White Cast Iron vs Gray Cast Iron vs Ductile Iron: Microstructure and Properties

Cast irons form a family of iron-carbon-silicon alloys with carbon content between 2 and 4 wt% — above the maximum solubility in austenite and therefore fundamentally distinct in solidification behaviour and microstructure from steels. The three commercially dominant types — white, gray, and ductile (spheroidal graphite) cast iron — differ not in carbon content but in the form in which that carbon exists in the final microstructure: as iron carbide (cementite), as interconnected graphite flakes, or as discrete graphite spheroids. This single structural variable governs the mechanical properties, failure modes, machinability, and application domains of each iron family in ways that no other factor approaches in significance.

The Iron-Carbon Phase Diagram and Cast Iron Solidification

Cast irons span compositions from approximately 2.0 to 4.5 wt% C with 1.0 to 3.0 wt% Si, placing them in the hypereutectic or near-eutectic region of the Fe-C-Si system. Solidification behaviour is governed by whether the iron follows the metastable Fe-Fe₃C phase diagram or the stable Fe-C (graphite) phase diagram. The two systems have slightly different eutectic temperatures (1147°C for Fe-Fe₃C; 1153°C for Fe-C graphite) and eutectic compositions, and both pathways are available depending on cooling rate, silicon content, and inoculation practice.

The full thermodynamic basis for these transformation choices is grounded in the iron-carbon phase diagram. The eutectoid reaction at 727°C in pure Fe-C (shifted to higher temperatures by Si in cast iron) governs the decomposition of austenite to ferrite plus pearlite or ferrite plus graphite during solid-state cooling, determining whether the final matrix is ferritic, pearlitic, or mixed.

Carbon Equivalent (CE) for cast irons:
CE = %C + (%Si + %P) / 3

Eutectic CE = 4.3 (for the Fe-C-Si stable system)
  CE < 4.3: Hypoeutectic iron (primary austenite dendrites form first)
  CE = 4.3: Eutectic iron (highest fluidity, minimum shrinkage)
  CE > 4.3: Hypereutectic iron (primary graphite or cementite precipitates first)

Typical CE ranges:
  White iron:   CE = 3.0–3.6 (hypoeutectic, metastable solidification)
  Gray iron:    CE = 3.8–4.5 (near-eutectic to slightly hypereutectic)
  Ductile iron: CE = 4.1–4.6 (near-eutectic to hypereutectic, Mg-treated)

Stability of Graphite vs Cementite

The stability of graphite versus cementite is the central question in cast iron metallurgy. Iron carbide (cementite, Fe₃C) is thermodynamically metastable with respect to iron and graphite at all temperatures below approximately 1147°C, but its decomposition rate is negligible at low temperatures. Silicon accelerates cementite decomposition by reducing the activity of carbon in the iron matrix, making it energetically more favourable for carbon to precipitate as graphite. Fast cooling suppresses graphite nucleation kinetics and favours cementite formation, even in silicon-rich compositions — this is why thin sections of gray iron can solidify white (“chilling”).

White Cast Iron: Composition, Microstructure, and Properties

Composition and Solidification

White cast iron is produced when solidification follows the metastable Fe-Fe₃C eutectic rather than the stable Fe-graphite eutectic. This is achieved by one or more of: low silicon content (<1.0 wt%), rapid cooling (thin sections, metal moulds), or addition of carbide-stabilising elements (Cr, Mo, V). The eutectic product is ledeburite, a two-phase mixture of austenite (which transforms to pearlite or martensite on further cooling) and cementite. At room temperature, the microstructure consists of pearlite colonies dispersed in a continuous network or plate-like cementite matrix — a microstructure of extreme hardness and brittleness.

Mechanical Properties

White cast iron’s defining mechanical characteristic is extreme hardness (500–800 HBW depending on composition) derived from the cementite phase (Vickers hardness approximately 840–1100 HV). This hardness makes white iron effectively unmachinable by conventional cutting — grinding is the only practical material removal method. The high cementite volume fraction simultaneously makes white iron extremely brittle: tensile strength is typically 140–180 MPa, compressive strength 1400–1800 MPa (the high compressive strength being characteristic of cementite-containing microstructures), and impact energy near zero. There is no measurable elongation or reduction in area in tensile testing.

