White Cast Iron
White Cast Iron
The term cast iron refers to those iron carbon silicon alloys which contain 1.8 % – 4 carbon (C) and usually 0.5 % – 3 % silicon (Si). Cast iron is an important engineering material with a number of advantages, mainly good castability and machinability and moderate mechanical properties.
White cast iron contains 1.8 % -3.6 % C, 0.5 % -1.9 % Si and 1 % – 2 % manganese (Mn). White cast irons are so called because when broken, the fracture surface is white. This is unlike the grey fracture surface normally associated with other cast irons which contain graphite.
White cast iron is a cast iron without any alloy addition and with low C and Si content such that the structure is hard brittle iron carbide (Fe?C, also called cementite) with no free graphite. A fast cooling rate prevents the precipitation of C as graphite. Instead the C, which is in solution in the melt, forms iron carbide. The structure of white cast iron consists of pearlite and ledeburite, a eutectic mixture of pearlite (converted from austenite) and cementite. Cementite is hard and brittle and dominates the microstructure of white cast iron. Thus, white cast iron is hard and brittle and has a white crystalline fracture because it is essentially free of graphite. Typical micro structure of white cast iron is shown in Fig 1.
Fig 1 Typical micro structure of white cast iron
White cast iron does not have the easy castability of other cast irons because its solidification temperature is generally higher, and it solidifies with C in its combined form as iron carbide.
White cast iron has a high compressive strength and excellent wear resistance, and it retains its hardness for limited periods even up to a red heat. It can be produced in selected areas of a casting—such as on the periphery of a cam—by causing localized rapid solidification of the iron. White cast iron at the surface of a casting is called chill. It is produced by making that portion of the mould—where the white cast iron is desired—of a material that can extract heat very rapidly, such as iron or graphite. In some cases the castings are designed and produced to have a white structure in certain areas and a grey or flake structure elsewhere to improve toughness. These cast irons are limited in application because of the lack of impact resistance and the difficulty in maintaining the structure in thicker sections.
Due to its large masses of carbides, especially when alloyed, white cast iron has an excellent resistance against wear and abrasion. It is commonly used for wear resistant surface. It is used for shot blasting nozzles, rolling mill rolls, crushers, pulverizers and ball mill liners. By chilling grey or ductile iron on the outside and letting it cool slowly inside, it is possible to produce parts with a hard surface of white cast iron with a ductile core (chilled cast).
The ductility of white cast iron can be improved by transformation to malleable iron on prolonged heating between 800 deg C and 900 deg C in a non oxidizing atmosphere. The cementite transforms to fine graphite clusters. Typical applications of malleable iron include connecting rods, gears and valves for heavy duty service.
White cast iron is brittle and almost impossible to machine. It is very hard and therefore difficult to cut. These cast irons are very hard and a cubic boron nitride cut off wheel is recommended for sectioning during sample preparation for microscopic analysis. Under microscope, the cementite can be observed at high magnification in the form of pearlite, from the transformation of the high temperature austenite dendrites. The matrix is ferrite.
High alloy white cast iron
High alloy white cast irons are different than the ordinary types of cast irons since they contain alloying elements. In these cast irons, the alloy content is well above 4 %, and hence they cannot be produced by ladle additions to irons of otherwise standard compositions. They are usually produced in foundries specially equipped to produce highly alloyed cast irons.
The high alloy white cast irons are mainly used for abrasion resistant applications and are readily cast into the parts needed in machinery for crushing, grinding, and handling of abrasive materials. The chromium content of high alloy white cast irons increases the corrosion resistance properties of these cast irons. The large volume fraction of primary and/or eutectic carbides in their microstructures provides the high hardness required for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these cast irons can be adjusted by the quantity of alloying element as well as heat treatment for developing the proper balance between the resistance to abrasion and the toughness needed to withstand repeated impact.
The hardness of high alloy white cast iron castings usually fall in the range of HB 450 to HB 800, whereas the hardness of low alloy white cast iron (alloying content less than 4 %) castings normally is in the range of HB 350 to HB 550.
The high alloy white cast irons fall into two major groups:
- Nickel (Ni) – chromium (Cr) white cast irons – These are low chromium alloy cast irons containing 3 % to 5 % Ni and 1 % to 4 % Cr. One grade of these cast irons contains 7 % to 11 % Cr.
