M. Tech. (FFT) Technology of Ferrous Casting Topic 3: Classification, Properties & Application of White & Malleable Cast Iron

WHITE CAST IRON

White CIs are hard and brittle and cannot be machined easily. White CI is the only member of the CI family in which carbon is present only as carbides. Because of the absence of graphite, it has a light appearance. The presence of different carbides makes white CIs extremely hard and abrasion resistant, but also very brittle. The microstructure of white CI contains massive cementite (white) and pearlite. White cast iron derives its name from the white, crystalline crack surface observed when a casting fractures. Most white cast irons contain less than 4.3% carbon, with low silicon contents to inhibit the precipitation of carbon as graphite. It is used in applications where abrasion resistance is important and ductility not required, such as liners for cement mixers, ball mills, certain types of drawing dies and extrusion nozzles. White cast iron is generally considered unweldable. The absence of any ductility that can accommodate welding-induced stresses in the base metal and heat affected zone adjacent to the weld results in cracking during cooling after welding.

Fig:- White Cast Iron Microstructure The typical microstructure of white cast iron, consisting of dendrites of transformed austenite (pearlite) in a white interdendritic network of cementite, Higher magnification of the same sample reveals that the dark areas are pearlite.

CLASSIFICATION OF WHITE CAST IRON • HYPO-EUTECTIC WHITE C.I :- Alloy having carbon equivalent between 2.11% to 4.3% • EUTECTIC WHITE C.I:- Alloy having carbon equivalent 4.3% • HYPER-EUTECTICWHITE C.I:- Alloy having carbon equivalent between 4.3% to 6.67%. WHITE C.I PROPERTIES • 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 with no free graphite. • 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 • white cast iron is hard and brittle and has a white crystalline fracture because it is essentially free of graphite

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 resistance 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.

APPLICATION:- 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 as the wear surfaces (impeller and volute) of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers, and the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).

MALLEABLE CAST IRON

Malleable iron is cast as white iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment, the brittle structure as first cast is transformed into the malleable form. Carbon agglomerates into small roughly spherical aggregates of graphite leaving a matrix of ferrite or pearlite according to the exact heat treatment used. Three basic types of malleable iron are recognized within the casting industry: blackheart malleable iron, whiteheart malleable iron and pearlitic malleable iron.

HEAT TREATING PROCEDURE Like similar irons with the carbon formed into spherical or nodular shapes, malleable iron exhibits good ductility. Incorrectly considered by some to be an "old" or "dead" material, malleable iron still has a legitimate place in the design engineer's toolbox. Malleable iron is a good choice for small castings or castings with thin cross sections (less than 0.25 inch, 6.35 mm). Other nodular irons produced with graphite in the spherical shape can be difficult to produce in these applications, due to the formation of carbides from the rapid cooling. Malleable iron also exhibits better fracture toughness properties in low temperature environments than other nodular irons, due to its lower silicon content. The ductile to brittle transition temperature is lower than many other ductile iron alloys. In order to properly form the spherical-shaped nodules of graphite (called temper graphite nodules or temper carbon nodules) in the annealing process, care must be taken to ensure that the iron casting will solidify with an entirely white iron cross section. Thicker sections of a casting will cool slowly, allowing some primary graphite to form. This graphite forms random flake-like structures and will not transform to carbide during heat treatment. When stress is applied to such a casting in application, the fracture strength will be lower than expected for white iron. Such iron is said to have a 'mottled' appearance. Some countermeasures can be applied to enhance the formation of the all white structure, but malleable iron foundries often avoid producing heavy sections. After the casting and heat treatment processes, malleable iron can be shaped through cold working, such as stamping for straightening, bending or coining operations. This is possible due to malleable iron's desirable property of being less strain rate sensitive than other materials.

Mechanical properties of malleable iron Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of nodular graphite and low-carbon metallic matrix. Because of the way in which graphite is formed in malleable iron, however, the nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates. Malleable iron and ductile iron are used for some of the applications in which ductility and toughness are important. In many cases, the choice between malleable and ductile iron is based on economy or availability rather than on properties. In certain applications, however, malleable iron has a distinct advantage. It is preferred for thin-section castings: • for parts that are to be pierced, coined, or cold formed, • for parts requiring maximum machinability, • for parts that must retain good impact resistance at low temperatures, and

• for parts requiring wear resistance (martensitic malleable iron only).

APPLICATION:- It is often used for small castings requiring good tensile strength and the ability to flex without breaking (ductility). Uses include electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, power line hardware, farm equipment, mining hardware, and machine parts. Before the widespread use of malleable iron in everyday items, heavy-duty bench vises were made from cast steel. The use of cast steel has rapidly declined in most application due to its poor castability, and hence high-quality cast vises (as opposed to forged ones), among other tools and machine elements, are now almost exclusively made of malleable Iron.