As can be seen from figure, the percentage of carbon present and the temperature define the phase of the iron carbon alloy and therefore its physical characteristics and mechanical properties. The percentage of carbon determines the type of the ferrous alloy: iron, carbon steel or cast iron.
Cast Irons
In materials engineering, cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%. Typically, cast irons contain from 2.14 wt% to 4.0 wt% carbon and anywhere from 0.5 wt% to 3 wt% of silicon. Iron alloys with lower carbon content are known as steel. The difference is that cast irons can take advantage of eutectic solidification in the binary iron-carbon system. The term eutectic is Greek for “easy or well melting,” and the eutectic point represents the composition on the phase diagram where the lowest melting temperature is achieved. For the iron-carbon system the eutectic point occurs at a composition of 4.26 wt% C and a temperature of 1148°C.
Cast iron, therefore, has a lower melting point (between approximately 1150°C and 1300°C) than traditional steel, which makes it easier to cast than standard steels. Because of its high fluidity when molten, the liquid iron easily fills intricate molds and can form complex shapes. Most applications require very little finishing, so cast irons are used for a wide variety of small parts as well as large ones. It is an ideal material for sand casting into complex shapes such as exhaust manifolds without the need for extensive further machining. Furthermore, some cast irons are very brittle, and casting is the most convenient fabrication technique. Cast irons have become an engineering material with a wide range of applications and are used in pipes, machines and automotive industry parts, such as cylinder heads, cylinder blocks and gearbox cases. It is resistant to damage by oxidation.
Types of Cast Irons
Cast irons also comprise a large family of different types of iron, depending on how the carbon-rich phase forms during solidification. The microstructure of cast irons can be controlled to provide products that have excellent ductility, good machinability, excellent vibration damping, superb wear resistance, and good thermal conductivity. With proper alloying, the corrosion resistance of cast irons can equal that of stainless steels and nickel-base alloys in many services. For most cast irons, the carbon exists as graphite, and both microstructure and mechanical behavior depend on composition and heat treatment. The most common cast iron types are:
- Gray cast iron. Gray cast iron is the oldest and most common type of cast iron. Gray cast iron is characterised by its graphitic microstructure, which causes fractures of the material to have a gray appearance. This is due to the presence of graphite in its composition. In gray cast iron the graphite forms as flakes, taking on a three dimensional geometry.
- White cast iron. White cast irons are hard, brittle, and unmachinable, while gray irons with softer graphite are reasonably strong and machinable. A fracture surface of this alloy has a white appearance, and thus it is termed white cast iron.
- Malleable cast iron. Malleable cast iron is white cast iron that has been annealed. Through an annealing heat treatment, the brittle structure as first cast is transformed into the malleable form. Therefore, its composition is very similar to that of white cast iron, with slightly higher amounts of carbon and silicon.
- Ductile cast iron. Ductile iron, also known as nodular iron, is very similar to gray iron in composition, but during solidification the graphite nucleates as spherical particles (nodules) in ductile iron, rather than as flakes. Ductile iron is stronger and more shock resistant than gray iron. In fact, ductile iron has mechanical characteristics approaching those of steel, while it retains high fluidity when molten and lower melting point.
Bronzes
The bronzes are a family of copper-based alloys traditionally alloyed with tin, but can refer to alloys of copper and other elements (e.g. aluminum, silicon, and nickel). Bronzes are somewhat stronger than the brasses, yet they still have a high degree of corrosion resistance. Generally they are used when, in addition to corrosion resistance, good tensile properties are required. For example, beryllium copper attains the greatest strength (to 1,400 MPa) of any copper-based alloy.
Types of Bronzes
As was written, bronzes are a family of copper-based alloys traditionally alloyed with tin, but can refer to alloys of copper and other elements (e.g. aluminum, silicon, and nickel).
