CAST IRON INTRODUCTION Greater amount of carbon It makes iron brittle Range 2.5 to 4 percent carbon Why it is called cast iron?
PROPERTIES Low ductility Can not be rolled Not malleable Lower strength than steel Melt readily Can be cast into complicated shapes which are usually machined to final dimensions Cheap Compressive strength CASTING IS THE ONLY PROCESS APPLIED TO THESE ALLOYS SO THEY ARE CALLED CAST IRON Types of cast iron Based on, Carbon content Cooling rate during and after freezing Heat treatment after casting Carbon form : Iron carbide or graphite Shape and distribution of free carbon
Types of cast iron White cast irons: cementite Malleable cast irons: free carbon in form of round particles irregular shape Gray cast irons: free carbon in form of graphite flakes Nodular cast irons: free carbon in form of compact spheroids round carbon particles regular in shape Alloy cast irons White cast iron Hypoeutectic alloys At eutectic temperature alloy consists of austenite-2%c liquid 4.3%C Ledeburite-coarse mixture because of high temp. Eutectic austenite get mixed with primary austenite leaving free cementite Proeutectoid cementite + cementite already present
White cast iron Large amount of cementite Hard and wear resistant Extremely brittle Difficult to machine Hardness 375 to 600 BHN Tensile strength 138 Mpa to 480 Mpa Compressive strength around 1400 Mpa Applications Ball mills Drawing dies Raw material for malleable cast iron
Gray cast iron Most widely used cast iron First solidify by forming primary austenite and cementite Graphitization is aided by high carbon, high temperature and graphitizing elements Controlling above factors graphite flakes can be generated Curved plates grayish fracture Strength depends on matrix Ferritic-softest and weakest, pearlitic
curved plates if we see the 3 rd dimension notch an indentation or incision on an edge or surface
Properties and applications of gray cast iron Gray fracture surface Good machinability compare to white cast iron Vibration damping capacity Max.Tensile strength around 295 Mpa Percentage elongation 3 to 4 % Gray iron should be first preference because of its cost üdiesel engine casting ücylinder block ügear box üclutch plates Silicon in cast iron Increases fluidity Graphitizer Iron graphite system
Sulfur Reverse of silicon Brittle carbides Iron sulfide Red short Reduces fluidity blowholes Manganese Greater affinity for sulfur than iron Eutectoid Carbide stabilizer Removes effect of sulfur Small, widely dispersed inclusions Two to three times the sulfur content Phosphorus Iron phosphide Brittleness Fluidity increases Thin castings
Heat treatment of gray iron Stress relieving Residual stresses Annealing for improving machinability above 1100 F(around 595 C) Normalizing for improving hardness and tensile strength - 1700 F(around 927 C) cooled in air Hardened by cooling rapidly, then it can be tempered Quenching with oil, water Graphite flake size
Inoculants aluminium, titanium, zirconium, silicon carbide
Malleable cast iron Cementite is not stable phase Tendency to decompose into iron and carbon This is why we can manufacture malleable cast iron Purpose of malleabilization is to convert all combined carbon in white iron into irregular nodules of graphite and ferrite. Elements that decompose Iron carbide silicone
Malleable cast iron Austenite of the metastable system can dissolve more carbon than can austenite of the stable system Graphitization results in graphite nucleus growth Spheroids called temper carbon Annealing two stage(rapid and slow) 20 to 70 hours at high temperature 750 to 900 Celsius Temper carbon + Ferrite Malleable cast iron Applications Automotive equipment Railroad equipment Chain hoist assemblies Pipe fittings
Pearlitic Malleable Iron If controlled quantity of carbon is retained as Iron carbide Strength and hardness Manganese Quenching with air in second stage annealing Tempering of pearlitic malleable irons improves machinability and toughness, lowers hardness Effect of magnesium on graphite shape
SG cast iron Gray cast iron is useful because of good machinability Strength is an issue Spheroidal shape is better for strength Magnesium Spheroidal graphite plus ferrite Processing advantages of gray cast iron with engineering advantages Effect of graphite nodularity on properties
Properties and applications of SG cast iron Ferritic SG iron Tensile strength 414 Mpa 18% elongation ücrankshafts ügears ürollers üpipes
Seasoning of cast iron Seasoning is a process by which a layer of animal fat or vegetable oil is applied and cooked onto cast iron or carbon steel cookware. Non stick surface Interaction of food with iron rusting Seasoning technique First, heat the cookware until water quickly boils when splashed onto its surface. Next, using a rag, wipe a thin coat of lard or oil onto the surface of the cookware. Finally, place the cookware upside-down in a moderately hot oven for 1-2 hours. It is a good idea to place a layer of aluminum foil under the cookware to catch drips of excess oil. Let the pan cool in the oven. At this point, the cookware should have a black sheen and be ready for cooking.