Casts Irons

  • Upload
    tao

  • View
    56

  • Download
    10

Embed Size (px)

DESCRIPTION

Casts Irons. - PowerPoint PPT Presentation

Citation preview

  • Titre - Title

    Mech 473 Lectures

    Professor Rodney Herring

  • Casts IronsThe cast irons are made and used by many industries including the automotive industry, farming industry (e.g., tractors), construction industry, etc to make the housings of crank shafts, piston rods, heads, intake and exhaust manifolds, transmission housing, starter motor housing, gear shafts and assembles, wheels, etc. A hard, good wear-resistant material with a reasonable amount of toughness is required. Cast iron is cheaply made using a blast furnace from Pig Iron since it contains a high carbon and Si content.Thus, the cast irons are not an iron-carbon alloy but an iron-carbon-silicon alloy of high C content between 3-3.75% C and high Si content of 1.5-3% Si.

  • Casts IronsSolidification RateThe stable Fe-C system is iron-graphite rather than iron-cementiteSo slow solidification from the melt favours the iron-graphite system while rapid solidification will favour the Fe-Fe3C system.In order to obtain some areas of Fe-Fe3C within a Fe-graphite casting, to make surface hard areas in a casting, nails with large heads (called chills) are inserted at the mould face to form local spots of cementite instead of graphite. The main factors determining the type of cast iron are: 1) The solidification rate 2) The composition

  • Casts IronsCompositionSilicon and carbon in solution in the melt both enhance the formation of graphite eutectic transformation at 1154oC.These elements are both naturally present in relatively high concentrations in pig iron, which is the major raw material mixed with iron and steel scrap for making cast iron.

    Typical Composition of Pig Iron

    C3.00 3.75S0.05-0.06Si1.5 3.00P0.3-1.5Mn0.1 1.0

  • Fe-Cementite Phase Diagram

  • Iron-Graphite Phase DiagramNote:Changes in eutectic and eutectoid compositions and temperatures

  • The Need for SiliconThe eutectic of graphite occurs at 1154 oC, This eutectic reaction is given by

    L4.26%C g2.08%C + Cgraphite

    The eutectic graphite reaction is competing with the eutectic reaction of Fe3C at 1148 oC.In the cast irons, Si is used to control the formation of the graphite phase.The eutectic reaction does not actually occur at 1154 oC nor 1148 oC nor 4.3 %C because of the addition of Si.

  • The Need for SiliconCast irons with a carbon equivalent, C.E., less than 4.3 wt% C are hypoeutectic.Cast irons with C.E. more than 4.3 wt% C are hypereutectic.C.E. close to 4.3 wt% induces the graphite reaction producing gray irons. Hypoeutectic cast irons will tend to have Fe3C and form white cast irons.

  • The Need for SiliconIn order to induce the eutectic graphite reaction to go to completion, a certain amount of Si is used to complete the formation of graphite from the available carbon.The addition of Si retards the Fe3C formation, which allows more time to form graphite.Si stabilizes the formation of graphite structures.It does this by increasing the undercooling of the eutectoid reaction. This encourages the formation of the ferrite and graphite phases from austenite. This is seen in the (Fe-2%Si)-C phase diagram.

    Do you recall what undercooling is?

  • (Fe-2%Si)-C Phase Diagram0.05%Cundercooling1.5%C3.66%C

  • (Fe-2%Si)-C Phase DiagramCast irons contain 1.15 2.85% Si and are thus conveniently described in terms of a pseudo-binary (Fe+2%Si)-C phase diagram.The g-field is severely constricted compared to the Fe-C diagram.The maximum solubility of C in g-Fe is 1.5%C instead of 2.11% C.The eutectic is 3.66%C instead of 4.26%CThus, the eutectic is given by

    L3.66%C g1.5%C + Cgraphite

    The eutectic temperature is increased to 1154 from 1148 oC.

  • (Fe-2%Si)-C Phase DiagramThe eutectoidThe eutectoid is 0.60%C instead of 0.77%C.The eutectoid temperature is increased to 765 from 727 oC.The maximum solubility of C in a-Fe is increased to 0.05% - from 0.02%C.The eutectoid reaction is given by:

    g0.60%C a0.05%C + Cgraphite

  • The Need for SiliconThe Si is converted to an equivalent carbon and the desired total carbon content is about 4.3 wt% C .This is expressed as:This equation is taken from the ranges of carbon and Si in ferrous alloys for different types of cast irons.

