The main defects of gray iron castings caused by smelting and pouring and their causes and prevention and control are analyzed as follows:
Part B
Cause analysis:
1. The chemical composition of the molten iron does not meet the requirements, resulting in a large solid shrinkage value, such as low carbon and high sulfur.
2. The casting contains low-melting-point slag inclusions, which reduces the high-temperature strength (because hot cracking occurs near the end of solidification, mainly due to mechanical hindrance of shrinkage at the hot section of the casting).
Prevention methods:
1. Control the reasonable chemical composition and try to keep the sulfur content in the original molten iron as low as possible.
2. When pouring, avoid slag from entering the mold cavity.
(6) Cold cracking
Characteristics and detection methods:
The crack has a relatively clean or slightly dark red oxidized surface.
The detection method is the same as hot cracking.
Cause analysis:
1. The chemical composition of the molten iron does not meet the requirements, resulting in a large solid shrinkage value
2. The phosphorus content in the molten iron is too high, which increases the brittleness and thus reduces the tensile strength of the cast iron (because cold cracking occurs after the casting is cooled, mainly at the stress concentration point at the junction of the thick and thin parts of the casting, due to thermal stress).
Prevention methods:
1. Control the reasonable chemical composition.
2. Generally, Wp in the molten iron should be below 0.15%.
(7) Slag eyes
Characteristics and detection methods:
There is slag in the holes outside or inside the casting
It can be found by visual inspection, mechanical processing or magnetic flaw detection
Cause analysis:
1. There is too much slag in the molten iron or the slag in the molten iron ladle is not completely removed, and no attention is paid to slag blocking during pouring.
2. During pouring, slag is brought in due to interruption of flow.
Prevention methods:
1. Appropriately increase the temperature of the molten iron and add a small amount of dry sand to the molten iron ladle to facilitate the removal of slag. To prevent sulfur from being transferred back into the molten iron
2. Pre-remove the residue in the molten iron ladle.
3. When pouring, pay attention to blocking the slag and prevent the flow from being interrupted.
(8) Iron beans
Characteristics and detection methods:
There are small iron beads in the pores.
It can be found by inspecting the cross section of the casting and machining.
Cause analysis:
Because the pouring temperature of the molten iron is too low, the iron beans produced after the molten iron splashes can no longer be melted by the molten iron, and as a result, they are wrapped in the casting together with the external gas; or the iron beans are wrapped in the casting together with the CO generated by FeO+C Fe+CO.
Prevention methods:
1. Appropriate pouring temperature.
2. When pouring, the flow cannot be interrupted.
(9) Cold shut and insufficient pouring
Characteristics and detection methods:
There are incompletely fused gaps or local lack of meat on the casting, and the surrounding edges are rounded.
It can be found by visual inspection.
Cause analysis:
1. The molten iron temperature is too low, which reduces the fluidity of the molten iron.
2. When the carbon and silicon content in the molten iron is low and the sulfur content is high, the fluidity of the molten iron is also reduced.
3. When pouring, if the flow is interrupted or the amount of molten iron is insufficient, cold shut is likely to occur during the second pouring.
Prevention methods:
1. Properly increase the pouring temperature of the molten iron.
2. Control the appropriate chemical composition of the molten iron and try to reduce the sulfur content.
3. Pour it all at once and avoid pouring. And do not interrupt the flow during pouring.
(10) Too hard
Characteristics and discovery methods:
White cast iron structure appears at the edge and thin wall of the casting.
It can be found by cross-section observation, hardness test, and mechanical processing.
Cause analysis:
1. Low carbon-silicon equivalent.
2. Insufficient inoculation treatment.
Prevention methods:
1. Correct ingredients.
2. Appropriately increase the inoculation amount.
