The quality of castings has a great influence on the performance of mechanical products. For example, the wear resistance and dimensional stability of machine tool castings directly affect the precision maintenance life of machine tools; The size, accuracy and surface roughness of impeller, shell and hydraulic components of various pumps directly affect the working efficiency, energy consumption and cavitation development of pumps and hydraulic systems. The strength and cold resistance of cylinder block, cylinder head, cylinder liner, piston ring and exhaust pipe of internal combustion engine directly affect the working life of the engine.
Consumers are the supporters of the market, and their demand ability has become the only standard of enterprise production. Enterprises should set production capacity according to the market. Therefore, market research of enterprises is particularly important. It is a bridge connecting consumers and a paving stone for the final product sales.
The market investigation of casting products can make foundry enterprises fully understand the situation of casting consumer market before arranging production. Such as casting demand, demand type, demand manufacturer, product requirements and other information. In a large amount of information, foundry production enterprises can choose their own fields with broad consumer market for production; If there is no suitable one, the enterprise also has enough time to adjust the production and produce the casting products needed by the market, thus effectively preventing the overcapacity of castings.
There are many factors affecting the quality of castings. The first is the design technology of castings. When designing, besides determining the geometric shape and size of the casting according to the working conditions and metal material properties, the rationality of the design must also be considered from the perspective of casting alloy and casting process characteristics, that is, obvious size effect and solidification, shrinkage, stress and other problems, so as to avoid or reduce the occurrence of defects such as segregation, deformation and cracking of the casting. Second, there must be a reasonable casting process. That is, according to the structure, weight and size of the casting, the characteristics and production conditions of the casting alloy, the proper parting surface, modeling and core-making method are selected, and the casting rib, chilled iron, riser and gating system are reasonably set up. So as to ensure that high-quality castings are obtained. The third is the quality of casting raw materials. The quality of metal charge, refractory, fuel, flux, modifier, foundry sand, sand binder, paint and other materials is not up to standard, which will cause defects such as porosity, pinhole, slag inclusion and sand sticking. This will affect the appearance quality and internal quality of castings, and even make castings scrapped in serious cases. The fourth is the process operation. It is necessary to formulate reasonable process operation rules, improve the technical level of workers, and make the process rules be correctly implemented.
In casting production, the quality of castings should be controlled and checked. First of all, it is necessary to formulate technological procedures and technical conditions, and implement the control and inspection of raw and auxiliary materials to every specific product. Each working procedure is controlled and inspected in strict accordance with the process regulations and technical conditions. Finally, check the quality of finished castings. Should be equipped with reasonable testing methods and appropriate testing personnel. Generally speaking, the surface roughness of castings can be judged by comparing samples. Micro-cracks on the surface can be checked by coloring method and magnetic particle method. The internal quality of castings can be detected and judged by audio, ultrasonic, eddy current, X-ray and γ-ray.
The defects of sand casting castings are: cold insulation, insufficient pouring, air holes, sand sticking, sand inclusion, sand holes, sand swelling and so on.
1. Insufficient cold preservation, insufficient mold filling ability or poor mold filling conditions for pouring liquid metal. Before the cavity is full, the liquid metal will stop flowing, resulting in insufficient casting or cold insulation defects. Insufficient pouring will make the casting unable to obtain a complete shape; Although the casting can obtain a complete shape during cold insulation, the mechanical properties of the casting are seriously damaged due to incomplete fusion.
Prevent insufficient pouring and keep cold: improve pouring temperature and pouring speed.
2. Before the liquid metal crust, the pore gas did not escape in time, which led to the formation of hole defects in the casting. The inner wall of stomata is smooth, bright or slightly oxidized. After porosity is produced in the casting, its effective bearing area will decrease, and stress concentration will occur around the porosity, thus reducing the impact resistance and fatigue resistance of the casting. Porosity will also reduce the compactness of castings, leading to the rejection of some castings that need hydrostatic test. In addition, porosity also has a bad influence on the corrosion resistance and heat resistance of castings.
Prevent blowholes: reduce the gas content in molten metal, increase the permeability of sand mold, and increase the outlet riser at the highest part of the cavity.
3. A layer of hard-to-remove sand particles attached to the surface of sand-bonded castings is called sand-bonded. Sticking sand not only affects the appearance of castings, but also increases the workload of cleaning and cutting castings, and even affects the life of machines. For example, if there is sticky sand on the surface of casting teeth, it will be easily damaged. If the pump or engine and other machine parts stick to sand, it will affect the flow of fuel, gas, lubricating oil, cooling water and other fluids, and will pollute and wear the whole machine.
Anti-sticking sand: coal powder is added to the molding sand, and anti-sticking sand coating is painted on the surface of the mold.
