Over the past decade, our team has been deeply involved in the production of large mill housings for rolling mills, from cold rolling to hot rolling and vertical stands. These castings, some exceeding 480 metric tons in rough weight, demand rigorous control of sand casting defects. Through years of hands‑on experience, we have identified the most common defects—cracks, sand drop, slag inclusion, shrinkage porosity, and coarse grain—and developed systematic prevention and treatment strategies. This article shares our insights, emphasizing quantitative analysis and practical solutions for sand casting defects. The following image illustrates typical manifestations of such defects in large castings.
1. Crack Defects in Sand Castings
Cracks are among the most critical sand casting defects in large mill housings, often appearing at riser bases, large boss corners, window openings, and screw‑hole areas. The primary cause is localized stress exceeding the material’s strength, especially in regions with coarse grains, segregation, or pre‑existing microscopic flaws.
1.1 Root Cause Analysis
- Riser‑induced cracks: Large risers create a long solidification time, leading to coarse grains and harmful element segregation. The thermal contraction of the massive steel volume generates high tensile stress in the riser contact area.
- Corner cracks: At large bosses or abrupt section changes, the thinner parts solidify first, imposing contraction restraint on the thicker sections during cooling, resulting in hot tears.
- Window cracks: The large window openings create significant restraint against contraction. Traditional tie‑bars in the window become hot spots with severe chemical‑sand adhesion, inducing internal stress during cleaning.
- Screw‑hole cracks: The sand core for threaded holes suffers from intense thermal degradation, metal penetration, and subsequent difficult cleaning, causing excessive stress.
1.2 Preventive Measures
| Factor | Prevention | Rationale |
|---|---|---|
| Steel cleanliness | Control carbon content to lower limit in first pour; reduce carbon in topping steel; minimize non‑metallic inclusions | Reduces hot‑tear susceptibility and improves ductility |
| Section geometry | Increase corner radii; place anti‑crack ribs at transitions | Reduces stress concentration and accommodates contraction |
| Window design | Increase fillet radii; use multi‑layer compressible core materials | Decreases restraint and eases stress buildup |
| Screw‑hole core | Thicken steel jacket; embed reinforcing bars; add internal compressible layers | Resists thermal deformation and reduces cleaning stress |
| Heat treatment | Normalize to refine grains | Improves mechanical properties and stress relief |
The thermal stress in a restrained casting can be approximated by:
$$ \sigma = E \cdot \alpha \cdot \Delta T $$
where \( E \) is the elastic modulus, \( \alpha \) the coefficient of thermal expansion, and \( \Delta T \) the temperature difference between solidification and ambient. For a typical steel housing, \( E \approx 210\ \text{GPa} \), \( \alpha \approx 12 \times 10^{-6}\ \text{/°C} \), and a \( \Delta T \) of 800 °C yields a stress of about 2 GPa—far exceeding yield strength if contraction is fully restrained. Thus, reducing restraint via geometry and process modifications is essential for controlling sand casting defects.
2. Sand Drop, Sand Adhesion, and Slag Inclusion
These sand casting defects are most frequent in the cope portion of the mold. They arise from mold material degradation, poor compaction, or metallurgical impurities.
2.1 Causes
- Sand adhesion (burn‑on): When clay‑bonded silica sand or sodium silicate sand loses refractory quality due to prolonged thermal exposure, the sand surface melts and fuses with the casting.
- Sand drop (sand inclusion): Low mold compaction or over‑drying causes surface spalling; loose sand is washed into the cavity and trapped.
- Slag inclusion: Inclusions from non‑metallic slag in the melt or from tundish/tundish nozzle slag enter the mold.
2.2 Prevention
| Category | Action | Details |
|---|---|---|
| Raw material control | Qualify suppliers; perform periodic sand testing | Monitor grain fineness, clay content, and moisture |
| Mold coating and reinforcement | Apply magnesium‑oxide slurry; nail and staple the surface | Increases thermal resistance and mechanical stability |
| Pouring practice | Pour at lower temperature (just above liquidus); open all ladle nozzles simultaneously to ensure rapid fill | Minimizes mold erosion and reduces time for slag formation |
| Mold drying | Avoid over‑burning; remove delaminated areas and patch with chromite sand | Prevents surface cracking and spalling |
The critical temperature for sand adhesion can be estimated by the heat transfer equation during filling:
$$ q = h \cdot A \cdot (T_{\text{metal}} – T_{\text{mold}}) $$
Lowering \( T_{\text{metal}} \) reduces the heat flux \( q \), thereby delaying sand fusion. In practice, we control the pouring temperature to within 15 °C above the liquidus for large housings to suppress sand casting defects of this type.
3. Shrinkage Porosity and Macro‑shrinkage
Shrinkage defects are common sand casting defects in heavy sections. The large volume of steel (often exceeding 100 t) contracts by about 3–4 % during solidification. Inadequate feeding leads to internal cavities.
3.1 Location and Causes
- Under risers: If the riser modulus is insufficient or the feeding path is blocked by premature solidification, shrinkage occurs beneath the riser neck.
- Between risers: When the solidification fronts from two adjacent risers meet without proper chilling, a centerline shrinkage zone forms.
- Causes: Incorrect riser size, insufficient feed metal, delayed top‑up, or mis‑positioned internal chills.
