Cast Irons Engr. Waqas Javaid WEC

Overview of Cast Iron Iron with 2.0 to 4.5% carbon and 0.5 to 3% silicon. Lower melting point and more fluid than steel (better castability). Low cost material usually produced by sand casting. A wide range of properties, depending on composition & cooling rate. WEC

Composition of Cast Iron A typical cast iron contains 2.0 to 4.5% carbon, 1.0 to 3.0% silicon, less than 1.0% manganese, and less than 0.2% sulfur. WEC

Types of Cast Iron Gray cast iron - carbon as graphite White cast iron - carbides, often alloyed Ductile cast iron nodular, spheroidal graphite Malleable cast iron Graphite nodules are irregular clusters / tempered graphite Compacted graphite cast iron CG or Vermicular Iron WEC

Microstructures of Cast Iron Gray iron aFe and graphite flakes Nodular iron aFe and graphite spheres White iron cementite and pearlite Malleable iron a Fe and tempered graphite flakes low melting point, castable, cheap; however, can be brittle. Compacted Graphite Iron WEC

Gray Cast Iron Gray cast iron forms when Cooling is slow, as in heavy sections High silicon or carbon, Mn = 0.4 to 1.0 % Flake graphite in a matrix of pearlitic, ferrite or martensite Properties depend strongly on casting shape & thickness Low ductility - elongation 0.6% High conductivity, high damping Wide range of applications WEC

Properties of Gray Cast Irons Excellent compressive strength (compressive strength is typically 3-4 times tensile strength), Excellent machinability (graphite acts to break up the chips and lubricate contact surfaces), Excellent wear resistance (graphite flakes self-lubricate), and Out­standing sound and vibration-damping capacity (graphite flakes absorb transmitted energy). WEC

Properties of Gray Cast Irons Good corrosion resistance and the enhanced fluidity due to high silicon contents Thermal conductivity high The formation of the lower-density graphite reduces the amount of shrinkage, making possible the production of more complex iron castings The pointed edges of the flakes act as preexisting notches or crack initiation sites, giving the material a characteristic brittle nature resulting low impact resistance Ductility is low (0.6%) WEC

Properties of Gray Cast Irons The size, shape, and distribution of the graphite flakes have a considerable effect on the overall properties of gray cast iron. For maximum strength, small, uniformly distributed flakes are preferred. WEC

Applications of Gray Cast Irons Engines Cylinder blocks, liners, Transmission housing Brake drums, clutch plates Pressure pipe fittings, Machinery beds Furnace parts, ingot and glass moulds Machine tool basis & large equipment parts – subjected to compressive load & vibration WEC

Ductile or SG Iron Also known as spheroidal graphite (SG), and nodular graphite iron Far better ductility than gray cast iron WEC

Properties of SG Iron good ductility, high strength Strength higher than gray cast iron Ductility up to 6% as cast or 20% annealed Low cost Simple manufacturing process makes complex shapes Machineability better than steel Good toughness, wear resistance, low-melting-point castability, up to a 10% weight reduction compared to steel makes ductile iron an attractive engineering material WEC

Applications of SG Iron Automotive industry 55% of ductile iron in USA Crankshafts, front wheel spindle supports, steering knuckles, disc brake callipers Pipe and pipe fittings (joined by welding) WEC

White Cast Iron WEC

White Cast Iron White fracture surface No graphite, because carbon forms Fe3C or more complex carbides. Features promoting the formation of cementite over graphite are: low Carbon equivalent (1.8 to 3.6% carbon), 0.5 to 1.9% Silicon, 0.25 to 0.8% Manganese, & rapid cooling very hard, brittle & abrasion resistant. Often alloyed WEC

Uses of White Cast Iron Products such as gates, fences, parts of stove are manufactured by using white cast iron. In addition it is also used to manufacture malleable cast iron WEC

Malleable Iron WEC

Malleable Iron Produced by heat treatment (in the range of 900C) of white cast iron- cementite dissociate into its component element. Carbon converts into graphite nodules of irregular clusters (also referred to as Clump or popcorn graphite). Similar properties to ductile iron. depending on the type of heat treatment, various types of malleable iron can be produced. WEC

Properties of Malleable Iron Similar to ductile iron Good shock resistance Good ductility Good machineability Corrosion resistance WEC

Applications of Malleable Iron Similar applications to ductile iron Malleable iron is better for thinner castings Ductile iron better for thicker castings >40mm Vehicle components Power trains, frames, suspensions and wheels Steering components, transmission and differential parts, connecting rods Railway components Pipe fittings products such as door keys, gear wheel, and crank levers WEC

Compacted Graphite Cast Iron (CGI) WEC

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Compacted Graphite Cast Iron (CGI) CGI is produced by addition of an Mg-Ce-Ti CGI is characterized by a graphite structure that is intermediate to the flake graphite of gray iron and the nodular graphite of ductile iron, and possesses some of the desirable properties and characteristics of each. WEC

Properties of CGI Properties of CGI bridge the gap between Gray and Ductile irons Strength, stiffness, and ductility are greater than those of gray iron, While, castability, machinability, thermal conductivity, and damping capacity all exceed those of ductile. Impact and fatigue properties are good. WEC

Typical Properties of Pearlitic Gray, Compacted Graphite, and Ductile Cast Irons Property Gray CGI Ductile Tensile strength (MPa) 250 450 750 Elastic modulus (Gpa) 105 145 160 Elongation (%) 1.5 5 Thermal conductivity (w/mk) 48 37 28 Relative damping capacity (Gray = 1) 1 0.35 0.22 WEC

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Fracto-Graphs of Cast Irons WEC

Welding Weldability of cast iron is low and depends on the material type, thickness, complexity of the casting, and on whether machinability is important WEC

Weldability White cast iron - not weldable Small attachments only Grey cast iron - low weldability Welding largely restricted to salvage and repair Ductile and malleable irons - good weldability (inferior to structural steel) Welding increasingly used during manufacture WEC