- Chromium-molybdenum (Mo) white cast irons – These white cast irons contain 11 % to 23 % Cr, up to 3 % Mo and often additionally alloyed with Ni or copper (Cu).
There is also a third group of high alloy white cast irons. This group comprises the 25 % or 28 % Cr white irons, which may contain other alloying element additions of Mo and/or Ni up to 1.5 %. The nickel-chromium white cast irons are also usually classified as Ni-Hard types 1 to 4.
Ni – Cr white cast irons are also known as Ni hard irons. These white cast irons are being produced for over 50 years and are the oldest group of high alloy white cast iron of industrial importance. Ni hard white cast irons are very cost effective materials for use in crushing and grinding. These are martensitic white cast irons where Ni is the primary alloying element. Ni at levels of 3 % to 5 % is effective in suppressing the transformation of the austenite matrix to pearlite, thus ensuring that a hard martensitic structure (usually containing significant amounts of retained austenite) is developed upon cooling in the mould. Cr percentage in these alloy white cast irons are at levels ranging from 1.4 % to 4 %, to ensure that the irons solidify carbidic, that is, to counteract the graphitizing effect of Ni.
The optimum composition of a Ni-Cr white cast iron depends on the properties needed for the service conditions and the dimensions and weight of the casting. Abrasion resistance is generally a function of the bulk hardness and the volume of carbide in the microstructure. There are four types of Ni- Cr white cast irons.
The first type is called ‘Class I type A’ or ‘Ni-Hard 1’. This type of white cast iron is used when the principal requirement is abrasion resistance and resstance to impact loading is of secondary importance.
The second type is called ‘Class I type B’ or ‘Ni-Hard 2’. This type of white cast iron has higher toughness because of less carbide and is used in those areas where repeated impact is there.
The third type is called ‘Class J type C’ or Ni-Hard 3’. It is of special grade that has a Ni-Cr alloy composition. It is used for chill casting, specialized sand casting processes, and producing grinding balls and slugs.
The fourth type is called ‘Class I type D’ or Ni- Hard 4. It is a modified Ni-Cr iron that contains higher levels of Cr, ranging from 7 % to 11 %, and increased level of Ni, ranging from 5 % to 7 %. Content of C in the iron is varied based on the properties needed for the intended service.
The high Cr white cast irons have very good abrasion resistance. These white cast irons and are used effectively in slurry pumps, brick moulds, coal grinding mills, shot blasting equipment, and components for quarrying, hard rock mining, and milling. In some applications these alloyed white cast irons must also be able to withstand heavy impact loading. The cast irons in this group are recognized as providing the best combination of toughness and abrasion resistance attainable among the white cast irons.
As with most abrasion resistant materials, there is a tradeoff between wear resistance and toughness in the high Cr white cast irons also. By varying composition and heat treatment, these properties can be adjusted to meet the needs of most abrasive applications. Specification ASTM A 532 covers the compositions and hardnesses of two general classes of the high Cr white cast irons. The Cr-Mo white cast irons (Class II of ASTM A532) contain 11 % to 23 % Cr and up to 3.5 % Mo and are normally available either as cast with an austenitic or austenitic martensitic matrix, or heat treated with a martensitic matrix microstructure for maximum abrasion resistance and toughness. These white cast irons are usually considered the hardest of all grades of white cast irons. Compared to the lower alloy Ni-Cr white cast irons, the eutectic carbides are harder and can be heat treated to achieve castings of higher hardness. Mo, as well as Ni and Cu when needed, is added to prevent pearlite and to ensure maximum hardness.
The high Cr white cast irons (class III of ASTM A 532) represent the oldest grade of high Cr white cast irons, with the earliest patents dating back to 1917. These general purpose white cast irons, also called 25 % Cr and 28 % Cr cast irons, contain 23 % to 28 % Cr with up to 1.5 % Mo. To prevent pearlite and attain maximum hardness, Mo is added in all but the lightest cast sections. Alloying with Ni and Cu up to 1 % is also practiced. Although the maximum attainable hardness is not as high as in the class II Cr- Mo white cast irons, these alloy white cast irons are selected when resistance to corrosion is also desired.
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