Tin and Phosphor Bronze. In general, bronzes are a family of copper-based alloys traditionally alloyed with tin, commonly with about 12–12.5% tin. The addition of small amounts (0.01–0.45) of phosphorus further increases the hardness, fatigue resistance and wear resistance. Addition of these alloyants leads to applications such as springs, fasteners, masonry fixings, shafts, valve spindles, gears and bearings. Other applications for these alloys are pump impellers, piston rings, and steam fittings. For example, copper casting alloy UNS C90500 is a cast alloy of copper-tin, which is also known as gun metal. Originally used chiefly for making guns, it has largely been replaced by steel.- Silicon Bronze. Silicon bronze usually contains about 96 percent copper. Silicon bronze has a composition of Si: 2.80–3.80%, Mn: 0.50–1.30%, Fe: 0.80% max., Zn: 1.50% max., Pb: 0.05% max. Silicon bronzes have a good combination of strength and ductility, good corrosion resistance and easy weldability. Silicon bronzes were developed originally for the chemical industry because of their exceptional resistance to corrosion in many liquids. They are used in architectural product applications such as:
- Door fittings
- Railings
- Church doors
- Window frames
- Aluminium Bronze. The aluminum bronzes are a family of copper-based alloys offering a combination of mechanical and chemical properties unmatched by any other alloy series. They contain about 5 to 12% of aluminium. They have excellent strength, similar to that of low alloy steels, and excellent corrosion resistance especially in seawater and similar environments, where the alloys often outperform many stainless steels. Their excellent resistance to corrosion results from the aluminium in the alloys, which reacts with atmospheric oxygen to form a thin, tough surface layer of alumina (aluminium oxide) which acts as a barrier to corrosion of the copper-rich alloy. They are found in wrought and cast form. Aluminium bronzes are usually golden in color. Aluminium bronzes are used in sea water applications that include:
- General sea water-related services
- Bearings
- Pipe fittings
- Pumps and valve components
- Heat exchangers
- Beryllium Bronze. Copper beryllium, also known as beryllium bronze, is a copper alloy with 0.5—3% beryllium. Copper beryllium is the hardest and strongest of any copper alloy (UTS up to 1,400 MPa), in the fully heat treated and cold worked condition. It combines high strength with non-magnetic and non-sparking qualities and it is similar in mechanical properties to many high strength alloy steels but, compared to steels, it has better corrosion resistance.
- Bell Metal (High-tin Bronze). In general, bell metals refer usually to a high-tin bronzes that are a family of copper-based alloys traditionally alloyed with tin, commonly with more than 20% of tin (typically, 78% copper, 22% tin by mass). Bell metal is used for the casting of high-quality bells.
Properties of Bronze vs Cast Iron
Material properties are intensive properties, that means they are independent of the amount of mass and may vary from place to place within the system at any moment. The basis of materials science involves studying the structure of materials, and relating them to their properties (mechanical, electrical etc.). Once a materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and the way in which it has been processed into its final form.
Density of Bronze vs Cast Iron
Density of typical bronze is 8.7 g/cm3.
Density of typical cast iron is 7.03 g/cm3.
Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:
ρ = m/V
In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).
Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious, the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm3),
- Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Therefore it is determined by the mass number (number of protons and neutrons).
- Atomic Number Density. The atomic number density (N; atoms/cm3), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm3) of the material. The atomic number density (N; atoms/cm3) of a pure material having atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm3) is easily computed from the following equation using Avogadro’s number (NA = 6.022×1023 atoms or molecules per mole):
- Crystal Structure. Density of crystalline substance is significantly affected by its crystal structure. FCC structure, along with its hexagonal relative (hcp), has the most efficient packing factor (74%). Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium.
Mechanical Properties of Bronze vs Cast Iron
Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial and engineers must take them into account.
Strength of Bronze vs Cast Iron
In mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. Strength of materials basically considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. Strength of a material is its ability to withstand this applied load without failure or plastic deformation.
Ultimate Tensile Strength
Ultimate tensile strength of aluminium bronze – UNS C95400 is about 550 MPa.
Ultimate tensile strength of tin bronze – UNS C90500 – gun metal is about 310 MPa.
Ultimate tensile strength of copper beryllium – UNS C17200 is about 1380 MPa.
Ultimate tensile strength of gray cast iron (ASTM A48 Class 40) is 295 MPa.
Ultimate tensile strength of martensitic white cast iron (ASTM A532 Class 1 Type A) is 350 MPa.
Ultimate tensile strength of malleable cast iron – ASTM A220 is 580 MPa.
Ultimate tensile strength of ductile cast Iron – ASTM A536 – 60-40-18 is 414 Mpa (>60 ksi).
The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension. Ultimate tensile strength is often shortened to “tensile strength” or even to “the ultimate.” If this stress is applied and maintained, fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after the ultimate strength has been achieved. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for an aluminum to as high as 3000 MPa for very high-strength steels.
Yield Strength
Yield strength of aluminium bronze – UNS C95400 is about 250 MPa.
Yield strength of tin bronze – UNS C90500 – gun metal is about 150 MPa.
Yield strength of copper beryllium – UNS C17200 is about 1100 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behaviour termed a yield point phenomenon. Yield strengths vary from 35 MPa for a low-strength aluminum to greater than 1400 MPa for very high-strength steels.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of aluminium bronze – UNS C95400 is about 110 GPa.