4. When the thick and large flat castings are cast by green mold, the surface of the castings is prone to the defects of grooves and scars formed by sand inclusion.
Most of the places where sand inclusion occurs in castings are in contact with the upper surface of sand mold. Due to the radiation heat of molten metal, the upper surface of the cavity is easy to arch and warp. When the inclined sand layer is constantly washed by molten metal, it may be broken, left in its original place or brought into other parts. The larger the upper surface of the casting, the greater the volume expansion of molding sand and the greater the tendency of sand inclusion.
5. The sand holes are filled with holes on the casting or on the surface.
6. Defects caused by the displacement of the mold wall and the local expansion of the casting under the pressure of molten metal during sand expansion casting. In order to prevent sand from expanding, we should improve the strength of sand mold and the rigidity of sand box, increase the box pressing force or fastening force when closing the box, and appropriately reduce the pouring temperature to make the surface of molten metal crust early, so as to reduce the pressure of molten metal on the mold. The inspection of castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical properties test. For castings that have important requirements or are prone to problems in the casting process, nondestructive testing is needed. Non-destructive testing technologies that can be used for quality inspection of ductile iron castings include liquid penetrant testing, magnetic particle testing, eddy current testing, X-ray testing, ultrasonic testing and vibration testing.
1 detection of surface and near-surface defects of castings
1. 1 liquid penetrant detection
Liquid penetrant inspection is used to check all kinds of opening defects on the surface of castings, such as surface cracks, surface pinholes and other defects that are difficult to be found by naked eyes. The commonly used penetrant inspection is colored inspection, which is to soak or spray colored (generally red) liquid (penetrant) with strong penetrability on the surface of the casting. The penetrant penetrates into the opening defect, quickly wipes off the surface penetrant layer, and then spray an easy-to-dry developer (also called developer) on the surface of the casting. After the residual penetrant in the opening defect is sucked out, the developer is dyed to reflect the shape, size and distribution of the defect. It should be pointed out that the accuracy of penetrant detection decreases with the increase of the surface roughness of the detected material, that is, the brighter the surface, the better the detection effect, and the surface polished by a grinder has the highest detection accuracy, and even intergranular cracks can be detected. In addition to color detection, fluorescence penetrant detection is also a common liquid penetrant detection method, which needs to be equipped with ultraviolet lamp for illumination observation, and the detection sensitivity is higher than color detection.
1.2 eddy current detection
Eddy current testing is suitable for detecting defects below the surface, and the depth is generally less than 6 ~ 7 mm Eddy current testing can be divided into two types: coil placement method and penetrating coil method. When the specimen is placed near the coil with alternating current, the alternating magnetic field entering the specimen can induce a current (eddy current), which flows like a vortex in the direction perpendicular to the excitation magnetic field, and the eddy current will generate a magnetic field opposite to the excitation magnetic field, which will weaken the original magnetic field in the coil and cause the change of coil impedance. If the surface of the casting is defective, the electrical characteristics of the eddy current will be distorted, so that the defect can be detected. The main disadvantage of eddy current testing is that it can't directly display the size and shape of the detected defect, and generally only determine the surface position and depth of the defect. In addition, it is not as sensitive to detect micro-opening defects on the surface of workpiece as penetration detection.
1.3 magnetic particle testing
Magnetic particle inspection is suitable for detecting surface defects and defects several millimeters below the surface. The detection operation requires DC (or AC) magnetizing equipment and magnetic powder (or magnetic suspension). Magnetization equipment is used to generate magnetic fields on the inner and outer surfaces of castings, and show defects with magnetic powder or magnetic suspension. When the casting produces a magnetic field in a certain range, the defects in the magnetized area will produce a leakage magnetic field. When spraying magnetic powder or suspension, the magnetic powder will be absorbed, thus showing defects. The defects displayed in this way are basically defects crossing the magnetic field lines, but the long defects parallel to the magnetic field lines cannot be displayed. Therefore, it is necessary to constantly change the magnetization direction during the operation to ensure that all defects with unknown directions can be detected.
2 Detection of internal defects in castings
For internal defects, the commonly used nondestructive testing methods are radiographic testing and ultrasonic testing. Among them, X-ray flaw detection has the best effect, which can get an intuitive image reflecting the type, shape, size and distribution of internal defects, but ultrasonic flaw detection is very effective for large castings with large thickness, which can accurately measure the position, equivalent size and distribution of internal defects.
