Steel casting

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 defects such as blowhole defects and inaccurate angle positioning during the casting process, and the casing may break during long-term use.

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Feature

1. Advantages

One of the advantages of steel castings is the flexibility of design. Designers have the greatest freedom of design choices for the shape and size of castings, especially for parts with complex shapes and hollow sections. Steel castings can use the unique process of core grouping. to manufacture. Its forming and shape change are very easy, and the conversion speed from drawing to finished product is very fast, which is conducive to quick 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 process advantages of steel casting design:

The metallurgical manufacturing of steel castings has strong adaptability and variability. Different chemical compositions and microstructure control can be selected to meet the requirements of various projects; mechanical properties and use can be selected within a wide range through different heat treatment processes. performance, and has good welding performance and processing performance.

The isotropy of the cast steel material and the overall structure of the cast steel parts are strong, thus improving the engineering reliability. Combined with the advantages of a weight-saving design and short lead times, it has a competitive advantage in terms of price and economy.

The weight of steel castings can vary within a wide range. Those with a small weight can be investment castings of only tens of grams, while the weight of large steel castings can reach several tons, tens of tons, or even hundreds of tons.

2. Disadvantages

The organization is uneven. After the liquid metal is injected into the mold, the first layer of liquid metal that is in contact with the mold wall quickly solidifies into finer grains due to the fastest drop in temperature. As the distance from the die wall increases, the influence of the die wall gradually weakens, and the crystals grow into columnar crystals parallel to each other along the direction perpendicular to the die wall. In the central part of the casting, the heat dissipation has no significant directionality and can grow freely in all directions until they contact 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.

The organization is not dense. The crystallization of liquid metal proceeds in the way of branch growth, and the liquid metal between the branches is finally solidified, but it is difficult to fill the branches with the metal liquid, resulting in the general lack of compactness in castings. In addition, if the liquid metal injected into the mold shrinks during cooling and solidification without sufficient replenishment, it may also form loose or even shrinkage cavities. Graphite in iron castings often appears in larger size flakes, spheres, or other shapes, and can also be regarded as a non-dense structure.

The surface is rough. Surfaces are generally rough, not comparable to machined surfaces, and have complex shapes.

Application

Due to the characteristics of steel castings, almost all industrial sectors need steel castings, in ships and vehicles, construction machinery, engineering machinery, power station equipment, mining machinery and metallurgical equipment, aviation, and aerospace equipment, and 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 vary greatly due to different specific conditions in various countries.

There are many varieties of steel castings, too many to list. Here is a brief description of the use of steel castings in several major industrial sectors.

1.Power station equipment

Power station equipment is a high-tech product, and its main parts run continuously for a long time under high load. Many parts in thermal power stations and nuclear power station equipment also need to withstand the corrosion of high temperature and high-pressure steam, so the reliability of the parts is affected. 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 rolling stock

Railway transportation is closely related to the safety of people’s life and property, therefore. Ensuring safety is paramount, and some key components of rolling stock, such as wheels, side frames, bolsters, couplers, etc., are traditional steel castings.

The switch used for railway switches is a component that bears strong impact and friction. The working conditions are extremely harsh and the shape is very complex.

3. Construction, construction machinery, and other vehicles

The working conditions of construction machinery and construction machinery are very poor, most of the parts are subject to high loads or need to withstand impact wear, a large part of which are cast steel parts, such as driving wheels, load-bearing wheels, rocker arms in action systems, crawler shoes, etc.

Generally, steel castings are rarely used in automobiles, but a lot of steel castings are also used in the action parts of special off-road vehicles and heavy-duty trucks.

Production

Smelting of cast steel. Cast steel must be smelted by electric furnaces, mainly electric arc furnaces and induction furnaces. According to the different lining materials and slag used, it can be divided into an acid furnace and an alkaline furnace. Carbon and low alloy steels can be smelted in any type of furnace, but high alloy steels can only be smelted in alkaline furnaces.

Casting process. The melting point of cast steel is high, the fluidity is poor, and the molten steel is easily oxidized and inhaled. At the same time, its volume shrinkage rate is 2 to 3 times that of gray cast iron. Therefore, the casting performance of cast steel is poor, and defects such as insufficient pouring, pores, shrinkage cavities, hot cracks, sand sticking, and deformation are prone to occur. To prevent the occurrence of the above-mentioned defects, corresponding measures must be taken in the process.

The molding sand used for the production of steel castings should have high refractoriness and sand resistance, as well as high strength, air permeability, and concession. The raw sand is usually made of silica sand with larger and uniform particles; to prevent sand from sticking, the surface of the cavity is mostly coated with a coating with higher refractoriness; when producing large pieces, it is mostly used in sand molds or water glass sand faster than casting molds. To improve the strength and concession of the casting mold, various additives are often added to the molding sand.