3.2 Prevention via Modulus Calculation
The Chvorinov’s rule governs riser design. The solidification modulus \( M \) is defined as:
$$ M = \frac{V}{A} $$
where \( V \) is volume and \( A \) is cooling surface area. For a riser to feed a casting section, the riser modulus must be greater than the casting modulus by a safety factor (typically 1.2–1.3). The feeding distance \( L \) can be related to the modulus:
$$ L = K \cdot M $$
where \( K \) is a constant dependent on steel composition and casting geometry. In our practice, we compute each hot‑spot modulus using a finite‑difference approach and verify the total feed metal requirement:
$$ V_{\text{riser}} \geq \beta \cdot V_{\text{casting}} $$
with \( \beta \) the shrinkage allowance (≈ 0.04 for steel). When the initial steel weight is insufficient, we schedule a top‑up with exothermic powder and insulating sleeves to extend feeding time.
| Parameter | Formula | Example for a 480 t housing |
|---|---|---|
| Shrinkage volume | $$ V_{\text{shrink}} = 0.04 \times V_{\text{casting}} $$ | ~19.2 m³ |
| Riser modulus required | $$ M_{\text{riser}} \ge 1.2 \times M_{\text{section}} $$ | Typical section modulus 0.15 m → riser modulus ≥ 0.18 m |
| Feeding distance | $$ L = 0.3 \times M_{\text{section}} \ (\text{in meters}) $$ | For M=0.15 m, L≈0.045 m (requires multiple risers) |
2.3 External Chills
Chills increase the local cooling rate, promoting directional solidification. The chill thickness \( t \) is approximated by:
$$ t = \left( \frac{2 \cdot \lambda \cdot \Delta T}{\rho \cdot L_f} \right) \cdot \sqrt{\tau} $$
where \( \lambda \) is thermal conductivity, \( \rho \) density, \( L_f \) latent heat, and \( \tau \) solidification time. Proper placement and dimensioning of chills are critical to avoid hot‑spot formation and thus prevent sand casting defects related to shrinkage.
4. Coarse Grain Structure
Coarse grains are inherent sand casting defects in massive sections due to slow cooling. In ultrasonic testing, the lack of back‑wall echo often indicates extreme grain coarseness.
4.1 Causes
- Thick sections (>500 mm) solidify at extremely low cooling rates (0.1–0.5 °C/min), allowing dendrites to grow large.
- Heat treatment non‑uniformity: the ends near the furnace wall heat faster than the center, leading to incomplete austenitization and non‑uniform grain refinement.
4.2 Prevention
| Method | Description | Mechanism |
|---|---|---|
| Micro‑alloying with Ti | Add 0.03–0.05 % Ti into the melt | Forms fine TiN particles that act as heterogeneous nucleation sites, refining as‑cast grains |
| Controlled normalizing | Use front‑zone and rear‑zone temperature control in furnace; hold at 920–950 °C for sufficient time | Ensures uniform transformation and grain refinement throughout the cross‑section |
The grain size \( d \) is related to cooling rate \( \dot{T} \) by:
$$ d = C \cdot \dot{T}^{-n} $$
where \( C \) and \( n \) are material constants. For steel, \( n \approx 0.5 \). Increasing \( \dot{T} \) by even a small amount (e.g., from 0.2 °C/min to 0.4 °C/min) can reduce grain size by ~30 %. In practice, we achieve this by optimizing mold coating and using internal chills where feasible, thereby mitigating sand casting defects from coarse grains.
5. General Treatment of Sand Casting Defects
When sand casting defects are discovered after casting or rough machining, we follow a strict protocol to avoid secondary damage:
- Mechanical removal first: Grind or machine away defects to sound metal. For cracks, drill stop holes at both ends before removal.
- Thermal gouging: If required, preheat the local area to 150–200 °C uniformly to reduce thermal stress. Use oxy‑fuel or carbon‑arc gouging.
- NDT verification: After defect removal, perform magnetic particle (MT) and dye‑penetrant (PT) inspection to ensure no hidden cracks.
- Welding repair: Use low‑hydrogen electrodes; preheat and interpass temperature control; post‑weld stress relief.
- Re‑heat treatment: If ultrasonic testing shows no back‑wall echo, a second normalizing cycle can refine the grain structure, effectively eliminating this type of sand casting defect.
| Defect Type | Primary Treatment | Key Precautions |
|---|---|---|
| Crack | Drill stop holes, gouge, MT check, weld repair | Even preheat; avoid rapid cooling |
| Sand drop / adhesion | Grind to clean metal, blend smoothly | Do not over‑heat the base metal |
| Shrinkage cavity | Excavate completely, weld fill with matching filler | Ensure full penetration; post‑weld NDT |
| Coarse grain (UT no echo) | Re‑normalize with controlled heating cycle | Monitor furnace temperature uniformity |
Conclusion
Large mill housing castings present unique challenges in controlling sand casting defects due to their enormous size, high hydrostatic pressure, and massive steel volume. Through systematic analysis of cracks, sand adhesion, slag, shrinkage, and coarse grains, we have developed a set of preventive measures and treatment protocols that rely on fundamental metallurgical principles and empirical data. The key is to combine proper riser design (using modulus calculations), mold material quality control, optimized pouring parameters, and appropriate post‑casting heat treatment. By rigorously applying these strategies, the occurrence of sand casting defects can be minimized, ensuring the structural integrity and performance of these critical components. Our experience confirms that attention to detail at every stage—from pattern design to final NDT—is essential for success in heavy castings.