Development of duplex stainless steel in China
Based on the development history of duplex stainless steel in the world, the development of duplex stainless steel in research, production, application and standards in China in recent years is described. The investigation shows that in recent years, the output of duplex stainless steel in China has increased considerably, and its steel composition has gradually evolved into a structure with 2205 (SAF 2205, also known as Sandvik SAF 2205, is a Sandvik-owned trademark for a 22Cr duplex (ferritic-austenitic) stainless steel.) as the main component and multi-steel series as the cooperative development. Especially after 2019, the economical duplex stainless steel and super duplex stainless steel have developed rapidly.
In recent years, the application of duplex stainless steel in China has not only been further expanded in the traditional petrochemical industry, but also in the fields of oil and gas transportation, chemical ship manufacturing, nuclear power, construction and so on. Duplex stainless steel in China is used not only in domestic projects, but also in the Middle East, Eastern Europe and other regions because of its continuous improvement in output and quality.
- Development of Duplex Stainless Steel
- Research Progress of Duplex Stainless Steel in China in Recent Years
- Products industry and construction industry
- Development of Duplex Stainless Steel Standards in China
- The future development direction of duplex stainless steel in China
Since France obtained its first patent in 1935, the development of duplex stainless steel has gone through three generations in the twentieth century. The first generation of duplex stainless steel is represented by 329 steel developed in the 1940s in the United States. It contains high chromium and molybdenum. It has good local corrosion resistance, but has high carbon content (<0.1% C). The 3RE60 steel developed in Sweden in the mid-1960s is already an ultra-low carbon duplex stainless steel. Since the 1970s, the development of secondary refining technology and the discovery of the important role of nitrogen in maintaining phase equilibrium and improving corrosion resistance have become an important milestone of duplex stainless steel. The second generation duplex stainless steel with ultra-low carbon and N content has been developed, which represents 2205 grades of steel. On this basis, by further increasing the alloy content and PREN value, the third generation duplex stainless steel, super duplex stainless steel, was developed in the late 1980s. Its PREN value is more than 40, and its representative brands are SAF2507 (SAF 2507, also known as Sandvik SAF 2507, is a Sandvik-owned trademark for a 25Cr duplex (ferritic-austenitic) stainless steel.), UR52N+, ZERON100, etc. The higher content of chromium, nickel, molybdenum and nitrogen alloy elements in this kind of steel, the better balance between ferrite and austenite. It has better corrosion resistance and higher strength.
After entering the 21st century, super duplex stainless steel and economical duplex stainless steel have become two important development directions of duplex stainless steel. Super duplex stainless steel has higher alloy elements, higher strength and better corrosion resistance. Economical duplex stainless steel has the characteristics of low nickel content, no molybdenum or only a small amount of molybdenum. Low cost makes economical duplex stainless steel a strong competitor for 304, 316 austenitic stainless steel and even 2205 duplex stainless steel. At the same time, it also becomes an important development direction and growth point of duplex stainless steel.
After more than 80 years of development, although the annual output of duplex stainless steel only accounts for less than 1% of the total output of stainless steel, duplex stainless steel has become an indispensable part of the stainless steel family, parallel with martensite, ferrite and austenitic stainless steel. At the same time, in view of the structure characteristics of duplex stainless steel with about half of austenite and ferrite phases, reasonable matching of austenite and ferrite forming elements is needed in composition design, and excessive alloy content will bring more difficulties to the prevention of harmful phases, hot working and smelting. Duplex stainless steel has developed to today, and has formed three generations of duplex stainless steel and economical duplex stainless steel. Phase stainless steel, super duplex stainless steel and other relatively complete series.
As early as the 1970s, China began the research and development of duplex stainless steel, and established the development direction of N-containing duplex stainless steel. With the continuous development of duplex stainless steel in the world, duplex stainless steel in China is also experiencing a similar stage of development, and constantly in line with the international standards, narrowing the distance with the international, China’s production of duplex stainless steel is increasing. At the same time, after 2000, the research, production, application and standard formulation of duplex stainless steel have been greatly developed.
Dual-phase stainless steel has high alloy content and the two-phase structure characteristics of austenite and ferrite, which give it high strength and excellent corrosion resistance. It also brings corresponding difficulties to its processing, production and structure control.
