Cast Irons: Grey, White, Ductile, Malleable, and Compacted Graphite Iron

Cast iron is not a single material — it is a family of iron-carbon-silicon alloys whose dramatically different properties arise from one controlling variable: the form in which carbon appears in the solidified microstructure. This guide covers all five principal cast iron families — grey iron, white iron, ductile (spheroidal graphite) iron, malleable iron, and compacted graphite iron (CGI) — with detailed treatment of composition, graphite morphology, matrix structures, mechanical properties, production practice, and industrial applications.

Composition, Carbon Equivalent, and Classification

Cast irons are iron-based alloys containing 2.0–4.5 wt% C and 0.5–3.5 wt% Si, with Mn typically 0.1–1.2 wt%, and small amounts of S and P as residual impurities. The wide property range across cast iron families originates not from major compositional differences but from how carbon partitions during solidification and subsequent solid-state transformation: as graphite or as iron carbide (cementite, Fe₃C).

The carbon equivalent (CE) is the most important single-number composition descriptor for cast iron:

Silicon and phosphorus behave as graphitising agents and lower the effective eutectic carbon content, which is why their combined contribution is included in CE. High CE improves fluidity (castability) but reduces tensile strength by increasing graphite volume fraction.

Role of Alloying Elements

Beyond the primary C-Si-Mn-S-P system, intentional alloying elements modify microstructure and properties in cast irons:

Grey Cast Iron

Grey iron is the most widely produced and lowest-cost cast iron. Its characteristic grey fracture surface results from the presence of graphite flakes, which absorb and scatter light rather than reflecting it. Grey iron accounts for roughly 70% of all cast iron tonnage globally, serving applications from engine blocks to machine tool bases.

Graphite Flake Morphology — ASTM A247

ASTM A247 classifies graphite in grey iron by type (A through E, describing the distribution and orientation of flakes) and by size (1 through 8, on a logarithmic scale from 100 mm to <0.01 mm). Understanding graphite type is critical to selecting grey iron for specific service conditions:

Matrix Structures in Grey Iron

The properties of grey iron are determined jointly by graphite type/size and matrix structure. The matrix forms by solid-state transformation of austenite below 727°C (the A₁ temperature). Typical matrix types include:

• Fully ferritic: Achieved by slow cooling or annealing. Soft, machinable, but lower strength (UTS ~150 MPa). Rare in as-cast grey iron.
• Pearlitic: Most common in as-cast grey iron. Gives good strength (UTS 200–350 MPa) and wear resistance.
• Mixed ferrite-pearlite (bull’s-eye): Ferrite envelopes around graphite, pearlite in bulk. Intermediate properties.
• Martensitic or bainitic: Produced by alloying (Ni, Cr, Mo) and quenching. High hardness (400–600 HV) for wear applications.

Mechanical Properties and ASTM A48 Grades

ASTM A48/A48M classifies grey iron by minimum tensile strength, measured on separately cast test bars (Class 20 to Class 60 in US customary; Class 100 to Class 400 in SI). Compressive strength is typically 3–4× the tensile strength due to graphite flake crack-blunting in compression. Elongation is typically <1%.

White Cast Iron

White iron forms when cooling rates are fast enough, or compositions lean enough in graphitising elements (low Si, high Cr), that all carbon is retained as iron carbide rather than precipitating as graphite. The iron-carbon phase diagram shows that at compositions above the eutectic (CE > 4.3), the equilibrium phases include cementite (Fe₃C) as a primary phase at the eutectic temperature of 1148°C. This produces the ledeburite microstructure.

Microstructure of White Iron

Below 727°C, austenite in ledeburite transforms to pearlite (or martensite if cooling is rapid), producing the typical white iron microstructure of:

• Hypoeutectic white iron: Primary austenite dendrites (transformed to pearlite or martensite) in a ledeburite matrix of pearlite + carbide.
• Eutectic white iron (4.3% C): Pure ledeburite — intimate eutectic mixture of austenite (→pearlite) and primary cementite.
• Hypereutectic white iron: Large primary cementite plates in ledeburite matrix. Extremely hard (1000 HV for cementite). Used for chilled iron rolls and grinding media.

