Brief introduction of four major pipeline materials for thermal power generating units

(HY-industry technical centre)

In order to comprehensively improve the overall quality of professional talents, the company has recently played a leading role in the company’s professional leaders, and regularly organizes technical training for professionals every Friday. Considering that our department employees are often on a business trip and cannot attend technical training on site, we will apply for lecture materials as much as possible to relevant experts to facilitate learning. This week’s training was led by the chief expert Ferguson Lu, which introduced the four major pipeline materials for thermal power plants.

Overview

1, four major pipelines

  • The high-pressure water supply pipeline system of the thermal power plant, the main steam pipeline system, the reheating section steam pipeline system, and the reheating section steam pipeline system are referred to as the four major pipelines. The use case is shown in the following diagram. The feed water pump sends the boiler water to the economizer, the water wall and the superheater to the temperature and pressure parameters of the main steam through the high pressure water supply pipe system, and sends the high pressure cylinder to the high pressure cylinder of the steam turbine through the main steam pipeline system. After the cylinder is completed, the temperature and pressure of the discharged steam are significantly reduced. The reheating section is sent back to the boiler reheater for reheating to the reheat steam temperature pressure parameter, and the hot steam passes through the reheating section. The system sends the intermediate pressure cylinder of the steam turbine to continue work.

2, unit type

Various types of unit piping parameters and unit thermal efficiency values
Unit type Main steam(Mpa/℃) High pressure water supply(Mpa/℃) Reheat cold section(Mpa/℃) Reheat hot section(Mpa/℃) Thermal efficiency(%)
Medium pressure boiler unit 3.5/435 7.46/175 / / 27
High pressure unit 9.8/510 17.15/230 / / 33
Ultra high pressure unit 13.73/540 22.56/240 4/350 4/540 35
Subcritical unit 17.36/540 28.22/270 4/350 4/540 38-42
Supercritical unit 25.4/560 33-38/280 5/375 5/560 41-44
Ultra-supercritical unit 27/600 36-50/290 6/400 6/600 44-48
700°C advanced ultra-supercritical unit 31/700 /300 7/450 7/700 46-51
  • For the same type of unit, the above parameter values ​​are different for each unit, indicating only the trend.

3,Main factors affecting the selection of pipeline materials

  • The main influencing factors of pipe materials are material yield strength, tensile strength, creep strength/lasting strength, steam oxidation resistance, smoke resistance, linear expansion coefficient, and machinability (such as welding, hot and cold processing) at design temperature. Forming), price, etc.

  • In general, the yield strength and tensile strength of steel decrease with increasing temperature. For high temperature use conditions, the metal material will also undergo creep rupture under stress less than the yield strength. According to ASME B31.1, When carbon steel is above 427 °C, it will produce obvious creep phenomenon. Adding alloying elements Cr, Mo, V, W and trace elements Ti, B, Nb, N, etc., can improve the creep resistance of steel, but improve The alloying element content will deteriorate the processing properties of the steel, such as welding, bending, and the like.

  • The design allowable stress of the steel used below the creep temperature is based on the yield strength and tensile strength;

  • The design allowable stress of the steel used above the creep temperature is based on creep strength or permanent strength;

  • The coefficient of linear expansion is an important index considering the thermal expansion and contraction performance of pipeline materials. It is hoped that the pipeline material has a small coefficient of linear expansion to reduce the thermal stress caused by thermal expansion and contraction of the pipeline.

  • At the same time, it is hoped that the pipeline has a higher allowable stress, so as to reduce the weight of the pipeline and facilitate the arrangement of the pipeline;

  • The choice of piping materials also needs to consider economics.

4,Evolution of four major pipeline materials

  • Main steam and reheat section (including high pressure bypass and low pressure bypass inlet piping)

For the medium-pressure unit and the following main steam pipelines, the design temperature does not exceed 450 °C, the main selection of materials is 20, 20G, domestic materials can meet the requirements.

