Cast IronsFabrizio D’Errico

Politecnico di Milano Dept. Mech. Engineering

Main reference: F.C.Campbell, Elements of Metallurgy and Engineering, ASM International, 2008 (Chapter 24)

Cast Iron general features• The ductility of cast irons is very low.• They are cheaper. • They cannot be rolled, drawn, or worked at room

temperature:– Most cast irons are not malleable at any temperature.

• However, they melt readily and can be cast into intricate shapes that are usually machined to final dimensions

Types of cast iron: white and grey• The form in which carbon is present in cast irons is cementite

(Fe3C) or graphite.• Carbon is present as cementite in white cast irons ( “white cast

iron” derives from the white and lustrous appearance of the fracture surface).

• Carbon is present in graphite in graycast iron• Carbon form largely determines cat iron properties

White cast iron• White cast irons are extremely hard and have

good wear resistance• They are too brittle and cannot be used in

applications with dynamic or impact loads

Gray cast irons

• In gray cast irons, carbon is present as graphite, making it much more ductile than white cast iron.

• Gray cast iron is so called because of the gray, fibrous appearance of the fracture surface.

• The properties of the gray cast irons are largely influenced by the shape, size, and distribution of the graphite.

Graphitizing effect • The form of carbon (Fe3C or

graphite) in cast irons is determined primarily by modifying the composition(graphitizing effect of added elements) and controlling the cooling rate during casting.

• Silicon is the most powerful graphitizing element: it is the most important alloying element in the gray cast irons

Silicon content range in cast irons

• Compositional range of carbon and silicon for common cast irons, as compared with steel.

Effect of silicon additions tothe Fe-C phase diagram

Silicon additions of 2.4 and 4.8 wt%decrease:• eutectic and

eutectoid carbon contents;

• decreased C content in perlite;

• Increase eutectoidic and etectictransformationtemperatures;

• the maximum solid solubility of carbon in austenite.

The equivalent carbon number• Since both carbon and silicon influence the nature of iron castings.• It is necessary to develop an approximation of their impact on

solidification.• The carbon equivalent (CE) is a parameter that accounts for the influence

of composition on microstructure according to:

CE < 4.3%, hypoeutectic behavior during solidificationCE = 4.3, eutectic behavior during solidificationCE > 4.3%, hypereutectic behavior during solidification

CE number and mechanical properties

• Increase in Si increases graphitization effect, lowers the C content in matrix (C is left in graphite form).

• Decrease in C content in matrix, increase the ferrite content against perlite.

• The result is general decrease in mechanical properties as the Si increase: usually as CE (namely Si, too) increases, the mechanical properties decreases

Strengthening of cast iron: alloying elements

• Since the strengths of cast irons depends on content of ferrite against pearlite, alloying elements that suppress the formation of ferrite and increase the amount of pearlite are added for increasing the strength.

• Alloying elements such as chromium, molybdenum, and tungsten are used for this purpose.

Cooling rates and carbon form in cast irons

• Slow cooling rates favor the formation of graphite, while faster rates promote more cementite.

• Finally, the formation of graphite rather than cementite is favored by:– High carbon contents– High silicon contents– Slow cooling from solidification– Absence of carbide formers, such as Cr and Mo

Cast Iron microstructures• With the exception of white cast iron (refer to

Fe-C diagram), all graycast irons have microstructures that consist of a graphite phase in a matrix that can be ferritic, pearlitic, bainitic, tempered martensitic, or combinations thereof.

Graphite morphology in gray cast iron

• Strength is also highly dependent on the graphite morphology and distribution.

• Coarse graphite flakes act as stress concentrations and decrease the tensile strength.

• Fracture occurs along the graphite flakes, so that the crack propagates almost entirely through the graphite

Effect of cooling rate on part sections microstructure

• The cooling rate, like the chemical composition, can significantly influence the as-cast structure and therefore the mechanical properties.

• The cooling rate of a casting is primarily a function of its cross-sectional size

Mechanical properties• Gray cast irons do not obey to Hook’s law• They are cheap material, easy to cast

because of its low melting point and no liquid-to-solid shrinkage.

• Easy to machine, because the soft• graphite flakes cause the chips to break into

many small chips. • Soft graphite flakes give gray cast irons very

good damping properties.

Gray pearlitic cast iron

Ferritic graycast iron

Ductile Cast Iron

• Known as nodular iron, spheroidal iron, and spheruliciron, is cast iron in which the graphite is present as tiny nodules or spheroids

• Spheroidal graphite particlesform during solidification because of the presence of small amounts of specific alloying elements, usually magnesium or cerium

Malleable Cast Iron• The starting material for fabricating

malleable cast iron is a hypoeutectic white cast iron with a low silicon content.

• A double heat treatment transforms the white iron into a malleable (high ductile) cast iron:1. graphitization: they are heated at 800 to 970 °C, so the Fe3C decomposes, and temper graphite is formed;2. cooling: Austenite is then properly cooled down to be transformed into ferrite, pearlite, or martensite,.

Ductile and malleable cast ironapplications

ADI (Austempered Cast Iron)

• It offers the combination of high fatigue resistance, wear resistance, high toughness, together with good machining properties.

ADI (Austempered Cast Iron)• A special heat treatment gives the

final microstructure and optimized mechanical properties