Cast Iron Foundry Practices 3. Metallurgy of grey irons

Transcription

1 MME 345, Lecture 36 Cast Iron Foundry Practices 3. Metallurgy of grey irons Ref: Heine, Loper and Rosenthal. Principles of Metal Casting, Tata McGraw-Hill, Topics to discuss today 1. Graphite morphology 2. Metastable nature of iron iron carbide system 3. Solidification of Fe-C-Si alloy 4. Chemical composition effects 5. Properties of grey iron 6. Heat treatment of grey iron

2 Graphite Morphology classification of graphite flake size and shape The properties of grey iron castings are influenced by the shape and distribution of the graphite flakes. The standard method of defining graphite forms is based on the system proposed by the American Society for the Testing of Metals, ASTM Specification A247, which classifies the form, distribution and size of the graphite. Shape Size Distribution ASTM A247 ISO R-945 3/26 Summary and description of ASTM and equivalent ISO classification of graphite shapes ASTM Equivalent Type (a) ISO Form (b) Description I VI Nodular (spheroidal) graphite II VI Nodular (spheroidal) graphite, imperfectly formed III IV Aggregate, or temper carbon IV III Quasi-flake graphite V II Crab-form graphite VI (c) I Flake graphite (a) As defined in ASTM A 247; (b) As defined in ISO/R (E); (c) Divided into five subtypes base on graphite distribution; uniform flakes; rosette grouping; superimposed flake size; interdendritic, random orientation; and interdendritic, preferred orientation. 4/26

3 Six forms: I flake graphite II crab-form graphite III quasi-flake graphite IV aggregate or tempered carbon V nodular graphite, imperfectly formed VI nodular graphite reference diagrams for different graphite form / shape as specified in ISO/R 945 5/26 longest flakes 4 in. or more in length longest flakes 1 to 2 in. in length longest flakes 1/4 to 1/2 in. in length longest flakes 1/16 to 1/8 in. in length longest flakes 2 to 4 in. in length longest flakes 1/2 to 1 in. in length longest flakes 1/8 to 1/4 in. in length longest flakes 1/16 in. or less in length graphite flake sizes as specified in ASTM A247 6/26

4 Type A: Random orientation, uniform distribution The preferred type for engineering applications. This type of graphite structure forms when a high degree of nucleation exists in the liquid iron, promoting solidification close to the equilibrium graphite eutectic. Type B: Rosette grouping The eutectic cell size is large because of the low degree of nucleation. Fine flakes form at the centre of the rosette because of undercooling, these coarsen as the structure grows. Type C: Superimposed flake sizes, random orientation Structures occur in hypereutectic irons, where the first graphite to form is primary kish graphite. It may reduce tensile properties and cause pitting on machined surfaces. Type D: Interdendritic segregation, random orientation Type E: Interdendritic segregation, preferred orientation Both are fine, undercooled graphites which form in rapidly cooled irons having insufficient graphite nuclei. Although the fine flakes increase the strength of the eutectic, this morphology is undesirable because it prevents the formation of a fully pearlitic matrix. Occurs in hypoeutectic alloys. reference diagrams for the distribution of graphite (Form 1) as specified in ASTM A247 7/26 The Metastable Nature of Fe-Fe 3 C System chemical composition, structure and properties of grey iron vary over broad limits range of alloy composition and properties produced are better understood by considering grey iron metallurgy, particularly the metastable nature of iron carbide under normal conditions, a hypoeutectic Fe-C alloy (>4.3%C) freezes with austenite dendrite and ladeburite (austenite-carbide eutectic), which at room temperature transform into pearlite dendrite and transformed ladebutite (pearlite-carbide mixture) a eutectic Fe-C alloy consists only the transformed eutectic iron carbide becomes unstable 1. in contact with graphite at elevated temperature 2. at prolonged exposure to high temperature 3. in presence of certain elements in the alloy conversely, nucleation of graphite is prevented and metastable carbide persisted if 1. the cooling is rapid 2. the alloy contains certain elements 8/26

5 Solidification of Fe-C-Si Alloy presence of Si in the alloy is the single most important composition factor that promote graphitisation in grey iron Three important stages of graphitisation: 1. During solidification 2. By carbon precipitation from austenite (solid state) 3. During eutectoid transformation (solid state) 9/26 Graphitisation During Solidification Size, shape and distribution of graphite flakes developed. Segregation, undercooling and rapid cooling promotes type D/E-type graphites. Suppression of eutectic freezing (by chilling for example) form white iron. Factors to consider: Section size, Superheat, Inoculation. Section Size Large, randomly nucleated flakes (type A/B) low nucleation rate, slow cooling rate, rapid graphitization Small flakes moderate undercooling, moderate nucleation with still time for diffusion and graphitization No flakes (chilled / white iron) severe undercooling (prevents graphitization) Superheating (heating liquid above 1510 C) Undercooling would most likely to occur Produce type D/E flakes Chill / mottled iron would also occur if not inoculate properly. Inoculation (additions to molten iron) Produce marked change in graphite type by preventing undercooling The effect is the most pronounced when added to superheated liquid Only % FeSi or other graphitizing agent addition produces type A graphite 10/26

