Casting is a process in which a metal is smelted into a liquid that meets certain requirements and poured into a mold, and cooled, solidified, and cleaned to obtain a casting having a predetermined shape, size, and performance. Casting blanks are near-formed to achieve machining-free or small-scale processing, reducing costs and reducing time to some extent. Casting is one of the fundamental processes in the modern manufacturing industry.
There are many kinds of castings. According to the modeling method, it is customarily divided into: ordinary sand casting, including wet sand type, dry sand type and chemical hardening sand type; according to modeling materials, it can be divided into special castings with natural mineral sand as the main modeling material (such as Investment casting, clay casting, shell casting, negative pressure casting, solid casting, ceramic casting, etc.) and special casting with metal as the main casting material (such as metal casting, pressure casting, continuous casting, Low pressure casting, centrifugal casting, etc.)
According to the forming process, it can be divided into gravity casting: sand casting, permanent mold casting, pouring molten metal into the cavity by gravity; pressure casting: low pressure casting, high pressure casting. The molten metal is instantaneously pressed into the casting cavity by an additional pressure.
The casting process usually includes:
(1) Preparation of molds, molds can be divided into sand type, metal type, ceramic type, mud type, graphite type, etc. according to the materials used, which can be divided into disposable type, semi-permanent type and permanent type according to the number of times of use. Advantages and disadvantages are the main factors affecting the quality of castings;
(2) Melting and casting of cast metal, the cast metal mainly consists of cast iron, cast steel and cast non-ferrous alloy;
(3) Casting treatment and inspection. The casting treatment includes removing foreign objects on the surface of the core and casting, cutting out the riser, burrs and burrs, and heat treatment, shaping, anti-rust treatment and roughing.
The casting process can be divided into three basic parts, namely casting metal preparation, mold preparation and casting processing. Cast metal refers to the metal material used for casting castings in casting production. It is an alloy composed mainly of a metal element and added with other metal or non-metal elements. It is customarily called cast alloy, mainly cast iron. Cast steel and cast non-ferrous alloys.
Metal smelting is not only a simple melting process, but also a refining process that allows the metal to be poured into the mold to meet the expected requirements in terms of temperature, chemical composition and purity. For this reason, various inspection tests for the purpose of controlling quality are carried out during the smelting process, and the liquid metal can be allowed to be poured after reaching various specified indexes. Sometimes, in order to achieve higher requirements, the molten metal is treated outside the furnace after being discharged, such as desulfurization, vacuum degassing, refining outside the furnace, gestation or deterioration treatment. Commonly used equipment for smelting metals are cupola, electric arc furnace, induction furnace, electric resistance furnace, reverberatory furnace and the like.
Process flow introduction
With the advancement of technology and the booming of the foundry industry, different casting methods have different mold preparation contents. Taking the most widely used sand casting as an example, the mold preparation includes two major tasks: modeling material preparation and modeling core making. Sand casting, various raw materials used for molding core, such as foundry sand, sand binder and other auxiliary materials, as well as molding sand, core sand, paint, etc., which are formulated by them, are collectively referred to as modeling materials. The task of preparing materials is to follow castings. The requirements, the nature of the metal, the selection of the appropriate raw sand, binder and auxiliary materials, and then they are mixed into a certain type of molding sand and core sand in a certain proportion. Commonly used sand mixing equipment are a roller-type sand mixer, a counter-flow sand mixer and a blade grooved sand mixer. The latter is designed for mixed chemical self-hardening sand, continuous mixing and fast speed.
The shape core is based on the requirements of the casting process, based on the determination of the modeling method and the preparation of the molding material.
The accuracy of the casting and the economics of the entire production process depend mainly on this process. In many modern foundry workshops, the styling core is mechanized or automated. Commonly used sand-type styling equipments include high, medium and low pressure molding machines, sand blasting machines, boxless injection molding machines, core shooting machines, cold and hot core boxes.
After the castings are removed from the cast-cooled mold, there are gates, risers, metal burrs, drapes and mold lines. The sand-cast castings are also adhered to the sand and must be cleaned. Equipment for performing such work includes a sander, a shot blasting machine, a pouring riser, and the like. The sand falling of sand castings is a process with poor working conditions. Therefore, when selecting the modeling method, it should be considered to create convenient conditions for the falling sand cleaning. Some castings are subject to special post-processing requirements such as heat treatment, shaping, anti-rust treatment, roughing, etc.
Casting is a relatively economical method of forming blanks, which is more economical for parts with complex shapes. Such as the cylinder block and cylinder head of a car engine, ship propellers and fine art. Some hard-to-cut parts, such as nickel-base alloy parts of gas turbines, cannot be formed without casting.
In addition, the size and weight of the cast parts are wide, and the metal types are almost unlimited. The parts have general mechanical properties, and also have comprehensive properties such as wear resistance, corrosion resistance and shock absorption. Other metal forming methods such as forging , rolling, welding, punching, etc. can not be done. Therefore, the number of blank parts produced by the casting method in the machine manufacturing industry is still the largest in terms of quantity and tonnage.
Materials commonly used in foundry production are various metals, coke, wood, plastics, gas and liquid fuels, modeling materials, and the like. The equipment required is a variety of furnaces for metallurgy, various sand mixers for sand mixing, various molding machines for core making, core machines, sand falling machines for cleaning castings, and shot blasting. Machine and so on. There are also machines and equipment for special casting and many transportation and material handling equipment.
Casting production has different characteristics from other processes, mainly due to its wide adaptability, the need for materials and equipment, and pollution of the environment. Casting produces dust, harmful gases and noise that pollute the environment. It is more serious than other mechanical manufacturing processes and requires measures to control it.
For casting and mechanical structural design engineers, heat treatment is a very meaningful and highly valuable method for improving the quality of materials. Heat treatment can change or affect the structure and properties of cast iron while achieving higher strength. , hardness, and improve its resistance to abrasion and so on.
Due to different purposes, there are many types of heat treatment, which can be mainly divided into two categories. The first type is the tissue structure, which does not change or should not change through heat treatment. The second is the change of basic organizational structure. By. The first heat treatment procedure is mainly used to eliminate internal stresses which are caused by different cooling conditions and conditions during the casting process. The structure, strength and other mechanical properties are not significantly changed by heat treatment. For the second type of heat treatment, al-Qaeda has undergone significant changes and can be broadly divided into five categories:
(1) Softening annealing: The purpose is mainly to decompose the carbide, lower the hardness thereof, and improve the processing property. For the spherical stone-milled cast iron, the purpose is to obtain a ferrite structure having a high fertility.
(2) Normalization treatment: It is mainly used to improve or to obtain a uniform distribution of mechanical properties of a cast iron-completed cast product.
(3) Quenching: Mainly to obtain higher hardness or wear strength, and at the same time to a very high surface wear resistance.
(4) Surface hardening treatment: mainly to obtain a surface hardened layer, and at the same time obtain a very high surface wear resistance.
(5) Precipitation hardening treatment: mainly in order to obtain high strength, the elongation does not change drastically.
The trend in the development of foundry products is to require castings with better overall performance, higher precision, less margin and a smoother surface. In addition, the requirements for energy conservation and the voice of society to restore the natural environment are also increasing. To meet these requirements, new casting alloys will be developed, and new smelting processes and new equipment will emerge.
As the degree of mechanization automation in foundry production continues to increase, more will be developed towards flexible production to expand the adaptability to different batches and varieties. New technologies for energy conservation and raw materials will be given priority. New processes and new equipment with little or no pollution will be the first priority. Quality control technology will have new aspects in the detection, non-destructive testing and stress measurement of various processes. development of.