Engineering Materials Haseeb Ullah Khan Jatoi Department of Chemical Engineering UET Lahore

Iron and Steel

Metal Alloy An alloy is a mixture or metallic solid solution composed of two or more elements. An alloy will contain one or more of the three: a solid solution of the elements (a single phase); a mixture of metallic phases (two or more solutions); an inter-metallic compound with no distinct boundary between the phases. May or may not be homogenous depending upon thermal history of the material May be substitution alloy, or interstitial alloy Brass (Copper + Zinc) Pewter (Tin 85-99% + Copper + Antimony + Bismuth) Phosphor Bronze (Copper + Tin + Phosphorus) Amalgam ( Mercury + Metal)

Phase Diagram To understand design and control of heat treating procedures To understand properties and microstructure of alloys Components (Pure constituents) System (E.g. Iron-Carbon System) Solubility Limit (May be temp dependent or composition etc.) Phase (homogenous portion of system having uniform physical and chemical properties) Homogenous and heterogeneous Most metallic alloys are heterogeneous

Microstructure of alloy Microstructure is subject to direct microscopic observation, using optical or electron microscopes Depends on: Alloying elements present Their concentration Heat treatment of alloy (Temperature, heating time, cooling rate)

Phase Diagram Temperature , Pressure, and Composition are three parameters that are played One component diagram (Unary) Two component diagram (Binary) Multi-component diagram

Single phase

Binary system

L = homogeneous liquid solution composed of both copper and nickel α= substitution solid solution consisting of both copper and nickel Liquidus and solidus Two extremities represent melting temperatures of two components

Information from phase diagram Phases present Phase composition (Tie Line) Phase amounts (Lever Rule)

Exercise

Microstructure (Cooling or solidification)

Eutectic phase diagram α= solid solution rich in copper or pure copper β = solid solution rich in silver or pure silver Eutectic (easily melted) point and eutectic reaction Solvus, solidus, liquidus, invariant point

Metal Alloys Classification

Ferrous Alloys Iron is the prime constituent Produced largely and abundantly used as engineering construction materials ‘All-purpose alloys’, but one big disadvantage i.e. Corrosion. Why prefer ferrous alloys? Iron-containing compounds exist in abundant quantities within the earth’s crust ( ores are cheaper) Metallic iron and steel alloys may be produced using relatively economical extraction, refining, alloying, and fabrication techniques They are extremely versatile (can be fashioned to have versatile desired properties)

Iron and Steel production Iron making – Iron is reduced from its ores Steel making – Iron is then refined to obtain desired purity and composition (Alloying)

Iron Ores required in Iron making The principal ore used in the production of iron and steel is hematite (Fe2O3) Other iron ores include magnetite (Fe3O4), siderite (FeCO3), and limonite (Fe2O3-xH2O, where x is typically around 1.5) Iron ores contain from 50% to around 70% iron, depending on grade (hematite is almost 70% iron) Scrap iron and steel are also widely used today as raw materials in iron and steel making

Iron Ores

Other Raw Materials in Iron-making Coke Supplies heat for chemical reactions and produces carbon monoxide (CO) to reduce iron ore

Hot gases (CO, H2, CO2, H2O, N2, O2, and fuels) Limestone (CaCO3) Used as a flux to react with and remove impurities in molten iron as slag Hot gases (CO, H2, CO2, H2O, N2, O2, and fuels) Used to burn coke

Iron-making in a Blast Furnace Blast furnace - a refractory-lined chamber with a diameter of about 9 to 11 m (30 to 35 ft) at its widest and a height of 40 m (125 ft) To produce iron, a charge of ore, coke, and limestone are dropped into the top of a blast furnace Hot gases are forced into the lower part of the chamber at high rates to accomplish combustion and reduction of the iron

Blast Furnace

Chemical Reactions in Iron-Making Using heated air (O2) with coke: 2C + O2 heat + 2CO Using hematite as the starting ore: Fe2O3 + CO 2FeO + CO2 CO2 reacts with coke to form more CO: CO2 + C (coke) 2CO This accomplishes final reduction of FeO to iron: FeO + CO Fe + CO2

Iron from the Blast Furnace Iron tapped from the blast furnace (called pig iron) contains over 4% C, plus other impurities: 0.3-1.3% Si, 0.5-2.0% Mn, 0.1-1.0% P, and 0.02-0.08% S Further refinement is required for cast iron and steel A furnace called a cupola is commonly used for converting pig iron into gray cast iron For steel, compositions must be more closely controlled and impurities brought to much lower levels

Steel-making Since the mid-1800s, a number of processes have been developed for refining pig iron into steel Today, the two most important processes are – Basic oxygen furnace (BOF) – Electric furnace Both are used to produce carbon and alloy steels

Basic Oxygen Furnace (BOF) Accounts for almost 70% of steel production in U.S Adaptation of the Bessemer converter Bessemer process used air blown up through the molten pig iron to burn off impurities BOF uses pure oxygen Typical BOF vessel is typically 5 m (16 ft) inside diameter and can process 150 to 200 tons per heat Cycle time (tap-to-tap time) takes almost 45 min

Basic Oxygen Furnace

Electric Arc Furnace Accounts for 30% of steel production in U.S. Scrap iron and scrap steel are primary raw materials Capacities commonly range between 25 and 100 tons per heat Complete melting requires about 2 hr; tap-to-tap time is 4 hr Usually associated with production of alloy steels, tool steels, and stainless steels Noted for better quality steel but higher cost per ton, compared to BOF

Electric arc furnace

Casting Processes in Steel-making Steels produced by BOF or electric furnace are solidified for subsequent processing either as cast ingots or by continuous casting – Casting of ingots – a discrete production process – Continuous casting – a semi-continuous process

Casting of Ingots Steel ingots = discrete castings weighing from less than one ton up to 300 tons (entire heat) Molds made of high carbon iron, tapered at top or bottom for removal of solid casting The mold is placed on a platform called a stool After solidification the mold is lifted, leaving the casting on the stool

Continuous Casting Continuous casting is widely applied in aluminum and copper production, but its most noteworthy application is steel-making Dramatic productivity increases over ingot casting, which is a discrete process For ingot casting, 10-12 hr may be required for casting to solidify Continuous casting reduces solidification time by an order of magnitude

Steel An alloy of iron containing from 0.02% and 2.11% carbon by weight Often includes other alloying elements: nickel, manganese, chromium, and molybdenum Steel alloys can be grouped into four categories: 1. Plain carbon steels 2. Low alloy steels 3. Stainless steels 4. Tool steels

Next Lecture Steel and its application in chemical engineering