Ferrous Alloys

Classification of Metallic Alloys

What is Pig Iron? Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Pig iron usually contains 3-4% of carbon, 2-4% of silicon, 1-2% of manganese and 1-1.2% of phosphorous which makes it very brittle and not useful directly as a material except for limited applications.

Cast Iron Cast iron is mainly an alloy of Fe and C just like steel Composition dictates whether an alloy will be called steel or cast iron Cast irons contain a greater amount of carbon than that necessary to saturate austenite at the eutectic temperature Therefore, cast irons contain between 2 and 6.67 percent C High C  brittleness Commercially used types contain 2.5 to 4 % C The ductility of cast iron is very low Cannot be rolled, drawn, or worked at room temperature However, they melt readily and can be cast into complicated shapes which are usually machined to final dimensions Since casting is the only suitable process applied to these alloys, they are known as cast irons.

Uses of Cast Irons

Forms of Carbon in Fe-C alloys In all these, C can be said to be in a combined from Is there any possibility of “free” carbon? In interstitial solid solution As cementite (Fe3C)

Meet the True “Fe-C” phase diagram!

Graphitization and cast iron Cementite is a metastable phase If conditions are suitable cementite breaks up into iron and graphite Fe3C  Fe (α) + C (graphite) This is called graphitization Graphitization is favored by Silicon content >1% and high C % Slow cooling through critical temperatures The suitable conditions are never met in steels When there is possibility of graphite formation then the class of material is referred to as cast iron In fact most cast irons contain graphite

Types of cast iron Although the composition of cast irons are similar, their structure and property can be vastly different depending on heat treatment, cooling rate and additions Any property differences between types of CI can be traced back to the microstructure The essential question one needs to ask is – “What is the form of carbon and what is its shape?” Depending on the characteristics of carbon, thus, there are four basic types of cast iron Gray cast iron White cast iron Malleable cast iron Ductile/nodular cast iron Just one among these don’t contain graphite - white cast iron

Types of cast iron

Gray Cast Iron C content  2.5-4 % Si content  1-3 % Strong graphitizing tendency Graphite exists as flakes Fracture surface appears gray due to flakes – That’s why its called Gray cast iron ! Matrix can be ferritic, pearlitic or mix of two depending on cooling rate Fast cooling through TC  more pearlite Slow cooling through TC  more ferrite

Stress concentration at these points Properties Properties of gray cast iron can be largely attributed to the presence of graphite flakes 1.Gray CI has low strength and almost zero ductility in tension, but has better properties in compression Graphite has very low strength (120-270 MPa) The graphite flakes can be thought to be voids – the strength is so low it is as if there is nothing there ! That’s why the sharp corners of the flakes act as notches/stress raisers when under tensile stresses Small applied loads get magnified at these notches and causes failure at a much lower stress level than would be expected in the absence of the flakes Stress concentration at these points

Properties 2.Good wear resistance Presence of graphite lubricates contacting surfaces Results in low friction coefficient and high wear resistance 3.Good damping capacity Graphite absorbs vibrations effectively, making its damping characteristics desirable One of the reasons for usage as the base of heavy machinery

Properties 4.Very good casting properties Silicon increases fluidity of the melt so intricate shapes can be formed Graphite in the microstructure lessens the degree of shrinkage, making casting a much simpler and easier process for producing GCI products These properties imply that gray cast iron can be used in components which do not face considerable tensile stress. In fact they are used in a lot of components that do not have to carry tensile loads, specially in wear applications. Wear resistance, good castability, vibrational characteristics and very low cost make this material attractive.

White Cast Iron Contains low silicon (<1%) Graphitizing tendency low If cooling rate is sufficiently high graphite cannot form – thick cementite exists Extremely hard and brittle Used as rollers for rolling mill, as pressure plates for jaw crushers etc.

White iron is produced by "chilling" selected areas of a casting in the mold, which prevents graphitic carbon from precipitating out Chills produce castings with white-iron working surfaces and cores that are a tougher and more easily machinable gray or ductile iron. During chilling, that portion of the casting that is to resist wear is cooled by a metal or graphite heat sink (chill) in the mold. When the molten iron contacts the chill, it solidifies so rapidly that the iron and carbon cannot become dissociated.

rollers Jaw crushers Railway brakes

Malleable Cast Iron Made by heat treating white cast iron Solid state diffusion takes place to rearrange the shape and form of graphite Driving force exists because cementite is metastable with respect to graphite Perfect nodules cannot form because of solid state transformation Property is better than normal cast iron Shows better strength and some ductility/malleability Representative applications include connecting rods, transmission gears, and differential cases for the automotive industry, and also flanges, pipe fittings, and valve parts for railroad, marine, and other heavy- duty services.

Pipe fittings Valve flange Connecting rod

Nodular Cast Iron Add Mg/Ce in the cast iron melt before casting It is important to remember that no heat treatment will make graphite nodules – they must be formed directly from the liquid state. Desulphurization is necessary before adding Mg. Otherwise Mg is lost as MgS. Graphite still forms but in a different shape – nodules, Matrix can be ferritic/pearlitic Property is completely different from gray cast iron Much higher tensile strength (380-480 MPa) and elongation (10-20%); approaching that of steel Typical applications for this material include valves, pump bodies, crankshafts,gears, and other automotive and machine components.

Crankshaft Pump bodies Frames