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Heat Treatment of Steel Castings in Dual Phase Region
Time：
2017-08-08 00:00
Investment casting takes the final product as a copy, presses the paraffin material into a precise and smooth wax mold, coats multiple layers of refractory material on the wax mold, and forms a mold shell after the slurry is dried and hardened, and then solidifies in the mold shell. The wax mold is melted to make it flow out, then the shell is roasted to make it firm and dry, and finally the molten liquid metal is poured into the shell. After the liquid metal cools and solidifies in the shell, it becomes a precise and smooth casting. This casting process can produce sophisticated and complex metal parts or exquisite handicrafts that are close to the final shape of the product and can be used directly without or with little processing. Therefore, it is a near-net shape metal liquid forming process. Compared with the sand casting method, it has the following characteristics:
1. The casting has high dimensional accuracy and low surface roughness, which can realize less or no cutting;
2. It can cast thin-walled parts or very light weight castings; it is especially suitable for casting castings with patterns, characters or grooves;
3. It can cast particularly complex castings, or combine and weld many parts.
China is one of the birthplaces of investment casting technology. According to research, lost-wax casting appeared in the Spring and Autumn Period 2,500 years ago. Unearthed in Suizhou, Hubei in 1978, the Zunhe plate of the tomb of Marquis Yi of Zeng is a representative work of early lost wax casting. The modern investment casting process began to take shape in the early 20th century. During the Second World War, due to the needs of national defense and aviation development, the United States, Britain and other countries first adopted the method of investment casting to produce jet turbine engine blades. Since the 1980s, the output, output value and quality of investment casting around the world have greatly increased and improved. At present, investment casting is developing in the direction of precision, large-scale, complex and integral casting.
Investment casting in my country began to be used in industrial production in the 1950s. It first gradually developed from the aerospace and aviation sectors to all industrial sectors, and was widely used.
The development status of China's precision casting industry
1. The scale of the precision casting industry in mainland China
Although the precision casting industry in mainland China has been established in the mid-1950s and has gradually been popularized and applied, the real rapid development has been in the ten years from the early 1990s to the present, especially in recent years. At present, there are two levels of investment casting processes in China. The first type of enterprises adopts the investment casting process of medium temperature mold material molding and silica sol or ethyl silicate binder shell making, that is, the international common investment casting process. . The second type is the shell-making process using low-temperature mold material and water glass binder. The dimensional accuracy and surface roughness of the castings are worse than those of the first type, which cannot reach the international investment casting level. According to the situation in the mainland, there is also a composite process of using medium-temperature or low-temperature mold materials to mold, the surface layer is made of silica sol, and the reinforcement layer is made of water glass shell, which can reduce costs while ensuring high dimensional accuracy and surface quality. According to statistics, in 2001, there were more than 350 manufacturers using the first category, with an annual output of 75,000 tons and an annual output value of 3.28 billion yuan; there were more than 1,350 manufacturers of the second category (including composite technology), with an annual output of nearly 200,000 tons and an annual output value of 3.28 billion yuan. The output value is 1.5 billion yuan. In 2002, the output reached 321,000 tons, and the output value reached 5.49 billion yuan. The output in 2002 was 6.4 times that of 1988. The output value is 16.6 times that of 330 million in 1988.
2. Technological level, casting quality and product category are viewed from the use and quality level
At present, the castings produced by precision casting in my country are mainly: aviation and gas turbine castings produced by the first-class process, the quality has reached the international level, there are 50 manufacturers, the annual output is 5,000 tons, and the annual output value is 400 million yuan; the first-class and The quality inspection items and standards of commercial precision castings produced by the composite process are lower than those of aviation parts. There are about 300 companies with an annual output of 70,000 tons and an annual output value of 2.88 billion yuan; the dimensional accuracy and surface roughness produced by the second type of process There are about 1,350 investment casting blanks with low requirements, with an annual output of nearly 200,000 tons and an annual output value of 1.5 billion yuan. It can be seen from the proportion distribution of castings at three quality levels that the output of high-level aviation precision castings is very small, accounting for only 1.8% of the total output, while general commercial precision castings account for 25.6%, and the quality does not meet international standards. Castings accounted for 72.6% of the total output. Compared with the market structure of investment castings in advanced countries and regions such as the United States, the United Kingdom, Europe and Japan, there is a clear gap, and the proportion of high-quality precision castings is still very low.
