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An Introduction to Grey Iron

These are iron-carbon alloys (and a form of cast iron) with carbon contents in excess of 2%, generally in the range 2 to 4% with the addition of about 1% silicon. They differ from steels in that the carbon present exceeds the solubility limit of 1.7%. This carbon is present in the form of austenite while the excess exists as graphite at room temperature. Consequently, grey irons are sometimes referred to as steels with graphite in them.

Structure

The structure of grey irons often display three phases, ferrite, pearlite or martensite. The carbon content of the matrix rarely exceed 1%. Metallographic sections usually reveal graphite as flakes, however it is actually present in rose-like structures. The distribution of these structures can strongly influence the properties of the metal. The best grey irons have a steel matrix with graphite flakes of varying size and uniform distribution.

The matrix structures also influence properties as follows:

  • Ferrite matrix – low strength
  • Pearlite matrix – higher strength
  • Martensite matrix – high strength, higher hardness, when high carbon grey irons are quench hardened.

The silicon is added to the composition to aid the formation of graphite. Control of other factors such as cooling rate is also necessary to prevent the formation of cementite-rich phases.

Grades of Grey Irons

As the most important factor influencing properties of grey irons is their microstructure, that is how they are most commonly classified. ASTM 48 is perhaps the most widely used system as follows:

Table 1. Grades of grey iron alloys.

Class no.

Min. Tensile Strength

MPa

psi

20

138

20000

25

172

25000

30

207

30000

35

241

35000

40

276

40000

45

310

45000

50

344

50000

55

380

55000

60

414

60000

Although often left off, full designations should have the suffix A, B, C or S which is indicative of the size of the test specimen.

Key Properties

Mechanical Properties

The tensile strengths are dictated by the class of the material. Grey irons are brittle, and have little plastic deformation and thus the yield strength and tensile strengths are almost identical. Further to this, grey irons exhibit little if any strain under tensile loadings and do not follow Hooke’s law very well. This is because microslip is experienced due to the presence of the graphite in the structure.

As these materials do not obey Hooke’s law, the stress-strain curve is not linear, making it difficult to calculate the modulus of elasticity.

The small amount of elastic and plastic deformation these materials exhibit is indicative of low ductility. This in turn leads to low toughness.

Compressive strengths of grey irons are their strong points. It is not uncommon for the compressive strength to be up to five times the tensile strength, while the shear strength may only be 1 to 1.5 times the tensile strength.

Grey irons are also generally quite hard and increase in hardness with increasing class (i.e. from 20 to 60).

Fatigue properties are similar to carbon steels and are typically about 40% of the tensile strengths.

Wear Resistance

Wear resistance is another key design property of gery irons. While they are comparable to medium carbon steels in terms of abrasion, fretting and some forms of corrosive wear, the graphite helps resist metal to metal wear. This is indeed the case when the mating material is a hardened steel, where the graphite provides lubrication and a low wear interface. Consequently, they resist seizing in applications such as screw thread and the like.

Physical Properties

The graphite present in the grey irons is influences damping capacity. This is the ability to suppress elastic deformations or vibrations. In this case, the graphite is thought to absorb vibrations.

Dimensional stability is also somewhat unstable due to the presence of the graphite. Mechanisms responsible for this include pearlite transforming into ferrite resulting in growth, internal oxidation of graphite also resulting in growth. Maintaining operating temperatures below approximately 400°C minimise these effects.

Thermal Properties

As their structures are similar to those of plain carbon steels, properties such as thermal conductivity and thermal expansion are very similar.

Electrical Properties

The presence of graphite influences electrical properties of grey irons, and all grey irons will have higher electrical resistivities compared to steels. Grey irons with coarse graphites in their structures have higher resistivities compared to those with finer graphites.

Magnetic Properties

The actual graphite structure and the type of grey iron influence properties such as permeability, coercive force and hysteresis. All grey irons exhibit ferromagnetism except austenitic grades.

Corrosion Resistance

Under most conditions, the corrosion resistance of grey irons is superior to that of carbon steels.

When corrosion onset begins, some of the matrix may actually dissolve leaving graphite sitting proud of the surface. As the graphite is more noble than the matrix, it effectively increases the formation of a protective barrier layer, which may be more resistant to corrosion by the corrodant.

Heat Treatments

The same processes that apply to carbon steels apply to grey irons. However, they are more complex than carbon steels due to the presence of silicon and must therefore be considered as ternary alloys.

Normailising

Normalising is usually used to increase strength and hardness by refining grain size. However, grey irons do not rely on grain size to dictate these properties, and thus normalising is used to increase strength by reducing segregation in the matrix by relieving internal stresses.

Annealing

Annealing is used to relive internal stresses and improve machinability. It can be used to convert pearlite or free cementite and combined carbon to graphite and ferrite. Annealing will also reduce the tensile strength by as much as 25%.

Stress Relieving

Internal stresses may be present for reasons including:

  • Uneven cooling due to differing cross sectional areas
  • Uneven cooling brought about by mould chill or heat treatment
  • Shrinkage of castings onto solid cores.

Stress relieving at temperatures of 400-620°C will remove approximately 75% of internal stresses. Further stress reduction can be performed at 650°C, which may relieve all residual stresses, but can potentially forming graphite as the expense of pearlite. To maximise strength a heat treatment in the range 540 to 565°C is usually recommended.

Quench Hardening

Quench hardening produces stronger, harder and more wear resistant grey irons. However, increased strength only comes after tempering as quench hardening actually reduces strength. Quench hardening can also induce an expansion of up to 0.5%.

Due to the fact that grey iron has a carbon content greater than 2% they can be quench hardened, however, optimal hardening is achieved when a combined carbon content in the range 0.5 to 0.7% is present. Consequently, the optimum microstructure for hardening is a pearlite matrix with fine graphite grains and these materials should be used for producing hardened grey iron castings.

Grey irons with a ferrite and graphite structure require a long soak at austenitising temperatures This is because there is not enough carbon in the ferrite to produce a fully martensitic structure and carbon must be taken into the matrix by dissolving free graphite. A consequence of this is that these materials cannot be flame or induction hardened as there is insufficient exposure to austenitising temperatures to allow dissolution of the graphite.

Common tempering temperatures are in the range 370 to 430°C, but are reliant on the actual type of grey iron being treated.

Design Aspects

Due to the poor tensile strength of grey iron, it should be loaded compressively in service.

Also, the poor toughness of grey irons means that shock loadings should be avoided where possible.

Applications

Engine and Machine Parts

Due to the low rates of metal-to-metal wear, grey iron has been a popular metal for use in engine blocks. Low wear rates also have seen grey irons used for gears, cams and machine ways which should be stress relieved before final machining. Flame hardening should be used when maximum wear resistance is desired.

Pipes

Grey irons have been used extensively in the past for pipes for water and sewage as they are resistant to attack by alkalis, seawater and some acids.

Machine Bases

Due to their ability to absorb vibrations, grey irons are often used for machine bases.

This information has been sourced, reviewed and adapted from materials provided by Fonderie Saguenay.

For more information on this source, please visit Fonderie Saguenay.

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