nodular cast iron
Pig iron is an iron-carbon alloy containing more than 2% carbon, and industrial pig iron generally contains 2.5%-4% carbon, and contains elements such as C, SI, Mn, S and P. It is a product made of iron ore by blast furnace smelting. According to the different forms of carbon in pig iron, it can be divided into steelmaking pig iron, casting pig iron and ductile iron.
Spherical precipitated graphite cast iron. The splitting effect of spherical graphite on metal matrix is less than that of flake graphite, which makes the strength of cast iron reach 70 ~ 90% of the matrix structure strength, the tensile strength can reach 120kgf/mm2, and it has good toughness. Except iron, the chemical composition of nodular cast iron is usually: carbon content is 3.6 ~ 3.8%, silicon content is 2.0 ~ 3.0%, the total content of manganese, phosphorus and sulfur does not exceed 1.5%, and there are some nodulizers such as rare earth and magnesium.
Ductile iron-characteristics
nodular cast iron
Ductile iron castings have been used in almost all major industrial sectors, which require high strength, plasticity, toughness, wear resistance, severe thermal and mechanical impact resistance, high or low temperature resistance, corrosion resistance and dimensional stability. In order to meet these changes in service conditions, there are many brands of ductile iron, which provide a wide range of mechanical properties and kissing properties.
According to ISO 150 1083, most ductile iron castings are mainly produced in non-alloy state. Obviously, this range includes high strength grades with tensile strength greater than 800 N/mm and elongation of 2%. At the other extreme, it is high plastic grade, with elongation greater than 17% and corresponding low strength (the lowest is 370 N/mm spoon. Strength and elongation are not the only factors for designers to choose materials. Other decisive and important properties include yield strength, elastic modulus, wear resistance and fatigue strength, hardness and impact properties. In addition, corrosion resistance, oxidation resistance and electromagnetic performance may be the key for designers. In order to meet these special purposes, a group of austenitic ductile iron was developed, which is usually called nickel-resistant ductile iron. These austenitic nodular cast irons are mainly alloyed with zinc, chromium and manganese, and are listed in international standards.
Ductile Iron-Application
nodular cast iron
The application scope of ductile iron can be divided into four departments: (1) pressure pipes and fittings; (2) Automotive applications; (3) Agricultural, road and building applications; (4) General engineering.
1: pressure pipes and fittings
When ductile iron is used as pipes, most iron pipes and fittings are produced by major industrial countries, and their manufacture, design and use are in line with national and international standards and implementation procedures (I50253 1). It has been proved for a long time that ductile iron pipes are superior to gray iron pipes in conveying water and other liquids. The main reason for this change lies in the combination of strength and toughness of ferritic ductile iron, which enables the pipeline made of this material to withstand higher working pressure and can be hastily loaded and unloaded during laying. The gas cushion pipeline must be able to withstand the requirements of multiple purposes, that is, it can withstand the excavation near the pipeline, municipal engineering construction and transportation. The high strength, high shear and simple installation and connection technology of ductile iron pipeline here prove that the choice of this material is decisive.
2. Automotive applications
In terms of production tonnage, the automobile industry is the second largest user of ductile iron castings, with the most projects. Ductile iron is mainly used in three places in automobiles: (1) power source-engine parts; (2) Power transmission includes gear train, gear and shaft sleeve; (3) Vehicle suspension, braking and steering devices. The crankshaft of power source is a part that bears changing bending, torsion and shear loads, and it will cycle one billion times in its service life. As early as 19 152, automobile design engineers immediately considered the possibility of adopting ductile iron after discovering the method of magnesium treatment. Almost all the crankshafts of Ford Motor Company are made of this material. Most gasoline engines in the world are equipped with nodular cast iron crankshafts instead of forged steel crankshafts. This application is considered as a typical example of value engineering.
In order to prolong the service life of the engine, the manufacturers of automobile diesel engines have carefully considered the choice of crankshaft materials. When the power-to-weight ratio of high-strength bainite isothermal strain fireball iron is increased, the expanded use of turbocharger affects the design basis of the material, and the exhaust pipe temperature with turbocharger is increased to 500-70℃. In this case, oxidation and creep strength become very important. Ductile iron with excellent performance is replacing gray iron in exhaust pipe. With the increase of temperature, silicon-aluminum alloy ductile iron will be further applied.
Malleable cast iron castings used in automobile power transmission are sometimes used as transmission parts of automobiles, and there is a strong trend to apply ductile iron to disc clutches, differential housings, rear axles and wheel covers. G.Fiseher foundry in Sehaffhausen, Switzerland has many applications in this field. They work closely with designers to transform steel castings, forged steel castings and malleable cast iron castings into ductile iron castings. Suspension Now, iron is often used as suspension parts such as spring hooks, as well as the main safety parts (brake calipers) and steering joints of braking systems.
