Scattered porosity: Distributed relatively uniformly across the entire or most of the casting section.
Subsurface porosity: Appears 1–3 mm below the casting surface as dense, fine pores.
Detection: Visible inspection, machining, shot blasting, or magnetic particle testing.
(2) Causes
High gas content in molten iron combined with low pouring temperature, preventing gases from rising and escaping:
High gas content or heavily rusted/surface greasy charge materials.
Subsurface pinholes are mainly caused by hydrogen. Silicon reduces oxygen content in cast iron but increases hydrogen content, so high-silicon cast iron is prone to hydrogen porosity. Aluminum or aluminum oxides in the charge also promote pinholes.
Wet ladle.
Improper inoculant preparation.
(3) Prevention Methods
Properly manage charge materials; clean or treat heavily rusted or greasy materials before use.
Re-melt high-gas content materials before use.
Add an appropriate amount of rare earth before pouring to remove gases.
Control suitable molten iron and pouring temperatures.
Preheat ladles, tundishes, and the pouring system.
Avoid interrupted pouring.
Fully preheat inoculants.
Ignite to release gases during pouring.
2. Composition, Microstructure, and Property Defects
(1) Characteristics and Detection Methods
Castings are too hard or too soft.
Macro- and microstructure of the section do not meet standards or technical requirements.
Detection: Section observation, chemical analysis, metallography, and hardness testing.
Correct batching and prevent material contamination.
Control appropriate overheating temperature.
Follow operational procedures and properly handle pre-inoculation.
3. Shrinkage Cavities (Distributed)
(1) Characteristics and Detection Methods
Numerous small cavities inside the casting with rough surfaces; water leakage occurs under hydrostatic pressure.
Detection: Machining or magnetic particle testing.
(2) Causes
High phosphorus content expands the solidification range. Low-melting phosphorus eutectics solidify last and are not adequately fed, causing microscopic shrinkage. This is especially notable for high-grade gray iron (low carbon) with larger volumetric shrinkage.
Pouring too quickly prevents sufficient feeding of critical areas.
(3) Prevention Methods
Control phosphorus (ωp) below 0.15% and stabilize iron chemical composition.
Pour at a moderate speed to allow full feeding.
4. Shrinkage Cavities (Concentrated)
(1) Characteristics and Detection Methods
Irregularly shaped, rough-surfaced cavities at hot spots.
Detection: Visual inspection, machining, or magnetic particle testing.
(2) Causes
Large volumetric shrinkage and improper chemical composition, particularly in high-grade low-carbon cast iron.
Excessive pouring temperature increases liquid shrinkage.
(3) Prevention Methods
Properly control iron chemistry, keeping sulfur low (generally ωs ≤ 0.12%).
Control suitable pouring temperature.
For large castings, feed extra molten iron at risers.
Increase inoculant amount if necessary.
5. Hot Cracks
(1) Characteristics and Detection Methods
Cracks with dark or nearly black oxidized surfaces.
Detection: Visual inspection, backlighting, magnetic particle testing, pressure testing, or kerosene penetration.
(2) Causes
Improper chemical composition causing large solidification shrinkage (low carbon, high sulfur).
Low-melting inclusions reduce high-temperature strength; hot cracks occur near the end of solidification at hot spots due to mechanical restraint.
(3) Prevention Methods
Control reasonable chemical composition, minimizing sulfur content.
Avoid slag entering the mold cavity during pouring.
6. Cold Cracks
(1) Characteristics and Detection Methods
Cracks with relatively clean or slightly dark red surfaces.
Detection: Same as hot cracks.
(2) Causes
Improper chemical composition causing large solidification shrinkage.
Excessive phosphorus increases brittleness, reducing tensile strength. Cold cracks occur during cooling, mainly at thick-to-thin transitions due to thermal stress.
(3) Prevention Methods
Control chemical composition, keeping sulfur content low.
Maintain phosphorus (ωp) below 0.15%.
7. Slag Inclusions
(1) Characteristics and Detection Methods
Slag in external or internal cavities of the casting.
Detection: Visual inspection, machining, or magnetic particle testing.
(2) Causes
High slag content in molten iron or incomplete slag removal in ladle; poor attention to slag during pouring.
Entrained slag due to interrupted pouring.
(3) Prevention Methods
Raise molten iron temperature slightly and add dry sand in ladle to collect slag; use lime/ash when desulfurizing to avoid sulfur re-entering molten iron.
Remove residual slag in the ladle beforehand.
Pay attention to slag control and avoid interrupted pouring.
8. Iron Beads
(1) Characteristics and Detection Methods
Small iron beads in pores.
Detection: Sectioning or machining.
(2) Causes
Low pouring temperature: splashed iron beads cannot melt back and get trapped with gases.
Surface oxidation reacts with carbon in molten iron (FeO + C → CO), entrapping the beads.
(3) Prevention Methods
Maintain suitable pouring temperature.
Avoid interrupted pouring.
9. Cold Shuts and Incomplete Filling
(1) Characteristics and Detection Methods
Unfused seams or local underfills with rounded edges.
Detection: Visual inspection.
(2) Causes
Low molten iron temperature reduces fluidity.
Low carbon and silicon, high sulfur content also reduce fluidity.
Interrupted pouring or insufficient initial pour causes cold shuts during secondary pouring.
(3) Prevention Methods
Increase pouring temperature appropriately.
Control chemical composition and reduce sulfur content.
One-time full pour; avoid interrupted or secondary pouring.
10. Over-Hardness
(1) Characteristics and Detection Methods
White iron microstructure at edges and thin walls.
