Malleable iron

Malleable (Latin temperare = moderate ) is an iron -carbon material, which unlike other grades of cast iron due to its chemical composition and the solidification process after the metastable system of the iron - carbon diagram graphite- free solidified and as for the time being hard, brittle Temperrohguss in the mold arises. A subsequent heat treatment, annealing, causes a structural transformation. The cementite in cast structure is brought to decay only after particularly long annealing time. The resulting graphite is called temper and is distinguished by its characteristic nodules form. Through this form the Temperkohleflocken interrupt the connection of the metallic matrix is not so unfavorable and with potential notch effect as the graphite flakes in cast iron with lamellar graphite. That is the main reason why malleable better mechanical properties than normal cast iron with lamellar graphite and can therefore be referred to as tough and easy to machine. Based on the fracture appearance of malleable iron is divided into black and white malleable.

  • 3.1 standardization
  • 3.2 Chemical composition
  • 3.3 manufacture
  • 3.4 Microstructure
  • 3.5 Properties and Uses

Temperrohguss

The structure of the Temperrohgusses consists of pearlite and ledeburite. It is achieved by adjusting the chemical composition as a function of the wall thickness of the parts to be cast. For all Tempergusssorten the sum of the content of carbon and silicon from 3.7 to 3.8 percent shall prevail. At high silicon contents and the strong, slowly cooling parts often occur already during the solidification of graphite excretion. This nests arranged like slats lead to septic failure. The tapping has such influence on the macro structure, because the higher it is, the more species-specific or foreign nuclei are melted and the melt solidifies thus exogenous. Also, high levels of carbon ( 2.6%) cause an exogenous solidification of Primäraustenits.

White malleable

Standardization

The white malleable is 1692 (old) and (new in 09.97 ) standardized in DIN in DIN EN 1562. The old short name is GTW and the new is GJMW. The symbol consists of ( EN ) GJ for cast iron, M ( malleable cast iron: cautery ) and W (white: white), including mechanical properties and / or chemical composition must be added to the designation. If necessary, additional requirements may be specified, eg EN- GJMW - 350th In DIN EN 1562 five varieties are recognized:

Chemical composition

Guidelines of the chemical composition of the Temperrohgusses

  • Carbon: 2.8% - 3.4% ( relatively high)
  • Silicon: 0.4 % - 0.8 % ( relatively few )
  • Manganese: 0.4 % - 0.6 %
  • Sulfur: 0.12% - 0.25 %
  • Phosphorus: 0.1%

Carbon and silicon must be matched to each other (the sum of carbon and silicon should not exceed 3.8% ), that even the strongest cross sections of a Tempergussstücks have after solidification a white, graphite -free structure.

Production (annealing )

In order to obtain a white malleable cast iron, the Temperrohguss ( untereutektisches white cast iron) is annealed ( " Glühfrischen "). This is done in order to reduce the carbon content in the cast piece as far as possible. Thus, the casting in the edge region is somewhat tougher. The raw casting at 1000 ° C for about 60 - 120h in an oxidizing atmosphere annealed ( heated in the gas stream ). The following reactions take place:

  • Reaction 1 ( in the interior of the casting ):
  • Reaction 2 ( on the surface of the cast part ):
  • Reaction 3 ( actual decarburization - self-running process )

Of cementite ( Fe3C ) of the casting is divided into the first reaction in the three - iron, and a carbon atom. This carbon reacts to the casting surface with the oxygen and thereby removed from the cast part ( reaction 2). As part of an effort by a concentration balance further diffuses the carbon from the cast to the edge of the casting and combines with the oxygen in the surrounding air. This is a gradual decarbonization of the work place ( reaction 3). At the same time, the remaining carbon agglomerate together to Temperkohleknöllchen in the core of the casting. Decarburization of the workpiece is highly dependent on the duration of the annealing process and the wall thickness of the casting. A uniform decarburization occurs only with a wall thickness of 2-3 mm, with thicker castings is only a surface decarburization and a decomposition of the cementite ( Fe3C ) to iron and temper instead.

Microstructure

The structure of the white malleable with wall thicknesses less than 3 mm is made of a ferritic matrix and very little or no Temperkohleknöllchen ( in the middle). For wall thicknesses greater than 3 mm, the microstructure of white malleable divided into three areas:

  • The decarburised border zone consisting of ferrite. The surface often contains an interspersed with oxides hem.
  • The transition region consisting of a ferritic- pearlitic matrix and some Temperkohleknöllchen.
  • The core zone consisting of a pearlitic matrix and Temperkohleknöllchen.

