Cast Iron PDF 2024-2025

20 Dec 2024 Cast iron 20 Dec 2024 Technology of Metals 2024-25 1 1 Cast iron FIRST CLASSIFICATION: REFINING...

20 Dec 2024 Cast iron 20 Dec 2024 Technology of Metals 2024-25 1 1 Cast iron FIRST CLASSIFICATION: REFINING or FIRST CASTING CAST IRONS for the production of steel (80 ÷ 85% of the cast iron deriving from the blast furnace). CAST IRONS from FOUNDRY or SECOND CASTING (15 ÷ 20%). Characteristics: Fe-C alloys (2.01 ÷ 4%) generally containing also Si (0.5 ÷ 3%); Typical foundry alloys: high fluidity and castability (low melting temperature); Good machinability on machine tools (excluding white cast irons); 20 Dec Inexpensive 2024 Technology of Metals 2024-25 2 2 Technology of Metals 2024-25 1  20 Dec 2024 20 Dec 2024 Technology of Metals 2024-25 3 3 Cast iron During solidification, a cast iron can follow the stable iron-graphite diagram (red lines) or During solidification, a cast iron can follow the stable iron-graphite diagram (red lines) the metastable iron-cementite diagram (black lines). The carbon present in cast irons can or the metastable iron-cementite diagram (black lines). The carbon present in cast irons be free as graphite or combined as cementite (Fe3C). can be free as graphite or combined as cementite (Fe3C). 20 Dec 2024 Technology of Metals 2024-25 4 4 Technology of Metals 2024-25 2  20 Dec 2024 Cast iron The factors that influence the appearance of carbon in cast iron are: chemical composition cooling rate eventual heat treatment It is also possible to perform particular treatments of the cast iron before or during casting. Elements such as Si, and low cooling rates are graphitizing factors, that is carbide formation is hindered 20 Dec 2024 Technology of Metals 2024-25 5 5 Cast iron The classification of cast irons is based on the metallographic structure which depends on: how C is combined (either combined like cementite or free as graphite) the morphology and distributiongrafite.  WHITE CAST IRON  GREY CAST IRON  DUCTILE CAST IRON (contains spheroidal graphitic constituents)  MALLEABLE CAST IRON  ALLOY CAST IRON Originally the name was attributed on the basis of the appearance and color of the fracture surfaces of the various types of cast irons. 20 Dec 2024 Technology of Metals 2024-25 6 6 Technology of Metals 2024-25 3  20 Dec 2024 Cast iron 20 Dec 2024 Technology of Metals 2024-25 7 7 METASTABLE Fe-Fe3C DIAGRAM: Solidification of an eutectic Eutectic micostructure: composition LEDEBURITE austenite matrix and Fe3C globules Austenite rejects C, which precipitates (as iron carbide upon combination with the iron in the matrix) on the globules already present in the Ledeburite Transformed Ledeburite: matrix consisting of structural elements deriving from the transformation of austenite –Martensite, or Bainite or Ferrite- and ledeburite globules Eutectic Cast iron(C=4,3%) At R.T. 100% of Transformed ledeburite 20 Dec 2024 Technology of Metals 2024-25 8 8 Technology of Metals 2024-25 4  20 Dec 2024 Cooling of a liquid with the eutectic composition 20 Dec 2024 Technology of Metals 2024-25 9 9 METASTABLE DIAGRAM Fe-Fe3C: Cooling of an ipoeutectic iron alloy Solidification starts with the formation of austenitic cystals. Austenite and ledeburite Austenite rejects C, which precipitates (as iron carbide upon combination with the iron in the matrix) on the globules already present in the Ledeburitic zones, as well originating platelets of secondary cementite at austenitic grain boundaries The austenite to pearlite transformation follows upon further cooling Ipoeutectic composition (%C tra 2 e 4,3 %). At R.T. 100% pearlite + Transformed Ledeburite. 10 20 Dec 2024 Technology of Metals 2024-25 10 Technology of Metals 2024-25 5  20 Dec 2024 Cooling of an ipoeutectic liquid 20 Dec 2024 Technology of Metals 2024-25 11 11 METASTABLE Fe-Fe3C DIAGRAM : Cooling of an Ipereutectic Composition Alloy Fe3C (primary cementite) is the first phase to nucleate from the undercooled liquid. Ledeburite and primary cementite. Austenite rejects C, which precipitates (as iron carbide upon combination with the iron in the matrix) Fe3C che va ad ingrossare i globuli della ledeburite. Cementite and transformed ledeburite. Ipereutectic Composition At R.T. 100% Cementite + Transformed Ledeburite. 