Nonetheless, all aspects of China's investment casting process have made great progress. There are also many new materials, new processes, and new equipment that have a great impact on the development of investment casting, such as water-soluble cores, ceramic cores, and metal materials. Improvements, large-scale investment casting technology, titanium alloy investment casting, directional solidification and single crystal casting, filtration technology, hot isostatic pressing, rapid prototyping technology, computer application in investment casting, and mechanized automation The development of investment casting Not only small castings can be produced, but also larger castings can be produced. The outline size of the largest investment casting is nearly 2m, while the minimum wall thickness is less than 2mm. At the same time, investment castings are becoming more precise. In addition to linear tolerances, parts can also achieve higher geometric tolerances. The surface roughness value of investment castings is also getting smaller and smaller, which can reach Ra0.4um.
3. The supporting business is gradually complete
Due to the rapid development of the precision casting industry, it has driven the rapid development of related supporting businesses. Mainly in mold processing, equipment manufacturing, raw and auxiliary material production. According to preliminary statistics, specialized factories that mainly manufacture precision casting molds with processing equipment have been formed in areas where precision casting plants are concentrated, such as Guangdong, Shandong and Wuhan, and there are many manufacturers of special equipment; The number of factories and suppliers has grown to nearly 100, and since the mold materials, zircon powder, abrasive belts and other auxiliary materials and some advanced equipment required for the production of the first type of precision casting process cannot be produced domestically, several Thousands of tons of raw and auxiliary materials. However, because my country's basic industry and resource conditions are much better than other developing countries, while the development of the precision casting service industry, it will also extend and develop their markets to Southeast Asian countries and even the world.
4. The gap and advantages between the industry and the international level are compared with the international investment casting level
In China's precision casting industry, the output of aviation precision castings produced by the first type of process is very small, and there are not many high-quality precision castings. Most of the pipe fittings, hardware, harness and auto parts that do not require high precision are produced. There are a single type of alloy, especially It is because the proportion of superalloys and non-ferrous alloys is too small, and the level of precision casting in my country is still very backward.
However, China's labor costs are relatively low, raw materials are abundant, and it has a large advantage in product price. As long as the product quality is improved, it will still be very competitive in the world. At the same time, the rise of the domestic high-tech industry, the development of the machinery industry towards precision and miniaturization, and the successive promulgation of some regulations and local regulations in the construction industry, the replacement of cast iron pipe fittings with cast copper and stainless steel castings has provided unprecedented business opportunities for the precision casting industry .
At the same time, the joining and rapid development of China's private precision casting industry has injected fresh vitality into China's precision casting industry, and has become the strongest and most realistic competitor of the national precision casting industry. From the 1980s to the early 1990s, most of these enterprises imported silica sol shell-making technology from Taiwan, Japan and other places in the form of joint ventures and sole proprietorships. They were mainly distributed in coastal areas and surrounding areas, and most of their products were sold to the international market. At the same time, because of its advanced system, flexible mechanism and efficient management, the benefits are very good. Although it has only developed for more than 20 years, it has become a new force in China's precision casting industry to undertake export business. On the other hand, due to its high level of equipment and facilities, its craftsmanship and quality are at an advanced level in China, basically reaching the general level of foreign countries. There are direct export orders and market alliances, and the product value remains at a high level, which is basically in line with international standards. In line. In terms of technical level and marketing channels, they are all powerful competitors of state-owned investment casting enterprises. In recent years, some of the second and third types of technology enterprises, represented by Dongfeng Precision Casting Co., Ltd. and other manufacturers, are accelerating their promotion to the international market regardless of the level of technology, product quality and labor productivity, and will continue to improve the level of China's precision casting industry. Active efforts.
In recent years, more and more steel castings are used at low temperature, many of which are pressure-bearing parts at low temperature. Most of these castings have strict requirements on low temperature impact toughness.
Among the various requirements for steel castings, for many steel foundries, the most difficult thing to achieve is the impact toughness. The measures usually taken to improve impact toughness are nothing more than the following:
◆Reduce carbon content;
◆Properly increase the ratio of manganese and carbon (Mn/c);
◆Add alloying elements, such as Ni, Cr, Mo, etc.;
◆Reduce sulfur content;
◆Reduce the inclusion of sulfur in steel and improve the morphology of sulfide inclusions;
◆Adopt out-of-furnace refining process;
◆Improved heat treatment
Among the above measures, improving heat treatment is the first choice that should be paid attention to, because:
◆It will not lead to major adjustment of the casting process;
◆No need to add equipment;
◆The increase in production cost is very small.
