As was written, cast irons are one of the most complex alloys used in industry. Because of the higher carbon content, the structure of cast iron, as opposed to that of steel, exhibits a carbon-rich phase. Depending primarily on composition, cooling rate, and melt treatment, the carbon-rich phase can solidify with formation of either a stable (austenite-graphite) or a metastable (austenite-Fe3C) eutectic.
With a lower silicon content (containing less than 1.0 wt% Si – graphitizing agent) and faster cooling rate, the carbon in cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The product of this solidification is known as white cast iron (also known as chilled irons). 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. It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of gray cast iron. This type of casting, sometimes referred to as a “chilled casting” has a harder outer surface and a tougher inner core.
White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications where wear resistance is desirable, such as on the teeth of excavators, impellers and volutes of slurry pumps, shell liners and lifter bars in ball mills.
For example, Ni-Cr-HC martensitic white cast iron (Nickel-Chrome-High Carbon Alloy), ASTM A532 Class 1 Type A, is martensitic white cast iron, in which nickel is the primary alloying element because, at levels of 3 to 5%, it is effective in suppressing the transformation of the austenite matrix to pearlite, thus ensuring that a hard, martensitic structure will develop upon cooling inthe mold. This material may also be called Ni-Hard 1. Ni-Hard 1 is an abrasion resistant material used in applications where impact is also a concern as the wear mechanism.
Properties of White Cast Iron – Ni-Cr-HC Martensitic White 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.
Mechanical Properties of White Cast Iron – Ni-Cr-HC Martensitic White 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 White Cast Iron – Ni-Cr-HC Martensitic White 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 martensitic white cast iron (ASTM A532 Class 1 Type A) is 350 MPa.
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.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of martensitic white cast iron (ASTM A532 Class 1 Type A) is 175 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.
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