The depth of decarburization shall be determined by a mitgeglühte wedge sample. Your metallographic polished section provides information on the structure formation. Improper annealing structural defects can occur. For example, the graphite nests can lead to so-called " lazy break", they have emerged already in the raw casting. It can also be a Rückentkohlungserscheinung occur while divorce on the edge carbides on ferrite from in the form of secondary cementite, possibly ledeburite.

Properties and Uses

Tempergusswerkstoffe are preferred because the procedure in casting production. The limitation of the piece weight from a few grams to 100 kilograms production reasons. Also, the maximum wall thickness of 20-30 mm. The tensile strength increases with the thickness, since the pearlite content is increased. By appropriate remuneration treatments, the quality-determining characteristics with high accuracy and high uniformity can be set. (eg close toughest areas, good machinability, high strength and good castability, weldability and also verzinkbar ). The properties of the white malleable are dependent on the wall thickness. They are divided by:

  • Mechanical properties such as: good elongation at break ( wall thickness -dependent)
  • Good tensile strength ( increases with pearlite )
  • Good vibration resistance
  • Well malleable, ductile
  • High toughness
  • Physical properties such as: good machinability
  • Good welding performance
  • Well verzinkbar
  • High surface quality
  • Good corrosion resistance (due to oxide layers on the rim )
  • Can be thermochemically curing ( hardening )

Application

Thin-walled castings of good vibration resistance for machining transfer lines; due to the ductility it is used for components that dynamic stresses ( swinging or jerky ) are exposed and must withstand high mechanical forces ( chassis and steering parts of motor vehicles, subject to documentation safety components, adjusting and fastening elements for the Schaltungsbau ); Fittings and valves for pipeline construction ¸ numerous components for the electrical industry due to the thermal, electrical and magnetic properties; supporting elements of high voltage underground and overhead lines; Switching, control and transmission elements in mechanical and agricultural engineering; due to the good castability and the possibility of very thin-walled structures with reproducible accuracy are to be mentioned properties; For the manufacture of locks and hinges; Workpieces made ​​of malleable offer many different ways to create specific properties targeted in the component area where they are needed (many other materials has replaced it).

Black malleable

Standardization

The black malleable 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 " ( cautery ) and B stands for " Black" (black).

Chemical composition

The malleable iron has a hypoeutectic composition generally. Due to the metastable solidification of Temperrohgusses the carbon is present in bound form as cementite ( Fe3C ) and is thus free graphite. The Temperrohguss has a silver-white fracture and is hard and brittle, it is practically suitable for industrial use. By tempering the cementite decomposes and dissolves in the basic structure, which is at the annealing temperature of austenite. The molten iron, which is used for the production of black malleable iron has the following composition:

  • Carbon 2 - 2.9%
  • Silicon: 1,2 - 1,5 % ( relatively high)
  • Manganese: 0,4 - 0.6%
  • Sulfur: 0.12 to 0.18 %
  • Phosphorus: about 0.1%

The carbon content is less, and the Si content is higher than the white malleable.

Production

For the production of pig iron, steel scrap, ferro alloys and recycled material is first ( from the casting - and gating system of the castings ) led to the premelting in the ( hot wind) cupola. To set the required casting temperature and the chemical composition of the arc furnace or induction furnace is downstream ( duplexing ). During annealing is annealed at a neutral atmosphere in two stages. Because of the neutral atmosphere in this case, the cast iron is not decarburised. The cementite decomposes due to the high carbon - silicon content and completely in ferrite and temper: Fe3C → 3Fe C.

The temper is caused by the resignation of elemental carbon during annealing in the form of nodules or flakes. The appearance of this node depends on the manganese - to-sulfur ratio. Thus, the material achieves steel-like properties of ductility. The first stage of heat treatment is also called the first Grafitisierungsstufe. Eutectic Carbide disintegrate and dissolve at 940 ° C - 960 ° C in a period of about 20 h in the basic structure ( austenite ). It also separates elemental carbon, as mentioned above, as Temperknoten from. The structure now consists of austenite and temper.

