How to avoid shrinkage cavity and porosity defects during solidification of steel castings
In order to avoid shrinkage cavity and porosity defects in the solidification process of steel castings, the last solidified part of the casting must be led out of the casting body, which requires the formation of a temperature gradient of sequential solidification in the casting to make the liquid metal solidify from a lower temperature, and the last solidified part is in the vent.
Characteristics of steel casting gating system
The characteristics of steel casting gating system are high melting point, poor fluidity, large shrinkage and easy oxidation. Liquid metal is required to fill the mold quickly and stably.
The cast steel gating system has the following characteristics:
- 1. The volume shrinkage of cast steel is large, and the casting is prone to shrinkage cavity, shrinkage porosity, crack and deformation. For castings with large difference in thickness and wall thickness, the principle of directional solidification is adopted to set the gating and riser system. The liquid is best to flow through the riser into the mold cavity to strengthen the feeding capacity of the riser; For shell castings prone to crack and deformation, the inner sprue shall be evenly distributed at the thin wall of the casting, and the mechanical obstruction of the sprue to the casting shall be minimized.
- 2. The cast steel has high pouring temperature and is easy to oxidize, so the ladle leakage pouring is usually used. The slag retaining function of ladle leakage casting is good, and the requirements for slag retaining function of the gating system are not high. Therefore, the sectional area of the gating system is large, which is open, without high slag retaining function, but the mold should be filled quickly and stably. The pouring pressure of ladle leakage is large and it is easy to damage the sprue. Therefore, the pouring system should strive to be simple, firm and impact resistant. The sprue of large and medium-sized castings and the runner and ingate with liquid steel flow exceeding 1t shall be composed of refractory brick pipes. The gating system of small steel castings can be composed of water glass, resin sand or all surface sand, and ensure sufficient strength.
The gating system is a process of distributing the cast steel liquid. The setting of the gating system shall minimize the scouring of the casting, minimize the turbulence of the liquid steel in the mold cavity, and try to follow the law of sequential solidification to make the liquid steel rise smoothly in the mold cavity, of which the setting of the inner gate is the most important. The pouring system shall shorten the solidification time of the casting as far as possible and implement distribution according to demand. For the parts requiring large amount of steel water, it needs to be added. If it is designed from the shape of the casting, it is generally set with a layer at the height of 400-500 and a layer at the length direction of 400-500. In this way, the time for the steel to flow through the sprue is long, but the time to enter the casting is short, which is conducive to the solidification of the lower part first and the feeding of the lower part in the middle, The upper part is fed to the middle part to form a state of sequential solidification. Imagine that if the bottom injection type is simply considered, the molten steel in the mold cavity is constantly moving within 90 seconds of pouring, and the imaginary stable rise and layer by layer solidification caused by layered pouring are better than others, The hot spot of the gate brought by the bottom injection inner gate and the cold steel water of the upper riser have a great influence on the yield of the riser and the solidification of the casting. Therefore, it is reasonable to use bottom injection for some castings with low height and large hot spot, but it has a great impact on the complete use of bottom injection for castings with small hot spot and high height.
How to avoid shrinkage cavity and porosity defects during solidification of steel castings?
The following methods are commonly used in production:
When pouring general small steel castings or small castings with simple structure, the presence or absence of riser has little effect, because the steel castings themselves have a certain feeding capacity. When the steel casting is more complex, the function of riser is more obvious. There are two types of risers: exposed riser and concealed riser. The open riser is exposed to the air and has a fast cooling speed. It solidifies after pouring for a period of time, which isolates the metal liquid in the riser from the outside and reduces the feeding efficiency of the riser. For this, a part of the metal liquid can be poured from the riser in the final stage of pouring to strengthen the feeding effect of the riser. The position of the riser shall be determined according to the wall thickness and cooling of the casting, and shall be set at the last solidification position of the casting“
Select the appropriate position of inner gate
The position of the inner gate has a great influence on whether the casting has shrinkage cavity and shrinkage porosity defects, because the appropriate position of the inner gate can form sequential solidification and avoid defects.
- (1) When the height of the casting is small and the horizontal dimension is large, the introduction position shall generally ensure the transverse sequential solidification of the casting, and the inner gate shall be set at the thick part of the casting to introduce the alloy liquid from the thick part.
- (2) When the wall thickness of the casting is large and uniform, in order to ensure the simultaneous solidification of the casting as a whole and avoid insufficient pouring, the alloy liquid shall be evenly introduced from around the casting through more internal gates, and risers shall be set at the last solidification of each area of the casting for feeding.
- (3) When the casting has a certain height, it shall first ensure the sequential solidification from bottom to top and the simultaneous solidification in the horizontal direction. The position of the inner gate shall make the temperature distribution in the horizontal direction as uniform as possible. Generally, the inner gate shall be set at the thin wall of the casting, and the cold iron shall be placed at the thick wall. In addition, on the premise of not damaging the sequential solidification of castings, the number of inner gates should be more and evenly distributed to avoid local overheating.
- (4) For castings with complex shape and multiple hot joints, the method of combining inner gate and riser is generally used for feeding. The gating system design mostly adopts bottom injection or side injection, that is, the smaller hot joints of castings are placed at the bottom or side of the gating system, and the inner gate is set at these hot joints. During pouring, metal is injected smoothly from the bottom of the mold, The gas and impurities in the mold can be easily discharged, and a riser is set at the large hot joint on the top of the casting for feeding.
Control pouring speed
Theoretically, when the metal liquid enters the sand mold, the heat loss is directly proportional to the contact time between the metal liquid and the mold wall, and is directly proportional to the ratio of the surface area and volume of the metal liquid. The pouring speed affects the time when the liquid metal contacts the mold wall, so controlling the pouring speed can change the temperature difference in the casting. The slower the pouring speed is, the greater the temperature difference in the casting is. However, the speed should not be too slow, otherwise it is easy to form defects such as cold insulation and insufficient pouring. The large plane casting should not be poured slowly, otherwise it will lead to sand falling defects due to long-time heating.
Modify casting structure
For steel castings with complex structure and poor casting processability, shrinkage cavity and porosity cannot be completely eliminated only from the design of gating system. In order to obtain high-quality castings, the casting structure can be changed appropriately in consultation with the machining unit, so as to improve the technical performance of castings. There are two main methods:
- (1) Increase process subsidies. In order to ensure sequential solidification and facilitate riser feeding, increase process subsidies between riser and hot joint, which are generally removed during machining. Due to the existence of process subsidies, the feeding channel is increased, so that the feeding channel solidifies later than the hot spot, so that the castings realize sequential solidification.
- (2) Increase the thickness of the metal layer reserved on the machining surface of the casting and ready to be removed by cutting, which is called machining allowance. Excessive machining allowance will waste metal and machining hours and increase the cost of parts. Therefore, the machining allowance should be as small as possible, but in order to meet the needs of the casting process, sometimes it should be appropriately increased and gradually increased towards the gate, which can play the role of process subsidy and strengthen feeding.
Of course, different castings have different problems. I won’t list them one by one. Different methods are adopted according to different technical requirements and structures of castings. I just want to throw a brick to attract jade. These are my personal views. If you can’t find anything, please correct it at any time.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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