Malleable Cast Iron

Chemical composition

Malleable cast iron is obtained from a hypoeutectic white cast iron with a controlled chemical composition, where the carbon content typically ranges between 2.0% and 2.9% and silicon between 0.9% and 1.9%. The exact composition is adjusted according to the part thickness and final desired properties, and in many standards it is left to the manufacturer’s discretion.

Element	Typical range (%)
Carbon (C)	2.00 – 2.90
Silicon (Si)	0.90 – 1.90
Manganese (Mn)	0.15 – 0.50
Sulfur (S)	0.02 – 0.20
Phosphorus (P)	≤ 0.10
Chromium (Cr)	≤ 0.10

The phosphorus content is limited to less than 0.1% in parts that will undergo additional treatment or that will operate at low temperatures. Sulfur and manganese must be balanced to ensure completely white solidification without formation of primary graphite.

Mechanical properties

The mechanical properties depend on the matrix microstructure, which can be ferritic (blackheart malleable), pearlitic, or mixed, according to the annealing cycle applied after casting. The tensile strength of the ferritic grade EN‑GJMB‑300‑6 is 300 MPa for nominal diameters of 12 to 15 mm.

Blackheart malleable (ferritic) according to EN 1562

Grade	Nominal diameter (mm)	Rm (MPa)	Rp0.2 (MPa)	Elongation A (%)	Brinell hardness (HB)
EN‑GJMB‑300‑6	12 – 15	300	—	6	≤ 150
EN‑GJMB‑350‑10	12 – 15	350	200	10	≤ 150

Pearlitic malleable (indicative values)

Grade	Nominal diameter (mm)	Rm (MPa)	Rp0.2 (MPa)	Elongation A (%)	Brinell hardness (HB)
EN‑GJMB‑450‑6	12 – 15	450	270	6	150 – 200
EN‑GJMB‑550‑4	12 – 15	550	340	4	180 – 230
EN‑GJMB‑650‑2	12 – 15	650	430	2	210 – 260
EN‑GJMB‑700‑2	12 – 15	700	530	2	240 – 290

The yield strength in ferritic grades is around 60‑65% of the tensile strength. Malleable cast iron retains good ductility even in thin sections and exhibits a lower ductile-to-brittle transition temperature than other nodular cast irons due to its lower silicon content.

Physical and thermal properties

The density of malleable cast iron is approximately 7.20 to 7.35 g/cm³. The properties vary slightly according to the matrix, offering a balanced combination of strength and toughness.

Property	Typical value (ferritic)	Typical value (pearlitic)	Units
Density	7.20 – 7.35	7.25 – 7.40	g/cm³
Elastic modulus (E)	165 – 175	170 – 185	GPa
Poisson’s ratio	0.26	0.26	—
Coefficient of thermal expansion (20‑100 °C)	10.5 – 11.0	11.0 – 12.0	10⁻⁶ /K
Thermal conductivity at 20 °C	50 – 55	40 – 45	W/(m·K)
Specific heat (20‑200 °C)	460 – 500	460 – 500	J/(kg·K)
Maximum continuous service temperature	400	350	°C

The thermal conductivity of the ferritic matrix is 20‑25% higher than that of the pearlitic matrix, which favors heat dissipation in components such as machinery housings and pipe fittings.

Heat treatments

Malleable cast iron is produced by a malleabilizing heat treatment applied to white cast iron castings. The process consists of a prolonged annealing that transforms the metastable cementite into tempered carbon nodules (temper graphite) within a ferritic or pearlitic matrix.

The typical ferritic (blackheart) malleabilizing cycle comprises two stages:

1. First-stage graphitization – Heating to 900‑980 °C and holding for 30‑60 hours in a neutral atmosphere. The cementite decomposes into austenite and graphite nodules.
2. Second-stage graphitization – Controlled cooling to 720‑760 °C and holding for 20‑40 hours to decompose the austenite into ferrite and additional graphite. Final cooling to room temperature is carried out slowly.