High-Chromium White Irons

Alloyed white irons with 12–30 wt% Cr replace unstable Fe₃C with (Fe,Cr)₇C₃ or (Fe,Cr)₂₃C₆ carbides, which are harder (~1600 HV for M₇C₃) and more thermodynamically stable. These grades (ASTM A532 Class I, II, III) are the dominant material for pump impellers, slurry liners, crusher wear parts, and cement mill liners. Molybdenum (1–3 wt%) and nickel (1–6 wt%) are added to promote martensite in the matrix without destabilising the carbide.

Ductile (Spheroidal Graphite) Iron

Ductile iron — also called spheroidal graphite (SG) iron, nodular iron, or simply DI — was developed in 1943 independently by Keith Millis (INCO, USA) and Morrogh (BCIRA, UK). The critical discovery was that adding magnesium to molten iron before casting caused graphite to nucleate and grow as spheres rather than flakes. This transformed cast iron from a brittle engineering material to one rivalling medium-carbon steel in tensile strength and ductility, yet retaining all the castability advantages of the iron-silicon system. For a deeper treatment of graphite nucleation mechanisms, see the related article on solidification and nucleation.

Magnesium Treatment and Nodularisation

Successful ductile iron production requires precise Mg control. The mechanism is now well established: sulfur and oxygen are surface-active elements that adsorb preferentially on graphite growth ledges and promote the anisotropic layer-by-layer growth responsible for flake morphology. Magnesium scavenges both S and O from the melt:

Magnesium is added as a ferro-silicon-magnesium (FeSiMg) master alloy, typically containing 4–10 wt% Mg. Common treatment methods include the sandwich process (FeSiMg placed in a pocket of the treatment ladle, covered by iron), wire injection (cored wire fed into the ladle at controlled speed), and the in-mould process (FeSiMg alloy placed in a reaction chamber in the mould runner system). Inoculation — addition of a FeSi-based inoculant immediately before pouring — is mandatory to provide graphite nucleation sites and prevent undercooled graphite.

Graphite Spheroid Quality

Spheroid count and nodularity are the two key microstructural quality indicators in ductile iron. ASTM A247 specifies graphite form VI as the target (spheroids). ISO 945-1 classifies nodularity on a 0–100% scale; production-quality ductile iron requires nodularity above 80%. Nodularity below 80% indicates incomplete nodularisation (excess S or O, fading Mg) and results in a progressive drop in elongation and toughness.

Grades and Matrix Structures — ASTM A536

Like grey iron, the matrix structure controls tensile properties. Matrix is controlled by alloy additions and heat treatment:

Austempered Ductile Iron (ADI)

ADI is produced by austenitising ductile iron at 850–950°C and then quenching into a salt bath held at 250–400°C for isothermal transformation to ausferrite (a mixture of acicular ferrite and high-carbon, stabilised austenite). The resulting microstructure — described in the bainite microstructure guide in the steel context — delivers exceptional strength-to-weight ratios:

ADI Grade 1 and 2 compete directly with forged steel in automotive differential and drive train gears at significantly lower production cost, owing to near-net-shape casting capability.

Malleable Cast Iron

Malleable iron is produced by a two-stage process: first casting the component as white iron (ensuring all carbon is locked as cementite), then annealing for an extended cycle to decompose the cementite. The resulting graphite morphology — irregular, rounded clusters called temper carbon — is less perfect than ductile iron spheroids but far superior to grey iron flakes in terms of stress concentration. The process was developed in the early 18th century and preceded ductile iron by nearly 200 years.