For high-pressure units, ultra-high pressure units, sub-critical units, the design temperature of the main steam and reheating section is increased to 540 °C, and the imported materials are A335P22 (US grade, European grade 10CrMo9-10) as the preferred material, and domestically produced materials are 12Cr1MoV. As the material of choice, the introduction of the unit in Eastern Europe also selects 15Cr1Mo1V, and the Alstom unit selects X20CrMoV121. Later, due to the A335P91 (US grade, European grade X10CrMoVNb9-1) steel has been more and more recognized, the main steam pipeline mostly uses P91 material, after replacing the P22 with P91, the main steam pipeline wall thickness is reduced from 85mm to 40mm, due to reheat The heat section pressure is low, and the comprehensive benefit of using P91 instead of P22 is not obvious. Only the reheating section of individual power plants uses P91, such as Shizuishan Power Station.

For supercritical units, the temperature of the main steam and reheat section is increased to 560 ° C, and the material is basically A335P91. Individual power plants also have a design temperature of 540 ° C, and the material still uses A335P22.

For ultra-supercritical units, the temperature of the main steam and reheating section is increased to 600 °C. Starting from Huaneng Zhejiang Yuhuan Project, the materials are all selected A335P92 (US grade, European grade X10CrWMoVNb9-2). In Europe, E911 is used, and Japan prefers to use P122. These steels have their own advantages and are relatively new materials. They are still being researched, explored and improved in use. At present, the temperature of the reheating section of the ultra-supercritical unit is increased to 610 °C, and it is also increased to 620 °C, such as the Tianji Phase II power station. At present, the Binhai Phase II power station is trying to increase to 630 °C, because there are no other materials that are more mature than P92. It is expected that the material will still use A335P92.

For 700°C advanced ultra-supercritical units, many organizations in Europe, Japan, the United States and China are currently researching and discussing, and the current research will be reported in the third part.

  • High pressure water supply pipeline

For subcritical and below units, the design temperature of high pressure feed water does not exceed 300 °C. The materials used are mainly carbon steel, domestic materials are 20, 20G, and imported materials are ST45.8/III (German grade). In the later stage of unit construction, 15NiCuMoNb5-6-4 (European grade, also referred to as WB36, American grade A335P36) was promoted. Because its strength is much higher than ST45.8/III, the wall thickness of the main pipeline of 300MW subcritical unit high pressure water supply is 55mm. Dropped to 25mm. Due to its high strength and good weldability, the material is currently the main material of choice for high-pressure water supply pipes for supercritical and ultra-supercritical units.

According to the standard, when the pipe wall thickness exceeds 3 inches (76.2 mm), the design allowable stress will be reduced to some extent.

  • Reheating section pipe (including high pressure bypass outlet pipe)

For supercritical units and the following reheating section steam pipes, the design temperature does not exceed 400 °C, mainly carbon steel materials, small units generally use domestic 20, 20G or ST45.8/III or A106B (US brand) no The seam tube has a thin wall thickness due to low pressure. For pipes with larger diameters (558.8mm and above), the cost of using seamless pipes is too high, so the electrofusion welded pipe A672B70CL32 (US grade, strength equivalent to A106C, slightly higher than A106B) is commonly used.

For ultra-supercritical units, the design temperature of the reheating section of some units is increased to 400 ° C (also required to be 415 ° C, 427 ° C), and A691 11⁄4 Cr CL22 material (or other tubes of comparable performance) is selected. Meet the requirements of creep resistance.

5,Division of the four major pipeline procurement bidding sections

  • The main tenders included in the bidding for the four major pipeline systems include four major pipelines (straight pipe materials), high-temperature and high-pressure pipe fittings (three-way, elbows, large and small heads, etc.), factory piping, and pipe hangers.

  • Among them, the four pipelines (straight pipe material) are divided into three sections (purchasing unit):

  • Inner diameter tube and large diameter outer diameter seamless tube, mainly main steam and reheating heat section are generally designed according to inner diameter tube. High pressure water supply pipeline is designed according to outer diameter seamless tube. Its main diameter is large, and it can be produced and supplied at home and abroad. There are not many manufacturers of pipelines, so they are purchased separately as a tender.

  • Small and medium-diameter outer diameter seamless pipe, generally 426mm in diameter and below, mainly used for the branch pipe of high-pressure water supply pipe, relative to the inner diameter pipe of the upper standard section and the large-diameter outer diameter seamless pipe, the manufacturer has more, and can be used as a separate purchase. One bid for purchase.

  • The electric fusion welded pipe is mainly used for the reheating and cooling section piping system. It is generally produced by a special manufacturer of electrofusion welded pipes, and also needs to be purchased separately as a standard section.

