Challenges facing cast iron today and future trends
[Today's cast iron challenges and future development trend] Throughout the global casting production development trend, at present and in the future cast iron is still the most important casting alloy. With technological progress and development, the performance of cast iron will be further improved, cast iron production will certainly grow. Due to the unique advantages of cast iron, cast iron as a highly competitive casting materials, will continue to occupy an important position.]
Overview of the development of cast iron
Cast iron is a casting material that is both ancient and new, traditional and modern. It is recognized by the world that China first smelted iron ore into molten iron and cast it into cast iron parts around 600 BC. At the latest in the middle and late Warring States period, the division of labor between smelting pig iron and casting ironware had begun. For a long time before the Industrial Revolution, China's iron smelting level and casting production level had been leading the world, and exported to Southeast Asia and other places. Cast iron smelting technology was introduced to Europe between 1200 and 1450 AD. However, until the Industrial Revolution, cast iron technology developed very slowly, and people's understanding of cast iron was limited to gray iron, white iron, and mottled iron judged by the color of the fracture. In the early days of Europe and the United States, it was believed that cast iron was not suitable for tensile tests, and only pressure tests were performed. The pursuit of high mechanical properties has always been the mission of cast iron workers around the world. In 1860, the tensile strength of cast iron recorded in Europe and the United States was only 60-100 MPa. During World War I, the tensile strength was increased to 120-140 MPa by adding scrap steel to the smelting process. In 1922, inoculated cast iron was invented, and the tensile strength was increased to 300 MPa. After that, alloying increased the tensile strength of inoculated cast iron to 400 MPa. Although the tensile strength of whiteheart and blackheart malleable cast iron invented in 1722 and 1826 respectively has been greatly improved, only small thin-walled parts can be produced. The tensile strength of ductile iron invented in 1948 can reach 600 MPa and has an elongation of 3%. The tensile strength of alloy ductile iron can reach 800-900 MPa. The tensile strength of ADI developed in the 1970s can reach 1,200 to 1,400 MPa, and up to 1,600 MPa (ASTM A897M-2003). The highest tensile strength of ADI recorded is 1,700 MPa, with an elongation of 3%. In the past 140 years, the tensile strength of cast iron has increased nearly 20 times. Figure 1 shows the development of cast iron strength over the years and the development of different cast irons and strength over the years. The invention of ductile iron in the 1940s greatly improved the performance of cast iron. The subsequent appearance of vermicular cast iron and ADI, silicon solid solution strengthened ferritic ductile iron, and various special application wear-resistant, heat-resistant, and corrosion-resistant cast irons further enriched the cast iron family members, expanded the application range of cast iron, and the output of cast iron parts continued to increase. By the 1990s, the performance of cast iron had improved 20 times compared with the early days.
(a) cast iron strength with the development of the years
(b) Different cast irons (LG low-grade and high-grade gray cast iron, DI ductile iron, ADI austempered ductile iron) and their strength development over time
Figure 1 Cast iron development
Not only has the performance of cast iron improved significantly, global production has also increased significantly. Since 1966, affected by the world economic development cycle, political climate, and emergencies, world casting production has increased and decreased, but the overall trend has been upward. The total output increased from 63 261 475 t in 1966 to over 100 million t in 2013 to 103 223 514 t, and to a record high of 112 738 168 t in 2018, an increase of 78%. Production fell slightly in 2019. In 2020, affected by the worldwide new coronavirus epidemic, production fell back to 105 505 602 t, Figure 2; in 1966, global ductile iron production was 1 930 700 t, reaching 28 128 422 t in 2018, an increase of nearly 15 times, and fell back to 23 593 945 t in 2020, as shown in Figure 2.
