Two-phase zone heat treatment

Investment casting is based on the final product being a copy of a paraffin-like material, pressed into a precise smooth mold, wax, coated with a multi-layered refractory material, for drying and curing the suspension to form a shell, and then solidifying in the shell. Then melt the molten metal into the shell, the liquid metal in the shell upon cooling after solidification to the precision and smoothness after casting. This casting process can produce complex metal parts or exquisite handmade products that are complex and close to the final product form, which can be used without machining or with minimal machining. Therefore, this is a process of obtaining a liquid metal surface. Compared with the sand casting method, it has the following characteristics:

1. High-precision casting size, low surface roughness, can be achieved with less or no machining;

2. Cast thin-walled parts or very light castings, especially suitable for casting with patterns, text, or groove-type castings;

3. Can cast particularly complex castings or a combination of many parts, welding.

China is one of the founders of investment casting technology. According to research, 2500 years ago, the Spring and Autumn period lost wax casting. In 1978, the Zeng Houyi tomb in Hubei Suizhou unearthed a Zun and a dish, which are representative of early wax casting. The modern investment casting process began in the early 20th century. During World War II, due to the needs of national defense and the development of aviation, the United States, Britain, and other countries first used the investment casting method to produce jet turbine blades. Since the 1980s, the production volume, product quality, and quality of investment casting worldwide have increased and improved significantly. Current investment casting is developing in the direction of precision, large-scale, complex, and component casting.

China's investment casting in the 1950s began to be used in industrial production, primarily from the aerospace industry, aviation sector, gradually developed for the entire industrial sector, widely used.

The state of development of the precision casting industry in mainland China

1. The scale of precision casting in China

Although China's casting industry was established in the mid-1950s and gradually popularized, the real rapid development occurred in the 20th century, the early 1990s, more than a decade, especially in recent years. Currently, China's investment casting has two types of horizontal technology: the first type of enterprises using medium-temperature casting material and the casting process of silica dioxide or ethyl silicate glue, that is, the international common casting process, the second category is the use of low-temperature casting material and water glass adhesive technology, the casting size accuracy and surface roughness than the first category of poor, less than the level of international investment casting. According to the mainland, as well as the use of medium or low-temperature mold forming material, the surface layer of silica sol, reinforced layer using composite process of fiberglass, to ensure high dimensional accuracy and surface quality to reduce costs. According to statistics, in 2001, the use of the first category of more than 350 manufacturers, the annual output of 75,000 tons, the annual output value of 3.28 billion yuan, the second category of technology manufacturers (including composite technology) - more than 1350, the annual output of about 200,000 tons, the output value of 1.5 billion. In 2002, the output reached 321,000 tons, the output value reached 5.49 billion yuan. The output in 2002 was 6.4 times that in 1988. While the output value is 19.6 billion in 1988, 16.6 times.

2. Process level, casting quality, and product category from the level of use and quality

Currently, China's precision casting production: the use of the first-class production of aviation and gas turbine castings, the quality of international level, manufacturers have 50, the annual output of 0.5 million tons, the annual output value of 400 million yuan; The production of composite commercial castings, the quality of control items and standards lower than the air, is about 300, the annual output of 70,000 tons, the annual output value is 2.88 billion yuan, the use of the second type of product size accuracy and surface roughness requires low casting, about 1350, the annual output of about 200,000 tons, the annual output value is 1.5 billion. Of the three types of quality castings, the distribution share can be seen, the high-level production of aviation castings is very small, only 1.8% of the total output, general commercial castings account for 25.6%, and the quality does not meet international standards, water glass casting accounted for 72.6% of the total output. Overall, and compared with the United States, Britain, Europe, and Japan and other developed countries and regions in the market structure of investment casting, there are significant gaps, the proportion of high-quality castings is still very low.