In the design of the gating system and riser. Since the cast carbon steel tends to solidify layer by layer and has large shrinkage, the principle of rigid sequential solidification is often used to set the gating system and riser. To prevent the occurrence of shrinkage cavities and shrinkage porosity. Generally speaking, steel castings should be provided with risers. Cold iron is also used more. In addition, the bottom pouring system with a simple shape and large cross-sectional area should be used as far as possible, so that the molten steel can fill the mold quickly and smoothly.

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 a particularly complex structure. Such castings have poor plasticity, large casting stress, and easy cracking. Normalizing is mainly used for steel castings with w(C)≤0.35%. Such steels have low carbon content, good plasticity, and are not easy to crack when cooled.

Common defects

Although the defects of steel castings during the casting process are similar to those caused by ingot casting, they are still process defects. Common process defects include pores, inclusions, shrinkage holes, porosity, and cracks.

Stomata (air bubbles): Stomata (air bubbles) are voids formed due to excessive gas content in the molten metal, and the model is wet and has poor air permeability. The pores in the casting are divided into single dispersed pores and dense pores.

Inclusions: Inclusions are divided into two categories: non-metallic inclusions and metallic inclusions. Non-metallic inclusions are the products formed by the chemical reaction of metal and gas during smelting or the inclusions formed by mixing refractory materials and molding sand into molten steel during casting. Metal inclusions are inclusions formed by dissimilar metals occasionally falling into molten steel and failing to melt.

Shrinkage cavities: Shrinkage cavities are defects formed due to the lack of replenishment of volume shrinkage when the molten metal cools and solidifies. Shrinkage holes are mostly located near the pouring riser and at the largest part of the section or the sudden change of the section.

Looseness: Due to poor smelting and improper mold shape, fine grain boundary cracks or fine voids are generated in the middle of the wall thickness of the steel casting, resulting in a loose structure. This part of the grain The bond between them is rather weak (cloudy shadows are formed on radiographic negatives).

Cracks: Cracks refer to defects caused by excessive low-melting-point impurities and excessive internal stress (thermal stress and tissue stress) in molten steel during the cooling process, which cause local cracking of the casting. At the sudden change of the cross-sectional size of the casting, the stress concentration is serious, and cracks are prone to occur.

To sum up, the distinctive feature of process defects in steel castings is complex shape; the use defects of steel castings are mainly fatigue cracks, including mechanical fatigue cracks and thermal fatigue cracks.

Detect

Difficulties in detection

Poor ultrasonic penetration
Coarse grains and complex interfaces such as uneven structures enhance the scattering of ultrasonic waves, and the energy attenuation is large so that the detectable thickness is smaller than that of forgings.

Lots of clutter
When the sound wave is scattered on the interface of uneven, non-dense structure and coarse grains, the scattered signal intensity is large and is received by the probe; the reflection of the sound wave by the rough casting surface will form clutter; these will be displayed on the oscilloscope screen It is a cluttered forest echo (also called a grass echo), which may drown the defect echo and hinder the identification of the defect echo.

Poor surface coupling conditions
The surface of steel castings is rough, which is not conducive to the coupling of sound, the surface hardness is large, and grinding is difficult.

Defect quantification is difficult
Due to the large attenuation of sound waves by steel castings and the complex shape of defects, the quantitative evaluation of defects based on artificial defects has a large error, and it is more difficult to use calculation methods to quantify defects.

The above is exactly the difficulty of casting inspection, and these difficulties make casting inspection subject to certain restrictions. But on the other hand, due to the low-quality requirements of castings, a single defect is allowed to have a large size and a large number, and at the same time, the parts where casting defects appear are highly regular, so casting inspection still has a certain value.

Detection method

For small and medium-sized castings (especially investment castings), which are small in size, light in weight, and less in processing, they can be magnetized in at least two roughly perpendicular directions on a stationary magnetic particle flaw detector. It is best to use direct current or pulsating direct current, and use the wet continuous method to test. The direct current method, rod penetration method, magnetic flux method, and coil method are all available.

For larger, heavier castings, magnetize local or subregions in at least two approximately 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 local or sub-regional castings. Inspection should generally be carried out in two mutually perpendicular directions.

To prevent the casting in contact with the electrode from burning out, it is recommended to take the following measures: when the contact is not in complete contact with the surface of the casting, do not connect the current, and only remove the contact when the current has been disconnected. And use adequately clean and suitable contacts. For machined smooth surfaces, the yoke method should be used.

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.

If the casting defects exceed the acceptance criteria and are rejected, but when excavation (shovel) and repair welding are allowed, the repair welding area should also pay attention to controlling the generation of delayed cracks.

The inspection should be done with the naked eye, and magnifying glass of no more than 3 times can be used only in the inspection of 001 and 01 quality levels.

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