3. Agricultural, road and building applications
Modern and economical agricultural methods require the provision of agricultural machinery that is reliable in the required state and has a long service life. Ductile iron castings widely used in the whole agricultural industry include various tractor parts, plowshares, brackets, clamps and pulleys. Typical parts are two rear axle housings of agricultural vehicles. It used to be made of steel. Road paving and construction industries need a considerable amount of equipment, including bulldozers, drivers, cranes and compressors, and ductile iron castings are used in these areas.
4. Universal application
nodular cast iron
The machine tool industry uses the engineering characteristics of ductile iron to allow the design of complex machine tool parts and castings with a weight exceeding 10. Tons of heavy machine castings. Applications include injection molding templates, forging cylinders and pistons. Ductile iron has high tensile strength and yield strength and good machinability, and can produce lighter castings and maintain their rigidity. Similarly, the strength and toughness of ductile iron make it an ideal material for various hand tools, such as wrenches, pliers and measuring tools.
The paper machine industry makes use of the high strength and high elastic modulus of nodular cast iron. For example, compared with gray cast iron, the elastic modulus of nodular cast iron is 60% higher, and the weight can be reduced when designing the pressing roller and drying roller. Similarly, nodular cast iron (with appropriate addition of Ni-In alloy) rolls used in steel mills have higher performance than steel and chilled cast iron.
5: Valve manufacturing
Valve manufacturers are the main users of ductile iron (including austenitic ductile iron), and their applications include the successful transportation of various acids, salts and alkaline liquids. An interesting example of the application of ductile iron in valves is the ball valve core produced by Parkfield factory in England. The original material of the large ball valve core for 1420mm gas pipeline is forged steel, with a diameter of 24 10mm and a weight of 17 ton. Basically, the redesigned ductile iron ball valve core weighs only n tons. Other benefits include reducing machining allowance and machining time and improving overall accuracy. Completely ferritic ductile iron (150 1085 brand 370-27) is suitable for these ball valve plugs. The KrefeldSiempelkamp foundry in Germany actively promotes the use of ferritic ductile iron (GGG40) as a transport and storage container for used nuclear fuel rods (still containing 50% uranium -235). In order to gain recognition for the application of this nodular cast iron, Ssempelkarnp requires that the casting container weighing about 85 tons be subjected to the most stringent and worst load test ever applied to iron castings. These tests included dropping 85 tons of castings from a height of 9 meters onto a cement base with a temperature lower than 40℃ and weighing 65,438+0,000 tons, and firing a large shell from a tank, hitting the castings almost at the speed of sound. In all these tests, the ductile iron container has withstood all rigorous tests, and only surface damage can ensure that the radiation seal is completely reliable. After simulating the fault state of the transport plane and the plane crash, it has been confirmed that the nodular cast iron is safe, and the designed nodular cast iron caster container may obtain the highest "Pradikat" safety level-B(L) certificate from Berlin and Braun-Schweitzer elg Administration. This is an excellent example of the potential market recognized by the foundry industry.
Ductile Iron-Manufacturing Steps
(1) Strict requirements on chemical composition, the carbon and silicon content of raw molten iron is higher than that of gray cast iron, and the manganese, phosphorus and sulfur content in nodular cast iron is reduced.
(2) The tapping temperature of molten iron is higher than that of gray cast iron, which makes up for the loss of molten iron temperature during spheroidization and inoculation.
(3) spheroidizing, that is, adding spheroidizing agent into molten iron.
(4) carrying out inoculation treatment.
(5) Ductile iron has poor fluidity and large shrinkage, which requires higher pouring temperature and larger gating system size, uses more risers and chills, and adopts the principle of sequential solidification.
(6) carrying out heat treatment
Development Prospect of Ductile Iron —— Application of Ductile Iron
nodular cast iron
When the tonnage of nodular cast iron increases and the market penetration rate is amazing, this material will never be regarded as having played its full potential. Based on this, it is suggested that foundries that do not produce nodular cast iron at present reconsider the possibility in this respect. Energy-saving requirements lead to the redesign of parts to achieve lightweight and high efficiency, which inevitably reminds designers to pay attention to materials. Ductile iron is increasingly considered to have high strength-weight characteristics and can be produced at relatively low cost.
Therefore, it is expected that with the substitution of gray cast iron, malleable cast iron and silver castings, we can see the continuous increase in the production tonnage of nodular cast iron. The recently published publications will help the factories in this field, although the calculated values will be improved and improved. However, when the temperature of molten iron is lower than1450 "C, the inoculation effect is very poor, and the RG value is almost unchanged. It can be seen from Table 3 that the quality index of inoculated cast iron is higher than metallurgical coke by 18%, but the relative hardness is reduced by 3%.
