Gray Iron vs Ductile Iron: Klíčové rozdíly

1. Zavedení

Šedé železo vs. tažné železo 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, Machinability, castiability, a nákladová efektivita.

2. Co Je Litina?

Litina 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, křehký, 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:

• Železo (Fe) – the primary element
• Uhlík (C) – 2.0–4.0%, promoting castability and influencing hardness and brittleness
• Křemík (A) – 1.0–3.0%, which promotes graphite formation during solidification
• Stopové množství mangan (Mn), síra (S), a phosphorus (Str) may also be present

Key Characteristics of Cast Iron:

• Vynikající castiability: Flows well into complex molds, making it ideal for intricate shapes
• Good Machinability: 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
• Nákladově efektivní: Inexpensive to produce in large quantities

Common Types of Cast Iron:

3. What Is Gray Iron?

Šedé železo, také známý jako šedá litina, 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 graphite flakes in its microstructure.

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

Mikrostruktura

The defining feature of gray iron is its flake graphite structure embedded within a matrix of ferit, perlit, nebo kombinace obou.

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

• Vynikající Tlumení vibrací
• Dobrý tepelná vodivost
• Vysoký compressive strength

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

Grades and Standards

Gray iron is classified by pevnost v tahu, often designated using standards like ASTM A48. Examples include:

• Třída 20 (CL20): Low strength, excellent machinability
• Třída 30 (CL30): General-purpose use
• Třída 40 (CL40): Vyšší pevnost, suitable for load-bearing parts

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

Key properties:

• Vysoká pevnost 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 motoru a hlavy válců
• Brake discs and drums
• Machine tool beds and bases
• Gearboxes and housings
• Pumps and valves

4. Co je tvárná litina?

Tvárná litina, také známý jako tvárná litina nebo železo s kuličkovým grafitem (SGI), is a type of cast iron that offers significantly improved mechanical properties over gray iron—especially in terms of tažnost, pevnost v tahu, a Odolnost vůči dopadu.

The key distinction lies in the shape of the graphite within the metal’s microstructure. V tvárné litině, graphite forms as kulovité uzliny, 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.”

Mikrostruktura

• Nodular Graphite: Sférické částice (5–20 μm diameter) that minimize stress concentration, allowing plastic deformation.
• Matice: Tailored via heat treatment—ferritic (Dukes), 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% prodloužení
• 80-55-06 → Higher strength, moderate ductility
• 100-70-03 → Very high strength, low ductility

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)

Key properties:

• 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:

• Automotive components: klikové hřídele, ovládací ramena, axle housings
• Municipal water and wastewater systems: ductile iron pipes and fittings
• Heavy equipment: rychlostní stupně, spojky, závorky, Strukturální části
• Energy sector: Huby větrné turbíny, Hydraulické systémy
• Railroad and mining equipment: track parts, ložiska

5. Srovnání chemického složení

Both alloys are primarily composed of iron (Fe), as well as carbon (C) a křemík (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:

• Tavení a legování: Gray iron is typically melted in cupola furnaces or electric induction furnaces. The base composition includes iron, uhlík (mostly as graphite), a křemík.
  Alloying elements such as manganese, síra, and phosphorus are controlled to optimize castability and graphite formation.
• Metody lití: The most common process is lití písku, favored for its flexibility and cost-effectiveness, especially for complex or large components like engine blocks, strojové základny, and brake drums.
• Tuhnutí: Graphite forms as flakes within the iron matrix during cooling, providing excellent vibration damping but leading to brittleness.
• Machinability: 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.
• Metody lití: Ductile iron is often cast using lití písku nebo Investiční obsazení for precision parts.
  Controlled cooling rates and composition adjustments ensure nodular graphite formation and mechanical properties.
• Microstructure Control: The spherical graphite reduces stress concentrations and increases ductility and toughness.
• Tepelné zpracování: Ductile iron can be heat-treated (žíhané, normalizovaný, or austempered) Zvýšení mechanických vlastností, including tensile strength and fatigue resistance.
• Machinability: 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. Odolnost proti korozi a trvanlivost

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:

• Corrosion Behavior: 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, urychlující lokalizovanou korozi.
• Surface Protection: To enhance durability, gray iron components often receive protective coatings such as painting, práškový povlak, nebo pozinkování.
  V některých případech, specialized corrosion-resistant alloys or linings are applied for aggressive environments.
• Trvanlivost: While gray iron has excellent wear resistance, corrosion can reduce the lifespan of components in outdoor or wet applications without adequate protection.

Tažné železo:

• Vylepšená odolnost proti korozi: 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.
• Vylepšená ošetření povrchu: 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.
• Katodická 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. Srovnání nákladů

• Suroviny: Gray iron costs $1–$3/kg; ductile iron costs $1.5–$4.5/kg (30–50% higher) due to Mg/Ce nodulizers.
• Zpracování: Gray iron requires no post-treatment; ductile iron may need annealing ($0.2–$0.5/kg extra).
• Lifecycle Cost: Ductile iron often offers lower long-term costs in high-stress applications (NAPŘ., potrubí: 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: Šedé železo (NAPŘ., bloky motoru, lathe beds).
• Need Strength/Ductility: Tvárná litina (NAPŘ., klikové hřídele, potrubí).
• Cost-Sensitive, Low-Stress Apps: Šedé železo (NAPŘ., poklopy šachet).
• Dynamic Loads/Impact Risk: Tvárná litina (NAPŘ., komponenty zavěšení).

13. Závěr

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, dynamic, and impact-prone scenarios.

Their differences, rooted in graphite morphology, make them irreplaceable in modern engineering, ensuring their continued relevance in automotive, infrastruktura, and machinery.

Časté časté

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?

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