Čo je zhutnená grafitová liatina na odlievanie piesku?

1. Zavedenie

Sand casting has powered the iron foundry industry for centuries, enabling the production of complex geometries at relatively low cost.

Recently, Compacted Graphite Iron (CGI)—also known as vermicular graphite iron—has emerged as a material bridging the gap between traditional gray cast iron and ductile iron.

By combining desirable properties of both, CGI offers higher tensile strength and thermal conductivity than gray iron, yet retains superior castability and damping compared to ductile grades.

V tomto článku, we examine “what is sand casting with CGI?“ through metallurgical, spracovanie, mechanický, and economic lenses.

We aim to present a comprehensive yet practical resource for foundry engineers, design professionals, and materials researchers interested in harnessing CGI’s benefits.

2. Compacted Graphite Iron (CGI): Metallurgy and Properties

Compacted (vermicular) graphite iron (CGI) occupies an intermediate position between gray iron and ductile iron:

its unique graphite morphology yields a combination of strength, stiffness, and thermal properties not attainable in other cast irons.

Graphite Morphologies: From Gray to Ductile to CGI

Graphite in cast iron appears in three primary morphologies. Each influences mechanical and thermal behavior:

• Šedé železo: Flake graphite provides crack‐arresting behavior under vibration but limits tensile properties.
• CGI: Vermicular graphite appears as short, compact “worms” (compactness factor ≥ 60 %), enhancing strength and conductivity while retaining acceptable damping.
• Ťažko: Graphite occurs as nearly perfect nodules; this maximizes ductility but reduces damping and thermal conduction compared to CGI.

Chemical Composition and Alloying Elements

Chemically, CGI resembles ductile iron but requires tighter control of certain elements, especially magnesium and sulfur, to achieve the desired vermicular graphite form.

Typical target composition (EN-GJV-450-12) appears below:

Key Points:

• Maintaining Mg between 0.035 % a 0.055 % (± 0.005 %) is essential; falling outside this window shifts graphite morphology.
• Síra must remain extremely low (< 0.01 %)—even 0.015 % S can tie up Mg as MgS, preventing vermicular graphite formation.
• Kremík levels above 2.5 % encourage graphite flake growth and a more ferritic matrix, improving thermal conductivity but potentially reducing strength if excessive.

Mikroštruktúra: Vermicular Graphite in a Ferritic/Pearlitic Matrix

The as‐cast microstructure of CGI depends on solidification rate, očkovanie, and final heat treatment. Typical features include:

• Matrix Control: A ferritic matrix (≥ 60 % ferit) yields thermal conductivity of 40–45 W/m·K,
  zatiaľ čo ferrite–pearlite mixes (30 % - 40 % perlit) push yield strength to 250 - 300 MPA without excessive embrittlement.
• Vermicular Graphite Nodule Count: Cieľ 100 - 200 vermicular flakes/mm² in sections ~ 10 mm hrubá. Lower counts reduce strength; higher counts risk transitioning to nodularity.

Mechanické vlastnosti (Sila, Stiffness, Únava)

CGI’s mechanical properties combine strength, stiffness, and moderate ductility. Representative values (EN-GJV-450-12, normalizované) appear below:

Observations:

• Vysoký Youngov modul (E ≈ 175 GPA) leads to stiffer components—advantageous in engine blocks and structural parts requiring minimal deflection.
• Fatigue resistance (≈ 200 MPA) makes CGI suitable for cyclical loads (Napr., cylinder heads under thermal cycles).
• Tvrdosť can be tailored via matrix composition: pure ferritic CGI (~ 115 HB) excels in wear applications; pearlitic CGI (~ 180 HB) is chosen for higher strength needs.

Thermal Conductivity and Damping Capacity

CGI’s unique graphite form and matrix produce distinctive thermal and vibrational characteristics:

• Tepelná vodivosť: High conductivity (40 W/m · k) accelerates heat dissipation from hot spots in engine blocks and turbocharger housings, reducing thermal fatigue risk.
• Damping: CGI’s damping factor (0.004 - 0.006) absorbs vibrational energy better than ductile iron, aiding noise, vibrovanie, and harshness (NVH) control—especially in diesel engines.
• Koeficient tepelnej expanzie: CGI’s expansion (≈ 11.5 x 10⁻⁶/°C) matches steel engine liners closely, minimizing thermal stresses at the liner/block interface.