In 1978, E. G. Nisbett, R. D. ASP, and D. E. Morgan et al. published the results of a study on dual-phase region heat treatment of wrought carbon steels. For carbon steel forgings used in nuclear power plants, after heat treatment in the dual-phase region, the impact toughness is approximately doubled, while the tensile strength is only slightly decreased.
Since then, the cast steel industry in Canada and some European companies have conducted research on the heat treatment of cast steel in the dual-phase region, and obtained satisfactory results.
1. What is heat treatment in the dual-phase region
The heat treatment in the dual-phase region of the casting is based on the original normalizing or quenching of the casting, plus a second normalizing or quenching treatment that only heats to the dual-phase region.
The dual-phase region is between A1 and A3. The steel is not completely austenitized in this range, and the structure is composed of α and γ phases, so it is called the dual-phase region. Eutectoid steels do not have a dual phase region, so this treatment is only suitable for hypoeutectoid steels. The lower the carbon content of the steel, the wider the range of its dual-phase region.
The microstructure of hypoeutectoid steel with ferrite and pearlite is heated to a temperature above A1, and the phase transformation in the structure is roughly as follows:
a. Just over A1 (727℃)
The ferrite remains basically unchanged, and the pearlite gradually transforms into austenite. New austenite grains nucleate on the boundaries of pearlite clusters, and the flaky carbides in pearlite coarsen and become spherical. As the transformation progresses, carbon diffuses from the spherical carbides into the growing austenite. In this way, new ferrite grains are formed in the original spheroid. The time required for this transformation to occur in pearlite varies with the alloying element content and temperature in the steel, but is approximately 90% complete in 1 hour in the dual-phase region. The austenite formed by pearlite has an initial carbon content close to the eutectoid (ie about 0.77%).
b. Heating to a higher temperature in the dual-phase region
If the steel is heated to a higher temperature, the austenite region will further expand into the original ferrite, and the carbon content of the austenite will gradually decrease to close to the composition of the steel. Some small and isolated austenite may also be formed in the original ferrite.
When the steel castings are kept warm in the dual-phase region and then cooled, the resulting austenite is transformed into pearlite and ferrite. The main effect of the treatment is to refine the grain structure, thereby improving the toughness of the steel.
2. Main points of heat treatment process
For important steel castings, a homogenization annealing should be carried out before heat treatment to eliminate the Widmanstatten structure. The temperature of homogenization annealing is generally 100-150 ℃ above A3, and the furnace is cooled after holding at this temperature. For general steel castings, this treatment may not be carried out.
Except for steel castings with special requirements, heat treatment processes such as annealing, normalizing or quenching and tempering are generally used.
The annealed steel castings generally do not have high requirements on mechanical properties. For such castings, it is not necessary to use the dual-phase zone treatment.
For steel castings that are normalized or quenched and tempered, if the impact performance is required to be improved, the experimental study of the dual-phase region treatment can be carried out. The following opinions can be used as a reference during the test.
a. The case of normalizing treatment
Carbon steel castings and low alloy steel castings are generally normalized or normalized and tempered. During normalizing treatment, the heating temperature should be 30-50℃ above A3, and the holding time should be determined according to the thickness of the casting section, generally 1 hour per 25mm thickness.
When heat treatment in the dual-phase zone is performed, the normalized castings are reheated to a temperature above A1 and below A3, and air-cooled after heat preservation. The specific value of A3 varies depending on the carbon content and alloying element content of the steel, which can be found in the heat treatment manual. The specific temperature value of the dual-phase region treatment should be determined by the test results, and samples can be used in the laboratory first. Do different treatments to select the temperature with the best effect.
If tempering is required after normalizing, the tempering should be carried out after the dual-phase area treatment.
b. The case of using quenching and tempering treatment
For low-alloy steel castings with high mechanical properties, a quenching and tempering process should be adopted, and the quenching and tempering treatment includes two processes of quenching and tempering.
When quenching steel castings, the heating temperature is generally 20-30 ℃ above A3.
The heat treatment of the quenched and tempered steel in the dual-phase region should be carried out after quenching. The selection of the treatment temperature is the same as that of the aforementioned normalizing, and after holding at the selected temperature, it is also quenched and cooled, and the quenching medium is the same as that of the first quenching.