Following the development trend of duplex stainless steel, in recent years, the research of duplex stainless steel has attracted more and more attention in China.
With the deepening of the understanding of duplex stainless steel, the research of Chinese duplex stainless steel workers is also gradually deepening. In recent years, the following progress has been made in thermoplasticity, harmful precipitates and the control of N alloy elements.
2.1 Thermoplasticity of Duplex Stainless Steel
Due to the different hardness of two-phase structure in duplex stainless steel at high temperature and different softening mechanism during hot deformation, there are uneven stress and strain distributions in austenite and ferrite. During hot deformation, cracks nucleate and propagate at the phase boundary of duplex stainless steel . The thermoplasticity of duplex stainless steel is not only closely related to steel grades, but also affected by strain rate, deformation temperature, austenite morphology and other factors , , . In recent years, thermoplasticity has been one of the research hotspots of Chinese duplex stainless steel workers.
The thermoplasticity of several typical duplex stainless steels (2101, 2205, 2507) in the deformation temperature range of 950 ~1200 ~C was studied by hot stretching method. It can be seen from Fig. 1 that the tensile strength of duplex stainless steel decreases gradually with the increase of deformation temperature. At the same deformation temperature, the increase of alloy content significantly improves the tensile strength of steel. At the same time, with the increase of deformation temperature, the section shrinkage rate of duplex stainless steel increases gradually, and maintains a relatively high level between 1100 and 1200 degrees C. At the same deformation temperature, the section shrinkage of 2507 is significantly lower than 2101 and 2205. Although the content of Cr and Mo in 2205 is higher than 2101, the content of Mn in 2101 is about 5%. Therefore, the section shrinkage of 2101 and 2205 is almost the same. Generally speaking, for duplex stainless steel, it has good thermoplasticity between 1100 and 1200 C. The research results lay a foundation for determining reasonable hot deformation temperature range in actual production process according to the factors of hot deformation mode, processing steel grades and specifications.
On this basis, by optimizing the smelting process, Chinese metallurgical enterprises have effectively reduced the oxygen content in steel to less than 30 ppm, which lays a good foundation for the hot processing of duplex stainless steel by means of low sulphur, adding rare earth and boron elements.
Figures.1 2101, 2205 and 2507 thermal tensile test results
2.2 Study on Harmful Precipitates of Duplex Stainless Steel
In duplex stainless steel, a large number of undesirable secondary phases are formed in the temperature range of 300-1000 ℃. There are not only common precipitates such as M7C3 and M23C6 in austenitic stainless steel, but also Cr2N, CrN, R, PI and alpha’phases. The diffusion rate of alloying elements in ferrite is much higher than that in austenite, and the enrichment of chromium and molybdenum in ferrite phase is beneficial to the nucleation of intermetallic phases containing these two elements in ferrite phase. For this reason, the transformation of structure often occurs in ferrite phase, and the precipitation reaction is much faster than that in austenite.
Most of these harmful phases contain higher Cr, Mo and N. Their precipitation not only results in a significant decrease in corrosion resistance, but also brings great difficulties to the formation of steel. Among these phases, the most harmful is phase, except Cr2N and secondary austenite, the other phases are not very important , , , . In recent years, the precipitation characteristics of and Cr2N harmful phases and their harm to the properties of duplex stainless steel have been further recognized.
The results show that the presence of Mo promotes the precipitation of phase obviously, and the tetragonal_phase is the key harmful phase for the duplex stainless steel containing Mo (including the second generation, super and super duplex stainless steel). The precipitation of phase at the interface of austenite/ferrite and ferrite/ferrite not only adversely affects the corrosion resistance of duplex stainless steel, but also significantly affects its mechanical properties, especially impact toughness. As shown in Fig. 2, the effect of phase precipitation at different solution temperatures on the plasticity and toughness of 2507 and 2707 duplex stainless steels can be seen from Fig. 2. For 2507 duplex stainless steels, the impact toughness of steel deteriorates sharply when the solution temperature is below 1020 C. Even if the solution temperature reaches 102C, about 3.2% phase can still be precipitated in the steel, resulting in a decrease in the impact toughness of steel. The impact toughness of phase stainless steel is more sensitive to phase. For 2707 duplex stainless steel, there are still 5.4% phase in the structure up to 1050 C, which is harmful to the impact toughness of the material. When the phase is dissolved above 1100 C, the impact toughness of the duplex stainless steel can reach the normal level.
