitic stainless steels and duplex steels, but higher than martensitic stainless steels. However, due to its low production cost compared to other stainless steels, it is widely used in chemical and petrochemical applications for areas where corrosion resistant media and strength are not critical. For example, it is widely used in the environment of sulfur-containing petroleum, hydrogen sulfide, room temperature nitric acid, carbonic acid, hydrogen ammonia mother liquor, high-temperature ammonia water produced by urea, urea mother liquor, and vinyl acetate and acrylonitrile produced by vitamin nylon.
Martensitic stainless steel generally has a Cr content of between 13% and 17%, a high C content of between 0.1% and 0.7%, and a high strength, hardness and wear resistance, but low corrosion resistance. It is mainly used in petroleum and petrochemical applications in environments where corrosive media is not strong, such as components requiring higher toughness and impact loads, such as turbine blades, bolts and other related components.
Austenitic stainless steel Cr content is between 17%-20%, Ni content is between 8%-16%, C content is generally less than 0.12%, mainly by adding Ni element to expand the austenite transformation zone, thus at room temperature The austenite structure is obtained. Austenitic stainless steel is superior to other stainless steels in corrosion resistance, plasticity, toughness, processing properties, weldability, and low temperature performance. Therefore, it is the most widely used in various fields, and its usage accounts for about all stainless steel. About 70% of the amount. In the petroleum and petrochemical field, austenitic stainless steels have great advantages in highly corrosive media and low-temperature media, such as internal components with high corrosion resistance, especially in intergranular corrosion resistant environments, such as heat exchangers/pipe fittings. Low-temperature liquefied natural gas (LNG) conveying pipeline, urea, sulfur ammonia and other production containers, flue gas dedusting and desulfurization devices.
Duplex stainless steel is developed on the basis of single-phase stainless steel, and its Ni content is generally about half of the austenitic stainless steel Ni content, which reduces the alloy cost. Austenitic stainless steel has excellent corrosion resistance and high comprehensive performance. It solves the disadvantages of weak corrosion resistance of ferrite and martensitic stainless steel and insufficient strength and wear resistance of austenitic stainless steel. In the petroleum and petrochemical field, it is mainly used in marine oil platforms resistant to seawater corrosion, acidic components and equipment, especially in pitting resistant components.
Precipitation-strengthened stainless steel mainly obtains high-strength properties through the precipitation strengthening mechanism. At the same time, it sacrifices its own corrosion resistance. Therefore, it is rarely used in corrosive media, and is generally used in petrochemical machinery mining and other industries.
2 stainless steel pipe in petroleum and petrochemical applications
The petroleum and petrochemical industry is the pillar industry of the national economy, and it plays a pivotal role in the national economy. In the past 20 years, stainless steel pipes have been greatly improved in terms of production technology, whether they are seamless pipes or welded pipes. The stainless steel pipes produced by some domestic manufacturers have reached the level that can completely replace imported products, and the localization of steel pipes has been realized.
In petroleum and petrochemical, stainless steel pipes are mainly used in pipeline transportation systems, including high pressure furnace tubes, piping, petroleum cracking tubes, fluid conveying tubes, and heat exchange tubes. Stainless steel is required to perform well in wet and acid service conditions.
2.1 Application of Stainless Steel Seamless Pipe for Large Diameter Thick Wall High Pressure Hydrogen
In order to adapt to the processing of low-quality crude oil and meet the requirements of environmental protection, domestic refining and chemical enterprises continue to optimize the processing structure of refinery units and adjust the product structure. Among them, high-pressure hydrogenation units such as hydrocracking and hydrotreating have developed rapidly in recent years, and the processing capacity of the equipment It is also constantly improving. The main features of the hydrogen pipeline are large diameter and thick wall.
For the selection of high-pressure hydrogen materials, due to its high temperature and high pressure conditions, TP321/H, TP347/H, etc. are generally used as materials for high-pressure hydrogen pipes at home and abroad. The above two stainless steel materials are stabilized by the addition of Ti and Nb. The element has high high temperature corrosion resistance and high temperature mechanical properties.