  • The Need for SiliconIf Phosphorus is added to the cast iron, the C.E. is:

  • Effect of Other Elements on the Fe-C EutecticBeside Si and C the g-Fe-Graphite eutectic is also promoted by the following elements:Ni Mg Al Ti Zr Cuwhich are expensive so not generally added.Carbide forming elements such as Cr & Mo stabilize the carbide phase and thus promote the (g-Fe + Fe3C) eutectic.Mn is also a strong carbide (Fe3C) forming solute but its effect on the eutectic depends on the presence or absence of Sulphur. Sulphur is present in cast irons from the coke used to smelt the pig iron.It does not actually form a carbide but strongly promotes carbide formation, e.g., 0.01%S can offset the graphitizing effect of 0.15% Si.

  • Effect of Other Elements on the Fe-C EutecticS has a similar affinity for Mn forming MnS, which itself is neutral - but the first additions of S remove some of the carbide-stabilizing Mn and thus can cause the amount of graphite to increase.Similarly the first additions of Mn can remove the S from solution and again increase the amount of graphite.Phosphorus can act in two ways:1) Physically it forms a phosphide eutectic with a lower melting point than FeC (graphite) which increases the time available for Si to promote graphite.2) Chemically it promotes carbide formation so large amounts increase carbide formation.The various alloying elements must thus be carefully balanced to obtain a desired grey or white cast iron.

  • The Need for SiliconFor Si contents > 2%The eutectic is composed of (gFe + C (graphite)) when the alloy is cooled slowly. These Fe-C alloys are the grey cast irons.The eutectic is composed of (gFe + Fe3C) when the alloy is chill cast. These alloys are the white cast irons.

  • Casts IronsWhen a sample of cast iron is fractured the exposed surface may be grey white- or a mottled grey/white mixture.A sooty grey fracture indicates that the microstructure is composed of graphite flakes in a ferrite matrix.This graphite is relatively weak as the fracture goes from flake to flake so the grey fracture is mostly exposed graphite.Gray cast iron contains small, interconnected graphite flakes in an alpha iron matrix. It has low strength and low ductility.Gray cast iron is the most commonly used cast iron for engine blocks.

  • Casts IronsA white fracture means that the microstructure consists of cementite and ferrite with the fracture going along or through the brittle white areas of cementite.White cast iron contains massive amounts of cementite.When it fractures its surface appears white, hence the name. White cast irons are very hard and resistant to wear.A mottled colour means that graphite flakes are present in some areas, while cementite is present in others.

  • Casts IronsSchematic drawings of five types of cast irons a) gray iron, b) white iron, c) malleable iron, d) ductile iron and e) compacted graphite iron.We will discuss these in detail.

  • Casts IronsSketch in a) and photograph in b) of the flake structure of graphite in gray cast iron.

  • Time Temperature Transformation (TTT) Diagram of Cast IronLater we will discuss TTT diagrams in detail.

    In fact well use TTT diagrams to make steel alloys.

  • Forms of Graphite FlakesMany forms of cast iron exist depending on its solidification. The iron-carbon eutectic can solidify by one or other of the two reactions:liquid austenite + graphiteliquid austenite + cementiteIf a cast iron is superheated to destroy the graphite nuclei undercooling will result and the lower temperature (g-Fe+Fe3C) will form.However if there is sufficient Si and/or C present the cementite will break down to austenite plus graphite the sequence is thus:liquid austenite + cementite then cementite austenite + graphiteThis secondary graphite is quite distinct from the eutectic graphite and forms between the austenite dendrites.What does superheated mean?

  • Dendritic Growth MechanismImages of dendritesLiquid temperature ahead of growing dendrite is cooler than dendrite, which promotes formation of protuberances and nodules.Heat of fusion, DHf heats liquid metal ahead of dendrite, slowing its growth.

  • Eutectic GraphiteUniform flakes random orientationRosette graphite by inoculationNon-uniform flakes random orientationSecondary GraphiteD. Interdendritic random orientationE. Interdendritic preferred orientationABCDEWhat is inoculation?