20 Dec 2024 Technology of Metals 2024-25 12 12 Technology of Metals 2024-25 6  20 Dec 2024 Cooling of an Ipereutectic Composition Alloy 20 Dec 2024 Technology of Metals 2024-25 13 13 ROLE OF SILICON  Reduces the amount of C in the eutectic and in the eutectoid;  Increases the amount of T of the stable eutectic (grafite) 20 Dec 2024 Technology of Metals 2024-25 14 14 Technology of Metals 2024-25 7  20 Dec 2024 ROLE OF SILICON During a generic cooling, the metastable eutectic could only be reached with a much higher supercooling than the stable one. Solidification of the stable eutectic (graphite) is favored Grafitizing effect 20 Dec 2024 Technology of Metals 2024-25 15 15 CARBON EQUIVALENT Without Si, the composition of the eutectic is 4.3% C. As the Si content increases, the C content of the eutectic decreases. CARBON EQUIVALENT: CE = %C + %Si/3 When CE = 4,3%, the alloy is eutectic. Cast irons with the same CE value can be obtained with different C and/or Si contents. High values of CE promote graphite formation. 20 Dec 2024 Technology of Metals 2024-25 16 16 Technology of Metals 2024-25 8  20 Dec 2024 ROLE OF CHROMIUM Cr makes solidification more likely to take place according to the metastable system Fe-Fe3C. antigraphitizing Effect Graphite promoting elements: Si, Al, Cu, Ni, B ( 0,15%), Ti ( > 0,25%) 20 Dec 2024 Technology of Metals 2024-25 17 17 ternary Fe-C-Si diagram 1. Liquid 2. Eutectic Solidification 3-4. Segregation of Fe3C (white cast iron) o graphite (ductile or grey cast iron). 4-5. Eutectoid Trasform 20 Dec 2024 Technology of Metals 2024-25 18 18 Technology of Metals 2024-25 9  20 Dec 2024 Fe-C-Si Ternary diagram The differences between cast irons are due to the modalities in which the solidification process takes place and can be explained using the Fe-C-Si ternary diagram. Point 1: As the liquid lowers its temperature and reaches point 1, austenite dendrites begin to appear and grow until point 2 is reached. Point 2: The eutectic solidification begins as soon as the temperature reaches point 2. The eutectic compound can be a compound of austenite and Fe3C, or of austenite and graphite. In the first case a white cast iron is obtained, in the second case a gray or spheroidal cast iron is obtained. The formation of graphite in the presence of graphitizing elements such as a high silicon content or very slow cooling rates. When point 3 is reached the solidification is completed and the structure is considered to be austenite plus Fe3C, or austenite plus graphite. Point 3: In the cooling between points 3 and 4, the austenite segregates carbon being oversaturated. This excess of C precipitates as secondary cementite in white cast iron, and as graphite in gray or spheroidal cast iron. Point 4: The final solid state transformation takes place between points 4 and 5. This transformation of austenite is very complex and allows only a few generalizations. With very favorable graphitizing conditions, only ferrite is formed in gray or spheroidal cast irons. Under less favorable conditions, ferrite and pearlite are formed, and at most only pearlite. In any case, pearlite is formed in white cast irons. Point 5: Cooling below point 5 doesTechnology 20 Dec 2024 not produce relevant transformations. of Metals 2024-25 19 19 CONDITIONS FOR EUTECTIC REACTIONS IN CAST IRONS L austenite + graphite L austenite +F3C High CE Low CE High Si Low Si Cu, Ni Cr, Mo, V, Te Low cooling rates High cooling rates Thick sections in castings Thin sections in castings Inoculation (addition of Fe-Si No inoculation alloys, with low amounts of Ca, Al, Ba) Grey Cast Irons White Cast Irons 20 Dec 2024 Technology of Metals 2024-25 20 20 Technology of Metals 2024-25 10  20 Dec 2024 White Cast Irons C≅2,5÷3,5% Low CE Si ≤ 1% The presence of cementite (hard and brittle) gives rise to reflective fracture surfaces, hence the name given to this type of cast iron. The very high hardness (≥50 HRC) gives excellent wear resistance. The factors that favor the formation of white cast irons are:  relatively low C and Si contents  high cooling rates  thin sections. 20 Dec 2024 Technology of Metals 2024-25 21 21 White Cast Irons Carbon: affects Hardness:  2,5%C 375 HB  3,5%C 600 HB Silicon: the quantity, never high, is strictly connected with the thickness of the casting, to which the cooling rate is linked. Chromium: favoring the formation of carbides, neutralizes the graphitizing effect of Si, thus improving wear resistance. Molybdenum: effect similar to Cr. Promotes the formation of martensitic or bainitic matrices. Tellurium, Vanadium: stabilize the carbides. Nickel: graphitizing effect, but promotes the formation of martensite. 