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.
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