Wrinkle casting defect
Wrinkle defect is due to polystyrene foam in high temperature metal pyrolysis to form a large amount of carbon to save the ball outside the cavity. It often occurs on the cold end of the metal liquid flowing to the end or the liquid flow, usually on the top of the grinding ball. Solution:
1) Select appropriate materials for foaming grinding ball pattern. Low density casting special foam EPS has quick gasification and few residues.
2) Increase pouring temperature and pouring speed. Increasing the pouring temperature of grinding ball by 50% – 80% can speed up gasification. Increasing pouring speed can reduce heat loss.
3) Increase the vacuum. It is conducive to smoke exhaust and exhaust, improve gasification conditions, and make residues and gasification products escape quickly.
4) Improve mold permeability. Use coarse sand and coating with good air permeability.
Carbon black casting defect
The direct cause of carbon black is due to the high carbon content in nodular cast iron grinding ball. Generally, the C% of grinding ball is 3.3% ~ 3.8%. There will be the defect. Solution:
When the pouring and molding conditions are the same, the coating with good wettability and air permeability should be adopted in the process. When pouring, the air extraction should be increased, and the expandable polymethylmethacrylate resin bead EPMMA material should be used. The amount of pattern binder should be strictly controlled to effectively prevent the formation of carbon black.
Inclusion and slag inclusion casting defects
Inclusion and slag inclusion casting defects are caused by:
1) The amount of residual magnesium in nodular cast iron is high, which improves the formation temperature of oxide film on the surface of nodular cast iron. At a higher temperature, oxide film is formed, and the molten iron is oxidized twice during pouring to form slag inclusion.
2) The original molten iron has high sulfur content and generates many sulfides (MGS, ce3s4 and CES), which promotes the formation of inclusions.
3) Low pouring temperature is not conducive to the floating and removal of slag inclusion.
Solution:
- Ensure the pouring temperature > 1300 ℃, which is conducive to the accumulation and floating of inclusions and easy to remove.
- On the premise of ensuring spheroidization, try to reduce the amount of residual magnesium and reduce the sulfur content of raw molten iron.
- Rare earth magnesium alloy is used as spheroidizing agent, and rare earth is used to reduce the conjunctival temperature of molten iron oxide film.
- Add filter screen and slag collecting bag in the sprue during pouring.
Porosity casting defect
① During the gas generation process of lost foam casting cracking, the gas enters the casting and produces pores. In the process of mold filling, turbulence is generated, or in the case of top injection and side injection, part of the pattern is surrounded by metal liquid for cracking, and the generated gas cannot be discharged from the metal, resulting in pores, which are large and mostly accompanied by carbon black.
Prevention and control measures: improve the process, replace layer by layer in the process of pouring and filling, avoid turbulence and improve the pouring temperature; Improve the permeability of coating and sand mold.
② The appearance and coating layer are not dry enough to cause pores. If the pattern contains moisture, the coating is not dry well or the content of foaming agent is too high, a large amount of gas is generated during pouring, which is very easy to produce back spray. In this case, pores are most likely to occur; If the coating is not dried enough and the moisture content is too high, it is very easy to form invasive pores at this time.
Prevention measures: the appearance must be dry; The coating must be dry.
③ Too much pattern binder causes air holes. The air generation of the binder used in pattern combination bonding is too large. During liquid metal pouring and filling, a large amount of gas is generated locally, which can make the liquid metal roll during the filling process. At this time, the gas cannot be discharged in time, and the casting will produce gas holes.
Prevention and control measures: select pattern binder with low gas generation; On the premise of ensuring adhesion, the less the amount of binder, the better.
④ When pouring, the air is drawn in to form pores. In the pouring process of lost foam casting, if the sprue cannot be filled, the air will be drawn in. If these drawn in air cannot be discharged in time, the casting will produce pores.
Prevention measures: adopt closed gating system. During pouring, keep a certain amount of liquid metal in the intersection cup to ensure that the sprue is full.
Sand sticking casting defect
There are two reasons for sand sticking defects of castings:
a. The paint coating falls off or cracks, and the metal liquid infiltrates into the molding sand, which is easy to form mechanical sand sticking; When the coating selection does not match the liquid metal, and there are fine sand particles and dust in the dry sand, chemical sand sticking will be formed. b. The influence of negative pressure on the flow capacity of molten metal during pouring. The greater the negative pressure, the better the fluidity of liquid metal, and the easier it is to form bonded sand.
Solution:
For lost foam casting Castings, sand sticking defects can generally appear in various parts of the casting under different conditions: under the condition of no negative pressure, sand sticking mostly appears at the bottom or side of the casting, as well as the hot spot area of the casting and the area where molding sand is not easy to compact; During negative pressure casting, it can appear on all sides, especially at the corner of the casting and the overheating during casting of string casting.