Malleable cast iron
Malleable ( latin temperare , moderate ' ) is a cast iron species, which the basis of their chemical composition and the solidification process according to the metastable system iron-carbon diagram solidifies Graphite and as for the time being hard , brittle arises Temperrohguss in the mold. A subsequent heat treatment , the tempering , causes a structural transformation. The cementite in the cast structure is only made to disintegrate after a particularly long annealing time. The resulting graphite is known as temper carbon and is characterized by its characteristic nodular shape. Due to this shape, the tempered carbon flakes do not disrupt the connection between the metallic base mass and with a potential notch effect as the graphite lamellas in cast iron with lamellar graphite . This is the main reason why malleable cast iron has better mechanical properties than normal cast iron with lamellar graphite and can therefore be described as tough and easily machinable. The malleable cast iron is divided into black and white malleable cast iron based on the appearance of the break.
Malleable cast iron
The structure of the malleable cast iron consists of perlite and ledeburite. It is achieved by adjusting the chemical composition depending on the wall thickness of the parts to be cast. For all malleable cast iron types, the sum of the carbon and silicon content of 3.7 to 3.8 percent is decisive. With high silicon contents and in strong, slowly cooling parts, graphite is often precipitated during solidification. These lamellas arranged like nests lead to the foul break. The tapping temperature has such an influence on the macrostructure, because the higher it is, the more native or foreign germs are melted and the melt thus solidifies exogenously . High carbon contents (2.6%) also cause an exogenous solidification of the primary austenite.
White malleable cast iron
standardization
The white malleable cast iron is standardized in DIN 1692 (old) and in DIN EN 1562 (new since 09.97). The old short name is GTW and the new one is GJMW. The abbreviation consists of (EN-) GJ for cast iron, M for ( malleable cast iron: annealing iron) and W for ( white: white), among other things, mechanical properties and / or chemical composition must be added to the abbreviation. If necessary, additional requirements can be specified, for example EN-GJMW-350. Five types are recorded in DIN EN 1562:
| Abbreviation *** | number | Sample diameter | minimum tensile strength R m | minimum elongation A 3.4 | minimum yield strength R p0.2 |
|---|---|---|---|---|---|
| EN-GJMW-350-4 (GTW-35-04) | EN-JM1010 | 6 mm | 270 N / mm² | 10% | ka * |
| 9 mm | 310 N / mm² | 5% | ka | ||
| 12 mm | 350 N / mm² | 4% | ka | ||
| 15 mm | 360 N / mm² | 3% | ka | ||
| EN-GJMW-360-12 (GTW-S38-12) ** | EN-JM1020 | 6 mm | 280 N / mm² | 16% | ka * |
| 9 mm | 320 N / mm² | 15% | 170 N / mm² | ||
| 12 mm | 360 N / mm² | 12% | 190 N / mm² | ||
| 15 mm | 370 N / mm² | 7% | 200 N / mm² | ||
| EN-GJMW-400-5 (GTW-40-05) | EN-JM1030 | 6 mm | 300 N / mm² | 12% | ka * |
| 9 mm | 360 N / mm² | 8th % | 200 N / mm² | ||
| 12 mm | 400 N / mm² | 5% | 220 N / mm² | ||
| 15 mm | 420 N / mm² | 4% | 230 N / mm² | ||
| EN-GJMW-450-7 (GTW-45-07) | EN-JM1040 | 6 mm | 330 N / mm² | 12% | ka * |
| 9 mm | 400 N / mm² | 10% | 230 N / mm² | ||
| 12 mm | 450 N / mm² | 7% | 260 N / mm² | ||
| 15 mm | 480 N / mm² | 4% | 280 N / mm² | ||
| EN-GJMW-550-4 (GTW-55-04) | EN-JM1050 | 6 mm | ka | ka | ka * |
| 9 mm | 490 N / mm² | 5% | 310 N / mm² | ||
| 12 mm | 550 N / mm² | 4% | 340 N / mm² | ||
| 15 mm | 570 N / mm² | 3% | 350 N / mm² |
- * Due to difficulties in determining the yield strength on small samples, the values and the measurement method must be agreed between the manufacturer and the buyer.
