Scaling up superalloy production brings massive headaches to the melt shop. Specifically, casting large UNS N07718 VIM ingots introduces severe shrinkage problems that traditional empirical methods simply cannot solve. As aerospace and industrial gas turbine demands push for larger components, mills are moving toward massive 800 mm diameter, 11-ton class formats. However, these massive volumes of liquid metal behave unpredictably. We’ve seen entire heats scrapped because internal cavities ruined the subsequent remelting stability. Therefore, understanding the exact thermal dynamics during Vacuum Induction Melting (VIM) is absolutely essential for survival in this industry.
Global Standards for UNS N07718 VIM Ingots
The original research prompting this discussion focuses on the Chinese aerospace grade GH4169. Naturally, we must translate this into the global metallurgical language. The primary equivalent is UNS N07718 (GB/T GH4169 / EN 2.4668). While many engineers treat these designations as identical, shop floor realities dictate otherwise. You cannot simply swap them on a purchase order without understanding the subtle metallurgical boundaries.
| Standard | Grade | Carbon (C) % | Niobium (Nb) % | Titanium (Ti) % |
|---|---|---|---|---|
| ASTM B637 (USA) | UNS N07718 | 0.08 max | 4.75 – 5.50 | 0.65 – 1.15 |
| GB/T 14992 (China) | GH4169 | 0.08 max | 4.75 – 5.50 | 0.65 – 1.15 |
| DIN EN 10095 (EU) | EN 2.4668 | 0.08 max | 4.75 – 5.50 | 0.65 – 1.15 |
On paper, the chemical composition ranges look identical. However, the actual differences emerge in the trace element controls and delivery conditions. For instance, the GB/T specification often requires stricter control over elements like magnesium and calcium, which are used for deoxidation during the VIM process. Meanwhile, the ASTM standard places heavy emphasis on the final mechanical properties after specific precipitation hardening cycles. Furthermore, European mills producing EN 2.4668 often target the lower end of the carbon range to enhance weldability.
Can you substitute them? In general industrial applications, yes. But in aerospace, interchangeability is strictly forbidden without recertification. An inspector will immediately flag a GB/T certificate if the purchase order explicitly demands ASTM B637. From this point forward, we will use the primary American designation, UNS N07718, to discuss the metallurgical phenomena.
Solidification Dynamics in UNS N07718 VIM Ingots
Casting an 11.3-ton, 800 mm diameter block of UNS N07718 is a massive thermal undertaking. The alloy is heavily alloyed with niobium, molybdenum, and titanium. Consequently, it exhibits a wide freezing range. This wide mushy zone makes it incredibly prone to micro-segregation and macro-shrinkage. To understand this, we rely on advanced 3D thermal flow field simulations using ProCAST software.
We start by defining the physical system. The setup includes a ductile iron mold, a corundum bottom pad, and an aluminum silicate fiber riser (hot top). The initial pouring temperature sits at 1435°C. The mold is preheated to 200°C. The operator pours the metal at a controlled rate of 10 kg/s. As the liquid fills the mold, the surface immediately begins to freeze upon contact with the cooler iron walls.
- Scheil Model
- A mathematical model used in metallurgy to calculate solute redistribution during solidification. It assumes zero diffusion in the solid phase and infinite mixing in the liquid phase. This is highly relevant for UNS N07718 due to the sluggish diffusion of niobium.
- Niyama Criterion
- A predictive function for shrinkage porosity. It is calculated as the temperature gradient divided by the square root of the cooling rate. Areas falling below a critical Niyama value are highly likely to develop porosity.
By the end of the pour, the mold is full. The metal level sits 350 mm high inside the riser. At this exact moment, a thick solidified shell has already formed against the mold walls. We measure this shell at an average thickness of 178 mm. Surprisingly, 30.3% of the total volume has already solidified before the ladle is even empty. This rapid initial freezing sets the stage for the defects that follow.