  • According to the type of unit, high-pressure pipe fittings can be divided into one bidding section and can also be divided into two bidding sections. Due to the difficulty in processing the main steam and reheating section pipe fittings of supercritical or ultra-supercritical units, some owners even import them. Therefore, the high-pressure pipe fittings for the main steam and reheating section piping systems of supercritical or ultra-supercritical units are purchased separately as a tender. The pipe fittings of the reheating section and the high-pressure water supply pipe system are separately packaged as a bidding section.

  • The work of the industrial piping is to redesign the piping system according to the layout of the design institute and the actual size of the pipelines and fittings, and complete the processing of the groove joints of all the pipeline joints in the factory, and as far as possible under the conditions of transportation, Pipes and fittings are combined and welded at the factory, sometimes with bends, to improve the quality of the four pipe installations and the reliability of the pipe operation. In general, the factory piping work of a project is subcontracted as a tender.

  • For pipe hangers and hangers, including spring hangers, rigid hangers, root beams, dampers, etc., they are generally purchased as a tender.

Discussion on the materials of WB36, P91 and P92 for supercritical and ultra-supercritical units

Introduction

  • The main steam and reheat piping system of the supercritical unit adopts SA335 P91 (referred to as P91) steel pipeline, and the high pressure water supply pipeline system adopts 15NiCuMoNb5-6-4 (referred to as WB36) steel pipeline. Before the supercritical unit, these two materials are already in the 300MW class units are even more commonly used in lower-class units, such as Huaneng Yangliuqing Power Plant, Shaanxi Xigu Thermal Power Plant, Chengdu Jialing Thermal Power Plant, etc. The application of supercritical units to SA335 P91 and WB36 steel is only a continuation of the application of these two materials in power plants.

  • For the main steam and reheating section of the super-supercritical unit (including bypass inlet), most of the new material SA335 P92 (P92) is used. The piping materials of other systems are almost unchanged. Now there are a few power plants. The design temperature of the main steam line is reduced to 590 ° C, and P91 is also used as the material of the main steam piping system.

  • Since the introduction and use of such new materials around 1996, people have become more and more aware of these new materials, and the technical management of the application process of these new materials has become more and more standardized. Many research institutes and manufacturing companies have carried out relatively comprehensive and systematic research from the welding performance research of these new materials, to these new material pipe fittings, pipe bending processing, etc., and have achieved a lot of research results. Now let me talk about my experience.

WB36

1,About chemical components

  • WB36 steel is a kind of copper-nickel-molybdenum alloy steel. Copper can improve the strength after tempering. However, copper alloy steel has the characteristics of large brittleness. The addition of nickel with half of copper content can avoid this problem, and it can further increase the strength. After a small amount of molybdenum, it can effectively slow down the formation of grain boundaries and precipitate ferrite before cooling. Compared with other materials of the same purpose, such as SA106, WB36 has basically the same restrictions on the formation of elements P, S, etc. of low melting point impurities. Since the material is less sensitive to trace elements and low melting point impurity forming elements, the ingot source becomes Relatively easy, this provides a good precondition for the localization of the material.

2,About heat treatment

  • According to the data, the critical heating temperature of the austenitizing of WB36 steel is Ac1≈725°C, and the critical heating temperature of complete austenitization is Ac3≈870°C; the recommended heat treatment process is normalizing (quenching)+tempering, ie: positive Fire (quenching): 900-980 ° C, the normalizing heat preservation time is determined according to the wall thickness; tempering: 610-680 ° C, tempering holding time ≥ 30 min.

  • Under normal circumstances, the structure of WB36 steel is ferrite + bainite, and the content of bainite is about 40-60%. According to the typical CCT curve of WB36 steel, if it is cooled from austenitizing temperature to about 600 °C. When the duration is no more than 3 minutes, 100% bainite structure can be obtained; if it is cooled from the austenitizing temperature to about 600 °C for a duration of not more than 1 hour, no more than 50% ferrite can be obtained. Body content of ferrite + bainite structure.

