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1. Zavedení

Grey Iron, or Grey cast iron—distinguished by its flaky graphite microstructure—combines cost‑effectiveness, vibration damping, a excellent machinability.

Originating in the early 19th century for steam‑engine cylinders, grey cast iron has since powered applications from automotive brake drums to industrial machine bases.

Today, it remains a foundational material across automobilový průmysl, heavy machinery, potrubí, a domestic sectors thanks to its unique blend of properties.

2. What Is Grey Cast Iron?

Grey cast iron is a type of cast iron that is easily recognizable by the grey color of its fractured surface, which results from the presence of graphite flakes in its microstructure.

These graphite flakes give grey iron its characteristic properties, including excellent damping capacity, good machinability, and relatively low cost.

It is the most commonly used form of cast iron and plays a foundational role in both traditional and modern manufacturing industries.

Classification and Grades of Grey Cast Iron

ASTM A48 Classification (U.S. Standard)

The ASTM A48 standard classifies grey cast iron into grades by minimum tensile strength, measured in ksi (1 ksi = 6.89 MPA).

ASTM Grade	Minimum Tensile Strength (MPA)	Typical Microstructure	Common Applications
Class 20	138 MPA	Predominantly ferritic	Counterweights, decorative castings
Class 30	207 MPA	Ferritic–pearlitic	Engine blocks, Čerpadlo
Class 40	276 MPA	Mostly pearlitic	Brake drums, flywheels, machine beds
Class 50	345 MPA	Fine pearlitic, low ferrite	Cylinder liners, high-load brackets

V 1561 Klasifikace (European Standard)

The European standard EN 1561 uses the “EN-GJL” prefix (GJL = Graphit Gusseisen mit Lamellenstruktur, or “lamellar graphite cast iron”) followed by the tensile strength in MPa.

EN Grade	Min. Pevnost v tahu (MPA)	Tvrdost (BHN)	Typical Application
EN-GJL-150	150	~150	Ornamental parts, light covers
EN-GJL-200	200	~160–170	Gear housings, transmission cases
EN-GJL-250	250	~180–200	Cylinder blocks, large castings
EN-GJL-300	300	~220–240	Brake rotors, heavy-duty housings

Typical Chemical Composition Range (% po váze)

Živel	Typický rozsah (%)	Function in Grey Iron
Uhlík (C)	2.5 - 4.0	Promotes graphite flake formation; increases castability
Křemík (A)	1.8 - 3.0	Graphitizer; aids carbon precipitation and improves fluidity
Mangan (Mn)	0.2 - 1.0	Strengthens matrix; promotes pearlite formation
Fosfor (Str)	≤ 0.12 (max 0.5)	Improves fluidity; excessive amounts cause brittleness (steadite)
Síra (S)	≤ 0.12	Generally undesirable; forms iron sulfide inclusions
Železo (Fe)	Váhy	Matrix base metal

4. Physical & Mechanické vlastnosti

Grey cast iron exhibits a distinctive combination of physical and mechanical properties due to its graphite flake microstructure embedded in a ferrous matrix.

These properties make it highly suitable for a wide range of structural and thermal applications, particularly where vibration damping, tepelná vodivost, and castability are essential.

Mechanické vlastnosti

The mechanical behavior of grey cast iron is heavily influenced by the graphite flake morphology, matrix type (ferritic, pearlitic, or mixed), and section thickness.

Vlastnictví	Typical Value Range	Notes
Pevnost v tahu	150–350 MPa	Varies by grade (NAPŘ., ASTM A48 Class 20 to Class 50)
Compressive Strength	3–4× tensile strength	High due to graphite flake orientation
Tvrdost	130–250 BHN	Increases with pearlite content
Prodloužení	~0.5–1%	Very low due to stress concentrations at flake tips
Modulus of Elasticity	70–100 GPa	Lower than steel due to graphite flakes disrupting stress transfer

Note: Unlike steel, grey iron exhibits virtually no ductility and fails in a brittle manner under tensile loading.

Fyzikální vlastnosti

Vlastnictví	Typická hodnota	Significance
Hustota	6.9–7.2 g/cm³	Slightly lower than steel (~7.85 g/cm³)
Tepelná vodivost	35–55 W/m·K	Much higher than ductile or malleable iron; ideal for heat dissipation
Specific Heat Capacity	~460 J/kg·K	Comparable to other ferrous metals
Coefficient of Expansion	~10.5–11.5 × 10⁻⁶ /K	Mírný; important for dimension-critical thermal applications
Damping Capacity	10× that of steel	Excellent vibration and noise absorption
Bod tání	1140–1200°C	Lower than steel; enhances castability

Unique Functional Advantages

• Superior Damping Capacity: Thanks to the internal friction created by graphite flakes, grey iron absorbs vibration far better than steel or ductile iron.
  This makes it ideal for engine blocks, machine tool beds, and brake components.
• Good Thermal Conductivity: Its ability to transfer heat efficiently makes grey cast iron a preferred material for cookware, radiator components, and brake discs.
• Excellent Machinability: The presence of graphite acts as a built-in lubricant, reducing tool wear and enabling higher cutting speeds.
  Pearlitic grades are harder but still more machinable than many steels.