At present, domestic large-diameter thick-walled hydrogen pipelines are mainly manufactured by hot perforation + cold rolling / cold drawing. The processing process is shown in Figure 1.
The pipe made by hot perforation + cold rolling / cold drawing has the characteristics of good surface, high dimensional accuracy and uniform wall structure compared with steel pipes manufactured by other methods.
For the high-pressure hydrogen-producing steel pipe, due to the particularity of the working medium, the requirements for the raw material of the steel pipe are high. Therefore, the requirements of the design institute for the high-pressure hydrogen pipeline are: S≤0.015%, P≤0.030%, non-metallic inclusion A Class B, C, and D are not higher than 1.5. Ultrasonic inspection is required for the finished pipe, and the artificial contrast defect is not more than 5% of the nominal wall thickness of the pipe.
2.2 Application of stainless steel welded steel pipe for low temperature LNG
Due to the development of society, people’s awareness of environmental protection has increased, and clean energy has received more and more attention. LNG is a clean and efficient energy source that plays an important role in national production and life. Therefore, LNG receiving stations and LNG carriers have sprung up. LNG is to cool gaseous natural gas to -162 ° C under normal pressure to cause it to condense into a liquid. Therefore, the piping for LNG transportation must have high low temperature performance.
For low-temperature LNG pipes, most of them are made of ultra-pure, low-carbon, low-sulfur, low-phosphorus stainless steel at home and abroad. In recent years, double-grade stainless steel has been very popular among LNG users, especially TP304/304L, TP316/316L and other applications. Shuangzheng Steel not only has L-grade corrosion resistance and low temperature performance, but also has high mechanical properties.
At present, the mainstream low-temperature LNG welded stainless steel pipes at home and abroad are generally processed by automatic unit welding forming process, UOE forming process and JCO forming process. The main processes are shown in Figure 2 and Figure 3, respectively.
The automatic welding unit is a fast, efficient and automatic method of producing welded pipes in the case of not thick wall. At present, most of the plates are formed by the rollers, and then welded, heat treated, etc., and some welding units are also It integrates advanced technology such as on-line ultrasonic welding, on-line welding ultrasonic, and automatic seam tracking technology to provide high-efficiency manufacturing operations for LNG long-distance pipeline manufacturing and reduce the manufacturer’s production period.
UOE forming technology is currently the most widely used, most mature and most recognized quality low-temperature LNG welded pipe production process in the world, and its main process technology has been finalized.
The JCO molding process is a new molding process in recent years. This molding technology is an organic combination of stepping pre-bending and tube CNC bending.
For the LNG stainless steel welded pipe, since the use environment is in a low temperature environment of -162 ° C, it is necessary to have a high low temperature impact property for the LNG pipe. At present, most design institutes, research institutes, and manufacturers require that the low-temperature impact performance of LNG pipes is not less than 80J, and the lateral expansion amount is not less than 0.38mm according to ASME B 31.3.
For stainless steel welded pipes, the weld is used as the weak link of the pipe. The quality of the weld directly affects the safety of the pipeline and even the pipeline. The welding coefficient is one of the important factors to evaluate the quality of the weld. For the low temperature LNG welded pipe, the welding coefficient is Ej=1.0, and the welded joint must be a full welded joint. After the welded joint is completed, all welds must be 100% ray-tested. The weld must be free of defects such as incomplete penetration, no weld inclusions, no undercuts, and no cracks to ensure the stability of the welded joint at low temperatures.
Oil pipelines are the bulk of stainless steel pipes. Stainless steel pipes play an important role in equipment manufacturing, oil recovery, oil refining and transportation in the petroleum industry. In recent years, the country has increased the development of petroleum resources. At the same time, as the world’s largest net oil importing country, with the increasing demand for petroleum, the petroleum-related industries will further develop and the demand for stainless steel pipes will continue. increase. In 2017, China’s steel industry has shown signs of recovery. Domestic leading stainless steel pipe companies have increased their cooperation with PetroChina and Sinopec to increase their market share. At the same time, domestic steel pipe enterprises have also carried out activities with foreign oil companies to push China’s stainless steel pipe manufacturing to the world platform.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)