  • In order to obtain a larger proportion of bainite structure, it is necessary to increase the cooling rate as much as possible. The wall thickness of the pipe, the amount of furnace during heat treatment, and the method of furnace loading determine the cooling rate of the pipe (pipe fitting). When the wall thickness is large, water quenching is required to increase the cooling rate of the pipe (pipe fitting). For each manufacturing enterprise, the heat treatment conditions are not consistent. Now most of the enterprises’ heat treatment process evaluation tests are mainly limited to the process verification test of heating temperature and holding time. It is recommended that the pipe manufacturing enterprises also carry out the cooling mode test to achieve the ideal organization. And mechanical properties. The upper and lower limits of the normalizing (quenching) heat treatment should be combined with the upper and lower temperatures of the tempering heat treatment. How the combination should be obtained by experiment.

P92

(1) About chemical components

  • P92 steel is based on the P91 steel to reduce the Mo content, while adding W element, etc., through improvement, P92 steel has a higher creep rupture strength, can withstand higher operating temperatures.

  • P92 steel, like P91 steel, has stricter control of C, P, S, Al, etc. than ordinary P22 heat-resistant steel, and adds trace amounts of N and Nb elements. The smelting of steel ingots is the key to producing quality. The stable and reliable P92 steel ingot is a prerequisite for the localization of P92 steel pipe.

(2) About heat treatment

  • The critical heating temperature of austenitization of P92 steel is Ac1=800-835°C, and the critical heating temperature of complete austenitization is Ac3=900-920°C; the recommended heat treatment process is normalizing (quenching)+tempering, namely: Normalizing (quenching): 1040-1080 ° C, tempering: 730-780 ° C.

  • According to the CCT curve of P92 steel, the metallographic structure under normal normalizing state should be quenched martensite or quenched martensite + small amount of carbide, only when the cooling rate is relatively slow (from normalizing temperature to 600 °C) At least 4 hours is required to obtain the structure of ferrite + carbide + quenched martensite.

Current research and development of 700 ° C advanced ultra-supercritical unit materials

1 Introduction

  • Increasing steam temperature and pressure parameters is an effective way to improve the thermal efficiency of thermal power generating units. Ultra-supercritical units are based on the technology upgrade of conventional supercritical power generation. Compared with other power generation technologies, their technology inheritance and feasibility are higher. Since the 1990s, it has developed rapidly.

  • As early as 1994, the ultra-supercritical power generation technology with the initial steam temperature of 580-600 °C was still under development. At the same time, the Danish ELSAM Power Company proposed a research proposal, that is, whether the steam temperature of the unit can be developed through moderate materials. Further increase to 700 ° C, the pressure is increased to 35-37.5 MPa, so that the unit efficiency is increased to 50%. This research proposal directly led to the launch of the 1998 European AD700 (Advanced Ultra-Supercritical Power Generation Program) project. The project was completed in 2010. It has carried out the feasibility study of 700 °C grade power generation technology and material basic performance, material verification and preliminary design, component verification, etc., but the demonstration unit originally planned to start construction in 2010 was postponed. In addition to the AD700, with the development of research work, a large number of 700 °C power generation technical support projects have been launched within the EU, including MARCKO, COORETEC-TD1, COMTES700, COMTES700 TurBINE Valve, GKM HWT 725I, NRWPP700, 725HWT GKMII, ENCIO. NextGenPower, MACPLUS, etc. These projects are funded by the European Union, EU industry alliances, national, local governments, enterprises, etc.

  • While the 700°C plan in Europe has achieved significant technological breakthroughs, the Vision21 and Future Gen plans previously proposed by the United States have been slow and have been difficult to achieve the expected goals for a long time, in order to avoid losing competitive advantage in the field of coal-fired power generation. The US Department of Energy adjusted its target. In 2001 and 2004, it launched a research project on ultra-supercritical boiler materials and steam turbine materials with a steam parameter of 38.5 MPa/760 °C, and carried out key technical research in this field.

  • Japan began the feasibility study of “700°C ultra-supercritical power generation technology” as early as 2000, but the conclusions based on the technology and market at that time did not support the development of 700°C power generation technology. With the progress of research in Europe and the United States, in 2006, the Japan Energy Integrated Engineering Research Institute conducted a case study on the transformation of old units with 700-degree A-USC technology. In August 2008, Japan officially launched the “Advanced Ultra Supercritical Pressure Power Generation (A-USC)” project, which aims to eventually achieve a steam temperature of over 700 °C. In Japan, although the A-USC program at the national level was launched late, Japanese companies have already begun to develop key technologies, especially new materials, and therefore have a good foundation.