High-Chromium White Irons (ASTM A532)

The commercially important white irons are not plain Fe-C alloys but chromium-alloyed grades per ASTM A532. Chromium additions of 11–35 wt% convert the cementite phase from Fe₃C to M₇C₃ (chromium-rich carbide, hardness 1600–1800 HV), which is harder and more stable than Fe₃C and significantly improves wet abrasion resistance. Three classes are defined in A532:

Class I (Ni-Hard): 2.7–4.5 wt% Ni, 1.4–4.0 wt% Cr, 2.4–3.6 wt% C. The Ni promotes martensite matrix transformation; Cr stabilises cementite. Used in pumps, pipe fittings, grinding media.

Class II (Chromium): 7–15 wt% Cr, 2.0–3.3 wt% C, lower Ni. Intermediate abrasion and corrosion resistance. Used in slurry pumps, impellers, and roll surfaces.

Class III (High Chromium): 18–28 wt% Cr, up to 3.5 wt% C. The highest chromium grades with M₇C₃ primary carbides in a martensitic or austenitic matrix. Used for mill liners, crusher wear parts, and dredge pump components.

Gray Cast Iron: Composition, Microstructure, and Properties

Graphite Morphology: ASTM A247 Classification

ASTM A247 classifies graphite in cast iron by morphology (Types A–E for flakes; Type VI for the true spheroid of ductile iron), distribution, and size (grades 1–8, with 1 being coarsest). For gray iron, the graphite type has a decisive effect on mechanical properties:

Matrix Microstructure and Grade

The matrix of gray iron — the phases surrounding the graphite — is controlled by composition and cooling rate and can range from fully ferritic to fully pearlitic. Ferrite provides lower strength but better machinability and thermal shock resistance. Pearlite provides higher strength and hardness. The ASTM A48 class number directly reflects the matrix constitution: Class 20 irons are typically fully ferritic or have a ferritic-pearlitic matrix; Class 60 irons are nearly fully pearlitic with fine interlamellar spacing.

Inoculation — the addition of small amounts of ferrosilicon or calcium-silicon alloy to the melt just before pouring — is essential to producing Type A graphite distribution. Inoculants promote graphite nucleation sites, preventing undercooled graphite types (D and E) and white iron formation in thin sections. The nucleation mechanism relates directly to the grain boundary phenomena described in the Grain Boundaries Guide; inoculant particles (calcium silicate, aluminium oxide inclusions) serve as heterogeneous nucleation sites for graphite.

Mechanical and Physical Properties of Gray Iron

The nonlinear elastic behaviour of gray iron — absent from ductile iron and steel — arises from the opening of graphite flake tips under tensile load, which progresses continuously from the elastic onset, giving a stress-strain curve that has no clear linear portion. Gray iron has no defined yield point; its tensile “strength” is simply the fracture stress. In compression, the graphite flakes cannot open and the iron behaves more like a full matrix material, producing the 3–4× ratio of compressive-to-tensile strength characteristic of the family. This property makes gray iron ideal for bases, beds, and frames of machine tools where compressive loads dominate. The vibration damping capacity of gray iron exceeds that of steel by a factor of 20–25; the graphite flakes dissipate acoustic and vibrational energy by internal friction at the flake-matrix interfaces.

Ductile Cast Iron: Composition, Production, and Microstructure

Magnesium Treatment and Spheroidisation

Ductile iron was invented by Keith Millis at the International Nickel Company in 1943–1948 through the discovery that magnesium additions to molten iron cause graphite to solidify as spheroids rather than flakes. The mechanism of spheroidisation is still not fully settled in the literature, but the dominant model holds that Mg (and Ce) adsorb preferentially onto the prismatic (a-axis) crystal faces of hexagonal graphite, preventing lateral growth and forcing growth in the c-axis direction, producing a sphere rather than a flat plate. Oxygen and sulphur — which naturally adsorb onto basal planes and promote flake growth — must be scavenged to below approximately 0.02 wt% S and 0.02 wt% O before Mg treatment is effective.