2. 1 X-ray detection (microfocus X-ray)
X-ray detection generally uses X-ray or γ-ray as the ray source, so auxiliary facilities such as equipment are needed to generate rays. When the workpiece is exposed to the radiation field, the radiation intensity of the radiation will be affected by the internal defects of the casting. The radiation intensity emitted by the casting changes locally with the size and nature of the defect, forming a radiographic image of the defect, which is developed and recorded by a radiographic film, or detected and observed by a fluorescent screen in real time or detected by a radiation counter. Among them, the method of developing and recording radiographic film is the most commonly used method, that is, radiographic inspection. The defect image reflected by radiography is intuitive, which can show the shape, size, quantity, plane position and distribution range of the defect, but it can't generally reflect the depth of the defect, and it needs special measurement and calculation to determine it. X-ray computed tomography (CT) has appeared in the international casting network, which cannot be popularized because of the expensive equipment and high use cost, but this new technology represents the development direction of high-definition ray detection technology in the future. In addition, the micro-focus X-ray system with approximate point source can actually eliminate the blurred edges produced by larger focus equipment and make the image outline clear. The use of digital image system can improve the signal-to-noise ratio of the image and further improve the clarity of the image.
2.2 Ultrasonic testing
Ultrasonic testing can also be used to check internal defects, that is, using sound beams with high-frequency acoustic energy to reflect when hitting internal surfaces or defects to find defects. The magnitude of reflected acoustic energy is a function of the directionality and nature of the inner surface or defect and the acoustic impedance of the reflector, so the acoustic energy reflected by various defects or inner surfaces can be used to detect the existence position, wall thickness or depth of subsurface defects. As a widely used nondestructive testing method, ultrasonic testing has the following advantages: high detection sensitivity, and small cracks can be detected; It has strong penetration ability and can detect thick section castings. Its main limitations are: (1) the reflection waveform of broken defects with complex outline size and poor directivity is difficult to explain; Unwanted internal structures, such as particle size, microstructure, porosity, inclusion content or finely dispersed sediments, will also hinder waveform interpretation; In addition, reference should be made to the standard test block when testing.
How to repair casting defects;
The most fundamental point of view to solve the shrinkage defect of castings is "thermal balance", and the method is as follows:
(1) rapidly solidifies at the thick hot spot formed by the machine tool casting structure, which artificially causes the temperature field in all parts of the machine tool casting to be basically balanced. Internal and external chills are used, and zircon sand, chromite or special coatings with large heat storage capacity are used locally.
(2) Reasonable process design. The inner runner is located at the opposite wall of the machine tool casting and dispersed for many hours. The molten metal that enters the thick wall first solidifies, and then solidifies at the thin wall, which basically realizes the balanced solidification everywhere. For machine tool castings with uniform wall thickness, multiple internal runners and air outlets are used. There are many internal gates, which are evenly distributed to balance the overall heat. The air holes are thin and numerous, which can not only exhaust smoothly, but also dissipate heat.
(3) changing the position of the inner runner
(4) It is very important to choose molding materials with large heat storage capacity, and it is very important to produce wear-resistant products by EPC! The substitution of chromite sand for quartz sand with low heat storage capacity will achieve good results, especially after pouring.
(5) Low temperature rapid firing and open pouring system. Make molten metal fill the mold quickly, smoothly and evenly. This should be tailored.
(6) The mould strength of nodular cast iron machine tool is high, the surface hardness is ≧90, and the sand box hardness is large, which is beneficial to eliminating shrinkage cavity.
(7) When a riser is needed, move the hot riser first and leave the hot section. If the riser is placed on the hot joint, the riser size will increase, forming "thermal heating". If it is not done well, not only shrinkage porosity is difficult to remove, but also concentrated shrinkage cavity will appear, which will reduce the process yield.
(8) Inclined placement and alloying of dies are beneficial to both. Eliminating the shrinkage defects of machine tool castings is a complicated process of understanding and implementation. According to the basic principle of "heat balance", the Holmium casting is scientifically analyzed, a reasonable technological scheme is formulated, suitable molding materials and tooling are selected, and it is operated correctly and standardized. Then the shrinkage cavity defect of any machine tool casting can be solved.
Due to various factors, defects such as blowholes, pinholes, slag inclusions, cracks and pits often occur. Commonly used repair equipment includes argon arc welding machine, resistance welding machine, cold welding machine, etc. For casting defects with low quality and appearance requirements, argon arc welding machine can be used to repair them with high calorific value and high speed. However, in the field of precision casting defect repair, due to the great influence of argon arc welding heat, secondary defects such as casting deformation, hardness reduction, sand holes, local annealing, cracking, pinhole, wear, scratch, undercut or insufficient bonding force, internal stress damage and so on will be caused during repair. Cold welding machine just overcomes the above shortcomings. Its advantages mainly lie in its small heat affected zone and no need for preheating and cold welding repair at room temperature, so there is no deformation, undercut and residual stress, no need for local annealing and no change in the metal structure of the casting. Therefore, cold welding machine is suitable for repairing the surface defects of precision castings. The welding repair range of cold welding is the process of repeated melting and accumulation of φ 1.5-φ 1.2 mm welding repair points. In the process of repairing large-area defects, repair efficiency is the only factor that restricts its wide application. For large defects, it is recommended to adopt the composite application of traditional welding repair process and casting defect repair machine. But sometimes we don't have many defects, so we don't need to invest a lot of money. We can repair it with some mending agents, which is convenient and simple. For example, for iron materials, we can fix them with JS902, and then use them when they are not used up. This can save costs for our manufacturers, let our foundry enterprises invest more money to improve product quality and let users create more wealth.