Fig. 2 The amount of phase precipitation at different solution temperatures and its effect on the plasticity and toughness of 2507 and 2707 duplex stainless steels
For duplex stainless steel containing Mo, the higher the content of Cr and Mo, the greater the sensitivity of phase precipitation, which is not only reflected in the number and speed of phase precipitation, but also in the precipitation temperature of phase. The results show that the complete dissolution temperature of phase of 2205 duplex stainless steel is 980-1000 C, and the complete dissolution temperature of phase of 00Cr25Ni7Mo3N, which belongs to the second generation DSS, reaches 1000-1020 C, and the complete dissolution temperature of phase of 2507 and 2707 with higher Mo content reaches 1040 and 1070 C, respectively. Of course, under certain heat treatment conditions, the precipitation of phase can also be avoided in duplex stainless steel containing Mo, while other harmful phases can be precipitated, which has adverse effects on the properties.
Microstructure and properties of 2101 duplex stainless steel with different holding temperature and time were studied. The precipitation of Cr2N could lead to a significant decrease in impact toughness of 2101 duplex stainless steel, but its hazard was smaller than that of phase precipitated from Mo duplex stainless steel. Fig. 3 shows the comparison of impact toughness between 2101 and 2205 after solution at different temperatures. When the solution temperature is 900 C, the precipitation of a small amount of Cr2N and Cr23C6 in 2101 results in a decrease in impact toughness to a certain extent, but it can still be maintained at about 115J. However, the precipitation of phase in 2205 results in a decrease in impact toughness to only about 5J. Cr2N fully precipitated from 2101 duplex stainless steel after aging at 600 ~ 700 C (its morphology is shown in Fig. 4) can lead to a significant decrease in impact toughness of 2101 duplex stainless steel. As M23C6 is a face-centered cubic structure, its slip system is more than Cr2N of hexagonal structure  . Therefore, it can be considered that the decrease of impact toughness of 2101 aging at 600-700 C is mainly caused by the precipitation of Cr2N.
Fig. 3 Effect of solution temperature on impact toughness of 2101 and 2205
(a) Intragrystalline precipitation of Cr2N (b) Cr2N grain boundary precipitation
Fig. 4 precipitation of Cr2N in 2101 grain boundary and crystal after holding at 670 ℃ for 50h
Generally speaking, for Mo-containing duplex stainless steel, the most harmful phase is_phase. For Mo-free economical duplex stainless steel, the most harmful phase is Cr2N. In duplex stainless steel, the plasticity and toughness of with tetragonal structure is more harmful than that of Cr2N with hexagonal structure.
2.3 N-Alloying and Control of Duplex Stainless Steel
Since the discovery of the important role of N in maintaining phase equilibrium, improving corrosion resistance and mechanical properties in the 1970s, all duplex stainless steels developed internationally are N-containing steels. N alloying plays an important role in the development of duplex stainless steels. N alloying and its precise control have also become an important factor in the development and production of duplex stainless steels. Combining with the development of N-containing stainless steel in China, the factors such as product specifications and quantity of duplex stainless steel in actual production process, and the actual equipment and technology of relevant factories, the smelting methods of duplex stainless steel in China include vacuum induction, non-vacuum induction, electric furnace+AOD or electric furnace+VOD, etc. EAF+AOD is the main smelting method for the production of conventional and batch duplex stainless steel.
For different smelting processes, N alloying mainly adopts two ways, adding ferroalloys containing nitrogen and blowing nitrogen. Through the research on the law of N dissolution and removal in steel and the influence of different elements on the law of N dissolution in steel, in recent years, relevant research institutes and metallurgical enterprises have developed N alloying process in combination with actual smelting methods, and introduced advanced smelting control software, which has been able to realize the accurate control of N deviation in the process of dual-phase stainless steel smelting at (+100 ppm), for example, through Taiyuan Iron and Steel Co. The mathematical model of controlling N content by nitrogen alloying in AOD furnace was established to predict the saturated solubility of duplex stainless steel with different components at a certain temperature. The precise control of N content in duplex stainless steel products between 500 ppm and 3200 ppm can be achieved. The control precision is <50 ppm, which lays a good foundation for the structure and performance control of duplex stainless steel in China.