5. Casting Suitability for Grey Iron

Grey cast iron is one of the most castable metals in the foundry industry, renowned for its excellent fluidity, low melting temperature, and minimal shrinkage.

These characteristics make it ideal for producing complex geometries, large castings, and high-volume parts with reliable dimensional accuracy and surface finish.

Excellent Fluidity

Grey cast iron exhibits exceptional molten flow characteristics due to its relatively low pouring temperature (typically between 1,150–1,250°C) and graphite content.

This fluidity allows it to easily fill intricate molds and thin-walled sections (as thin as 3–5 mm), reducing the risk of cold shuts or misruns.

Low Shrinkage Rate

With a linear solidification shrinkage typically in the range of 0.8–1.0%, grey cast iron maintains superior dimensional stability.

This predictable shrinkage can be accurately compensated for in pattern design, minimizing defects and machining allowances.

Graphite Flake Structure Enhances Castability

The flake graphite in grey iron not only contributes to its mechanical damping and machinability but also assists in feeding during solidification, reducing the likelihood of internal shrinkage porosity.

It acts as a natural micro-riser, improving overall casting soundness.

High Thermal Conductivity

The high thermal conductivity (typically 50–60 W/m·K) promotes rapid heat dissipation during solidification, helping to control microstructure and reduce thermal cracking risk.

This is particularly advantageous in large castings or high-speed production environments.

Excellent Machinability Post-Casting

Due to the lubricating effect of graphite flakes and relatively low hardness (Brinell 150–250 HB), it can be easily machined without requiring extensive finishing processes.

This lowers post-processing costs and enhances production throughput.

Suitable Casting Methods for Grey Iron

6. Tepelné zpracování & Obrábění

Grey iron rarely undergoes quench‑and‑temper cycles; instead, foundries apply:

• Annealing/Stress Relief: 650–700 °C for 1–2 hours reduces residual stresses and improves machinability.
• Normalizace: Fine‑tunes matrix (ferrite vs. pearlite) for targeted hardness.

During machining, engineers favor:

• Carbide tooling at moderate speeds (50–80 m/min).
• Rigid workholding to offset low tensile strength.
• Coolant use to avoid built‑up edge; graphite flakes facilitate chip breaking.

Post‑machining, grey cast iron achieves povrchové povrchové úpravy as low as Ra 1.6 µm with minimal secondary operations.

7. Advantages and Disadvantages

Výhody:

• Vibration Damping: Až do 90 % better than steel, reducing noise and fatigue.
• Machinability: Graphite flakes act as chip breakers, lowering tool wear.
• Cost Efficiency: > 80 % recycled content and lower melting energy than steel.

Nevýhody:

• Low Tensile Ductility: < 2 % elongation limits shock‑loading use.
• Anisotropy: Flake orientation creates directional strength variations (~ 20 %).
• Křehkost: Lower impact resistance compared to ductile iron.

8. Aplikace & Performance

Grey cast iron’s property synergy drives its use in:

• Automobilový průmysl: Engine blocks, Hlavy válců, brake drums—leveraging thermal conductivity (~ 45 W/m · k) for heat dissipation.
• Heavy Machinery: Gear housings, machine tool bases—utilizing vibration damping to extend bearing life.
• Konstrukce & Piping: Manhole covers, valve bodies—benefiting from corrosion resistance in neutral waters and low cost.
• Domestic Goods: Cookware, radiators—ensuring even heat distribution and durability.

9. Comparison with Alternative Materials

Grey cast iron has long served as a foundational material in engineering and manufacturing, but it often competes with alternatives like ductile iron, ocel, aluminum alloys, a kompozity.

Each of these materials brings distinct benefits and trade-offs, making material selection highly application-dependent.

Below is a comparative overview that highlights where grey iron stands about its common substitutes.

Srovnávací tabulka: Grey Cast Iron vs. Alternative Materials

Ductile Iron vs. Grey Cast Iron

• Ductile iron offers much higher ductility and strength, making it suitable for pressure-containing or dynamic load applications.
  Však, grey cast iron still outperforms it in damping and cost-efficiency, especially in static structural parts.

Uhlíková ocel vs.. Grey Cast Iron

• Steel provides superior tensile properties and ductility, but is more expensive and harder to machine.
  Grey iron is preferred for parts requiring vibration control (NAPŘ., machine bases, housings).

Aluminum Alloys vs. Grey Cast Iron

• Hliník is significantly lighter and offers excellent corrosion resistance, making it ideal for transport and heat-sensitive components.
  Grey iron, na druhé straně, excels in applications needing rigidity and vibration absorption.

Composites vs. Grey Cast Iron

• While advanced composites can surpass grey iron in strength-to-weight ratio and corrosion resistance, they are far more costly and difficult to manufacture at scale.

10. Závěr

Grey iron endures as a cornerstone material kvůli jeho economic production, built‑in damping, a ease of machining.

By mastering its eutectic graphite formation, casting practices, a design guidelines, engineers can continue leveraging grey cast iron for reliable, cost‑effective solutions across industries—from the heart of an engine to the base of heavy machinery.

As emerging alloy modifications and hybrid manufacturing techniques evolve, grey cast iron will maintain its role in shaping tomorrow’s engineered components.

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