3. Casting quality inspection results
The quality inspection results of castings are usually divided into three categories: qualified products, repaired products and waste products.
3. 1 Qualified products refer to castings whose appearance quality and internal quality meet relevant standards or technical conditions for delivery and acceptance;
3.2 Repaired products refer to castings whose appearance quality and internal quality do not fully meet the standards and acceptance conditions, but are allowed to be repaired and can meet the requirements of standards and technical conditions for delivery and acceptance of castings after repair;
3.3 Scrap refers to castings whose appearance quality and internal quality are unqualified, which are not allowed to be repaired or still fail to meet the requirements of standards and technical conditions for delivery and acceptance of castings after repair. Waste products are divided into internal waste products and external waste products. Internal waste refers to the waste castings found in foundry or foundry workshop; External waste refers to the waste found after casting delivery, and its economic loss is far greater than internal waste. Factors affecting solidification mode of castings
There are many solidification methods for castings. During the solidification process of castings, the cross section is generally divided into three regions:1-solid region 2-solidification region 3-liquid region. The width of solidification zone has a great influence on solidification zone, and the solidification mode is divided according to it. First, intermediate solidification: the solidification of most alloys is between layer-by-layer solidification and paste solidification. 2. Layer-by-layer solidification: the alloy of pure metal and * * * crystal composition has no solidification zone during solidification, and the liquid phase and solid phase on the cross section are clearly separated by a boundary. With the decrease of temperature, the solid layer increases and the liquid layer decreases, reaching the center. Thirdly, paste solidification: the crystallization temperature range of the alloy is very wide. In a certain period of solidification, there is no solidification layer on the surface of the casting, and the solidification zone runs through the whole section, first pasting and then solidification. Relevant experts said that the factors affecting the solidification mode of castings can be summarized as follows: First, the temperature gradient of castings. When the crystallization temperature range of the alloy is fixed, the width of solidification zone depends on the temperature gradient of the inner and outer layers of the casting. The smaller the temperature gradient, the wider the solidification zone. Large temperature difference between inside and outside, fast cooling and narrow solidification zone. Second, the crystallization temperature range of the alloy. Small range: the narrower the solidification zone, the more inclined it is to solidify layer by layer. Such as sand casting, solidification of low carbon steel layer by layer, and solidification of high carbon steel paste.
The repairing agent for casting defects is a two-component, pasty, room-temperature curing polymer resin adhesive, and a polymer metal composite cold welding repairing material with metal and alloy as reinforcing filler. It has high bonding strength with metal, can basically keep the same color, and has the characteristics of wear resistance, corrosion resistance and aging resistance. The cured material has high strength and no shrinkage, and can be used for various mechanical processing. It has excellent properties such as wear resistance, oil resistance, water resistance and various chemical corrosion resistance, and can also resist high temperature 120℃.
use
The invention is a high-performance polymer metal material compounded by various alloy materials and modified toughened heat-resistant resin, which is suitable for repairing various metal castings and repairing and bonding various castings with defects larger than 2mm, such as blowholes, sand holes, pits, cracks, wear and corrosion. It is generally used to repair various casting defects that are not strict with color requirements, with high strength, and can be used for mechanical processing of various belt substrates. The performance of castings directly affects the processing quality, and the hardness value is an important index to determine the processing of castings.
1. Brinell hardness: it is mainly used to measure the hardness of castings, forgings, non-ferrous metal parts, hot rolled blanks and annealed parts, with the measuring range of ≯HB450.
2. Rockwell hardness: HRA is mainly used to measure the hardness and surface hardness of materials above HRC67, such as cemented carbide and nitrided steel. The measuring range is HRA >；; 70。 HRC is mainly used to measure the hardness of steel parts (such as carbon steel, tool steel, alloy steel, etc. After quenching or tempering, the measuring range is HRC20~67.
3. Vickers hardness: used to measure the hardness of thin parts and steel plate parts, and also used to measure the hardness of surface hardened parts such as carburizing, cyaniding and nitriding.