308 FTJ December 2012 Malleable irons are a class of cast irons with mechanical strength properties that are intermediate to those of grey or ductile cast irons. The microstructure provides properties that make malleable 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. They 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 iron. Another significant aspect is that the malleable properties can exist to the surface of the casting as opposed to in ductile (SG) irons where the cast surface can contain flake graphite, creating a situation where thin malleable iron castings can be stronger than SG. The lower silicon gives malleable iron better fracture toughness properties in low temperature environments than the standard nodular iron grades because the ductile to brittle transformation temperature is lower than many other ductile iron alloys. Malleable iron is first cast as white iron, with all the graphite as carbide in the structure, as opposed to flakes of graphite in grey cast iron or nodules (spheroids) of graphite in ductile cast iron. The iron is then heat treated (annealed) to produce the properties and metallurgy required. The annealing process breaks down the carbide to irregular graphite nodules, termed temper nodules (1) . Types of malleable cast iron There are two types of malleable iron, whiteheart (or decarburised) and blackheart (non-decarburised). Blackheart malleable irons are available with a ferritic (softer and more ductile) or a pearlitic (harder, less ductile). Both are produced by annealing in a non-decarburising atmosphere to produce a microstructure of either ferrite (or pearlite) and graphite so can be readily heat treated in standard heat treatment furnaces. There should be no flake graphite in the structure (2) . Whiteheart malleable cast irons are annealed in a decarburising atmosphere which reduces the carbon content of the surface layers to produce a carbon gradient from the edge (fully ferritic) to the centre (pearlite and graphite nodules). Therefore specialist heat treatment furnaces are required where the atmosphere can be carefully controlled. Since a pre-requisite of the production of malleable irons is that the iron must initially solidify white, this does necessarily place section thickness limits on the production. Thicker sections require the base metal to have low carbon and low silicon levels or alloying with a carbide stabiliser (3) . Blackheart ferritic and pearlitic grades combine high strength with ductility and are often selected as they are readily machinable whilst having significant ductility or are chosen for their good castability combined with toughness and machinability. The microstructure of a ferritic malleable iron should consist of temper carbon in a matrix of ferrite. Pearlitic or martensitic malleable irons contain a controlled amount of combined carbon, either as lamellar pearlite or tempered martensite (2) . The latter can be produced with a wide range of mechanical properties depending on the heat treatment, alloying and melting practice. Welding of pearlitic or martensitic grades is difficult as the high temperatures required can lead to the formation of white iron which is brittle. These grades can be welded if the surface layers are heavily decaburised. Brazing is also possible. Blackheart grades range from those with tensile strengths of 300MPa (M/mm 2 ), with minimum elongation levels of 10%, to those with strengths of 800MPa (M/mm 2 ) with elongations of 1%. In the EN (4) blackheart grades are designated with a B in the material symbol with the ISO standard (5) standard further differentiating pearlitic grades with the letter P in the designation. Whiteheart grades are designated with a W in both the EN (4) standard and the ISO (5) standard and grades range from those with strengths of 270MPa to 570MPa and corresponding elongation values of 10 to 3% - note that there is a cast section thickness consideration also. Applications for malleable cast iron Malleable irons remain the material of choice for pipe fittings but some further examples of malleable iron castings are shown in the images in this article which have been provided by Pennine Castings. The castings have been produced in greensand using high speed, vertically parted, boxless moulds and have then been heat treated by the foundry. Stephen Lown FICME, general manager of Pennine Castings Ltd said: “Malleable iron is a very under- rated material. We successfully produce a wide variety of castings in malleable iron. Its properties offer casting designers many options, reliable high strength in thin sections can enable weight savings and therefore cheaper castings. Its 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 iron is annealed, machinists can be confident not to encounter carbides leading to expensive tooling damage. The forgotten iron – malleable cast iron End lugs for roller shutter doors (ductile iron castings of the same section thickness in this application would tend to crack Pan handle (the same design can be used for pans of varying diameters as the casting can be bent to shape during assembly) Focus on Iron
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308 FTJ December 2012
Malleable irons are a class of cast irons with mechanical strength properties that are intermediate to those of grey or ductile cast irons. The microstructure provides properties that make malleable 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.They 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 iron. Another significant aspect is that the malleable properties can exist to the surface of the casting as opposed to in ductile (SG) irons where the cast surface can contain flake graphite, creating a situation where thin malleable iron castings can be stronger than SG.
The lower silicon gives malleable iron better fracture toughness properties in low temperature environments than the standard nodular iron grades because the ductile to brittle transformation temperature is lower than many other ductile iron alloys.