3. Čo je zhutnená grafitová liatina na odlievanie piesku (CGI)?

Odlievanie piesku with compacted graphite iron (CGI) follows the same overall steps as conventional iron sand casting,

mold preparation, taviace sa, nalievanie, stuhnutie, and cleaning—but modifies key parameters to produce CGI’s unique “vermicular” graphite morphology.

Defining the Process

Pattern and Mold Construction

• Pattern Design: Foundries create patterns (often from wood, epoxid, or aluminum) that include allowances for 3–6 % shrinkage typical of CGI alloys (solidus ~ 1 150 ° C, liquidus ~ 1 320 ° C).
• Sand Selection: Standard silica‐sand molds (priepustnosť > 200, AFS grain fineness ~ 200) work well,
  but enhanced binders—phenolic–urethane or furan—help resist CGI’s higher pouring temperature (~ 1 350–1 420 ° C).
• Cope and Drag Assembly: Technicians pack the drag around the lower half of the pattern, then remove the pattern and place cores (if needed) before ramming the cope.
  Careful vent placement ensures gas escape when high‐temperature CGI fills the cavity.

Melting and Metal Treatment

• Charge Composition: Typical melts use 70–80 % recycled scrap, 10–20 % pig iron or hot‐metal,
  and master alloys to fine-tune chemistry. Foundries aim for C 3.5 ± 0.1 %, A 2.5 ± 0.2 %, and S < 0.01 %.
• Magnesium and Rare-Earth Additions: Right before pouring, operators add 0.035–0.055 % Mg (alongside 0.005–0.015 % RE/Ce) in a covered ladle to form vermicular graphite rather than flakes or spheroids.
  They stir gently to distribute modifiers uniformly.
• Inoculation and De-Oxidation: Foundries inoculate with ~ 0.6–1.0 kg/T of ferrosilicon or barium-silicon inoculant to provide graphite nucleation sites.
  Súčasne, de-oxidants—such as FeSi—scavenge dissolved oxygen and minimize oxide inclusions.

Pouring and Mold Filling

• Superheat Management: Pouring temperature for CGI sits around 1 350–1 420 ° C (2 462–2 588 °F), roughly 30–70 °C above the liquidus.
  This extra superheat ensures complete filling of thin wall sections (až po 4 mm) but also increases the risk of sand erosion.
• Dizajn brány: Foundries use a tapered sprue and generous runner cross-sections, sized for a Reynolds number (Re) z 2 000–3 000—to minimize turbulence.
  Ceramic foam filters (30–40 ppi) often intercept any inclusions carried into the mold.
• Mold Venting: Because CGI fluidity rivals gray iron, proper venting—through bottom vents under risers and controlled permeability—prevents gas entrapment.
  Specialized risers (exothermic or insulated) feed molten metal into the last-to-solidify hot spots.

Solidification and Microstructure Control

• Graphite Nucleation: As the molten CGI cools from ~ 1 350 ° C do 900 ° C, vermicular graphite nucleates on inoculant sites.
  Foundries target a cooling rate of 0.5–2.0 °C/s in sections between 10–15 mm thick to develop 100–200 vermicular flakes per mm².
• Matrix Formation: Nižšie 900 ° C, the austenite-to-ferrite transition begins.
  Rapid cooling yields more pearlite (higher strength but lower thermal conductivity), while moderate cooling produces a primarily ferritic matrix (better heat dissipation).
  Foundries often normalize at 900 °C after shakeout to achieve a 60 % ferrite–40 % pearlite balance.
• Shrinkage Feeding: CGI shrinks by approximately 3.5 % upon solidification. Risers sized at 10–15 % of casting mass—positioned at strategic hot spots—mitigate shrinkage porosity.

Shakeout, Upratovanie, and Final Processing

• Shakeout: After 30–45 minutes of cooling, foundries break away mold sand using vibrating tables or pneumatic rams. Reclaimed sand undergoes screening and reclamation for reuse.
• Upratovanie: Otryskanie (for ferrous) or air-carbon arc cutting removes residual sand, sprues, and risers. Technicians inspect for surface cracks or fins before heat treatment.
• Tepelné spracovanie (Normalization): CGI castings typically normalize at 900 ° C (1 652 °F) for 1–2 hours, then air or oil quench.
  This step refines grain size and ensures consistent ferrite–pearlite distribution.
• Machining and Inspection: After normalization, castings reach final hardness (ferritic CGI ~ 115 HB; pearlitic CGI ~ 180 HB).
  CNC centers machine critical surfaces (tolerances ± 0.10 mm) and inspectors verify graphite morphology (vermicularity ≥ 60 %) via metallography.