Young’s modulus of elasticity of tin bronze – UNS C90500 – gun metal is about 103 GPa.
Young’s modulus of elasticity of copper beryllium – UNS C17200 is about 131 GPa.
Young’s modulus of elasticity of gray cast iron (ASTM A48 Class 40) is 124 GPa.
Young’s modulus of elasticity of martensitic white cast iron (ASTM A532 Class 1 Type A) is 175 GPa.
Young’s modulus of elasticity of malleable cast iron – ASTM A220 is 172 GPa.
Young’s modulus of elasticity ductile cast Iron – ASTM A536 – 60-40-18 is 170 GPa.
The Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to a limiting stress, a body will be able to recover its dimensions on removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position. All the atoms are displaced the same amount and still maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions and no permanent deformation occurs. According to the Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.
Hardness of Bronze vs Cast Iron
Brinell hardness of aluminium bronze – UNS C95400 is approximately 170 MPa. The hardness of aluminum bronzes increases with aluminum (and other alloy) content as well as with stresses caused through cold working.
Brinell hardness of tin bronze – UNS C90500 – gun metal is approximately 75 BHN.
Rockwell hardness of copper beryllium – UNS C17200 is approximately 82 HRB.
Brinell hardness of gray cast iron (ASTM A48 Class 40) is approximately 235 MPa.
Brinell hardness of gray cast iron martensitic white cast iron (ASTM A532 Class 1 Type A) is approximately 600 MPa.
Brinell hardness of malleable cast iron – ASTM A220 is approximately 250 MPa.
Brinell hardness of ductile cast Iron – ASTM A536 – 60-40-18 is approximately 150 – 180 MPa.
Rockwell hardness test is one of the most common indentation hardness tests, that has been developed for hardness testing. In contrast to Brinell test, the Rockwell tester measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The difference between depth of penetration before and after application of the major load is used to calculate the Rockwell hardness number. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale.
The Rockwell C test is performed with a Brale penetrator (120°diamond cone) and a major load of 150kg.
Thermal Properties of Bronze vs Cast Iron
Thermal properties of materials refer to the response of materials to changes in their temperature and to the application of heat. As a solid absorbs energy in the form of heat, its temperature rises and its dimensions increase. But different materials react to the application of heat differently.
Heat capacity, thermal expansion, and thermal conductivity are properties that are often critical in the practical use of solids.
Melting Point of Bronze vs Cast Iron
Melting point of aluminium bronze – UNS C95400 is around 1030°C.
Melting point of tin bronze – UNS C90500 – gun metal is around 1000°C.
Melting point of copper beryllium – UNS C17200 is around 866°C.
Melting point of gray cast iron – ASTM A48 steel is around 1260°C.
Melting point of martensitic white cast iron (ASTM A532 Class 1 Type A) is around 1260°C.
Melting point of malleable cast iron – ASTM A220 is around 1260°C.
Melting point of ductile cast Iron – ASTM A536 – 60-40-18 steel is around 1150°C.
In general, melting is a phase change of a substance from the solid to the liquid phase. The melting point of a substance is the temperature at which this phase change occurs. The melting point also defines a condition in which the solid and liquid can exist in equilibrium.
Thermal Conductivity of Bronze vs Cast Iron
The thermal conductivity of aluminium bronze – UNS C95400 is 59 W/(m.K).
The thermal conductivity of tin bronze – UNS C90500 – gun metal is 75 W/(m.K).
The thermal conductivity of copper beryllium – UNS C17200 is 115 W/(m.K).
The thermal conductivity of gray cast iron – ASTM A48 is 53 W/(m.K).
The thermal conductivity of martensitic white cast iron (ASTM A532 Class 1 Type A) is 15 – 30 W/(m.K).
The thermal conductivity of malleable cast iron is approximately 40 W/(m.K).
The thermal conductivity of ductile cast iron is 36 W/(m.K).
The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m.K. It is a measure of a substance’s ability to transfer heat through a material by conduction. Note that Fourier’s law applies for all matter, regardless of its state (solid, liquid, or gas), therefore, it is also defined for liquids and gases.
The thermal conductivity of most liquids and solids varies with temperature. For vapors, it also depends upon pressure. In general:
Most materials are very nearly homogeneous, therefore we can usually write k = k (T). Similar definitions are associated with thermal conductivities in the y- and z-directions (ky, kz), but for an isotropic material the thermal conductivity is independent of the direction of transfer, kx = ky = kz = k.
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