- ** Most suitable for welding
- *** Information in brackets according to old DIN 1692
Chemical composition
Guidelines for the chemical composition of malleable cast iron
- Carbon : 2.8-3.4% (relatively high)
- Silicon : 0.4-0.8% (relatively little)
- Manganese : 0.4-0.6%
- Sulfur : 0.12-0.25%
- Phosphorus : 0.1%
Carbon and silicon must be matched to one another (the sum of carbon and silicon should not exceed 3.8%) that even the thickest cross-sections of a malleable cast iron have a white, graphite-free structure after solidification.
Manufacturing (tempering)
In order to obtain a white malleable cast iron, the malleable cast iron (hypoeutectic white cast iron ) is annealed ("annealing"). This largely reduces the carbon content in the casting. This makes the casting a little tougher in the edge area. The raw casting is annealed at 1000 ° C for about 60–120 hours in an oxidizing atmosphere (tempered in a gas flow). The following reactions take place:
- Reaction 1 (inside the casting):
- Fe 3 C → 3Fe + C
- Reaction 2 (on the surface of the casting):
- C + O 2 → CO 2
- Reaction 3 (actual decarburization - self-running process)
- CO 2 + C → 2CO to this comes O 2 + 2CO → 2CO 2 again
The cementite (Fe 3 C) of the casting breaks down in the first reaction into three iron and one carbon atoms. This carbon reacts with the oxygen on the casting surface and is thereby withdrawn from the casting (reaction 2). In the course of striving to equalize the concentration, the carbon from the casting continues to diffuse to the edge of the casting and combine with the oxygen in the surrounding air. This results in a gradual decarburization of the workpiece (reaction 3). At the same time, the remaining carbon in the core of the casting agglomerates to form nodules of tempered carbon. The decarburization of the workpiece is heavily dependent on the duration of the annealing process and the wall thickness of the casting. Uniform decarburization only occurs with a wall thickness of 2-3 mm, with thicker castings only edge decarburization and the decomposition of the cementite (Fe 3 C) into iron and tempered carbon takes place.
Structure formation
The structure of the white malleable cast iron with wall thicknesses below 3 mm consists of a ferritic matrix and very little or no nodules of tempered carbon (in the middle). With wall thicknesses over 3 mm, the structure of the white malleable cast iron is divided into three areas:
- The decarburized edge zone, consisting of ferrite. The surface often contains a border interspersed with oxides.
- The transition area, consisting of a ferritic - pearlitic base matrix and some tempered carbon nodules.
- The core zone, consisting of a pearlitic base matrix and small tempered carbon nodules.
The depth of decarburization is determined using an annealed wedge sample. Their metallographic polished section provides information about the structure formation. Incorrect tempering can result in structural defects. For example, the graphite nests can lead to so-called "foul breakage", they were created in the raw casting. A back decarburization phenomenon can also occur, with carbides depositing on the ferrite edge in the form of secondary cementite, possibly ledeburite .
Properties and use
Malleable cast iron materials are preferred due to the process sequence used in casting production. The limitation of the piece weight from a few grams to 100 kilograms is due to the manufacturing process. The same applies to the maximum wall thickness of 20–30 mm. The tensile strength increases with the wall thickness, as the pearlite content increases. Appropriate tempering treatments are used to set the quality-determining properties with great accuracy and high uniformity (e.g. tight, hardest areas, good machinability , high strength and good castability , also weldable and galvanizable ).