(a) 1966-2020 global total castings production
(b) The development of several major materials castings
Figure 2 Global ductile iron production development
Competition and challenges facing cast iron
1.1 Cast iron and aluminum alloy
As aluminum alloy is face-centered cubic crystal structure, light specific gravity, good thermal conductivity, ductility and low-temperature performance, coupled with the continuous improvement of the performance of aluminum alloy, aluminum alloy in automotive and other applications for more than 50 years of steady growth, the global ratio of castings accounted for from 2% in 1966, increased to 12% in 2000, to about 16% in recent years, see Fig. 3. 1978 and before the aluminum alloy in China's castings in the Proportion share is very small, to 2000 accounted for 7.30%, to 2020 to reach 13.12%, see Figure 4. aluminum alloy ductility so that the car's overall frame structure can even be formed in one die-casting, the body will be from 70 parts to 1 part, thus greatly reducing all the robots to assemble these parts of the capital expenditure.
Figure 3 Stable growth of aluminum alloy application in the world
The global proportion of castings increased from 2.27% in 1966 to 12.43% in 2000 and about 16% in 2020.
Fig. 4 Stable growth of aluminum alloy application in China
In 1978, China's castings in the proportion of aluminum alloy is very small, not listed separately, in 2000 for 7.30%, 2020 increased to 13.12%. However, due to the low total cost of production of cast iron, casting performance and machinability, wear resistance, low gap sensitivity, recyclability, cast iron, especially creeping iron, ductile iron, especially ADI compared to aluminum alloys with high strength, high fatigue strength, cast iron is still widely used, still occupies the casting of more than 70% of the total output, is still the most important casting materials. Figure 5, Figure 6 is a comparison of ductile iron and aluminum alloy performance. In addition, cast iron has low energy content per unit mass and low greenhouse gas emissions, and the production of castings of the same quality cast iron consumes the lowest energy and greenhouse emissions. Figure 7 gives the energy and greenhouse gas emissions per kilogram of mass produced from different materials. When the cylinder block is made of aluminum alloy there is a reduction in weight and thus fuel savings and reduced emissions, but a detailed analysis of each step of the manufacturing process from life cycle analysis, i.e., from mining to on-road use, shows that there is no net benefit in terms of total manufacturing energy when changing from cast iron to aluminum alloy blocks when comparing fuel savings due to weight reduction. Total energy consumption and greenhouse gas emissions are more favorable when cast iron engines are designed for lightweighting.
(a) Typical stress/cycle (N) curves for cast aluminum alloy and ductile iron (DI)
(b) Typical specific stress/cycle curves for cast aluminum alloys and DI, r, density
Figure 5 Fatigue curves of cast aluminum alloy and ductile cast iron
(a) different solid solution treatment cast aluminum alloy
(b) Specific fatigue strength of ductile iron
Figure 6 Comparison of mechanical properties of ductile iron and aluminum alloy
SC-sand casting; DC-die casting; DI-ductile iron; F-ferritic; FP-ferritic-pearlitic; P-pearlitic; T-quenched and tempered; AUST- austempered. 355=Al7Si; 356=Al7Si0.4Cu; Right: effect of temperature on specific tensile strength of aluminum alloys and ductile iron (DI). uts, Ultimate Tensile Strength; r, density 357 = Al7Si0.8Cu.