To obtain a pearlitic matrix, the second stage is fully or partially suppressed and an air cooling or a subsequent tempering between 600 and 700 °C is applied. The whiteheart malleabilizing treatment is carried out in a decarburizing atmosphere, producing a ferritic surface with a carbon gradient toward the core.

The total absence of graphite before the treatment is critical; any indication of primary graphite (“mottling”) compromises the final properties. Parts thicker than 20‑25 mm require special precautions to avoid this defect.

Weldability and machinability

Malleable cast iron exhibits excellent machinability, especially in the blackheart ferritic grades, whose relatively low yield strength and soft matrix favor chip removal. Machinability is comparable or slightly superior to that of gray cast iron of similar hardness.

Weldability is limited. Brazing processes with brass or copper-phosphorus filler metals can be applied, obtaining satisfactory joints in components not subjected to high mechanical loads. Fusion welding is not recommended because the localized heat can transform the tempered graphite into cementite, generating brittle zones in the heat-affected zone and compromising joint integrity.

If fusion welding becomes unavoidable, uniform preheating to 500‑600 °C, nickel or iron-nickel electrodes, and controlled slow cooling are required, in addition to a subsequent stress-relief annealing. Even so, the strength and ductility in the welded zone will be lower than those of the base material.

Applications by industry

Malleable cast iron occupies a specific niche in small and medium-sized parts with wall thicknesses below 6.35 mm (0.25 inches), where other nodular cast irons present difficulties due to premature carbide formation during rapid cooling.

Agricultural machinery and tillage equipment

Plowshare supports, steering arms, drag chain links, forks, and low-speed gears.

Pipe and conduit fittings

Threaded connections up to 4 inches, fittings, elbows, tees, plugs, low-pressure flanges, and valve bodies for water, gas, and steam at moderate temperatures.

Railway and overhead line sector

Suspension clamps, catenary hardware, insulator supports, track components, and signaling parts where toughness and weather resistance are required.

Hand tools and hardware

Adjustable wrenches, vise jaws, mechanical jack bodies, hinges, fence brackets, and construction hardware where the ability to bend without fracture is exploited.

Electrical industry

Mechanism boxes, solenoid valve housings, switch arms, cable clamps, and electrical line accessories.

Malleable cast iron can undergo cold forming operations after casting, such as stamping for straightening, bending, or coining, an advantage over other brittle cast irons.

Equivalent designations

Standard	Basic ferritic grade	Medium pearlitic grade
EN 1562 (Symbol)	EN‑GJMB‑300‑6	EN‑GJMB‑550‑4
EN 1562 (Number)	EN‑JM1110	EN‑JM1150
ISO 5922	B30‑06	P55‑04
ASTM A47 (American)	Grade 32510	—
ASTM A220 (American)	—	Grade 50005
JIS G 5705 (Japanese)	FCMB 30‑06	FCMP 55‑04
GB/T 9440 (Chinese)	KTH 300‑06	KTZ 550‑04

Equivalence between standards is approximate and must be verified for each specific composition or property requirement. American ASTM numerical designations differ from European ones in nomenclature but are functionally interchangeable in most cases.

Comparison with similar materials

Property	Malleable cast iron ferritic	Ductile cast iron ferritic (GJS‑400‑15)	Gray cast iron (GJL‑250)	Carbon steel (0.2% C)
Tensile strength (MPa)	300 – 400	400 – 430	200 – 300	400 – 550
Yield strength (MPa)	180 – 230	250 – 290	—	250 – 350
Elongation (%)	6 – 12	15 – 20	< 1	25 – 35
Elastic modulus (GPa)	165 – 175	170 – 175	90 – 120	205 – 210
Brinell hardness (HB)	130 – 150	140 – 170	170 – 210	120 – 160
Low-temperature toughness	Excellent	Good	Poor	Good
Vibration damping	Good	Moderate	Excellent	Low
Machinability	Excellent	Very good	Excellent	Good
Weldability	Limited (brazing)	Limited	Very limited	Excellent
Minimum reliable thickness (mm)	3 – 6	8 – 12	3 – 5	—
Relative production cost	Medium‑high	Medium	Low	Low‑medium