The properties of the white malleable cast iron depend on the wall thickness. They are divided into:
- mechanical properties such as:
- good elongation at break (depending on wall thickness)
- good tensile strength (increases with the percentage of pearlite)
- good fatigue strength
- easy to forgive, malleable
- high toughness
- physical properties such as:
- good machinability
- good welding behavior
- easy to galvanize
- high surface quality
- good corrosion resistance (due to oxide layers on the edge zone)
- can be hardened thermochemically ( case hardening )
application
Thin-walled cast parts with good fatigue strength for machining on transfer lines; Due to its ductility , it is used for components that are exposed to dynamic loads (oscillating or jerky) and have to withstand high mechanical forces (chassis and steering parts of motor vehicles, safety components requiring documentation, adjusting and fastening elements for circuit construction); Fittings and armatures for pipeline construction, numerous components for the electrical industry due to their thermal, electrical and magnetic properties; load-bearing elements of high voltage and overhead lines; Shift, control and transmission elements in machine and agricultural machinery construction; Due to the good castability and the possibility of very thin-walled constructions with reproducible accuracy, properties are to be mentioned; For the manufacture of locks and fittings; Malleable cast iron workpieces offer a wide range of options for creating specific properties in the component area in which they are required (has replaced many other materials).
Black malleable cast iron
standardization
The black malleable cast iron is also standardized in DIN EN 1562. The old abbreviation GTS has also been replaced and is GJMB, GJ stands for cast iron, M for "malleable cast iron" and B stands for "black".
Chemical composition
Malleable cast iron generally has a hypoeutectic composition. Due to the metastable solidification of the malleable cast iron, the carbon is present in bound form as cementite (Fe 3 C) and is therefore graphite-free. The malleable cast iron has a silvery white fracture structure and is hard and brittle, making it practically unsuitable for technical use. The tempering causes the cementite to break down and dissolve in the basic structure, which consists of austenite at the annealing temperature. The molten iron used to make black malleable cast iron has the following composition:
- Carbon: 2–2.9%
- Silicon: 1.2-1.5% (relatively high)
- Manganese: 0.4-0.6%
- Sulfur: 0.12-0.18%
- Phosphorus: approx. 0.1%
The carbon content is lower and the Si content higher than in white malleable cast iron.
Manufacturing
For production, pig iron , steel scrap, ferro alloys and cycle material (from the casting and gate system of the castings) are first fed into the cupola furnace (with a hot blast) for pre- melting . To set the required casting temperature and the chemical composition, the electric arc furnace or induction furnace is connected downstream (duplex process).
Annealing takes place in two stages in a neutral atmosphere. Due to the neutral atmosphere, the cast iron is not decarburized. Due to the high carbon and silicon content, the cementite breaks down completely into ferrite and tempered carbon: Fe 3 C → 3Fe + C.
The tempered carbon is created by the precipitation of elemental carbon during annealing in the form of knots or flakes. The appearance of these nodes depends on the manganese-sulfur ratio. This gives the material ductility properties that are similar to steel.
The first stage of this heat treatment is also called the first graphitization stage. Eutectic carbides disintegrate and dissolve in the basic structure (austenite) at 940–960 ° C in a period of approx. 20 hours. As mentioned above, elemental carbon is also precipitated as annealing knots. The structure now consists of austenite and tempered carbon.
In the second stage, which is also known as the second graphitization stage, the basic structure is determined. In order to initiate the second stage, the temperature is lowered to approx. 800 ° C. If the temperature is then slowly cooled (at 3–5 ° C per hour) between 800 and 700 ° C or the temperature is kept between 760 and 680 ° C for several hours, a stable eutectoid transformation takes place. γ → α + C
The carbon thus has the opportunity to diffuse from the austenite to the already existing tempered carbon and to become its component. The structure then consists of ferrite (matrix) and graphite and any residues of the pearlite. The tempering carbon is evenly distributed over the entire cross-section of the sample. The material is very soft and consists of ferrite and graphite. Example: GJMB - 350 During rapid cooling between 800 and 700 ° C in air, the eutectoid area is passed through quickly and a eutectoid metastable solidified structure of pearlite is created.