Figure 7 Energy and GHG emissions per kilogram of mass produced from different materials
Energy Energy, GHG Greenhouse Gas Emissions, PigIron Pig Iron, Scrap Iron Scrap Iron, VirginAl Primary Aluminum, Scrap Aluminum Scrap, Ferro-Si Ferro-Silicon, Mn Manganese, Cu Copper
1.2 Challenges of complex high-performance castings
With the development of industry, modern equipment is increasingly powerful, for a variety of sizes, different wall thicknesses of cast iron parts demand more and more, and cast iron parts due to the continuous development of high-quality raw materials and generation method performance is getting better and better, as well as the use of a variety of new modeling materials and new modeling methods of a variety of complex cast iron parts can be manufactured. For example, in recent years by the world's clean energy, renewable energy demand, the demand for nuclear power and wind power generation is growing, especially for nuclear power plants, nuclear power spent fuel storage and transportation tanks and other large castings demand has become an urgent need. The rated output power of wind turbines has been increasing in recent years, with wind turbines already reaching 15 MW, and future wind turbine power could reach 21 MW or even higher. Much of the demand for more renewable energy can only be realized through the supply of castings. Ductile iron and ADI are ideally suited to the needs of these components. In recent years due to the rapid development of the wind power industry, the fastest development of wind power castings, the installed capacity of wind power in 2020 compared to the previous year increased 111.1 GW, the production of ductile iron for wind power is about 2.2 million t. In 2021, the global growth rate of wind power is accelerated, the estimate of ductile iron for wind power in the 2.5 million t. The most widely used in the wind power and nuclear power is the low-temperature ferrite ductile iron 400-18LT, and they are all thick and large parts (see Figure 8-10). Wind power planetary frame has long been used ADI, and the size is getting bigger and bigger (Figure 10). Thick large pieces of low-temperature ferritic ductile iron on the microstructure has strict requirements, there can be no broken lumpy graphite, low temperature (-20 or even -30, -40 ℃) impact performance must meet the strict requirements. In order to meet these requirements, it is necessary to strictly control all stages of the production process, including the selection of pure raw materials. Silicon solution-strengthened ferrite ductile iron is beginning to be used in wind power applications and is expected to account for more than 10% of wind power castings. Figure 11 shows Ningbo Riyue's award-winning silicon solid solution strengthened wind power castings. As wind turbine power increases, these castings may be larger in size, with tighter organizational requirements and higher performance requirements. With industrial development, modern equipment is becoming more powerful, the structure is increasingly large and complex. Especially large precision machine tools, the demand for large machine tool castings is also increasing. Figure 12 for the Beijing machine tool production of large precision machine tool bed. The current automotive, railroad, shipping and other transportation industries are in the center of energy saving, environmental protection, carbon reduction and the use of alternative fuels for change, the need to develop lighter, more efficient vehicles and means of transport, and casting can provide more efficient parts for automotive and other transportation industries, such as high-speed rail gearboxes, large V-type engine block, lighter power aluminum alloy and creep cast iron composite of the engine. There are also complex control valves and extremely thin ductile iron impeller's, Figures 13 to 14，Provides optimized strength and stiffness, reduces overall weight, and achieves energy saving and emission reduction. The bracket and cylinder are mechanically connected by standard washers and bolts.
Figure 8 Large wind turbine ductile iron base and hub
Figure 9 Hundred tons of nuclear spent fuel storage and transportation tanks
(a) ADI Planetary Racks undergoing austenitization at ADItreatments, Birmingham, UK
(b) Right Shiyan Aobei technology company for an enterprise handling 3.2tADI wind power planetary frame
Figure 10 ADI wind power planetary carrier
Fig. 11 Award-winning silicon solution-strengthened ductile iron wind turbine castings from Ningbo Riyue Company
Figure 12 Beijing a machine tool production of ductile iron beam LE901013A020, casting single weight 123 t
(a) High-speed rail gear box
(b) large V-engine cylinder block
(c) Ductile iron impeller of Ningxiaville (the thinnest point is only 0.2 mm)
Fig. 13 Complex structure ductile iron castings
Fig. 14 Ford aluminum alloy bracket and CGI block combination structure block
Cast iron future development trend
2.1 Production will continue to increase
As shown in Figure 2, in 2020, all countries in the world were affected by the COVID-19 epidemic. Due to the shutdowns and restrictions imposed by the government, the world's casting production fell back to 105,505,602 tons, a decrease of 6.4% from 2018; in 2020, the production of ductile iron fell back to 23,593,945 tons, a decrease of 16% from 2018. However, the foundry industry is the core industry of the entire manufacturing industry and a vital part of the global industrial value chain. The current production reduction is temporary, and the production of cast iron parts will inevitably increase in the future. In 2020, China felt the impact of the new coronavirus earlier than most countries. However, due to the correct response policies adopted by China, the casting production in 2020 increased by 6% after two consecutive years of stagnation. The total output was 51.95 million tons, of which gray cast iron was 21.75 million tons and ductile iron was 15.3 million tons, both reaching historical highs. Recently, Turner, Secretary General of the World Foundry Organization, said that in the near