Key Differences from Gray Iron Sand Casting

4. Advantages and Challenges of Sand Casting Compacted Graphite Iron (CGI)

Advantages of Sand Casting CGI

Enhanced Strength and Stiffness

CGI’s tensile strength (400– 450 MPa) exceeds gray iron by 50 %, while its modulus of elasticity (170–180 GPa) surpasses gray iron by 50 %.

V dôsledku, CGI castings exhibit less deflection under load—particularly valuable for engine blocks and structural components.

Improved Thermal Conductivity

With thermal conductivity of 40–45 W/m·K, CGI transfers heat 20–30 % faster than gray iron.

This allows quicker engine warm-up, reduced hot spots, and better resistance to thermal fatigue in cylinder heads and liners.

Balanced Damping

CGI’s damping factor (~ 0.005) falls midway between gray (~ 0.010) and ductile (~ 0.002) irons.

Následne, CGI absorbs vibration effectively—reducing NVH (hluk, vibrovanie, harshness)—while avoiding the high brittleness of gray iron.

Nákladovo efektívna výroba

Although CGI adds ~ 5–10 % material cost due to Mg/RE additions and tighter process control, it costs 20–30 % menej than ductile iron for equivalent performance.

Lower machining allowances—thanks to improved dimensional stability—further trim casting costs.

Challenges of Sand Casting Compacted Graphite Iron

1. Tight Melt Chemistry Control: Maintaining Mg within ±0.005 % je kritický. A slight deviation can revert graphite morphology to flake or spheroidal, necessitating full‐scale scrapping.
2. Higher Pouring Temperatures: CGI’s 1 350–1 420 ° C (2 462–2 588 °F) melt demands more robust mold binders and coatings to prevent sand erosion and scabbing.
3. Risk of Carbide Formation: Excess silicon or rapid cooling can produce cementite networks, embrittling CGIs; inoculation and controlled cooling are mandatory.
4. Porosity Management: CGI’s higher fluidity leads to greater aspiration of gases unless mold venting and degassing practices are exemplary.
5. Limited Global Foundry Expertise: Although CGI’s market share has grown (especially in automotive), only 20–25 % of iron foundries worldwide have mastered the specialized procedures, raising lead times.

5. Common Compacted Graphite Iron Applications via Sand Casting

• Automotive diesel engine blocks
• Cylinder heads and liners
• Exhaust manifolds and turbocharger housings
• Pump and compressor housings
• Gearbox and transmission housings
• Industrial engine components (Napr., genset blocks)
• Hydraulic valve bodies and pump blocks

6. Comparisons to Alternate Casting Materials

7. Záver

Compacted Graphite Iron (CGI) delivers better strength, stiffness, and thermal performance than gray iron—without the cost of ductile iron.

It requires tight control of chemistry, high pouring temperatures, and proper mold design to ensure vermicular graphite formation.

Already used in engine blocks and cylinder heads, CGI reduces weight by up to 10% and improves thermal fatigue life by 30%.

Advances in simulation and process control are expanding its use to turbochargers, exhausts, a čerpadlá.

With ongoing improvements in alloys and sustainable manufacturing, CGI is becoming a key material in modern, efficient engineering.

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Časté otázky

Why is sand casting used for CGI?

Sand casting is cost-effective for complex, veľký, and medium-to-high volume parts.

It accommodates CGI’s specific thermal and mechanical properties, especially in automotive and industrial components.

What are common applications of CGI sand castings?

Typical applications include diesel engine blocks, hlavy valcov, brzdové komponenty,

turbocharger housings, and structural machine parts—where strength and thermal stability are critical.

What are the key advantages of Sand Casting Compacted Graphite Iron?

CGI provides excellent strength-to-weight ratio, improved fatigue resistance, better heat dissipation, and lower cost than ductile iron in similar roles.

How does CGI affect machinability?

CGI is moderately machinable—harder and more abrasive than gray iron but easier than ductile iron. Advanced tooling and cutting strategies are recommended.

Is CGI suitable for high-temperature applications?

Áno. Its microstructure resists thermal fatigue and distortion, making it well-suited for components exposed to cyclic thermal loads, such as exhaust manifolds and cylinder heads.