When the measured temperature of molten iron is fully up to specification, yet the liquid metal appears viscous, is difficult to pour, and exhibits extremely poor fluidity, this is known in the foundry industry as the “false cold” (or apparent cold) phenomenon.
Since temperature has already been ruled out, the core of the problem lies in the alteration of the rheological properties of the molten iron. Simply put, the internal liquid structure and suspended particles are hindering the free movement of metal atoms. Below is an in-depth analysis and troubleshooting guide for “false cold” caused by non-metallic inclusions, abnormal slag, and trace elements:
I. Core Reasons Behind the “False Cold” Phenomenon
1. Excessive Non-Metallic Inclusions (The “Brake” Effect of Suspended Solids)
When large amounts of fine, high-melting-point non-metallic inclusions (such as Al₂O₃, SiO₂, and composite silicates) exist in the molten iron, they do not easily float to the surface due to the electromagnetic stirring effect of the induction coils. Instead, they remain uniformly suspended.
- Suspension Resistance: These solid or semi-solid inclusions disrupt the continuity of the liquid metal, drastically increasing the apparent viscosity of the molten iron.
- Heterogeneous Nucleation: Excessively fine inclusions act as premature crystallization nuclei during cooling. This causes the molten iron to thicken locally before it even reaches the solidus line.
2. Abnormal Slag (Slag Emulsification and High-Viscosity Entrainment)
The physical and chemical properties of the slag have a direct impact on metal fluidity:
- Slag “Emulsification”: If the slag has a high melting point and poor fluidity, or if the induction furnace’s electromagnetic stirring is too intense, the slag cannot aggregate effectively on the surface. Instead, it gets “entrained” deep into the melt, forming a problematic slag-liquid mixture.
- Severe Oxidation: Severe rust on the charge or prolonged holding at excessive furnace temperatures leads to severe oxidation. This spikes the FeO at SiO₂ levels in the molten iron. The resulting acidic or high-viscosity slag networks directly “lock up” the fluidity of the iron.
3. Increase in Trace Elements (Formation of High-Melting Phases & Altered Surface Tension)
Certain trace elements have very low solubility in molten iron, or they easily react with O, N, at C to form high-melting-point interstitial compounds:
- Titanium (Ti) and Aluminum (Al): Even trace amounts of Ti (>0.04%) easily combine with nitrogen and carbon to form extremely hard, high-melting-point Ti(C, N) particles. Al generates chain-like or cluster-like Al O. These are the primary culprits behind the thickening of molten iron.
- Chromium (Cr) and Vanadium (V): These elements hinder carbon diffusion and widen the solidification interval of the molten iron, causing it to prematurely enter a “pasty” or mushy state during pouring.
- Surface Tension Alteration: Certain elements significantly increase the surface tension of the molten iron, preventing it from properly filling the mold cavity, which macroscopically manifests as extremely poor fluidity.
II. On-Site Emergency Treatment and Radical Prevention
To address these issues, a combination of “immediate on-site conditioning” at “source control” must be applied:
1. Immediate On-Site Treatment (In-Furnace Measures)
- Deoxidation and Inclusion Modification (Calcium Treatment):
Add an appropriate amount of calcium-silicon (Ca-Si) alloy o rare-earth (RE) based modifiers to the furnace. Calcium or rare earths convert high-melting-point, stringer-type, or clustered Al₂O₃ and silicates into low-melting-point, easily floatable spherical calcium aluminates (such as 12 CaO · Al₂O₃), drastically reducing the viscosity of the molten iron.
- Utilize High-Efficiency Slag Coagulants:
Throw high-efficiency slag coagulants or fluxing agents rich in the SiO₂ – Al₂O₃ – F system onto the melt surface to lower the melting point and viscosity of the surface slag. This allows it to fully absorb the fine inclusions suspended in the upper layer of the molten iron. Afterward, completely skim off the slag.
- Balance Static Holding and Electromagnetic Stirring:
After adjusting the furnace composition, reduce the power or turn off the power completely to let the melt sit undisturbed for 2 sa 3 minutes. This allows the suspended inclusions driven by electromagnetic stirring to float up to the surface using their own buoyancy (enhanced by the calcium treatment). Avoid prolonged, violent, over-powered electromagnetic stirring.
2. Fundamental Preventive Measures (Source Governance)
| Control Dimension | Specific Control Measures |
| Raw Material Quality Control | * Strictly limit impurity elements like Ti, Al, As, and Sb in the pig iron and steel scrap. Specifically, avoid using coated steel sheets (such as aluminized or galvanized sheets) and scrap titanium alloy steels. * Control the ratio of return scrap / foundry returns. Strictly prohibit the use of heavily rusted scrap (which is rich in iron oxides). |
| Melting Process Optimization | * Control the holding time of molten iron at high temperatures to prevent over-oxidation and excessive silicon pickup from the furnace lining materials (like silica sand), which forms micro-inclusions. * Perform proper final deoxidation at the end of the melting cycle. |
| Furnace Lining & Covering | * Regularly inspect furnace lining erosion to prevent lining spalling from entering the molten iron and forming exogenous inclusions. * Use high-quality covering agents before tapping to isolate the melt from the air, preventing secondary oxidation in the tapping launder and pouring ladle. |