term, the impact on casting production is the supply of materials, whether it is chips affecting the industry's largest casting users, the lack of electricity for foundries, rising raw material prices, and insufficient transportation logistics, but these are short-term problems, either normal economic cycles, or minor problems caused by some of the reasons that have affected the past 12 months; none of them are systemic long-term problems. Javier Gonzalez, Secretary General-elect of the World Foundry Organization, said that looking at the production and order indexes, the outlook for this year and next year shows that the level of manufacturing, including the foundry industry, is expected to recover. We have seen that our foundries around the world have actually restarted activities this year and in the second half of 2020; depending on the industry they serve, many factories are at high capacity utilization levels. Modern Casting in the United States and the European Foundry Association both predict that the foundry industry and cast iron will continue to grow. The European Foundry Association reported that the European foundry industry sentiment has remained stable at a high level so far since June 2021, due to the very good order situation in the mechanical engineering sector. At the same time, the business sentiment index remained at a high level in November. The index rose slightly by 0.05 points to 1.80 points. In particular, the expectation of selling prices in the coming months is increasing the business sentiment index. Some major casting producers and raw and auxiliary material producers in various countries around the world, including China, have completed or are deploying new plant construction to prepare for future growth. As predicted by Turner, Secretary General of the World Foundry Organization and President of the American Foundry Association, the foundry industry will be glorious again, casting production will grow again, and cast iron production will grow again!
2.2 Performance continues to improve
Although the performance of cast iron has been greatly improved, the performance standards of ductile iron, vermicular graphite cast iron and ADI in China and the world have been improved since 2000. Many Chinese companies have produced 900-5 and 1000-5 ductile iron, which are higher than the existing standards; the performance of various grades of ADI is also much higher than ASTM and international and national current standards. my country is considering revising and improving relevant cast iron technical standards. At present, the research on the performance of cast iron is extensive and in-depth to the atomic level. It is believed that with the deepening of research and the improvement of technology, the performance of cast iron will be further improved. In addition to conventional performance, high-performance and high-demand safety-critical structural parts also require high and stable fatigue performance. The fatigue life of equipment structural parts is the basic ability to determine the safe and reliable service of equipment. Therefore, improving the fatigue strength of safety-critical cast iron structural parts has become an important direction for the development of cast iron. In order to detect and improve the fatigue strength of cast iron, some of the world's advanced foundries, including China, have even built fatigue test laboratories.
2.3 Develop castings with stronger functions, more complex structures, and more energy-saving and emission-reducing castings
The development of cast iron parts is the development of cast iron. Foundries and casting technicians have been at the forefront of the development of energy, transportation and other equipment for many years. The development of any new equipment is inseparable from castings. With the demand for energy conservation, environmental protection, sustainability, clean energy, reduction of CO2 emissions and carbon neutrality, the performance requirements for new and high-demand equipment are getting higher and higher, and the number is increasing. Casting is the industry that provides parts for these equipment. Wind power and nuclear power equipment in new energy, high-speed rail in transportation, and many parts of automobiles are cast iron parts. On September 16, according to media reports, Weichai in my country has developed the world's most efficient diesel engine with a thermal efficiency of 50.26%, which is a revolutionary breakthrough in the history of internal combustion engine development. There are many high-demand cast iron parts in internal combustion engines. The challenges of transitioning to a zero-carbon industry and to electric vehicles are both challenges and new opportunities for cast iron. Although electric vehicles do not have internal combustion engines, they still have many control, transmission and brake parts. Iron foundries need to work closely with equipment manufacturers, raw material suppliers and designers to develop more integrated, efficient, energy-saving and more functional castings. Thanks to 3D printing and computer technology, any complex shape of previously unimaginable metal castings can be developed and manufactured, not only replacing steel castings, forged steel parts, welded parts, but some even replace plastic parts. In addition, as the largest share of cast iron, gray cast iron parts will continue to develop steadily and retain their largest share due to their low price, good castability and machinability, good wear resistance, low notch sensitivity, and performance that can meet many applications including engine castings, brackets, housings, etc. The redesigned integrated high-grade gray iron 300 can even replace 36 steel welded composite parts, reducing costs and delivery time. In addition, as people's living standards improve, the demand for high-quality daily cast iron cookware is increasing. High-quality daily cast iron cookware with extremely low content of trace harmful elements, safety, health, beauty and convenience is becoming more and more popular and popular at home and abroad.