20 Dec 2024 Technology of Metals 2024-25 22 22 Technology of Metals 2024-25 11  20 Dec 2024 White Cast Irons Unalloyed white cast irons consisting of cementite and pearlite; not very impact resistant. White cast irons alloyed with Ni and Cr (eg Ni-Hard, 4%Ni - 1.5%Cr), have a martensite and carbide structure with outstanding wear resistance. White cast irons with a high Cr content (up to 30%), solidify with an austenite and chromium carbides microstructure; the matrix can thereafter vary from pearlite to a mixture of retained austenite and martensite, depending on both the composition and the cooling rate. 20 Dec 2024 Technology of Metals 2024-25 23 23 White Cast Irons Exceptional resistance to wear and abrasion, corrosion and oxidation. Low toughness. USES: rolling mill cylinders grinding balls facing plates of rock and mineral crushers dies for drawing extrusion nozzles tips for plowshares and plows 20 Dec 2024 Technology of Metals 2024-25 24 24 Technology of Metals 2024-25 12  20 Dec 2024 CHARACTERISTIC SHAPES OF GRAPHITE I) Thin lamellae with sharp points. II) Nodules with accentuated ramifications of lamellae. III) Thick lamellae with rounded tips. IV) Jagged flocculi. V) Compact Flocculi. VI) Spheroids 20 Dec 2024 Technology of Metals 2024-25 25 25 Grey Cast Irons C=2.5÷3.5% Si=1÷3.5% They solidify in a structure consisting of graphite flakes dispersed in a metallic matrix Their name derives from the characteristic color of the fracture surfaces. The most industrially used type of cast iron, also because it is very economical. Grey cast iron in coarse pearlitic matrix 20 Dec 2024 Technology of Metals 2024-25 26 26 Technology of Metals 2024-25 13  20 Dec 2024 Grey Cast Irons DISTRIBUTION OF GRAPHITE ELEMENTS 20 Dec 2024 Technology of Metals 2024-25 27 27 Grey Cast Irons DISTRIBUTION OF GRAPHITE ELEMENTS 20 Dec 2024 Technology of Metals 2024-25 28 28 Technology of Metals 2024-25 14  20 Dec 2024 Grey Cast Irons DISTRIBUTION OF GRAPHITE ELEMENTS 20 Dec 2024 Technology of Metals 2024-25 29 29 Grey Cast Irons CONSEQUENCES RELATED TO THE PRESENCE OF GRAPHITE LAMELLAS TYPE A: High UTS and toughness. It is what you are trying to achieve. TYPE D: Less desirable from the point of view of UTS and wear resistance. Strength depends also from the structure of the matrix. The pearlitic structure confers a higher UTS than the same cast irons with ferritic matrix 20 Dec 2024 Technology of Metals 2024-25 30 30 Technology of Metals 2024-25 15  20 Dec 2024 Grey Cast Irons Mechanical Properties of Grey Cast Irons 20 Dec 2024 Technology of Metals 2024-25 31 31 Grey Cast Irons Microstructural Characteristics 20 Dec 2024 Technology of Metals 2024-25 32 32 Technology of Metals 2024-25 16  20 Dec 2024 DESIGNATION GREY CAST IRON UNI EN-1561 EN : indicates that the cast iron is standardized GJ : cast iron symbol L : graphite (lamellar) structure symbol 100 : Characteristic mechanical property 20 Dec 2024 Technology of Metals 2024-25 33 33 Properties of Grey Cast Irons Good castability and low shrinkage Excellent compressive strength Excellent machinability Good thermal conductivity Excellent vibration damping capacity Good wear resistance and low friction (graphite acts as a solid lubricant) Inexpensive USE: machine tool bases, compressors, valves, radiators, engine blocks, exhaust manifolds, cylinders for rolling mills. 20 Dec 2024 Technology of Metals 2024-25 34 34 Technology of Metals 2024-25 17  20 Dec 2024 EFFECT OF THE GRAPHITE MORPHOLOGY ON STRENGTH Influence of graphite morphology on stress-strain curves of different cast irons. 20 Dec 2024 Technology of Metals 2024-25 35 35 DUCTILE CAST IRON Graphite in the form of spheroids. Significant improvement of all mechanical properties, including ductility. Good resistance to fatigue and wear. Lower-melting material than steel, but with comparable mechanical characteristics. More common ferrous material after gray cast iron and steel 20 Dec 2024 Technology of Metals 2024-25 36 36 Technology of Metals 2024-25 18  20 Dec 2024 DUCTILE CAST IRON Microstructural features 20 Dec 2024 Technology of Metals 2024-25 37 37 DUCTILE CAST IRON Some elements (magnesium, calcium, rare earths) act as spheroidizers of graphite. Effect of Magnesium on the microstructure of cast iron. 20 Dec 2024 Technology of Metals 2024-25 38 38 Technology of Metals 2024-25 19  20 Dec 2024 DUCTILE CAST IRON 20 Dec 2024 Technology of Metals 2024-25 39 39 DUCTILE CAST IRON Spheroidization 1. Desulphurization: aim: reduce sulfur (S