History of nodular cast iron
In 1947, H.Morrogh of Britain found that when cerium was added to gray cast iron with * * * crystals, graphite was spherical when its content was higher than 0.02wt%. In 1948, A.P.Ganganebin and others in the United States pointed out that spherical graphite was obtained by adding magnesium to cast iron and then inoculating ferrosilicon. Since then, large-scale industrial production of ductile iron has begun.
nodular cast iron
As a new engineering material, nodular cast iron has an amazing development speed. 1949, the world output of ductile iron was only 50,000 tons, 1960 was 535,000 tons, 1970 increased to 5 million tons, 1980 was 7.6 million tons, and 1990 reached 9/kloc-0. In 2000, it reached150,000 tons. The production and development of nodular cast iron in industrialized countries are particularly fast. 75% of the world's ductile iron production is produced by the United States, Japan, Germany, Italy, Britain and France.
The production of nodular cast iron in China started very early, and/kloc-0 was successfully developed and put into production in 950. The annual output of nodular cast iron in China is 2.3 million tons, ranking third in the world, second only to the United States and Japan. The rare earth magnesium nodulizer suitable for China's national conditions has been successfully developed, and the production technology and research work of as-cast ductile iron and austenite-bainite ductile iron have reached a high technical level.
(1) As-cast pearlitic ductile iron crankshaft and as-cast ferritic ductile iron automobile chassis parts were put into production in China No.2 Automobile Factory, Nanjing Automobile Factory and No.1 Automobile Factory respectively. This indicates that the production of as-cast ductile iron in China has reached a higher level. The adoption of technologies such as desulfurization outside the furnace, duplex melting, instantaneous inoculation, inoculation block technology, audio detection and rapid thermal analysis indicates that the gap between the technical level of mass production of automobile castings in China and the international advanced level is narrowing.
(2) The metallurgical factors of large section ductile iron (wall thickness greater than 120mm) and the corresponding production process measures were studied experimentally. Using proper amount of yttrium-based heavy rare earth composite nodulizer, forced cooling, sequential solidification, delayed inoculation and adding trace antimony and bismuth when necessary can prevent graphite distortion and loose structure in the center of ductile iron castings. At present, it is a large-scale complex structural member with a weight of 38 tons, a diesel engine block with a weight of 17.5 tons, a nodular cast iron roller with a cross section of 805mm, etc. Has been successfully manufactured.
(3) Research and application of austenite-bainite ductile iron. At the beginning of 1970s, almost at the same time, China, the United States and Finland announced that they had successfully developed austenite-bainite ductile iron with high strength and toughness (ADI for short internationally). The tensile strength of this material reaches 65,438 000 MPa, so it is widely used in gears and various structural parts. Compared with alloy steel, ADI has remarkable economic and social benefits.
(4) Ductile iron pipes and horizontal continuous casting ductile iron profiles. China has built several ductile iron pipe factories, and several ductile iron pipe factories will be built in recent years. In 2000, the annual output of centrifugal cast ductile iron pipes in China reached 900,000 tons. In addition, the horizontal continuous casting ductile iron profile production line developed in China has passed the national appraisal and has been put into production by many enterprises. Coupled with a production line imported from China, by 2002, the annual output of ductile iron profiles in China will reach tens of thousands of tons.
Ductile iron castings
(5) The mechanical properties and other properties of RE-Mg ductile iron are systematically tested, which provides relevant data for designers. The specific gravity, thermal conductivity and electromagnetic properties of rare earth magnesium ductile iron were measured, and the effects of graphite and matrix structure on the properties of ductile iron were studied in combination with metallographic standards. The properties of ferritic ductile iron under normal temperature, low temperature, static and dynamic conditions were systematically measured. In addition, the stress-strain properties, small energy multi-impact resistance and fracture toughness of rare earth magnesium ductile iron were also studied and used to guide production. Combined with the application of nodular cast iron gears, the bending fatigue strength and contact fatigue strength of nodular cast iron gears, as well as the pitting corrosion and spalling mechanism of nodular cast iron gears are systematically studied.
(6) Rare earth magnesium nodular cast iron. In addition to copper and molybdenum, nickel and niobium are also being studied in high strength low alloy ductile iron. Some domestic units have done a lot of systematic work in producing vanadium-titanium alloy ductile iron from natural vanadium-titanium pig iron. Although the performance of medium manganese ductile iron is not stable enough, it has achieved remarkable economic benefits in systematic research and production application.