  • The service temperature of many high-temperature components of 700°C advanced ultra-supercritical generator sets has far exceeded the resistance of existing heat-resistant steel for power plants, and nickel-based or iron-nickel-based superalloys with better heat-resistant steel properties must be used. Although superalloys have extensive application experience in aerospace, petrochemical, industrial gas turbines and other industries, thermal power plant components have different service conditions, decades of design life requirements and much larger than aero engines. The size makes the application of nickel-based alloy completely enter a new field, the original experience can not be fully transplanted, how to choose the right material for the high-temperature parts of the unit becomes the most critical technology. In the research and development plan of 700°C advanced ultra-supercritical power generation technology in foreign countries, the material development, performance test, process test and other materials research are the most important contents.

  1. Status of research on 700 °C unit materials

  • In the material research of the 700 °C unit, it mainly focuses on three aspects: material development and material modification, material performance test and process test, and on-site verification of materials and components.

  • Development and modification of materials:

  • In the 700°C R&D plan of each country, there are three main sources of materials:

  • Mature materials: These materials have been used in other industries, such as Inconel 617, 625, 718, 263, Haynes 282, Nimonic 105, Waspaloy, Udimet 720Li and other nickel-based alloys. However, due to the lack of experience in the application of thermal power plants, it is necessary to conduct targeted research on material properties and manufacturing processes.

  • Modified materials: The original mature material performance is not ideal, and the composition is re-adjusted. In addition to Inconel 617, in order to further increase the high temperature strength, the adjusted CCA617, Inconel 617B (617 mod), TOSIX, and 617B OCC are performed.

  • Newly developed materials: Materials developed for different component requirements of 700°C units, such as Inconel740 and its modified Inconel740H, Sancro 25, HR6W, LTS700, USC141, FENIX-700, etc.

  • In addition to the development of alloy materials, in the United States, due to the fact that boilers may be burned with abundant domestic high-sulfur coals, the development of boiler materials has placed greater emphasis on the development of coating materials to prevent high-temperature sulfur corrosion of boiler tubes.

3, foreign 700 ° C unit selection plan

  • Europe, the United States and Japan have proposed their 700 ° C unit selection program.

4. Research status of China’s 700 °C power generation

  • Since 2005, China has begun to pay attention to the international 700°C power generation technology research and development. Xi’an Thermal Engineering Institute started the 700°C unit key material pre-research project in 2008, which is the key to the European and US 700°C plans. Nickel-based alloys, including Inconel 740, Nimonic 263, Inconel 617, Inconel 625, Inconel718 and other six materials have been fundamentally studied, including microstructure and mechanical properties, 1000 hours of high temperature aging and tissue stability studies, etc. The research and development and production capacity of the alloy were fully investigated. The necessity and feasibility of the domestic organization of the 700 °C thermal power unit project were analyzed and demonstrated. In 2011, the high temperature steam oxidation test of the above alloy was carried out for 1000 hours.

  • China’s three major host equipment manufacturers (Shanghai, Dongfang, Harbin) also tracked the 700°C advanced ultra-supercritical technology development plan of foreign countries, especially the EU, through various channels. The Research Institute of Iron and Steel, the Institute of Metal Research of the Chinese Academy of Sciences and the Special Steel Plant cooperated in the development of some materials. The GH2984 alloy independently developed by the Metal Research Institute has long-term experience in ship boilers. At present, further composition optimization is carried out according to the requirements of the 700 °C thermal power unit. The G110Fe-Ni based alloy developed by the Iron and Steel Research Institute has a permanent strength of 100 MPa at 700 °C for 100,000 hours. The Beijing University of Science and Technology and the Shanghai Power Generation Equipment Institute have also been conducting research at 700 °C and have achieved certain results.

  • In June 2010, the China Energy Bureau organized the “National 700°C Ultra-Supercritical Coal-fired Power Generation Technology Innovation Alliance”, and planned to integrate domestic research and production forces to research and develop 700°C ultra-supercritical coal-fired power generation technology. The Ministry of Science and Technology also included the “Key Technology Research of 700°C Ultra-Supercritical Power Generation” in the 12th Five-Year Research Project. The current situation of the project is subject to further investigation.

