What Is Steel Casting?

Cast steel refers to parts made of cast steel, which have similar properties to cast iron, but have better strength than cast iron. Casting steel parts are prone to defects such as porosity defects and inaccurate angular positioning during the casting process, and the shell may break in long-term use.

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 forming cores. To make. It is very easy to change the shape and shape, and the conversion speed from the pattern to the finished product is very fast, which is conducive to the quick quotation response and shortening the delivery time. The perfect shape and quality design, the smallest stress concentration factor, and the strongest overall structure all reflect the flexibility and process advantages of steel casting design:

1) Strong adaptability and variability in metallurgical manufacturing of steel castings, you can choose different chemical composition and organization control to adapt to the requirements of various different projects;

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. Applications are particularly widespread. As for the application of steel castings in various industrial sectors, due to the specific conditions of different countries, the situation may vary greatly.

The variety of steel castings is endless. A brief description of the use of steel castings in several major industrial sectors is now made.

1. Power station equipment

Power plant equipment is a high-tech product, and its main parts are continuously operated under high load for a long time. Many components in thermal power plant and nuclear power plant equipment also need to withstand the corrosion of high temperature and high pressure steam, so the reliability of the components 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 vital to ensure safety. Some key components of rolling stock, such as wheels, side frames, bolsters, couplers, etc., are traditional steel castings.

The switch for railway switch is a component that withstands strong impact and friction. The working conditions are extremely harsh and the shape is very complicated.

3.Construction, engineering machinery and other vehicles

The working conditions of construction machinery and construction machinery are very poor. Most parts are subjected to high loads or need to withstand impact wear. A large part of them is cast steel, such as active wheels, load wheels, and swing arms in mobile systems , Track shoes, etc.

In general, steel castings are rarely used in automobiles, but special off-road vehicles and heavy goods vehicles also use a lot of steel castings. ^[1]

(1) Smelting of cast steel. Cast steel must be used

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

(1) Stomata (bubbles): Stomata (bubbles) are voids that are formed because the metal liquid contains too much air, and the model is humid and has poor air permeability. The pores in the casting are divided into single dispersed pores and dense pores.

(2) Inclusions: Inclusions are classified into non-metallic inclusions and metallic inclusions. Non-metallic inclusions are products formed by the chemical reaction between metals and gases during smelting, or inclusions formed by mixing refractory materials, molding sand, etc. into molten steel during pouring. Metal inclusions are inclusions formed by dissimilar metals that occasionally fall into molten steel and fail to dissolve.

(3) Shrinkage: Shrinkage is a defect formed due to the lack of volume shrinkage when the liquid metal is cooled and solidified. Shrinkage is mostly located near the sprue and at the largest cross-section or where the cross-section is abrupt.

(4) Looseness: due to poor smelting, inappropriate mold shape, etc., fine grain boundary cracks or fine voids are generated in the middle of the wall thickness of the cast steel, resulting in a loose structure. This part of the grain The bonding between them is quite weak (cloud-like shadows are formed on the radiographic negative film).

(5) Crack: Crack refers to the molten steel, due to excessive low-melting-point impurities during cooling, coupled with internal stress (thermal stress and structural stress) caused by the local cracking of Ambassador castings and formed defects. Where the section size of the casting is abrupt, the stress concentration is severe and cracks are easy to occur.

In summary, the prominent feature of the process defects in cast steel is the complex shape; the defects in the use of cast steel are mainly fatigue cracks, including mechanical fatigue cracks and thermal fatigue cracks. ^[2]

Difficulties in steel casting detection

1. Poor ultrasonic penetration

Complex interfaces such as coarse grains and non-uniform structures have enhanced the scattering of ultrasonic waves and have a large energy attenuation, so that the detectable thickness is smaller than that of forgings.

2. More interference clutter

When the sound wave is scattered on the uneven, dense structure and coarse grain interface, the scattered signal intensity is strong and received by the probe; the rough casting surface will form clutter on the reflection of the sound wave; these will be displayed on the oscilloscope screen It is a messy forest-like echo (also known as a grass-like echo), which will possibly overwhelm the defective echo and prevent the identification of the defective echo.

3 Poor surface coupling conditions

The surface of the cast steel is rough, which is not conducive to the coupling of sound, the surface hardness is large, and it is difficult to polish.

4 Difficult to quantify defects

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

The above is exactly the difficulty of casting detection, these difficulties make casting detection limited. On the other hand, due to the lower quality requirements of castings, the size and number of individual defects are allowed to be large, and at the same time, the location of defects in castings is regular, so the detection of castings has certain value. ^[2]

Testing method for steel castings

1) For small and medium-sized castings (especially investment precision castings), they are small in size, light in weight, and small in processing volume. They can be magnetized in at least two approximately perpendicular directions on a fixed magnetic particle flaw detector. It is best to use DC or pulsating DC current and check it by wet continuous method. Direct current method, rod-through method, magnetic flux method, and coil method are all available.

2) For larger and heavier castings, magnetize the parts or zones in at least two approximately perpendicular directions. It is best to use a DC or half-wave rectified portable or mobile magnetic particle flaw detector, using the contact method or the yoke method, the dry continuous method or the wet continuous method to detect the castings locally or partitioned. The test should generally be performed in two mutually perpendicular directions.

3) In order to prevent burning of the casting that is in contact with the electrode, it is recommended to take the following measures: Do not turn on the current when the contact is not in full contact with the surface of the casting, and only remove the contact when the current has been disconnected. And use enough clean and suitable contacts. For machined smooth surfaces, the yoke method should be used.

4) Due to the influence of casting stress, some cracks (cold cracks) will delay cracking, so it should not be detected immediately after casting, but should be detected after 1 to 2 days.

5) If the casting defect exceeds the acceptance criteria and is rejected, while digging (shovel) and repair welding are allowed, the repair welding area should also pay attention to controlling the occurrence of delayed cracks.

6) The naked eye should be used for testing, and only a magnifying glass no more than 3 times can be used for testing of 001 and 01 quality levels. ^[4]