2.4 The transformation of iron foundries to green, efficient, energy-saving, digital and intelligent
The development of iron foundries is the development of cast iron. Casting production plants are no longer the manual, labor-intensive and poor environmental factories of the past. However, mechanization and improvement of the working environment are far from enough. In order to maintain competitiveness, iron foundries must have a comprehensive and accurate level of control over the production process. It is understood that a foundry has 137 variables that need to be controlled (based on the evaluation of a foundry in Mexico). In order to be competitive, the process parameters must be controlled within the standard range of these 137 variables. Patricio Gil, former chairman of the American Foundry Society, believes that iron foundries are not only passively completing customer orders, but more importantly, providing customers with more services and value. A foundry that can achieve perfect quality, delivery and operation is considered a reliable supplier; a flexible and cost-effective foundry can be regarded as a competitive supplier; and a foundry that is willing to share costs and opportunities is a reliable supplier. Foundries that provide fully finished parts are suppliers with value-added space; suppliers that can provide support for part optimization are considered foundries with technical capabilities; finally, those that can provide component integration are considered suppliers that can help customers optimize the supply chain. Foundries must not only provide castings for new and high-demand equipment with energy conservation, environmental protection, sustainability, clean energy, reduced CO2 emissions and carbon neutrality, but the foundries themselves must also develop towards more energy conservation, environmental protection, sustainability, reduced CO2 emissions and carbon neutrality. Most foundries use electric furnace smelting, but electricity does not necessarily come from renewable energy. Although the demand for more renewable energy can only be achieved through the supply of castings, casting production still requires a lot of energy, which is a dilemma. It requires collaborative cooperation from all aspects of the country to solve. Another challenge facing iron foundries is to gradually develop towards digitalization and intelligence while maintaining competitiveness, so that raw material supply, casting quality, customer feedback needs, new product development, etc. can be connected and "intelligent" through advanced equipment, infrastructure, automation and other technologies, so as to be able to compete and win in the new reality. Fortunately, some foundries in my country have achieved partial intelligence and are moving towards a higher level of digitalization and intelligence.
Conclusion
Cast iron, which has experienced thousands of years of development, has recently experienced a brief stagnation and decline due to some political and economic factors and the impact of the new coronavirus epidemic. However, due to the unique physical properties of cast iron, a wide range of continuously improving mechanical properties, and the ability to cast various complex castings, as well as competitive price advantages, the foundry industry is the core industry of the entire manufacturing industry and a vital part of the global industrial value chain. Cast iron is still the most important casting alloy at present and in the future. With the advancement and development of technology, the performance of cast iron will be further improved, the production of cast iron parts will inevitably increase, and there will be more and more castings with stronger functions, more complex structures, and more energy-saving and emission-reducing. At present, if cast steel is added, the share of cast iron-based alloys in the world's casting production will exceed 81%. As far as casting materials are concerned, as Professor Stefanescu says, we are still in the Iron Age.