The main defects of gray iron castings caused by smelting and pouring and their causes and prevention and control are analyzed as follows:
Part A
(1) Porosity
Characteristics and detection methods:
Sieve-like pores: relatively evenly distributed on the entire or most of the cross-section of the casting.
Subcutaneous pores: densely distributed fine pores appear 1 to 3 mm from the surface of the casting.
It can be found by appearance inspection, mechanical processing, shot blasting or magnetic flaw detection.
Cause analysis:
When the gas content in the molten iron is high and the pouring temperature is too low, the precipitated gas does not have time to float up and overflow the casting.
1. The gas content of the charge itself is high, or it is severely rusted and has a lot of grease on the surface.
2. Subcutaneous pinholes are mainly caused by hydrogen. Silicon can reduce the oxygen content in cast iron, but can increase the hydrogen content, so high-silicon cast iron is prone to hydrogen pores. Pinholes are also prone to occur when the charge contains aluminum or alumina.
3. The molten iron bag is not dry.
4. The inoculant is not dry.
Prevention methods:
1. The charge should be properly managed. Charges with severe rust or excessive surface grease should be cleaned or treated before use.
2. Charges with high gas content should be remelted and regenerated before use.
3. An appropriate amount of rare earth can be added in front of the furnace to degas.
4. Control the appropriate temperature of the molten iron out of the furnace and the pouring temperature.
5. The furnace cylinder, front furnace and molten iron ladle must be dried.
6. Avoid interruption during pouring.
7. The inoculant should be fully preheated.
8. When pouring, ignition and air entrainment must be performed.
(2) Unqualified composition, structure and performance
Characteristics and discovery methods:
The material is too hard or too soft.
The macrostructure and microstructure of the casting cross section do not meet the standards or technical conditions.
It can be discovered by cross-section observation, chemical analysis, metallographic examination, hardness test, etc.
Cause analysis:
1. When the carbon-silicon equivalent is low, the material is hard, and when the carbon-silicon equivalent is high, it is soft.
2. Improper overheating of the molten iron.
3. Inadequate inoculation.
Prevention methods:
1. Correctly mix the materials and prevent material leakage during operation.
2. Control the appropriate overheating temperature.
3. Comply with the operating procedures and correctly handle the inoculation before the furnace.
(3) Shrinkage
Characteristics and detection methods:
There are many scattered small shrinkage holes inside the casting, and the surface is rough, which leaks water during the water pressure test.
It can be found by mechanical processing or magnetic flaw detection.
Cause analysis:
When the phosphorus content is high, the solidification range is expanded; at the same time, the low-melting point phosphorus eutectic cannot be supplemented during the final solidification, resulting in micro-shrinkage holes. Especially for high-grade cast iron (low carbon content), the body shrinkage rate is large, and more attention should be paid. The pouring speed is too fast, so that the parts that need to be supplemented for shrinkage cannot be supplemented with enough molten iron in time.
Prevention methods:
1. Wp is generally controlled below 0.15%, and the chemical composition of the molten iron is controlled to be stable.
2. When pouring, pour slowly to facilitate sufficient shrinkage compensation.
(4) Shrinkage holes
Characteristics and detection methods:
Irregularly shaped, rough-surfaced concentrated holes are produced at the hot spots of the casting.
They can be found by appearance inspection, machining or magnetic flaw detection.
Cause analysis:
1. Due to the large volume shrinkage rate, the chemical composition of the molten iron does not meet the technical requirements, especially high-grade low-carbon cast iron.
2. The pouring temperature is too high, which increases the liquid shrinkage value.
Prevention methods:
1. Correctly control the chemical composition of the molten iron. Try to keep Ws as low as possible, generally below 0.12%.
2. Control the appropriate pouring temperature.
3. For large pieces, molten iron can be poured at the riser.
4. Appropriately increase the inoculation amount.
(5) Hot cracking
Characteristics and detection methods:
The crack has a dark or almost black oxidized surface.
It can be detected by visual inspection, light transmission method, magnetic flaw detection, pressure test, kerosene penetration and other methods.
