Large Castings
Steel castings refer to parts made of cast steel, which have similar properties to cast iron but are stronger than cast iron. Steel castings are prone to shortcomings such as pore defects and inaccurate angular positioning during the casting process, and the casing may break in long-term use.
1. Advantages
One of the advantages of steel castings is the flexibility of design. Designers have the greatest design freedom in the shape and size of the castings, especially for parts with complex shapes and hollow sections. Steel castings can use the unique process of the core assembly.
To make. Its forming and shape change are very easy, and the conversion speed from drawing to finished product is very fast, which is conducive to rapid quotation response and shortening of delivery time.
The perfect design of shape and quality, the smallest stress concentration factor, and the strongest overall structure, all reflect the flexibility and technological advantages of steel casting design:
1) The metallurgical manufacturing of steel castings has strong adaptability and variability. Different chemical compositions and structure control can be selected to adapt to the requirements of various projects; the mechanical properties and use can be selected in a larger range through different heat treatment processes Performance and has good welding performance and processing performance.
2) The isotropy of the cast steel materials and the strong overall structure of the cast steel parts improve engineering reliability. Coupled with the advantages of reduced weight design and short delivery time, it has a competitive advantage in terms of price and economy.
3) The weight of steel castings can vary within a wide range. The small weight can be investment precision castings of only tens of grams, while the weight of large steel castings can reach several tons, dozens of tons, or even hundreds of tons.
2. Disadvantages
(1) Uneven organization. After the liquid metal is injected into the mold, the layer of liquid metal that first contacts the mold wall has the fastest temperature drop, so it solidifies quickly into finer grains.
As the distance from the mold wall increases, the influence of the mold wall gradually weakens, and the crystals grow into columnar crystals parallel to each other along the direction perpendicular to the mold wall. In the center of the casting, the heat dissipation has no significant directionality, and it can grow in all directions until it comes into contact with each other, so an equiaxed crystal region is formed. It can be seen that the structure in the casting is not uniform, and generally speaking, the grains are relatively coarse.
(2) The organization is not dense. The crystallization of liquid metal proceeds in the way of branch growth, and the liquid metal between the branches finally solidifies, but it is difficult for the branches to be completely filled by the liquid metal, which causes the general non-compactness of castings.
In addition, the liquid metal injected into the mold shrinks in volume during cooling and solidification without being sufficiently replenished, and may also form loose or even shrinkage holes. Graphite in iron castings often appears in larger-sized flakes, spheres, or other shapes, and can also be regarded as a non-compact structure.
(3) The surface is rough. The surface is generally rough and cannot be compared with the machined surface, and the shape is also more complicated Due to the characteristics of steel castings, almost all industrial sectors need to use steel castings in ships and vehicles, construction machinery, engineering machinery, power station equipment, mining machinery and metallurgical equipment, aviation, and aerospace equipment, oil wells, and chemical equipment, etc.
The application is particularly extensive. As for the application of steel castings in various industrial sectors, the situation may be quite different due to different specific conditions in various countries.
There are many varieties of steel castings. Here is a brief description of the use of steel castings in several major industrial sectors.
Application of steel castings
1. Power station equipment
Power plant equipment is a high-tech product, and its main parts are operated continuously for a long time under high load. Many parts of the thermal power plant and nuclear power plant equipment still need to withstand the corrosion of high temperature and high-pressure steam, so the reliability of the parts There are very strict requirements.
Steel castings can meet these requirements to the greatest extent and are widely used in power station equipment.
2. Railway locomotives and vehicles
Railway transportation is closely related to the safety of people's lives and property, therefore. It is very important to ensure safety. Some key components of rolling stock, such as wheels, side frames, bolsters, couplers, etc., are all traditional steel castings.
The switch used in railway switches is a component that withstands strong impact and friction. The working conditions are extremely harsh and the shape is very complicated.
3. Construction, construction machinery, and other vehicles
Large Double Helical Gears Made of Castings Steel
The working conditions of construction machinery and engineering machinery are very poor. Most of the parts are subjected to high loads or need to withstand impact and wear. A large part of them are steel castings, such as driving wheels, load-bearing wheels, and rocker arms in mobile systems. , Track shoes, etc.
Steel castings are rarely used in general automobiles, but a lot of steel castings are also used in the moving parts of special off-road vehicles and heavy trucks.
Produce
(1) Smelting of cast steel. Cast steel must be smelted in electric furnaces, mainly electric arc furnaces and induction furnaces. According to the lining material and the slag system used, it can be divided into acid furnace and alkaline furnace. Carbon steel and low alloy steel can be smelted in any furnace, but high alloy steel can only be smelted in an alkaline furnace.
(2) Casting process. Cast steel has a high melting point, poor fluidity, and molten steel is easy to oxidize and get gas. At the same time, its volume shrinkage is 2 to 3 times that of gray cast iron. Therefore, the casting performance of cast steel is poor, and it is prone to defects such as insufficient pouring, porosity, shrinkage cavity, thermal cracking, sand sticking, and deformation.
In order to prevent the above defects, corresponding measures must be taken in the process.
The molding sand used in the production of steel castings should have high refractoriness and anti-sticking properties, as well as high strength, air permeability, and retreat.
The raw sand usually uses large and uniform silica sand; in order to prevent sand sticking, the surface of the cavity is often coated with a higher refractory paint; when producing large parts, it is mostly used in sand or water glass sand faster than casting. In order to improve the strength and retreat of the mold, various additives are often added to the molding sand.