In recent years, the production and application of duplex stainless steel in China has been widely used in various industrial fields, such as pulp and paper making, petrochemical industry, chemical tanker, oil and gas industry on land and sea, pharmaceuticals and food industry, because of its special duplex structure and high alloy content, which endow it with high strength and excellent pitting and stress corrosion resistance.
With the continuous improvement of domestic knowledge and production level of duplex stainless steel, in recent years, China’s duplex stainless steel has made great progress in production, variety and application.
3.1 Production of Duplex Stainless Steel
Table 1 shows the output of crude stainless steel and duplex stainless steel in China from 2005 to 2014. In the past ten years, the crude steel output of stainless steel in China has increased from 3161,000 tons to 216,922,000 tons, and the crude steel output of stainless steel in China has increased from 12.9% to 52% in the world. At the same time, the output of duplex stainless steel in China increased from 828 tons in 2005 to 35741 tons in 2014, which increased more than three times. In 1999, according to incomplete statistics, the consumption of duplex stainless steel in China was only about 2000 tons. With the growth of duplex stainless steel production, its proportion in China’s stainless steel has also increased steadily, from less than 0.1% in 2005-2007 to about 0.2% in 2008-2014.
According to incomplete statistics, duplex stainless steel in China has been stable in the period of 2010-2014 at 20-40,000 tons per year. In 2014, the output of duplex stainless steel in Taigang, Yongxing Special Stainless Steel Co., Ltd. and Baosteel has reached about 25,000 tons, 29,000 tons and 26,000 tons respectively (see table 2).
At the same time, the output of duplex stainless steel in China has changed greatly. Among them, the output of sheet metal has increased greatly in recent years, especially in 2205 and 2101. Taiyuan Iron and Steel Co. is an important stainless steel production enterprise. Its duplex stainless steel products mainly consist of plates. Its crude steel output of duplex stainless steel in 2011-2014 is stable at about 15-25,000 tons. In terms of steel grades, although 2205 duplex stainless steel is still the main type of duplex stainless steel in China, the production of economical duplex stainless steel and super duplex stainless steel has increased rapidly since 2010.
As a duplex stainless steel production enterprise in China, Baosteel Special Steel covers duplex stainless steel round steel, pipes, plates and other profiles. It is a typical representative of a comprehensive production enterprise with multi-specification duplex stainless steel production capacity. The change of its duplex stainless steel products can reflect the overall situation of China. Fig. 5 shows the output of duplex stainless steel in Baosteel Special Steel from 2006 to 2014. Since the output of duplex stainless steel in Baosteel exceeded 10 million tons in 2010, the output of duplex stainless steel in Baosteel has increased rapidly, and the output of duplex stainless steel in 2014 has reached 6201 tons. At the same time, according to Fig. 5 (a), the production of economical duplex stainless steel and super duplex stainless steel has increased rapidly since 2010 and 2012, respectively. From Fig. 5 (b), it can be seen that the growth of production from 2010 to 2014 mainly comes from the growth of plate and pipe production. From Fig. 5 (c), the growth of plate production from 2010 to 2014 mainly comes from 2205 and 2101. The increase of steel grade output is very small. Fig. 5 (d) shows that the growth of pipe production mainly comes from the growth of 2205 and part of 2507 in 2010-2014. In addition, there are still the production of 00Cr18Ni5Mo3Si2 pipes of the first generation duplex stainless steel from 2006-2010, and there will be no production of 00Cr18Ni5Mo3Si2 pipes after 2011.
Figure.5 Production of Duplex Stainless Steel in Baosteel Special Steel from 2006 to 2014
For the production of duplex stainless steel pipes, Figures 6 and 7 show the actual production situation of Jiulite and Wujin stainless steel pipes in China duplex stainless steel production enterprises. Generally speaking, about 50% of the duplex stainless steel pipes of Jiulite are welded pipes and seamless pipes, while the seamless pipes are the main duplex stainless steel pipes of Wujin. Starting from 2010, the production of super duplex stainless steel S32750 began to increase, and the production of S32760 pipes began from 2012 to 2014.