Rapid cooling creates a martensitic structure. After tempering, it can still be tempered . At 600 ° C, for example, GJMB - 700 is produced, at 700 ° C GJMB - 450. At 620 ° C, the perlite is formed (globular cementite).
It is characteristic of black malleable cast iron that the structure is independent of the wall thickness except for a narrow edge zone 0.2 mm deep without tempering carbon due to the non-decarburizing annealing.
Structure formation
In the first annealing stage, the cementite of the Ledeburit breaks down at 950 ° C to form austenite and tempered carbon. During the second annealing stage, the austenite breaks down into ferrite and tempered carbon. The basic structure depends on the cooling rate in the eutectoid area.
- Ferritic basic structure
Through slow cooling between 700 and 800 ° C (for more details see production), the eutectoid transformation takes place under stable conditions. γ → α + C
The ferrite forms the matrix and the tempering carbon is evenly distributed if roughly the same cooling conditions applied in all areas of the sample. The less manganese and sulfur there are, the more compact the tempered carbon is. Manganese and sulfur prevent the graphite from agglomerating in a spherical shape, which results in the jagged and nodular formation of the tempered carbon. - Pearlitic basic structure
By heating to 700–800 ° C, rapid cooling (previous quenching, see production), the material solidifies metastable to pearlite. γ → α + Fe 3 C. Here the pearlite forms the basic structure. Even with this solidification, the tempering carbon can be designed differently. - Martensitic basic structure
When cooling down very quickly, the martensitic structure arises. The diffusion is suppressed by the very high cooling rate. The partial collapse of the space lattice creates a lattice that is distorted and tensioned by the carbon, and martensite is created. The tempering structure is created by tempering the martensitic structure or by controlled cooling on this structure. - Mixed structure
can also arise ferritic-pearlitic structure. This happens when the eutectic solidification is partially stable and metastable. Melt → γ + C (stable) and melt → γ + Fe 3 C (metastable).
The eutectoid transformation is metastable again. A structure with different amounts of pearlite and ferrite and tempered carbon is to be expected, depending on the cooling rate. The tempering charcoal can have different shapes, sizes and arrangements.
Properties and use
In general, black malleable cast iron has good castability, it is also easier to machine than GJMW (see Machinability of cast iron ), hardenable, heat treatable and surface hardenable (for flame and induction hardening). Among other things, it is used for pistons, gears, engine parts and thick-walled components such as engine housings.
- Ferritic GJMB-350
Although this structure has moderate toughness, it has good ductility and excellent machinability. This material is used where there are demands on machinability. It is suitable for thermophysical hardening after double heating. The hardness of the material corresponds to ≤ 150 HBW 30, which corresponds to ≤ 160 HV10. - Pearlitic GJMB-450
This material has better strength and similar toughness as GJMB-350. Hardening up to 600 HV10 is possible after prior double heating. The hardness of the material corresponds to 150–200 HBW 30, which corresponds to 160–210 HV10. - GJMB-550
The machinability of this material is not as good as that of the previous structures. But if you compare it with that of a forged steel of the same strength, it is excellent. Thermophysical hardening is even possible without double heating beforehand. The hardness of the material corresponds to 180–230 HBW 30, which corresponds to 190–240 HV10. - GJMB-650
Strength is the main requirement here. This material has short-brittle chips. Alternatively, it can be used for forged steels. The hardness of the material corresponds to 210–260 HBW 30, which corresponds to 220–270 HV10. - GJMB-700 tempering structure The
same properties and uses as for GJMB-650. The hardness of the material corresponds to 240–290 HBW 30, which corresponds to 250–300 HV10.
literature
- Malleable cast iron - a ductile cast iron material. Federal Association of the German Foundry Industry, Technical Publication, 2011
- Hermann Schumann, Heinrich Oettel: Metallography. 14th edition, Wiley-VCH Verlag.