Sivá liatina vs tvárna liatina: Kľúčové rozdiely

1. Zavedenie

Šedá železo vs ťažko are two of the most widely used types of cast iron, each offering unique properties and advantages that make them indispensable across a wide range of industries.

As members of the cast iron family—iron-carbon-silicon alloys formed by casting molten metal into molds—both materials are valued for their strength, machináovateľnosť, zlievateľnosť, a nákladová efektívnosť.

2. Čo je liatina?

Liatina is a group of iron-carbon alloys with a carbon content typically greater than 2%.

It is produced by melting pig iron—usually derived from iron ore—in a furnace and pouring the molten metal into molds to form desired shapes.

The result is a hard, krehký, and strong material that offers excellent castability and a wide range of mechanical properties depending on its specific formulation and treatment.

General Composition

The basic composition of cast iron includes:

• Žehlička (Fe) – the primary element
• Uhlík (C) – 2.0–4.0%, promoting castability and influencing hardness and brittleness
• Kremík (A) – 1.0–3.0%, which promotes graphite formation during solidification
• Stopové množstvá mangán (Mn), síra (Siež), a fosfor (P) may also be present

Key Characteristics of Cast Iron:

• Vynikajúca zlievateľnosť: Flows well into complex molds, making it ideal for intricate shapes
• Dobrá opracovateľnosť: Especially in certain grades like gray iron
• High Compressive Strength: Makes it suitable for bearing loads in structural applications
• Superior Vibration Damping: Reduces noise and movement in machines and equipment
• Nákladovo efektívne: Inexpensive to produce in large quantities

Common Types of Cast Iron:

3. What Is Gray Iron?

Šedá železo, tiež známy ako sivá liatina, is the most commonly used type of cast iron. It is named for the gray color of its fracture surface, which is due to the presence of grafitové vločky in its microstructure.

These graphite flakes create a discontinuity in the iron matrix, giving gray iron its characteristic appearance and mechanical properties.

Mikroštruktúra

The defining feature of gray iron is its flake graphite structure embedded within a matrix of ferit, perlit, alebo kombinácia oboch.

These flakes form during solidification and are responsible for the material’s:

• Vynikajúci tlmenie vibrácií
• Dobrý tepelná vodivosť
• Vysoký compressive strength

Však, the sharp edges of the flakes act as koncentrátor, which significantly reduce tensile strength and make the material brittle under tension or impact.

Grades and Standards

Gray iron is classified by pevnosť v ťahu, often designated using standards like ASTM A48. Príklady zahŕňajú:

• Triedny 20 (CL20): Low strength, výborná opracovateľnosť
• Triedny 30 (CL30): General-purpose use
• Triedny 40 (CL40): Vyššia pevnosť, suitable for load-bearing parts

Higher class numbers indicate higher tensile strength, typically achieved by adjusting cooling rates or alloy content.

Kľúčové vlastnosti:

• Vysoká pevnosť v tlaku
• Excellent damping capacity
• Poor ductility and impact resistance

Typical Applications of Gray Iron

Gray iron’s cost-effectiveness and performance in compression-dominated applications make it a go-to material for:

• Bloky motora a hlavy valcov
• Brake discs and drums
• Machine tool beds and bases
• Prevodovky a skrine
• Pumps and valves

4. Čo je tvárna liatina?

Tvárna liatina, tiež známy ako tvárnej liatiny alebo železo s guľôčkovým grafitom (SGI), is a type of cast iron that offers significantly improved mechanical properties over gray iron—especially in terms of ťažkosť, pevnosť v ťahu, a nárazový odpor.

The key distinction lies in the shape of the graphite within the metal’s microstructure. In ductile iron, graphite forms as spherical nodules, rather than flakes as in gray iron.

This round morphology minimizes stress concentration, allowing ductile iron to stretch or deform without fracturing—hence the name “ductile.”

Mikroštruktúra

• Nodular Graphite: Sférické častice (5–20 μm diameter) that minimize stress concentration, allowing plastic deformation.
• Matrix: Tailored via heat treatment—ferritic (tvárny), perlitické (silný), or bainitic (high strength and toughness).

Grades and Standards

ASTM A536 – Standard Specification for Ductile Iron Castings

• 60-40-18 → 60 ksi tensile, 40 ksi yield, 18% predĺženie
• 80-55-06 → Higher strength, mierna ťažnosť
• 100-70-03 → Very high strength, nízka ťažnosť

ISO 1083 – International designation for spheroidal graphite iron

• EN-GJS-400-15 (similar to ASTM 60-40-18)
• EN-GJS-700-2 (similar to ASTM 100-70-03)

Kľúčové vlastnosti:

• Much higher strength and ductility
• Greater impact resistance
• Better fatigue resistance, ideal for cyclic loading
• Retains some damping capacity, though less than gray iron

Common Applications of Ductile Iron

Thanks to its performance characteristics, ductile iron is widely used in:

• Automobilové komponenty: kľukové hriadeľ, ovládacie ramená, axle housings
• Municipal water and wastewater systems: ductile iron pipes and fittings
• Heavy equipment: ozubené kolesá, spojky, zátvorky, konštrukčné časti
• Energy sector: náboje veterných turbín, hydraulické systémy
• Railroad and mining equipment: track parts, ložiská

5. Porovnanie chemického zloženia

Both alloys are primarily composed of iron (Fe), as well as carbon (C) a kremík (A), but subtle differences and additives distinguish them:

6. Physical Property Comparison of Gray Iron vs Ductile Iron

7. Mechanical Property Comparison of Gray Iron vs Ductile Iron

8. Manufacturing and Casting

Both gray iron and ductile iron are produced using established casting methods, but their processing differs due to their distinct microstructures and mechanical requirements.