6 Graphitisation in the Solid State On slow cooling, graphite precipitates on previously existing flakes. On very slow cooling, austenite completely transforms into ferrite and graphite. Fine graphite flakes (formed during freezing) promote solid-state graphitisation. The commercial practice is to retain 100% or some portion of pearlite Proper balance between Mn and S assists to obtain pearlitic structure even when cooled in sand moulds Rapid solid-state cooling and presence of carbide-forming elements increase retention of combined carbon Fine graphite flakes (developed during solidification), regardless of type, promotes solid-state graphitisation The flakes serve as the precipitation centre for carbon 11/26 Chemical Composition Effects cast iron compositions Grade (BS 1452: 1990) Total carbon, % max. Silicon, % Manganese, % Sulphur, % max max max. Phosphorous, % max max max. Molybdenum, % Copper or Nickel, % Mechanical-property specifications are usually considered far more important than chemical specifications So, composition and foundry practice must be adjusted to obtained the desired strength class of grey iron 12/26

7 Element Carbon Silicon Sulphur and Manganese Phosphorous Effect Reported as total carbon: % TC = % Graphitic C + % Combined C For graphitisation, TC must have a minimum value ( 2.2%, value depends on Si content) Shifts eutectic and eutectoid points to the left. Eutectic %C = %Si / 3 CE = %C + %Si / 3 Promote graphitisation after carbon; a certain minimum level of Si is necessary to cause sufficient graphitisation during solidification and develop a satisfactory grey iron Low Si is not sufficient to causes graphitisation during solidification, but promote nucleation and graphitisation at high temperature in the solid state (malleableisation treatment) Both act as carbide stabiliser; presence in low level will cause complete graphitisation S alone form FeS and segregates along grain boundary, but with Mn, form MnS and precipitated throughout the matrix; the effect as carbide stabiliser is lost Relationship between S and Mn: %Mn = 1.7 %S form MnS %Mn= 1.7 %S ; Highest limit of Mn to promote ferrite & graphite %Mn = 3.0 %S ; Lowest limit of Mn to develop 100% pearlite Forms steadite and segregated along grain boundary Forms iron iron phosphide eutectic, thereby promoting eutectic formation 13/26 Properties of Grey Irons foundry properties for several reasons, grey irons are among the most easily cast of all alloys 1. Pouring Temperature wide working temperature ( C) permits easy manipulation, re-ladling, adequate time for pouring typical pouring temperature: C 2. Shrinkage and Feeding favourable freezing mechanism and low shrinkage characteristics higher yield (60 70% or even higher) feeding is not always easy; some casting designs are easily cast with commercially acceptable soundness with low CE value 14/26

8 3. Fluidity most fluid of ferrous alloys; intricate and thin sections can be produced the eutectic composition has the most fluidity the hypereutectic composition suffers extreme loss of fluidity due to graphite precipitation Composition Factor (CF) = %C + %Si / 4 + %P / 2 (for highest fluidity, CF = 4.55) Fluidity (inch) = 14.9 x CF T (T = pouring temperature in F) Fluidity related to pouring temperature and composition of grey and malleable cast iron 15/26 engineering properties Specification of grey irons Country Specification Designation Minimum Tensile Strength (MPa) France NFA FGL Germany DIN GC India IS FG Italy UNI G Japan JIS G FC Class Netherlands GOST Sch UK BS Grade USA ANS/ASTM A48-83 Grade 20A 25A 30A 35A 40A 45A 50A 60A International ISO Grade Equivalent Tonf/in Hardness ranges for grades of grey iron Grade BHN (10/3000) /26

9 from metallurgical standpoint, grey irons are viewed as microstructurally-sensitive alloys microstructure, chemical composition and mechanical properties are intimately related the processing parameters that influence structure, chemical composition variations and cooling rate also influence properties 17/26 C and Si are the most important composition factors affecting mechanical properties maximum strength obtained with a pearlitic matrix composition and structure effect maximum limit of strength by decreasing CE value is about psi; higher strength requires special alloy additions type A graphite produces maximum strength relationship between tensile strength and carbon equivalent value for various bar diameters tensile strength of 1.20-in.-diameter gray-iron bars as affected by carbon equivalent. 18/26

10 Addition of alloying elements has two effects: 1. effect on microstructure, metal matrix, and graphitisation process 2. effect on properties (increased strength, resistances to wear, corrosion, oxidation/scaling and abrasion)

11 cooling rate (section size) effect of cooling rate on properties is profound because of its influence on microstructure rapid cooling increased hardness and tensile strength (as long as no white or chilled iron or D-type graphite is produced) relationship between section size, CE value and structure slow cooling coarsening of graphite flakes and lamellar pearlite and appearance of ferrite, causing softening and weakening of grey iron with reduced wear resistance 21/26 variation of tensile strength with section thickness for several grades of iron 22/26

12 thin section casting has the possibility of misruns and chilled iron surface or hard spot so, certain minimum section thicknesses are desirable in grey iron castings ASTM Class Iron Suggested Wall Thickness, min. (inch) 20 1/8 25 1/4 30 3/8 35 3/8 40 5/8 50 1/2 60 3/4 dependence of grey iron properties on section thickness summary of relationships of CE, section size and properties of unalloyed grey iron 24/26

13 Heat Treatment of Grey Irons Since grey irons may be heated to austenite zone, heat treatments similar to steels can be applied Principal purposes to heat treat grey irons: 1. improve machinability sub-critical annealing to C for 2-4 hrs, followed by slow cooling; spherodisation of pearlite with some graphitisation 2. improve wear resistance hardening at C, followed by oil or water quenching and then tempering to suit the need 3. improve strength rarely used; hardening followed by tempering at C produces optimum tensile strength; may cause warpage or cracking 4. dimensional stability and stress relief often desirable; annealing or normalising can be used; a specific stress-relief anneal consists in heating to C for 1 hr or more, followed by slow cooling 25/26 Next Class MME 345, Lecture 37 Cast Iron Foundry Practices 4. Grey irons foundry practice