Nevertheless, all aspects of the casting process in China have made great progress, the development of investment casting has a greater impact on the development of new materials, new technologies, new equipment is also a lot, such as water-soluble core, ceramic core, metal. Such as 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 automation and mechanization. Not only can produce small castings, but also produce larger castings, the largest casting mold contour size of nearly 2 m, and the minimum wall thickness is less than 2 mm. At the same time, investment casting is also more complex, in addition to linear tolerance, parts can achieve high geometric tolerances. The surface roughness of investment casting is also getting smaller and smaller, reaching Ra0.4um.

3. Business support is gradually being completed

The rapid development of the foundry industry has led to the corresponding maintenance of rapid business development. Mainly in mold processing, equipment manufacturing, raw material production, and auxiliary materials. According to preliminary statistics, mainly for the production of foundry molds in a professional factory, it was concentrated in foundry production areas such as Guangdong, Shandong, and Wuhan, as well as other places for forming, special equipment plants, casting raw materials, and auxiliary materials. Plants and suppliers were developed to almost 100, and because the first type of casting process required for the production of molds, zircon powder, belts, and other accessories, as well as some modern equipment, is not yet domestic production, the annual import from abroad is up to 1000 tons of raw materials and auxiliary materials. However, due to China's main industrial and resource conditions compared to other developing countries being much better, the development of casting services, as well as Southeast Asian countries and the world, is expanding and developing its market.

4. The gap and advantage between the industry and the international level are compared with the level of international investment casting.

The application of China's first-class aviation casting product precision casting industry is very small, there are not many high-quality castings, most production requirements do not require pipe fittings, hardware, harnesses, and auto parts, single alloys, special Is the proportion of high-temperature alloys and non-ferrous alloys too small, the level of casting in China is still very backward.

However, China's labor costs are lower, it is rich in raw materials, and product prices have a greater advantage. If the quality of products is still very competitive internationally. At the same time, the growth of the domestic high-tech industry, machinery manufacturing to precision, miniaturization development, as well as the construction industry has introduced some laws and regulations and local rules, cast copper and stainless steel castings instead of cast iron pipe fittings for the foundry industry provide unprecedented opportunities,

At the same time, China's private foundry industry, which has joined and the rapid development of China's precision casting industry in new vitality, has become the country's most powerful casting industry, but also the most realistic competitors. Such enterprises from the 1980s to the early 1990s, mainly in the form of joint ventures and individual businesses, from Taiwan, Japan and other places, to introduce silicon dioxide shell technology, mainly in coastal and adjacent areas, most of its products are sold to the international market, But also thanks to its advanced system, flexible mechanism, efficient management, so the advantages are very good. Although only 20 years of development, but has become China's precision casting industry to undertake export business forces. On the other hand, due to the high level of equipment and capabilities, the level of technology and quality in the country is an advanced level, mainly reaching the general level of foreign countries, direct export orders and market alliances, product values remain high, basic and international standards. Regardless of the technical level or sales channels, they are state-owned enterprises, are strong competitors. And Dongfeng Precision Casting Co., Ltd. and other manufacturers in the second and third categories of technology enterprises in recent years, regardless of the level of technology, product quality and labor productivity, are accelerating to the international market and will continue to enhance the level of precision casting industry in China's positive efforts.

In recent years, for low-temperature steel castings, many of which are low-temperature support parts, such castings mainly have low-temperature impact toughness and have strict requirements.

In steel castings of various requirements for many cast steel manufacturers, the most difficult is the impact of impact toughness. Usually, to improve impact toughness and measures taken in addition to the following:

◆ Reduce carbon content;

◆ To improve the ratio of manganese to carbon (Mn/c);

◆ Add alloying elements such as Ni, Cr, Mo, etc.;

◆ Reduce sulfur content;

◆ Reduce sulfur inclusions in steel, improve sulfide inclusions in shape;

◆ Use the furnace reprocessing process;

Improved heat treatment

Among the above measures, improved heat treatment is the preferred option to be taken because:

◆ Does not lead to significant adjustment of the casting process;

◆ No need to add equipment;

◆ A small increase in production costs.