In the aspect of heat-resistant ductile iron, except medium silicon ductile iron, the effect of total Si+Al content on the growth resistance of rare earth magnesium ductile iron was systematically studied. RQTAL5Si5 heat-resistant cast iron developed in China, as a heat-resistant bar, has a service life three times that of gray cast iron, twice that of ordinary heat-resistant cast iron, and is equivalent to that of Japanese Cr25Ni 13Si2 heat-resistant steel. High-nickel austenitic ductile iron has also made progress and has been successfully applied to oil mining machinery, chemical equipment and industrial furnace equipment. In terms of acid-resistant ductile iron, the microstructure of rare earth high-silicon ductile iron produced in China is finer, more uniform and denser than that of ordinary high-silicon cast iron. As a result, the corrosion resistance was improved by 10% ~ 90%, and the mechanical strength was also significantly improved.
(7) The role of rare earth in nodular cast iron. Rare earth can spheroidize graphite. Since H.Morrogh first obtained nodular cast iron with cerium, many people have studied the spheroidizing behavior of various rare earth elements, and found that cerium is the most effective spheroidizing element, and other elements also have spheroidizing ability to varying degrees.
Combined with the national conditions, China has done a lot of research and development on spheroidization of rare earth elements, and found that it is difficult for rare earth elements to obtain complete and uniform spherical graphite like magnesium ductile iron for commonly used components (C 3.6 ~ 3.8 wt%, Si 2.0 ~ 2.5 wt%); Moreover, if the rare earth content is too high, all kinds of abnormal graphite will appear, and the tendency of white mouth will also increase. However, if it is a high-carbon hypercrystalline component (c > 4.0 wt%) and the residual amount of rare earth is 0. 12 ~ 0. 15 wt%, good spherical graphite can be obtained.
According to the situation of poor iron content, high sulfur content (cupola melting) and low tapping temperature in China, it is necessary to add rare earth. Magnesium is the dominant element in spheroidizing agent, and rare earth can promote graphite spheroidization on the one hand; On the other hand, it is necessary to overcome the influence of sulfur and impurity elements and ensure spheroidization. Rare earth prevents interference elements from destroying spheroidization. The results show that when the total amount of interference elements such as Pb, Bi, Sb, Te and Ti is 0.05wt%, the interference can be completely neutralized by adding 0.0 1wt% (residual amount) of rare earth, and the generation of abnormal graphite can be suppressed. Most pig iron in China contains titanium, and some pig iron contains titanium as high as 0.2 ~ 0.3 wt%. However, the rare earth magnesium nodulizer can make the residual amount of rare earth in iron reach 0.02 ~ 0.03 wt%, so it can still ensure good spheroidization of graphite. If 0.02 ~ 0.03 wt% Bi is added to nodular cast iron, the spherical graphite will be almost completely destroyed. If 0.0 1 ~ 0.05 wt% Ce is added later, the original spheroidizing state will be restored, because Bi and Ce form stable compounds.
Rare earth nucleation. The research after 1960s shows that the inoculant containing cerium can increase the number of balls in molten iron during the whole heat preservation period, so that the final structure contains more graphite balls and the tendency of white mouth is small. The study also shows that the inoculant containing rare earth can improve the inoculation effect of ductile iron and significantly improve the aging resistance. The reasons why adding rare earth can increase the number of graphite balls can be attributed to the following: rare earth can provide more crystal nuclei, but its crystal nucleus composition is different from FeSi inoculation; Rare earth can make the inactive crystal nucleus grow, and as a result, the total number of crystal nuclei in molten iron increases.
Ductile Iron-Historical Debate
nodular cast iron
Iron unearthed from Tieshenggou iron smelting site in Gongxian County, Henan Province in the middle and late Western Han Dynasty? Through metallographic examination, it has radial spherical graphite, and the spheroidization rate is equivalent to the first-class level of modern standards. However, it was not until 1947 that modern ductile iron was successfully developed. In ancient China, the silicon content of cast iron was low for a long time. In the Western Han Dynasty about 2000 years ago, spherical graphite in cast iron was obtained by softening and annealing low silicon pig iron castings. This is a great achievement of China's ancient cast iron technology and a miracle in the world metallurgical history.
Because of its excellent performance, nodular cast iron can sometimes replace expensive cast steel and forged steel, and is widely used in machinery manufacturing. The international metallurgical industry used to think that ductile iron was invented by the British in 1947. Some western scholars even claim that it is unthinkable to invent ductile iron without modern scientific and technological means. 198 1 year, China ductile iron experts studied 5 13 pieces of ancient Han and Wei ironware unearthed by modern scientific means, and drew a conclusion through a lot of data: spherical graphite cast iron appeared in Han Dynasty in China. Related papers were read at the 18 World Congress on the History of Science and Technology, which caused a sensation in the international foundry industry and the history of science and technology. After verifying this point in 1987, international experts in metallurgical history believe that the law of softening cast iron to make nodular cast iron was explored in ancient China, which is of great significance to the re-staging of world metallurgical history.