  • Sizes of Graphite Flakes Rapid solidification results in finer graphite flakes. But too rapid solidification will result in cementite unless there is a very high Si and C concentration.Graphite flakes are classified in sizes like ASTM grain sizes according to the maximum length observed at a magnification of 100x.No. 1 Longest flakes > 4 in (100 mm)No. 2 Longest flakes 2 - 4 in (50 100 mm)No. 3 Longest flakes 1 - 2 in (25 50 mm)No. 4 Longest flakes 0.5 1 in (12.5 25 mm)No. 5 Longest flakes (0.25 0.5 in (6.25 12.5)No. 6 Longest flakes (0.125 0.25 in (3.125 6.25)Etc.What does ASTM stand for?

  • Sizes of Graphite Flakes

  • What does BHNAnd UTS stand for?

    Note: C-Eq. (carbon equivalent)

  • Graphitization in the Solid State Both grey and white cast irons contain austenite. In hypereutectic irons the austenite is only in the eutectic. In hypoeutectic irons the austenite is the pro-eutectic constituent as well as being a constituent of the eutectic so the structure consist of austenite dendrites surrounded by interdendritic eutectic. This helps us to understand the structures we see. During slow cooling from the eutectic to the eutectoid temperature the proeutectic austenite will precipitate carbon. The graphite or carbide in the eutectic exerts a strong nucleation effect so that the precipitation of excess carbon from the proeutectic austenite results in the growth of the eutectic graphite or cementite.

  • Graphitization in the Solid StateDuring slow cooling from the eutectic to the eutectoid temperature the proeutectic austenite will precipitate carbon.

  • The Eutectoid Transformation in White Cast Irons On cooling through the eutectoid temperature the austenite transforms by:

    austenite ferrite + graphiteoraustenite ferrite + cementite (i.e., pearlite)

    In a hypoeutectic white cast iron the pearlite reaction will normally occur but austenite in the eutectic will precipitate carbide on to the cementite formed in the eutectic instead of forming pearlite between these carbide plates. That is, the pre-existing cementite acts as a nucleation site for the precipitation of cementite from austenite.(see next slide)

  • The Eutectoid Transformation in White Cast IronspearliteInterdendriticcarbideNucleation sites

  • The Eutectoid Transformation in Grey Cast Irons In a grey cast iron the type of eutectoid reaction will depend on the Si and C and on the rate of cooling.For a given composition With slow coolingthe eutectic will be austenite + coarse graphite and on passing through the eutectoid this austenite will transform to ferrite + graphite with the graphite depositing on the existing flakes.With rapid coolingthe eutectic structure will be austenite + fine graphite but on passing through the eutectoid the austenite will transform to a regular pearlite structure.(see next slide)

  • The Eutectoid Transformation in Grey Cast Irons

  • Malleable Cast Irons It is possible to have a composition that would form a grey cast iron on extremely slow coolingbut form a white cast iron on regular coolingReheating a regularly cooled sample and holding at high temperature will cause decomposition of the carbide to form graphite + austenite.The graphite formed by this process is quite different from eutectic graphite it grows in the form of compact aggregates rather than flakes.On slow cooling to room temperature the austenite will decompose to ferrite + graphite which will deposit on the previously formed aggregates so the alloy will no longer exhibit brittleness.

    (see next slide)

  • Malleable Cast Irons

  • Nodular or Spherulitic Cast Iron Cast irons with tensile ductility can be formed by adding Mg (or Ce Ca Li or Na) to a very low S alloy (0.01%) just before casting.These additives cause the graphite to form as tiny balls of spherules rather than flakes during the eutectic solidification of grey irons.Since Mg is above its boiling point at casting temperatures of 1400 1500 oC, it is necessary to add it in the form of a Mg-Ni alloy.Alternatively the cooling of a white cast iron can be arrested to permit the eutectic carbide to decompose to austenite + aggregates of graphite (like the malleable irons) an then rapidly cool through the eutectoid temperature to transform the austenite to pearlite.(see next slide)

  • Nodular or Spherulitic Cast IronThe pearlitic nodular irons have a greater strength than the ferritic nodular irons but still have reasonable ductility.graphitepearlite

  • Compositions and Properties of Ductile Cast Irons

  • The End

    Any questions or comments?