Malleable Cast Iron


Malleable Cast Iron

 Malleable cast iron is essentially white cast iron which has been modified by heat treatment. It is formed when white cast iron is heated to around 920 deg C and then left to cool very slowly. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by the gray cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable cast iron, since it is made from white cast iron.

The white cast iron is converted to malleable cast iron by a two stage heat treatment process to a condition having most of its carbon content in the form of irregularly shaped nodules of graphite, called temper carbon. The structure of malleable cast iron consists of ferrite, pearlite and tempered carbon as compared to the fracture inducing lamellar structure of gray cast iron.

Malleable cast irons are a class of cast irons with mechanical strength properties that are intermediate to those of gray or ductile cast irons. The microstructure provides it properties that make malleable cast irons ideal for applications where toughness and machinability are required, and for components that are required to have some ductility or be malleable so that they can be bent or flexed into position without cracking. Malleable cast iron besides less sensitive to cracking has a range of features, such as higher values of tensile strength, rupture strength and elongation, and high resistance to wear and strong shock resistance. These properties make it useful for a range of commercial purposes. Malleable cast iron is often used as an alternative to steel since it is cheaper to produce and use. Typical microstructure of malleable iron is given in Fig 1

Microstructure of maleable iron

Fig 1 Microstructure of malleable iron

 Chemical composition of malleable cast iron

The chemical composition of malleable cast iron generally is in the ranges as given in the Tab 1. Malleable cast iron also contains sometimes small amounts of chromium (0.01 % to 0.03 %), boron (0.0020 %), copper (up to 1.0 %), nickel (0.5 % to 0.8 %), and molybdenum (0.35 % to 0.5 %).

Tab 1 Chemical composition of malleable iron
   
Element Composition in %
   
Carbon 2.16 – 2.90
Silicon 0.90 – 1.90
Manganese 0.15 – 1.25
Sulphur 0.02 – 0.20
Phosphorus 0.02 – 0.15

There are two groups of malleable cast iron namely whiteheart malleable cast iron and blackheart malleable cast iron. These two groups are differentiated by chemical composition, temperature and time cycles of the annealing process, the annealing atmosphere and the properties and microstructure resulting there from.

  • Whiteheart malleable cast iron – It is a decarburized cast iron. The microstructure of whiteheart malleable cast iron depends on section size. Small sections contain pearlite and temper carbon in ferritic substrate. In the large sections there are three different zones namely (i) surface zone which contains pure ferrite, (ii) intermediate zone which has pearlite, ferrite and temper carbon, and (iii) core zone that contains pearlite, temper carbon and ferritic inclusions. The microstructure of this iron does not contain flake graphite.
  • Blackheart malleable cast iron – It is either ferritic or pearlitic. Ferritic blackheart malleable cast iron is softer and more ductile while the pearlitic blackheart malleable cast iron is harder and less ductile. Both are produced by annealing in a non decarburizing atmosphere. They are non-decarburized cast irons. The microstructure of blackheart malleable cast iron has a matrix essentially of ferrite. The microstructure of pearlitic malleable cast iron has a matrix, according to the grade specified, of pearlite or other transformation products of austenite. Graphite is present in the form of temper carbon nodules. The microstructure does not contain flake graphite.

 Designation system for malleable cast iron

Malleable cast irons are normally designated as per ISO 5922 – 1981. It consists of one letter designating the type of iron, two figures designating the tensile strength and two figures designating the minimum elongation. Letters designating the type of malleable cast iron are usually W for whiteheart malleable cast iron, B for ferritic blackheart malleable cast iron and P for pearlitic malleable cast iron.  This letter is followed by a space. The first two figures designating the minimum tensile strength, in N/ sq mm, of a 12 mm diameter test piece, divided by ten. For example if the minimum tensile strength is 400 N/sq mm, the designation is 40. The next two figures designating the minimum elongation (L0 = 3d) as a percentage of a 12 mm diameter test piece. A nought (0) shall be the first figure when the value is less than 10 %, for example if the minimum elongation is 8 %, the designation is 08, and if the minimum elongation is 12 %, the designation is 12. As an example the designation of a whiteheart malleable cast iron having a minimum tensile strength of 360N/sq mm and minimum elongation of 12 % when measured on a 12 mm diameter test piece is W 36 – 12.

Mechanical properties of malleable cast iron

Malleable cast iron like ductile iron possesses considerable ductility and toughness because of its combination of nodular graphite and low carbon metallic matrix. However the nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates.

Malleable cast 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 cast iron has a distinct advantage. It is preferred for thin section castings as given below.

  • 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
  • For parts requiring wear resistance (martensitic malleable iron only)

Ductile iron has a clear advantage where low solidification shrinkage is needed to avoid hot tears or where the section is too thick to permit solidification as white iron (Solidification as white iron throughout a section is essential to the production of malleable cast iron). Malleable cast iron castings are produced in section thicknesses ranging from about 1.5 mm to 100 mm and in weights from less than 0.03 kg to 180 kg or more.

The mechanical properties of different grades of malleable cast iron are given in Tab 2.

Tab 2 Mechanical properties of malleable cast irons
           
Designation Diameter of test piece Tensile strength 0.2 % proof stress Elongation Hardness
mm N/sq mm N/sq mm (L0 = 3d) HB
      % min  
W 35-04 9-15 340 – 360 5-3 230
W 38-12 9-15 320 – 380 170 – 210 15 – 8 200
W 40-05 9-15 360 – 420 200 – 230 8-4 220
W 45-07 9-15 400 – 480 230 – 280 10-4 220
B 30-06 12-15 300 6 150 max
B 32-12 12-15 320 190 12 150 max
B 35-10 12-15 350 200 10 150 max
P 45-06 12-15 450 270 6 150 -200
P 50-05 12-15 500 300 5 160 -220
P 55-04 12-15 550 340 4 180 -230
P 60-03 12-15 600 390 3 200 -250
P 65-02 12-15 650 430 2 210 -260
P 70-02 12-15 700 530 2 240 -290
P 80-01 12-15 800 600 1 270 -310

Applications

Malleable cast irons are often used as the material of choice for small castings or castings with thin cross sections which, in other irons, would tend to have chill (carbides in the surface layers due to the rapid cooling rates in thin sections). Castings of less than a gram in weight are successfully sand cast in malleable cast iron. Another significant aspect is that the malleable properties can exist to the surface of the casting as opposed to in ductile irons where the cast surface can contain flake graphite, creating a situation where thin malleable iron castings can be stronger than ductile iron.

Malleable cast irons ability to be welded to other ferrous materials makes it ideal for complex brackets that need to be integral to steel castings or fabrications. Because all malleable cast irons are annealed, machinists can be confident not to encounter carbides leading to expensive tooling damage.

Black heart malleable cast iron is mainly used to produce the iron casting parts with impact, shake or torsion functions. This cast iron is normally used to produce rear axle housing castings, spring bracket, low pressure valve body, pipe fittings, tools and wrenches. Pearlitic malleable cast iron has higher strength, hardness and abrasive resistance. So, it is mainly used to produce the abrasion resistant parts for motive power machinery and agricultural machinery. White heart malleable cast iron is seldom used because of its long malleablizing process.

In the past, many foundries used to produce malleable cast iron castings. However, with the development of ductile iron, most of the casting parts of malleable cast iron have been replaced by parts of ductile iron. Hence now only very few iron foundries still produce castings of malleable cast iron.