  1. Suggestions on the research of China’s 700 °C unit selection

China should start the development of 700°C ultra-supercritical coal-fired power generation technology. The research on materials and applications should bear the brunt. To study materials, we must first determine the candidate materials for China’s future 700°C unit. In view of the research status at home and abroad, the selection of Chinese units is selected. Suggestions are as follows (Recommended by Xi’an Thermal Engineering Institute):

(1) Self-selection based on foreign experience

  • The research on China’s 700°C unit started late, after years of research in Europe, the United States and Japan, and proposed and partially disclosed its material selection plan, which can greatly reduce the scope of material selection, but this does not mean that China is On the road to develop 700 °C power generation technology, the existing material selection plan can be completely copied.

  • First of all, by comparing and analyzing the selection schemes of Europe, the United States and Japan, it can be found that even if there are different choices for the same components, the difference comes from different considerations of the unit design, operating conditions, and domestic material production basis. In order to shorten the study time, save manpower and material resources, it is impossible for us to include all the materials in our candidate area for research. Secondly, the foreign material screening test is not perfect. The design of the material screening test program that has been published is based on the experience of heat-resistant steel. It does not fully consider the material properties of the nickel-based alloy and the operating environment of the power plant. A series of problems that may arise. For example, according to the research results of Xi’an Thermal Engineering Institute, some nickel-based alloys are very brittle when operating at around 700 °C. These materials used in power plant thick-walled parts will bring serious safety problems, but these materials appear in foreign rotors. , cylinders and other components within the selection range. In addition to high temperature mechanical properties and resistance to typical high temperature steam and flue gas corrosive atmospheres, the various environmental and media conditions that may occur during the various stages of manufacturing, installation, operation, and maintenance of the unit are nickel-based alloys. The impact should be taken seriously.

  • Since it is impossible to invest enough funds in the short-term to conduct comprehensive experimental research on various performance indicators of materials, China’s 700 °C material screening can make appropriate use of foreign countries in terms of long-term strength, steam oxidation corrosion, high-temperature flue gas corrosion, etc. Work, on the basis of which a set of screening test schemes is proposed, which selectively and reproducibly complements the performance basic data such as the test endurance strength, and the emphasis should be placed on the series of possible coupling of the material properties of the nickel-based alloy with the operating environment of the power plant. The problem.

(2) Make full use of domestic material development and production capacity

  • There is a complete nickel-based alloy development and production system in China. In particular, it has its own advantages in the development of nickel-based alloys. China’s 700°C ultra-supercritical research program should rely on its own material research and development system to maximize the research of materials with independent intellectual property rights, such as high-temperature components such as GH2984. Applications. However, it is undeniable that the research foundation of China’s nickel-based superalloys is still far from the European and American days. It is unlikely that all sources of materials will be completely solved in the short term, especially for large castings and forgings. If necessary, rely on the international market. For these materials and components, localization can be gradually realized when the capabilities are available.

(3)  To reflect the development direction of Chinese units

  • Each country’s thermal power generation technology has its development characteristics and direction. In the United States, the 760 °C unit was developed with the characteristics of high-sulfur coal in the country, and considered the technical problems that may arise in the future when combined with oxy-combustion technology. . In Europe, we are considering future research on blending biomass in the 700 °C group. Japan’s A-USC program considers the technical solution for retrofitting old units with 700 °C unit technology. China’s 700°C plan should also be consistent with the overall future direction of China’s entire thermal power generation industry.

(4) Strengthen the construction of the test platform

  • The screening of materials is carried out throughout the 700 °C program. On-site verification of materials and components is an important part of the material screening process and an important manifestation of the success of material screening. However, in the past, the test bench like COMTES700 has never been established in China. The establishment of high-temperature component verification test bench is an important measure to improve the research and development capability of China’s 700 °C power generation technology. Huaneng and Thermal Engineering Institute are focusing on improving the overall technology of China’s high-temperature materials R & D and production, power generation equipment manufacturing, thermal power generation and other industries. The improvement of the level has already considered and planned the construction of two 700°C high temperature component test platforms of different scales as early as 2012. The current situation needs to be tracked and investigated.

(5) Mutually beneficial cooperation

  • It is possible to consider appropriate cooperation with relevant foreign manufacturers as potential users to obtain relevant progress information and data in a timely manner. Avoid less detours.

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