In the design of the gating system and riser. Since cast carbon steel tends to solidify layer by layer and shrinks greatly, the principle of rigid sequential solidification is used to set up the gating system and riser. To prevent shrinkage and shrinkage. Generally speaking, risers are required for steel castings. Cold iron is also used more. In addition, a bottom-pouring pouring system with a simple shape and a large cross-sectional area should be used as much as possible to make the molten steel fill the mold quickly and smoothly.
(3) Heat treatment. The heat treatment of cast steel is usually annealing or normalizing. Annealing is mainly used for steel castings with w(C)≥0.35% or particularly complex structures. Such castings have poor plasticity, high casting stress, and easy cracking. Normalizing is mainly used for steel castings with w(C)≤0.35%. This type of steel has low carbon content, good plasticity, and is not easy to crack during cooling.
Common defects
Although the defects produced in the casting process of steel castings are similar to those produced by ingot casting, they are still process defects. Common process defects include pores, inclusions, shrinkage holes, porosity, and cracks.
(1) Pores (bubbles): Pores (bubbles) are voids formed due to excessive gas content in the molten metal, moisture, and poor air permeability of the model. The pores in the casting are divided into single dispersed pores and dense pores.
(2) Inclusions: Inclusions are divided into non-metallic inclusions and metallic inclusions. Non-metallic inclusions are the products formed by the chemical reaction between metal and gas during smelting or the inclusions formed by mixing refractory materials and molding sand with molten steel during casting. Metal inclusions are inclusions formed by dissimilar metals that occasionally fall into the molten steel and fail to melt.
(3) Shrinkage cavities: Shrinkage cavities are defects formed because the volume shrinkage of the molten metal cannot be supplemented during cooling and solidification. Shrinkage holes are mostly located near the pouring riser and the largest part of the cross-section or the sudden change of the cross-section.
(4) Porosity: due to poor smelting, improper mold shape, etc., fine grain boundary cracks or fine voids are generated in the middle of the wall thickness of the steel casting, and the loose structure is formed. This part of the grain The combination between them is quite weak (the formation of cloud-like shadows on the radiographic film).
(5) Crack: Crack refers to the defect formed by partial cracking of the casting due to excessive low melting point impurities during the cooling process and excessive internal stress (thermal stress and structural stress). Where there is a sudden change in the section size of the casting, the stress concentration is serious and cracks are easy to appear.
In summary, the significant feature of process defects in steel castings is their complex shape; the defects in use of steel castings are mainly fatigue cracks, including mechanical fatigue cracks and thermal fatigue cracks.
Detect
Difficulties in detection
1. Poor ultrasound penetration
Coarse crystal grains, uneven structure, and other complex interfaces, all enhance the scattering of ultrasonic waves, and the energy attenuation is large so that the detectable thickness is smaller than that of forgings.
2. Many interference clutter
When the sound wave is scattered on the uneven, non-dense structure and the coarse grain interface, the intensity of the scattered signal is larger and is received by the probe; the rough casting surface will form clutter on the sound wave reflection; these will be displayed on the oscilloscope screen It is a messy forest-like echo (also called grass-like echo), which may flood the defect echo and hinder the identification of the defect echo.
3. Poor surface coupling conditions
The surface of the steel casting is rough, which is not conducive to the coupling of sound, the surface hardness is large, and it is difficult to polish.
4. It is difficult to quantify defects
Due to the large attenuation of sound waves by steel castings and the complicated shape of defects, the quantitative evaluation of defects based on artificial defects has large errors, and it is more difficult to quantify defects by calculation.
The above is exactly the difficulty of casting inspection, these difficulties make casting inspection subject to certain restrictions. But on the other hand, due to the lower quality requirements of castings, a larger size and larger number of single defects are allowed, and the regularity of the places where casting defects appear is strong, so casting inspection still has a certain value.
Detection method
1) For small and medium-sized castings (especially investment precision castings), which are small in size, light in weight, and less processed, they can be magnetized in at least two substantially perpendicular directions on a fixed magnetic particle inspection machine.
It is best to use direct current or pulsating direct current, and use the wet continuous method for inspection. Direct current method, rod-through method, flux method, and coil method are all available.
2) For larger and heavier castings, magnetize parts or zones in at least two substantially perpendicular directions. It is best to use a portable or mobile magnetic particle flaw detector with DC or half-wave rectification, and use the contact method or yoke method, dry continuous method, or wet continuous method to detect parts or partitions of castings. Testing should generally be carried out in two mutually perpendicular directions.
3) In order to prevent burning of the casting in contact with the electrode, it is recommended to take the following measures: when the contact is not in full contact with the surface of the casting, no current is connected, and the contact is only removed when the current has been disconnected. And use sufficiently clean and suitable contacts. For smooth and clean surfaces that have been machined, the yoke method should be used.
4) Due to the influence of casting stress, some cracks (cold cracks) of steel castings will delay cracking, so they should not be tested immediately after casting but should be tested after 1 to 2 days.
5) If the casting defect exceeds the accepted standard and is rejected, and digging (shovel) and repair welding are allowed, the repair welding area should also pay attention to control the delayed cracks.
6) The inspection should be done with the naked eye, and a magnifying glass no more than 3 times can be used only in the inspection of 001 and 01 quality levels.
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