Figure.8 Production of Duplex Stainless Steel for Jiuli Special Material from 2011 to 2014
Figure.9 Production of duplex stainless steel in Wujin from 2010 to 2014
In addition, as the largest duplex stainless steel tube blank manufacturer in China, Yongxing Special Steel began to try the smelting and production of super duplex stainless steel 2707 in 2015. Its 2707 duplex stainless steel composition smelted by electric furnace + AOD + LF meets the requirements of the standard (see table 3).
3.2 Application of Duplex Stainless Steel
China’s duplex stainless steel has experienced increasing production, and its application field has been expanding. While consolidating and improving the use of duplex stainless steel in petrochemical and other major applications, in recent years, China’s duplex stainless steel has been expanding in oil and gas transportation, chemical ship manufacturing, nuclear power, construction and other fields. Duplex stainless steel is especially suitable for pitting corrosion, crevice corrosion and stress corrosion cracking in various corrosive media with high temperature, such as reducibility, strong oxidation and halogen ions, H2S and CO2. Petrochemical industry is still one of the most important application fields of duplex stainless steel. The early developed 00Cr18Ni5Mo3Si2 duplex stainless steel in China was first used as lining of atmospheric and vacuum distillation tower, coking fractionator, stripper, catalytic cracking absorption tower and stabilization tower in refineries.
At present, China’s 2205, 2507 duplex stainless steel has been widely used in methanol reactor, vinyl chloride (VCM) oxychlorination reactor, heat exchanger, atmospheric and vacuum condenser, feed water heater, catalytic cracking, hydrocracking and other devices , , .
At the same time, duplex stainless steel in China has been developed and applied in oil and gas transportation, chemical tankers, nuclear power, marine engineering and other fields in recent years.
From 2003 to 2004, 2205 duplex stainless steel pipes ,  produced by Baosteel Special Steel and Jiuli Special Steel were used in the West-to-East Gas Transmission Project of Kela 2 Gas Field in Tarim Basin, Xinjiang.
In the construction of chemical vessels in China, the S31803 plate for chemical liquid cargo vessel with a load of 18 500 tons built by Qingshan Shipyard in Hubei Province in 2002 was supplied by the French company Arcelor, and the plate for the second similar type vessel built in 2003 was supplied by AvestaAS. Sweden, Sweden. In 2009, the tank liner of the 9000t stainless steel chemical tanker “Chongqing” manufactured by Chuandong Shipyard first used 2205 duplex stainless steel , .
In AP1000 nuclear power plant, S32101 is used in refueling channels, spent fuel pools, refueling pools, reactor chambers, etc. . In 2010, the S32101 duplex stainless steel plate supplied by TISCO was used in the structural module of Taohuajiang Project, the first inland nuclear power plant in China.
In the aspect of coastal construction bridges, the combination products of duplex stainless steel rebar, connectors and bending parts of TISCO have passed the authoritative British CARES certification, and established the standards and application technical specifications of stainless steel rebar. More than 8,000 tons of duplex stainless steel rebar produced by TISCO has been successfully applied to the Hongkong-Zhuhai-Macao Sea-Crossing Bridge with the longest design life in the world and more than 120 years.
In addition, duplex stainless steel in China is also used in pulp and paper making, seawater desalination, desulfurization absorption devices, urban rail transit and other fields.
Duplex stainless steel in China has been used in foreign projects while meeting domestic engineering needs and replacing imports. For example, 2205 welded pipe made of Jiulite special material is used for PDO (Oman National Petroleum Company) gas gathering pipeline, 2205 duplex stainless steel cold plate made by Taiyuan Steel is used for 2098 sunshade umbrellas in Dubai landmark building, ADIC Project (Abu Dhabi Immigration Bureau Building). Manufacture of (23980m2) bracket.
In a word, with the continuous improvement of the understanding and production level of duplex stainless steel, China’s duplex stainless steel has made great progress in production, steel grades and varieties in recent years. China’s duplex stainless steel has got rid of the situation of small output and single variety around 2000. While the output is increasing, varieties are gradually diversified. At the same time, economical duplex stainless steel The proportion of phase stainless steel and super duplex stainless steel is increasing.