Magnesium treatment parameters:
  Base iron S before treatment: <0.020 wt% (desulphurisation required if higher)
  Mg addition (as FeSiMg master alloy): 1.0–1.8 wt% of melt weight
  Residual Mg in iron after treatment: 0.030–0.060 wt%
  Recovery of Mg from FeSiMg: ~50–70% (Mg loss to slag and vapour)

Inoculation after Mg treatment:
  FeSi75 or Ca-Si inoculant: 0.1–0.3 wt% addition to ladle or stream
  Purpose: promote graphite nucleation, prevent white iron in thin sections
  Inoculant fade: nuclei dissolve within 8–12 minutes; pour promptly

Grades and Mechanical Properties: ASTM A536

The Grade 60-40-18 (fully ferritic) condition is achieved by a full ferritising anneal at 900–955°C, which dissolves pearlite and converts austenite to ferrite during slow cooling, relocating the austenite carbon to the existing graphite nodules. This is a direct application of the annealing and normalising cycles discussed for steel, but at higher temperatures due to silicon raising A_c1 in cast iron. The pearlitic grades (80-55-06, 100-70-03) are produced by normalising or controlled as-cast cooling. The martensitic Grade 120-90-02 requires the full quenching and tempering cycle, producing a tempered martensite matrix around the undissolved graphite nodules.

Nodule Count and Nodularity

The quality of a ductile iron casting is assessed partly through nodularity — the percentage of graphite present as true spheroids (ASTM A247 Type V and VI) versus degenerate forms. A nodularity above 85% is typically required for Grade 60-40-18 and Grade 65-45-12 compliance; lower nodularity produces vermiular or compacted graphite morphologies that significantly reduce ductility. Nodule count (number of nodules per unit area in polished section) should be above 100/mm² for most applications; very low nodule counts indicate insufficient inoculation and risk of chunky graphite formation in heavier sections. The hardness testing methods article covers the Brinell and Vickers methods used for routine quality assurance of ductile iron castings.

Austempered Ductile Iron (ADI)

Austempered ductile iron represents the highest-performance form of cast iron available commercially. The process austenitises a ductile iron casting at 850–950°C to dissolve the pearlite and partially enrich the austenite with carbon from the graphite nodules, then quenches into a salt bath held at 250–400°C (below M_s but above M_f) for isothermal transformation. The resulting ausferrite matrix — a mixture of acicular ferrite and high-carbon retained austenite — provides a unique combination of strength, ductility, and toughness unavailable in any conventionally cooled microstructure.

The bainite microstructure article is directly relevant here: ausferrite is mechanistically related to lower bainite but formed in a system where graphite nodules act as carbon sources and sinks, making the transformation kinetics and microstructure distinct from bainite in steel. The martensite formation article explains why the retained austenite in ADI is stabilised by high carbon content (1.8–2.2 wt% C in the austenite) rather than transforming on cooling to room temperature.

Compacted Graphite Iron (CGI)

Between flake graphite (gray iron) and spheroidal graphite (ductile iron) lies compacted graphite iron (CGI, also called vermicular iron), in which graphite forms interconnected worm-like particles intermediate in aspect ratio between flakes and spheroids. CGI is produced by controlling Mg residual to 0.005–0.015 wt% — below the threshold for full spheroidisation. ISO 16112 governs the specification.

CGI has higher tensile strength and stiffness than gray iron (typically 300–450 MPa tensile strength and elastic modulus of 140–165 GPa versus 66–162 GPa for gray iron), retains most of the castability and thermal conductivity advantages of gray iron, and provides modest elongation (1–3%). These characteristics make CGI the preferred material for diesel engine cylinder blocks and heads where the combination of thermal fatigue resistance, high stiffness, and moderate strength required exceeds the capability of gray iron without reaching the cost of ductile iron.

Heat Treatment of Cast Irons

The graphite morphology in any cast iron is fixed permanently at solidification and cannot be altered by heat treatment — flakes remain flakes; spheroids remain spheroids. What heat treatment can change is the matrix microstructure surrounding the graphite.

Industrial Applications and Selection Guide

For corrosion considerations in cast iron selection — particularly relevant for water pipe, pump, and marine applications — the Corrosion Mechanisms article covers the electrochemical basis of graphitic corrosion in gray iron (selective leaching of iron matrix leaving graphite skeleton) and why ductile iron with its lower free graphite area-to-volume ratio is less susceptible. The Pitting Corrosion article addresses localised attack at graphite-matrix interfaces in aggressive aqueous environments. The Charpy impact test is routinely specified for ductile iron grades used in pressure service to verify adequate toughness at operating temperature.