Malleable iron is first cast as white iron, with all the graphite as carbide in the structure, as opposed to flakes of graphite in grey cast iron or nodules (spheroids) of graphite in ductile cast iron. The iron is then heat treated (annealed) to produce the properties and metallurgy required. The annealing process breaks down the carbide to irregular graphite nodules, termed temper nodules(1).
Types of malleable cast ironThere are two types of malleable iron, whiteheart (or decarburised) and blackheart (non-decarburised).
Blackheart malleable irons are available with a ferritic (softer and more ductile) or a pearlitic (harder, less ductile). Both are produced by annealing in a non-decarburising atmosphere to produce a microstructure of either ferrite (or pearlite) and graphite so can be readily heat treated in standard heat treatment furnaces. There should be no flake graphite in the structure(2).
Whiteheart malleable cast irons are annealed in a decarburising atmosphere which reduces the carbon content of the surface layers to produce a carbon gradient from the edge (fully ferritic) to the centre (pearlite and graphite nodules). Therefore specialist heat treatment furnaces are required where the atmosphere can be carefully controlled.
Since a pre-requisite of the production of malleable irons is that the iron must initially solidify white, this does necessarily place section thickness limits on the production. Thicker sections require the base metal to have low carbon and low silicon levels or alloying with a carbide stabiliser(3).
Blackheart ferritic and pearlitic grades combine high strength with ductility and are often selected as they are readily machinable whilst having significant ductility or are chosen for their good castability combined with toughness and machinability.
The microstructure of a ferritic malleable iron should consist of temper carbon in a matrix of ferrite. Pearlitic or martensitic malleable irons contain a controlled amount of combined carbon, either as lamellar pearlite or tempered martensite(2). The latter can be produced with a wide range of mechanical properties depending on the heat treatment, alloying and melting practice. Welding of pearlitic or martensitic grades is difficult as the high temperatures required can lead to the formation of white iron which is brittle. These grades can be welded if the surface layers are heavily decaburised. Brazing is also possible.
Blackheart grades range from those with tensile strengths of 300MPa (M/mm2), with minimum elongation levels of 10%, to those with strengths of 800MPa (M/mm2) with elongations of 1%. In the EN(4) blackheart grades are designated with a B in the material symbol with the ISO standard(5) standard
further differentiating pearlitic grades with the letter P in the designation.
Whiteheart grades are designated with a W in both the EN(4) standard and the ISO(5) standard and grades range from those with strengths of 270MPa to 570MPa and corresponding elongation values of 10 to 3% - note that there is a cast section thickness consideration also.
Applications for malleable cast ironMalleable irons remain the material of choice for pipe fittings but some further examples of malleable iron castings are shown in the images in this article which have been provided by Pennine Castings. The castings have been produced in greensand using high speed, vertically parted, boxless moulds and have then been heat treated by the foundry.
Stephen Lown FICME, general manager of Pennine Castings Ltd said: “Malleable iron is a very under-rated material. We successfully produce a wide variety of castings in malleable iron. Its properties offer casting designers many options, reliable high strength in thin sections can enable weight savings and therefore cheaper castings. Its 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 iron is annealed, machinists can be confident not to encounter carbides leading to expensive tooling damage.
The forgotten iron – malleable cast iron
End lugs for roller shutter doors (ductile iron castings of the same section thickness in this application would tend to crack
Pan handle (the same design can be used for pans of varying diameters as the casting can be bent to shape during assembly)
Focus on Iron
FTJ December 2012 309
“The iron, when left ‘white’ has a hardness of 400HB, making it ideal for castings in high wear environments such as plough blades and media for shakeout drums. Also, when in the white brittle state, the castings can easily be removed from runner systems, the savings often outweighing the cost of annealing; creating a cheaper alternative to SG.
“Casting designers and buyers should look again at malleable it may be the forgotten iron but it can offer
surprising design possibilities and cost savings.” www.penninecastings.co.uk
References1. Manual of Foundry Technology, Institute of British Foundrymen (ICME),
1997.2. Cast Irons, ASM Speciality Handbook, pub ASM, 1996, pp94-106.3. Key to Metals, http://www.keytometals.com 4. BS EN 1562:2012 Founding. Malleable cast irons. Pub BSI, March 2012.5. ISO 5922:2005 Malleable cast iron, Published ISO, 2005.
Top left: Lindapter beam clampBottom left: Jockey wheel bracket which is then welded onto trailer towbarTop right: Part of adjuster mechanism for lorry cab seats Bottom right: Cloth carrier used in textile industry
Focus on Iron
Hollow nipple casting used as explosive warnings on tracks in railway safety systems with Rawl anchor bolt
Part of Plus 8 scaffolding system; these end lugs can be squeezed onto the main assembly without cracking and will remain securely fixed
Clamp used in railway overhead electric power lines
Pliers for working lead on roofs where the ductility provides a spring grip with some ‘give’
Pipe connector for compressed air application
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