In the second stage, which is also referred to as second Grafitisierungsstufe, the basic structure is determined. In order to initiate the second stage, the temperature is lowered to about 800 ° C. If now slowly (at 3-5 ° C per hour ) between 800-700 ° C cooled or kept the temperature for several hours 760-680 ° C, so there is a stable eutectoid transformation. γ → α C The carbon thus has the opportunity to diffuse from the austenite to the existing temper and be an integral part thereof. The microstructure consists then of ferrite (matrix) and graphite and possible remnants of the perlite. The temper is uniformly distributed over the entire cross section of the sample. The material is very soft and consists of ferrite and graphite. Example: GJMB - 350 In the rapid cooling between 800-700 ° C in air of eutectoid range is passed through quickly and there is a metastable eutectoid solidified microstructure of pearlite.

Due to very rapid cooling results in a martensitic structure. After annealing can be started. For example, at 600 ° C produced GJMB - 700, at 700 ° C GJMB - 450 At 620 ° C, the pearlite formed ( globular cementite ) is.

It is characteristic of black malleable that the structure down to a narrow marginal zone of 0.2 mm depth without temper due to the nichtentkohlenden annealing is wanddickenunabhägig.

Microstructure

In the first annealing step decomposes the cementite of the ledeburite, at 950 ° C to austenite and temper. During the second annealing the austenite to ferrite and temper decays. The basic structure depends on the cooling rate in the eutectoid region.

  • Ferritic basic structure

By slow cooling between 700-800 ° C (for more details see Preparation ) takes place the eutectoid transformation under stable conditions. γ → α C The ferrite forms the matrix, and the temper is present uniformly distributed if the sample were considered in all areas about the same cooling conditions. Each of manganese and of sulfur are present, so as to temper the compact is formed. Manganese and sulfur prevent the graphite from to agglomerate in spherical shape, resulting in the rugged and nodular formation of the temper follows.

  • Pearlitic structure

By heating to 700-800 ° C, rapid cooling ( quenching headed see Preparation ) the material solidified metastable to pearlite. γ → α Fe3C. Here the pearlite forms the basic structure. Also in this freezing can be designed differently, the temper.

  • Martensitic basic structure

For very rapid cooling, the martensitic structure is formed. The diffusion is suppressed by the very high cooling rate. Due to the partial collapse of the space lattice creates a distorted and strained by the carbon lattice, it creates martensite. The remuneration structure is caused by the starting of the martensitic structure or by controlled cooling to this structure.

  • Mixed structure

It may also arise ferritic- pearlitic microstructure. This happens when the eutectic solidification takes place partially stable and metastable. Melt → γ C ( stable) and melt → γ Fe3C ( metastable ). The eutectoid transformation runs again metastable. It is expected that a structure with depending on the cooling rate varies much pearlite and ferrite and temper. The temper can have different shapes, sizes and configurations.

Properties and Uses

In general, black annealed has good castability, further it is easier to machine than GJMW curable, treatable and oberflächenhärtbar ( for flame and induction hardening ). He finds, among other things its application for pistons, gears, engine parts and thick-walled components such as motor housing.

  • Ferritic GJMB -350

Although this structure has a moderate viscosity, but has a good elasticity and an excellent machinability. This material is used where demands on the machinability are given. It is suitable for thermo-physical cures after a double warming. The hardness of the material corresponds to ≤ 150 HBW 30, which corresponds to ≤ 160 HV10.

  • Pearlitic GJMB -450

This material has better strength and toughness similar as GJMB - 350th Hardness up to 600 HV10 is possible after a previous double warming. 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 structure. But we compare it with that of a forged steel of the same strength, so it is excellent. Here is a thermo- physical curing is even possible without a previous double warming. The hardness of the material corresponds to 180-230 HBW 30, which corresponds to 190-240 HV10.

  • GJMB -650

Here the strength is mainly in demand. This material has short brittle chips. It can be alternatively used for forging steels. The hardness of the material corresponds to 210-260 HBW 30, which corresponds to 220-270 HV10

  • GJMB -700 treated structure

These same properties and uses as GJMB - 650th The hardness of the material corresponds to 240-290 HBW 30, which corresponds to 250-300 HV10.

Swell

  • Federal Association of the German Foundry Industry: Malleable - a ductile cast iron material, technical publication 2011
  • Hermann Schumann, Heinrich Oettel: Metallography - 14th Edition, Wiley -VCH Verlag.
Structural robustness Ferroalloy Cupola furnace Tempering (metallurgy)
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