Malleable cast iron occupies an intermediate position. Compared to gray cast iron, it offers far superior ductility and toughness, but at a higher cost. Compared to ferritic ductile iron, it better withstands thin sections (below 6‑8 mm) and exhibits a lower ductile-to-brittle transition temperature due to its lower silicon content. However, ductile iron surpasses it in elongation and in unit production cost because it does not require the long malleabilizing cycle.

Limitations and design considerations

The maximum wall thickness is limited to approximately 25‑30 mm to avoid the formation of primary graphite during solidification. In thicker sections, the cooling rate is insufficient to guarantee a completely white structure, leading to a defect known as “mottled iron” that degrades the final mechanical properties.

The malleabilizing cycle is long and energy-intensive: 48‑100 total hours depending on the grade and thickness. This factor makes the finished part more expensive compared to alternatives such as ductile iron or low-carbon steel, so its use is justified only when the following are simultaneously required:

• Thin wall thicknesses (less than 8 mm)
• Good ductility and low-temperature toughness
• Pressure resistance (leak tightness)
• Post-casting cold deformation capability

The sensitivity to strain rate is low, allowing cold stamping, bending, or coining operations without risk of brittle fracture, an advantage over other cast irons.

High-production malleable cast iron parts require rigorous control of composition, especially the Mn/S ratio and residual elements such as chromium, boron, or tellurium, which stabilize carbides and can inhibit graphitization during annealing.

Frequently Asked Questions (FAQ)

What is the yield strength of ferritic malleable cast iron?

Ferritic malleable cast iron grade EN‑GJMB‑350‑10 has a yield strength Rp0.2 of 200 MPa for nominal diameters of 12 to 15 mm. In the stronger pearlitic grades such as EN‑GJMB‑650‑2, the yield strength reaches 430 MPa.

What is the difference between malleable cast iron and ductile cast iron?

Both have graphite in spheroidal form, but in malleable iron the tempered carbon nodules are obtained through a 48‑100 hour heat treatment after white casting, while in ductile iron spheroidization is achieved directly in the liquid state by adding magnesium. Malleable iron performs better in thin sections (3‑6 mm) and has better low-temperature toughness, but its process is longer and more expensive.

How is malleable cast iron welded?

Fusion welding is not recommended for structural applications. The usual joining method is brazing with brass or Cu‑P fillers. If fusion welding is unavoidable, preheating to 500‑600 °C, Ni or Fe‑Ni electrodes, and slow cooling are required, although the joint strength will be reduced compared to the base material.

What is the maximum service temperature of malleable cast iron?

The maximum continuous service temperature is 400 °C for ferritic grades and 350 °C for pearlitic grades. Above these values, the tempered carbon tends to redissolve, degrading the structure and mechanical properties.

Which type of malleable cast iron offers the greatest ductility?

Blackheart malleable cast iron with a fully ferritic matrix (grades EN‑GJMB‑350‑10 or ASTM 32510) offers the highest elongation, up to 10‑12%. This grade is intended for applications where leak tightness pressure takes precedence over high mechanical strength.

In which applications is malleable cast iron preferred over gray cast iron?

It is preferred when ductility, impact resistance, and the ability to withstand deformation without fracture are needed. Typical examples are threaded pipe fittings, overhead line hardware, hand tools, and agricultural machinery parts requiring high toughness. Gray cast iron is more economical but does not tolerate any degree of plastic deformation or significant impacts.

References

• azom.com: https://www.azom.com/article.aspx?ArticleID=3864
• engineeringtoolbox.com: https://www.engineeringtoolbox.com/cast-iron-d_1163.html
• steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=1558