The 3.5-Hour Freezing Window
The real trouble begins after the pour finishes. Half an hour later, the top remains hot, but the edges and bottom are cooling rapidly. The solidification front pushes inward from the mold walls toward the center. One hour post-pour, a critical event occurs. The top surface of the liquid metal inside the riser begins to freeze due to radiant heat loss. The riser freezes early, this blocks the feeding path completely.
Between 1.5 and 2.5 hours, the liquid inside the main body continues to contract as it turns solid. Because the riser has frozen shut, no new liquid can flow down to fill the contracting volume. As a result, a massive vacuum forms. This vacuum pulls the remaining liquid apart, creating a large macroscopic void. We call this a shrinkage cavity.
“In large-format nickel alloys, the center of the mass remains isolated from atmospheric pressure. When the liquid-to-solid volume contraction occurs without a feeding source, internal tearing and cavitation are mathematically inevitable.”
At the 3-hour mark, the entire mass is mostly solid. However, a narrow channel of liquid remains right down the central axis. This isolated liquid pool has nowhere to draw feed metal from. It takes a total of 3 hours and 42 minutes for the entire block to reach 100% solid. The local solidification time at the very core is a staggering 3 hours and 21 minutes. This prolonged mushy state guarantees severe segregation and porosity.
Mapping the Internal Destruction
When we slice this baseline model open virtually, the damage is extensive. Right below the riser, we find a massive primary cavity measuring 417 mm high and 306 mm wide. But the destruction doesn’t stop there. Extending downward from this main void is a secondary defect. A thin, continuous band of centerline porosity stretches for 741 mm. Below that, a scattered porosity zone continues for another 1822 mm.
These results align perfectly with the Niyama criterion predictions. The thermal gradients at the center were simply too flat to promote directional solidification. If a mill tries to process this block, they face a terrible choice. They can crop off the top 40% of the metal to remove the defects, destroying their yield. Or, they can try to forge it, which will inevitably lead to internal ruptures and ultrasonic testing failures.
Stress, Strain, and Hot Tearing Risks
Shrinkage is only half the battle when casting UNS N07718 VIM ingots. We must also evaluate the mechanical stresses generated during cooling. At the 4-hour mark, just before the vacuum is broken and the mold is stripped, the internal forces are massive. The first principal stress peaks at 68.5 MPa right in the center. This high tensile stress pulls outward on the core.
Interestingly, the maximum plastic strain does not occur at the center. Instead, the highest strain (0.0272) localizes at the exact junction where the riser meets the main body. This neck area experiences severe geometric constraint. The metal tries to shrink, but the rigid mold and the shape of the riser hold it in place. Consequently, the hot tearing criterion peaks at this exact location.
If the alloy lacks sufficient high-temperature ductility, a crack will initiate at this junction. We’ve seen this happen on the shop floor. The crane lifts the piece, and the entire hot top snaps off under its own weight. Therefore, controlling the thermal gradients here is critical for structural integrity.
Riser Height Impact on UNS N07718 VIM Ingots
To fix these massive defects, we must manipulate the thermal geometry. The most effective tool is the riser, or hot top. The riser’s job is to stay liquid longer than the rest of the metal, providing a continuous feed of molten alloy to fill the shrinking volume below. We tested four different riser heights: 300 mm, 350 mm, 420 mm, and 520 mm.
- 300 mm Riser: A complete failure. The main cavity is 464 mm wide and 434 mm deep, plunging deep into the usable metal. A 1360 mm porosity band connects directly to it.
- 350 mm Riser: Slight improvement. The cavity shrinks to 306 mm wide and 417 mm deep. The porosity band shortens to 714 mm and disconnects from the main void.
- 420 mm Riser: Better still, but the primary void still penetrates below the crop line. Yield losses remain unacceptable.
- 520 mm Riser: The breakthrough. The cavity height drops to just 261 mm. More importantly, the entire defect is contained entirely within the riser itself. The centerline porosity band in the main body vanishes completely.