Blackheart Malleable Iron Process

Blackheart malleable (the dominant Western process) uses a neutral annealing atmosphere:

1. Stage 1 (first-stage graphitisation): Heat to 900–970°C for 15–40 hours. Primary cementite and ledeburite carbides decompose: Fe₃C → 3Fe + C (as temper carbon).
2. Cooling through eutectoid: Slow cool from 900°C to 700°C (∼3°C/hour) to decompose pearlitic cementite.
3. Stage 2 (second-stage graphitisation): Hold at 680–720°C for 30–50 hours to fully decompose pearlite to ferrite + temper carbon.

Total annealing time: 60–100 hours. This is the primary cost and lead-time disadvantage of malleable iron versus ductile iron.

Whiteheart Malleable Iron

Whiteheart malleable (continental European process) anneals white iron castings in contact with iron ore (haematite) in annealing boxes. Oxygen diffuses inward from the ore, decarburising the casting from the surface inward. Thin sections are fully decarburised (white, ferritic); thicker sections retain a pearlitic core. Whiteheart grades are specified in EN 1562. The process is now largely displaced by ductile iron but remains in use for thin-section components (<10 mm) in European foundries.

Grades and Standards

ASTM A47/A47M covers ferritic blackheart grades (32510 and 35018); ASTM A220 covers pearlitic malleable iron grades (40010 through 90001). Pearlitic malleable iron is produced by accelerated cooling through the eutectoid range (retaining pearlite) or by re-heat-treatment.

Compacted Graphite Iron (CGI)

CGI occupies the microstructural and property space between grey and ductile iron. Graphite grows as thick, worm-like (vermicular) particles — interconnected within eutectic cells but with rounded edges and blunt ends rather than the sharp flake tips of grey iron. This morphology is achieved by tightly controlling the residual Mg content at 0.005–0.020 wt% — above the threshold for full flake formation but below that required for complete nodularisation.

Production of CGI

CGI production is technically demanding because the graphite-morphology window is narrow (Mg residual ±0.005 wt%) and shifts with melt sulfur, oxygen, and temperature. Three approaches are used commercially:

• SinterCast process: Continuous thermal analysis with computer-controlled additions of Mg (via cored wire) and titanium/cerium to maintain target graphite morphology.
• Controlled-Mg wire injection: Low-Mg FeSiMg wire injected to achieve target residual, with in-process thermal analysis monitoring.
• Sandwich treatment with controlled Mg master alloy: Lower-cost but less precise; more susceptible to fading and variation.

Properties of CGI

The partial interconnection of graphite in CGI provides thermal conductivity close to grey iron (30–35 W/m·K vs. 40–55 W/m·K for grey iron and 13–17 W/m·K for ductile iron), making it ideal for components experiencing large thermal gradients. Tensile strength (300–500 MPa) and fatigue strength are substantially higher than grey iron.

These properties make CGI the material of choice for modern high-power diesel engine blocks (Volvo, MAN, Ford, DAF), exhaust manifolds, and cylinder heads, where grey iron lacks the mechanical strength for modern cylinder pressures (>200 bar) and ductile iron lacks the thermal conductivity and damping performance.

Heat Treatment of Cast Irons

All cast iron families except white iron respond to heat treatment. The same heat treatment operations described for steel apply in principle, with important differences in transformation kinetics (due to graphite as an internal carbon reservoir) and section sensitivity (due to variation in graphite type with section thickness). See the article on annealing and normalising of steel for the underlying metallurgical principles.

Stress Relief Annealing

Cast iron castings contain significant residual stresses from non-uniform cooling. Stress relief is performed at 500–600°C for 1–8 hours (time depending on section size) followed by slow furnace cooling to below 200°C. No microstructural change occurs at these temperatures; only elastic stress redistribution by creep and recovery.

Full Annealing (Ferritic Annealing)

To produce a fully ferritic matrix in grey or ductile iron: austenitise at 900–930°C for 1–2 h per 25 mm section thickness, then furnace-cool at ≤5°C/min through the eutectoid range (760–680°C) to avoid pearlite formation. This maximises machinability and ductility.