On this basis, China’s dual-phase stainless steel market expansion has also made remarkable progress. China’s dual-phase stainless steel has got rid of the single industry mainly used in petrochemical industry, and gradually expanded to the application in petrochemical, chemical transportation, nuclear power, marine and other fields. Moreover, China’s dual-phase stainless steel is also stepping out of China and exploring overseas markets.
Because duplex stainless steel has the advantages of high strength and chloride stress corrosion resistance of ferritic stainless steel, excellent toughness and welding performance of austenitic stainless steel. In recent years, in line with the international trend, the production of duplex stainless steel in China has developed very rapidly. With the increasing use of duplex stainless steel and the gradual substitution of imports, its application scope has been constantly expanded.
Although the research and development of duplex stainless steel in China started in the 1970s, there was no professional standard for duplex stainless steel before 2008. In China’s national standards, only four kinds of steel, such as 00cr18ni5mo3n, 1cr18ni11si4alti, 1Cr21Ni5Ti, 0cr26ni5mo2, have been listed in the national standards for quite a long time. In the newly issued brand and chemical composition of stainless steel and heat resistant steel (Standard No.: GB / t20878-2007) in October 2007, 11 brands of duplex stainless steel have been listed in the new standards, including some super duplex stainless steels. Steel grades (such as 022Cr25Ni7Mo4N, 022Cr25Ni7Mo4WCuN), but in “Stainless Steel Bar” (GB/T1220-2007), there are still only six duplex stainless steel grades, and there is no super duplex stainless steel. In YB/T2008-2007 “Stainless Steel Seamless Steel Tube Round Billet”, only one duplex stainless steel grade is listed. The number and quality index of duplex stainless steel grades in China’s standards have been listed. It can not meet the actual needs. In order to adapt to the rapid development of production and application of duplex stainless steel in China, it is necessary to formulate and expand professional standards for duplex stainless steel.
Therefore, on the basis of fully understanding the production status, technology level and application requirements of Austenite-ferrite duplex stainless steel at home and abroad, according to the production capacity of domestic enterprises and the user’s needs, the standard data of ISO, USA, Japan and Europe are compared and compared. In terms of specifications and sizes, China is mainly developed with reference to China’s National standards and actual conditions. The formulation of professional standards for duplex stainless steel has been published, including “Austenitic-ferrite duplex stainless steel seamless steel pipe” (GBT21833-2008) and “Austenitic-ferrite duplex stainless steel welded steel pipe” (GBT21832-2008). The standards have been approved and to be issued are “Austenitic-ferrite duplex stainless steel bar” (GBT31303-2014).
Among the several national standards formulated, different grades of duplex stainless steel are included according to the actual varieties and development stages. Among them, the Austenite-ferrite duplex stainless steel seamless pipe (GBT21833-2008) and the Austenite-ferrite duplex stainless steel welded pipe (GBT21832-2008) are listed in 12 and 7 grades of duplex stainless steel, including two super grades. Duplex stainless steel grades 022Cr25Ni7Mo4N (equivalent to S32750) and 022Cr25Ni7Mo4WCuN (equivalent to S32760). Austenite-ferrite duplex stainless steel rods (GBT31303-2014) are listed in 15 duplex stainless steel grades. According to the development, production and application of duplex stainless steel, the super duplex stainless steel 022Cr29Ni5Mo2N (equivalent to S32950) and economical duplex stainless steel 03Cr21Ni1MoCuN (equivalent to S32101) and 022Cr20Ni3Mo2N (S32003) were added.
5. The future development direction of duplex stainless steel in China is envisaged. Based on the actual situation of China (overcapacity, fierce market competition), the future development directions of duplex stainless steel in the world and China are discussed.
- 1) High-quality, through improving the purity of steel and product quality, continuously improve market competitiveness;
- 2) Low-cost, through process optimization and fine control of components, control costs and improve market competitiveness;
- 3) Multi-element alloying and customization of steel grade design;
- 4) More in-depth and detailed research. On the basis of the previous studies, a more in-depth study on the second phase precipitation and control was carried out to gradually realize the fine control of phase ratio and morphology.
- 5) According to the national strategy, the application of duplex stainless steel in China will be continuously expanded.
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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