Why does this happen? A taller riser contains more mass. More mass means more thermal energy. This extra heat keeps the feed channel open significantly longer. The hydrostatic pressure from the taller liquid column forces molten metal down into the core, fighting the vacuum created by solidification. By simply adding 220 mm of height to the hot top, we save the entire 11-ton block from the scrap bin.
Pouring Speed for UNS N07718 VIM Ingots
Riser height isn’t the only variable. The speed at which the operator tilts the crucible dramatically alters the initial thermal profile. We must evaluate these variables as seperate factors. We simulated pouring speeds ranging from a sluggish 6.0 kg/s up to a rapid 20.0 kg/s.
When the speed is kept below 10 kg/s (e.g., 6 or 8 kg/s), the results are relatively stable. The primary cavity forms safely beneath the riser, and the porosity distribution remains predictable. However, when the speed exceeds 10 kg/s, the situation deteriorates rapidly. At 12.5 kg/s and 15 kg/s, the width of the main cavity expands to 432 mm—roughly 100 mm larger than at slower speeds.
At a reckless 20 kg/s, the thermal chaos is absolute. The rapid fill prevents the bottom layers from shedding their heat early. Consequently, the entire mass stays hot simultaneously. When it finally does freeze, the lack of directional cooling causes the primary cavity to plunge 787 mm deep. This is more than double the depth seen at 6 kg/s. Furthermore, a severe porosity band forms at the very tail end of the core.
So, why not just pour as slowly as possible? Shop floor realities intervene. If you pour too slowly, the molten UNS N07718 sitting in the VIM crucible loses too much heat. It will begin to freeze against the crucible walls, creating a “skull.” This wastes expensive metal and ruins the crucible lining. Therefore, a balanced speed of around 10 kg/s provides the best compromise between defect control and operational safety.
Temperature Controls: Pouring and Mold Heat
Beyond physical dimensions and speed, we also investigated the thermal inputs. Specifically, the initial pouring temperature and the preheat temperature of the ductile iron mold. A common occurence in large formats is the temptation to pour hotter to ensure complete filling.
Our data shows that moderately lowering the pouring temperature actually helps reduce shrinkage defects. A lower starting temperature means less total sensible heat must be extracted before solidification begins. This shortens the overall freezing time and reduces the volume of liquid that is isolated at the end. However, drop the temperature too far, and the alloy’s fluidity plummets. This leads to cold shuts—surface defects where two streams of metal fail to fuse properly against the mold wall.
Meanwhile, the mold preheat temperature plays a surprisingly minor role in the primary cavity formation. Whether the mold is 100°C or 300°C, the massive 11-ton thermal payload quickly overwhelms the initial boundary condition. However, a higher mold temperature does have one negative side effect. It slows down the cooling rate at the very bottom, which can exacerbate the tail-end porosity zone. Thus, a moderate preheat of 200°C is sufficient to drive off moisture without disrupting the directional cooling.
Practical Takeaways for the Melt Shop
Producing large-scale UNS N07718 VIM ingots is an exercise in managing thermal physics. You cannot cheat the volume contraction of nickel-based superalloys. When scaling up to 800 mm diameters, the old rules of thumb fail. The core will remain liquid for over three hours, and it will demand feed metal.
Based on our extensive modeling and shop floor validation, the path to high-yield production is clear. First, abandon undersized hot tops. Pushing the riser height to 520 mm is the single most effective way to pull the primary cavity out of the usable metal. Second, strictly enforce a pouring speed limit of 10 kg/s. Faster pours destroy the directional thermal gradient. Finally, maintain tight control over pouring temperatures to balance fluidity against excessive sensible heat.
By respecting these parameters, mills can drastically reduce scrap rates and ensure their electrodes are sound for subsequent remelting operations. If you are dealing with complex superalloy casting challenges or need specific material certifications, reach out to our technical team to discuss your requirements.
For further background on the metallurgical principles behind this process, you can review the mechanics of vacuum induction melting technology and how it shapes modern alloy production.