Gray Iron Manufacturing:

• Tavenie a legovanie: Gray iron is typically melted in cupola furnaces or electric induction furnaces. The base composition includes iron, uhlíka (mostly as graphite), a kremík.
  Alloying elements such as manganese, síra, and phosphorus are controlled to optimize castability and graphite formation.
• Metódy obsadenia: The most common process is odlievanie piesku, favored for its flexibility and cost-effectiveness, especially for complex or large components like engine blocks, strojové základne, and brake drums.
• Tuhnutie: Graphite forms as flakes within the iron matrix during cooling, providing excellent vibration damping but leading to brittleness.
• Machináovateľnosť: Gray iron’s flake graphite structure acts as a lubricant during machining, making it easier to machine than ductile iron.

Ductile Iron Manufacturing:

• Melting and Treatment: Ductile iron starts from similar raw materials, melted in induction or electric arc furnaces.
  The key difference lies in nodulizing treatment—adding magnesium or cerium to the molten iron to transform graphite flakes into spherical nodules.
• Metódy obsadenia: Ductile iron is often cast using odlievanie piesku alebo odlievanie investícií for precision parts.
  Controlled cooling rates and composition adjustments ensure nodular graphite formation and mechanical properties.
• Kontrola mikroštruktúry: The spherical graphite reduces stress concentrations and increases ductility and toughness.
• Tepelné spracovanie: Ductile iron can be heat-treated (žíhané, normalizované, or austempered) na zlepšenie mechanických vlastností, including tensile strength and fatigue resistance.
• Machináovateľnosť: Slightly more challenging to machine due to its higher strength and toughness compared to gray iron but still good machinability when using appropriate tooling.

9. Odolnosť proti korózii a trvanlivosť

Corrosion resistance and long-term durability are critical factors when selecting between gray iron and ductile iron, especially for applications exposed to harsh environments.

Šedé železo:

• Korózne správanie: Gray iron is moderately resistant to corrosion in dry environments but is susceptible to rusting when exposed to moisture, especially in the presence of salts or acidic conditions.
  The graphite flakes can create micro-galvanic cells with the iron matrix, zrýchlenie lokalizovanej korózie.
• Ochrana povrchu: To enhance durability, gray iron components often receive protective coatings such as painting, práškové lakovanie, alebo pozinkovanie.
  V niektorých prípadoch, specialized corrosion-resistant alloys or linings are applied for aggressive environments.
• Trvanlivosť: While gray iron has excellent wear resistance, corrosion can reduce the lifespan of components in outdoor or wet applications without adequate protection.

Ťažko:

• Vylepšená odolnosť proti korózii: The spheroidal graphite structure in ductile iron reduces stress concentrations and creates a more uniform matrix, which tends to improve corrosion resistance compared to gray iron.
• Vylepšené povrchové úpravy: Ductile iron components commonly utilize protective coatings such as epoxy lining, zinc coatings, or polyurethane paints, especially for use in water and wastewater piping systems.
• Katódová ochrana: In underground or submerged applications, ductile iron pipes often incorporate cathodic protection systems to mitigate corrosion.
• Durability in Harsh Conditions: Thanks to its higher toughness and ductility, ductile iron withstands mechanical stresses during corrosion processes better than gray iron, contributing to longer service life under cyclic loading and corrosive environments.

10. Porovnanie nákladov

• Suroviny: Gray iron costs $1–$3/kg; ductile iron costs $1.5–$4.5/kg (30–50% higher) due to Mg/Ce nodulizers.
• Spracovanie: Gray iron requires no post-treatment; ductile iron may need annealing ($0.2–$0.5/kg extra).
• Náklady na životný cyklus: Ductile iron often offers lower long-term costs in high-stress applications (Napr., potrubia: 50-year lifespan vs. 30 years for gray iron).

11. Key Differences Between Gray Iron vs Ductile Iron

Understanding the fundamental distinctions between gray iron and ductile iron is crucial for selecting the appropriate material based on application requirements.

12. Choosing Between Gray and Ductile Iron

• Prioritize Damping/Vibration Control: Šedá železo (Napr., blok, sústružnícke lôžka).
• Need Strength/Ductility: Tvárna liatina (Napr., kľukové hriadeľ, potrubia).
• Cost-Sensitive, Low-Stress Apps: Šedá železo (Napr., obaly).
• Dynamic Loads/Impact Risk: Tvárna liatina (Napr., komponenty zavesenia).

13. Záver

Gray iron vs ductile iron, both types of cast iron, serve distinct roles: gray iron excels in low-cost, vibration-damped, and compressive-load applications, while ductile iron dominates high-stress, dynamický, and impact-prone scenarios.

Their differences, rooted in graphite morphology, make them irreplaceable in modern engineering, ensuring their continued relevance in automotive, infraštruktúry, a strojov.

Časté otázky

Is ductile iron stronger than steel?

Yes—ductile iron can rival low to medium carbon steels (~400–600 MPa), though it’s less ductile.

Can gray iron be heat-treated?

No—it retains brittleness due to graphite flakes and does not improve via heat treatment.

Why use gray iron for engine blocks?

Its excellent vibration damping, tepelná stabilita, and low cost make it ideal for engine components.

How long do ductile iron pipes last?

With proper coating and installation, they often achieve 50–100+ years of service.

Are both types recyclable?

Áno, both are 95% recyklovateľné, with recycled gray/ductile iron retaining 90% of original properties.