In 1978, E.G. Nisbett, R.D. ASP, and D.E. Morgan et al. published the results of two-phase heat treatment of forged carbon steels. Nuclear power plant with carbon steel forgings, two-zone heat treatment, impact toughness increased by about twice, and tensile strength only slightly decreased.

Since then, some of Canada's steel industry and some European steel two-zone heat treatment research companies, but also get satisfactory results.

1. What is two-zone heat treatment

Bimetallic heat treatment of castings is based on the normalization or quenching of castings, and then a second normalization or quenching process, which is only hot for the two-phase zone.

The two-phase zone between A1 and A3, the steel in this range is not completely austenitized, the organization consists of α plus γ two phases, the so-called two-phase region. Eutectic steel does not have a two-phase zone, so this treatment is only applicable to hypoeutectoid steel. The lower the carbon content in the steel, the wider the range of the two-phase zone.

The microstructure of ferrite plus pearlite hypoeutectoid steel, heated to a temperature above A1, has a phase transformation structure:

A. Only above A1 (727 ℃)

Ferrite remains virtually unchanged, pearlite gradually transforms into austenite. New austenite grains are nuclei at the boundary of the pearlite mass, and the lamellar carbides in the pearlite coarsen and become spherical. As the transformation process develops, carbon diffuses from the spherical carbide into the growing austenite. Thus, new ferrite grains form in the original sphere. The time required for such a change in pearlite depends on the alloying element content and temperature in the steel, but about 90% can be completed in the two-phase zone within 1 hour. Austenite formed from pearlite has an initial carbon content near the eutectoid composition (i.e., about 0.77%).

B. Heating to a higher temperature in the two-phase zone

If the steel is heated to a higher temperature, the austenite region will be further expanded to the original ferrite, the carbon content of the austenite will gradually decrease to a level close to the composition of the steel. The original ferrite may also form small and isolated austenite.

Casting steel in the two-phase region after isolation, and then cooling, the formation of austenitic low body and transformation into pearlite and ferrite. The main effect of the treatment is grain refinement, which can improve the strength of the steel.

2. Heat treatment points

Important steel castings should undergo homogenization annealing before heat treatment to eliminate the Widmanstätten structure. The homogenization annealing temperature is usually 100 to 150 °C above A3, and the furnace is cooled at this temperature. Ordinary steel castings cannot undergo such treatment.

In addition to the special requirements of cast steel, the general application of annealing, normalizing or quenching and other heat treatment processes.

Using annealed steel castings, the mechanical properties of the requirements are generally small, such castings do not require two-zone processing.

Using normalizing or quenched and tempered steel castings, such as requirements for improving impact efficiency, can be two-zone processing of experimental research, the following types can be used as a benchmark test.

A. Use normalizing treatment

Carbon steel castings and low alloy castings are usually used for normalizing or normalizing quenching process. Normalizing the treatment, the heating temperature should be greater than A3 30-50 ℃, the holding time to determine the thickness of the casting section, usually every 25 mm in 1 hour.

When heat treatment is performed in the two-phase zone, castings subjected to normalizing treatment are reheated to temperature A1 or above and A3 at a temperature below the air temperature. The specific value of A3 varies depending on the carbon content of the steel and the content of alloying elements. It can be found in the heat treatment manual. The specific temperature value of the two-phase treatment should be determined by the test results. For different processing, to choose the best temperature.

Such as normalizing after tempering, tempering should be carried out after the two-phase region.

B. Using quenching and tempering treatment

High mechanical properties of low alloy steel castings should be used for quenching and tempering, quenching and tempering, including quenching and tempering two processes.

Quenching cast steel, the heating temperature is usually above A3 20-30 ℃.

Quenched and tempered steel undergoes heat treatment in the two-phase zone and should be performed after quenching. The processing temperature is the same as normalization, and after quenching and cooling at the selected temperature, the quenching medium is the same as the first quenching.