Normalising

Air-cooling from 870–940°C produces a pearlitic matrix. Pearlite fineness depends on cooling rate and section size. Normalising increases hardness and strength compared to annealed iron but reduces toughness and machinability.

Quenching and Tempering

Ductile and alloy grey irons can be quenched from 840–900°C to produce martensitic structures (HRC 55–65), then tempered at 150–600°C to relieve brittleness. Higher tempering temperatures progressively reduce hardness and increase toughness, following a similar quenching and tempering response to that seen in steel.

Surface Hardening

Flame and induction hardening can be applied to grey and ductile iron components (camshafts, crankshafts, cylinder liners) to produce a hard surface layer (600–700 HV) whilst retaining a tough core. The graphite network acts as an internal carbon source, enabling rapid austenitisation at the surface without the need for external carbon supply. For induction hardening parameters and process design, refer to the induction hardening article.

Weldability of Cast Iron

Cast irons present significant weldability challenges due to their high carbon equivalent, low ductility, and sensitivity to rapid cooling (which produces brittle hard spots or martensite in the HAZ). The hydrogen-induced cracking risk in the HAZ is also elevated — see the hydrogen cracking guide for the underlying mechanisms. Ductile iron is more weldable than grey iron due to higher Si and the absence of stress-concentrating flakes, but pre-heat and post-weld heat treatment are still essential for most applications.

Key welding considerations for cast iron:

• Pre-heat: 150–300°C minimum for grey iron; 200–400°C for ductile iron. Hot welding at 500–650°C eliminates hardening but requires whole-part heating.
• Filler metal: ENiFe-Cl (55% Ni-Fe) electrodes recommended for SMAW; ENi-CI (pure nickel) for thin sections. Both have low carbon solubility, minimising carbide formation in the weld metal.
• Technique: Short stringer beads, immediate peening, avoid weaving. Interpass temperature control is critical.
• PWHT: Stress-relief anneal at 590–620°C for 1 h minimum after welding.

Industrial Applications by Cast Iron Type

The correct selection of cast iron type is an engineering decision that must balance castability, mechanical performance, machinability, cost, and the specific service environment. The summary below maps each cast iron family to its primary industrial applications:

Testing and Quality Assurance of Cast Iron

Cast iron quality assurance spans melt chemistry control, microstructural verification, and mechanical testing. Key techniques include:

Thermal Analysis

Real-time thermal analysis (TA) of small cups of liquid iron is the primary process control tool. The cooling curve reveals the eutectic arrest temperature (T_EUT), which reflects graphite nucleation potential and Mg effectiveness in ductile iron production. Recalescence undercooling (ΔT) below the equilibrium eutectic temperature indicates poor inoculation. Modern computerised TA systems (SinterCast, NovaCast) can calculate CE, predict graphite morphology, and trigger corrective additions before pouring.

Microstructural Examination

Graphite type (A–E), size (1–8), and nodularity (0–100%) are assessed by optical microscopy on polished and etched metallographic sections, following ASTM A247 or ISO 945-1. Etching with 2–3% Nital reveals the matrix structure (ferrite, pearlite, bainite, martensite). Image analysis software is used in production foundries for automatic nodularity measurement.

Mechanical Testing

Tensile testing uses separately cast test bars (B-bar for grey iron: 30 mm diameter; A-bar: 22 mm; S-bar: unmachined surface). Brinell hardness testing (3000 kg load, 10 mm ball) is the most common production control test. Charpy impact testing (CVN testing) is specified for pressure-containing ductile iron components (ASTM A536 supplementary requirements S1).

Non-Destructive Testing

Ultrasonic testing (UT) is used to verify nodularity in ductile iron — ultrasonic velocity in fully nodular iron (5.4–5.6 km/s) is significantly higher than in grey iron (4.2–4.8 km/s) due to the absence of elastic anisotropy from flake graphite. This is exploited for in-line production NDT of ductile iron castings. Radiographic testing (RT) detects porosity, shrinkage, and cold shuts in all cast iron types.

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