Iron and Steel Making

Metal Processing Overview

• Metal processing can be classified into:
  • Casting (Sand, Investment, Die)
  • Forming (Rolling, forging, extrusion, drawing)
  • Sheet forming, powder metallurgy and molding
  • Machining
  • Joining
  • Surface finishing

Iron and Steel Making: Raw Materials

• Iron ore is a raw material in iron and steel making
• Limestone (calcium carbonate) removes impurities from molten iron
• Coal (or coke) generates heat for chemical reactions and produces a reducing CO gas
• Coal is heated in a coke oven to form coke.

Iron and Steel Making: Processes

• The ironmaking process involves reactions of the three raw materials in a blast furnace
• The main reaction is the formation of CO (coke + O2), which reduces iron oxide to iron: Fe2O3 + 3CO → 2Fe + 3CO2
• Typical composition of iron includes high contents of C, Si, and Mn.
• The steelmaking process refines the molten metals from the ironmaking process by reducing C, Si, Mn, and other elements.
• Two typical furnaces used in steelmaking include:
  • Basic oxygen furnace (BOF) which uses O2 to refine the molten metal
  • Electric arc furnace (EAF) where the heat source is from the electric arc between the electrodes

Metal Shapes

• Molten metals go through a continuous casting process to produce Blooms, Slabs, and Billets

Blast Furnace

• Raw materials are blasted into the furnace and molten iron is taken out at the bottom
• Slag is a byproduct, usually metal oxides and SiO2

Electronic Arc Furnace (EAF)

• An EAF uses 350-425 kWh for every ton of steel produced
• Scrap (recycled) metal accounts for 80% of all electric arc furnace metal feedstock.

Casting: Sand Casting

• Sand casting is used for large parts, e.g., auto engine blocks
• A riser overflows to help compensate for shrinkage of the part during solidification.

Sand Casting: Advantages

• Good for large parts
• Almost any metal can be cast
• Low tooling cost

Sand Casting: Disadvantages

• Rough and coarse finishing which may require final finishing
• Poor tolerance and precision, cannot make complex parts
• Slow process with low production volume

Casting: Investment Casting

• Investment casting is used for low volume, complex shapes, e.g., jewelry, gas turbine blades

Investment Casting: Advantages

• Complex part shapes
• Excellent surface finish
• Almost any metal can be cast

Investment Casting: Disadvantages

• Slow process with low production volume
• Part size is limited
• Relatively high cost

Casting: Die Casting

• Die casting is used for high production volume, low melting-temp alloys, e.g., Mg, Pb, Sn, Zn etc.
• This process utilizes pressure to push molten metal into the die.

Die Casting: Advantages

• Excellent dimension accuracy
• Excellent surface finish
• High production rate
• Mostly semi- or fully automated process

Die Casting: Disadvantages

• High tool and die costs
• Limited part size
• Generally limits to nonferrous metals (low melting temp), e.g., Mg, Pb, Sn, Zn

Metal Forming: Hot and Cold Working

• Metal working includes hot work and cold work
• It involves permanent deformation of a material part into a desired shape (σy < stress < στ)

Hot Working

• Less force and large deformation
• A poor surface finish, e.g., oxide scales and tolerance
• The part is soft and ductile

Cold Working

• High strength but have some residual stress
• Good surface finish and tolerance
• Cannot deform as much as hot working

Metal Forming Overview

• Rolling: the process of plastically deforming metal by passing it between rolls (I-beams, rails)
• Forging: the working of metal into useful shapes by hammering or pressing, increasing strength via work hardening (wrenches, crankshafts)
• Extrusion: Parts with uniform cross section through length (rods, tubing)
• Drawing: pulling metal through a die by means of a tensile force applied to the exit side of the die; used for rods, wire, tubing, and nails

Hot and Cold Working in Steelmaking

• Hot Rolling reduces large thickness reduction
• Pickling removes scales from the steel surface
• Cold Reduction increases the strength of the steel sheet.

Thermal Processing: Annealing

• Annealing is a heat treatment where a material is exposed to an elevated temperature for an extended time and then slowly cooled
• It is used to relieve stresses, increase softness, ductility, and toughness, and/or produce a specific microstructure
• Annealing processes includes (1) heating to a desired temperature, (2) holding at that temperature, and (3) cooling
• Annealing of ferrous alloys involves Normalizing, Full Anneal, and Spheroidizing

Thermal Processing: Normalizing

• Involves heating at least 55°C above A3 or Acm, holding until alloy transforms to austenite completely, and cooling in air
• Normalizing produces fine pearlite and decreases the average grain size of steels that have been plastically deformed by, for example, rolling
• Utilizes a smaller and more even distribution of grain size is = stronger

Thermal Processing: Full Anneal

• Involves Heating 30°C above A3 or A1, holding until alloy transforms to austenite or austenite + Fe3C, and slow furnace cool
• Full anneal produces coarse pearlite which is relatively soft and ductile

Thermal Processing: Spheroidizing

• Involves heating the alloy at a temperature below A₁ and holding at that temperature for 15 to 25 h
• Spheroidized steels have maximum softness and ductility and are easily machined or deformed.

Thermal Processing: Precipitation Hardening

• Uniform dispersion of nanoscale precipitate is dispersed in the softer more ductile matrix
• Particles impede dislocation motion and increase the yield strength
• An example is solution heat treatment (get a solid solution), Pt B: quench to room temp. (retain a solid solution), Pt C: reheat to nucleate small a particles within phase
• Other alloys that can be precipitation hardened include Ni-based super alloy, Titanium alloy, Be-Cu

Sheet Metal Processing

Cutting

• Blanking involves making the cut piece a workpiece
• Punching creates round or shaped holes and make the cut piece scrap
• Notching removes a small metal piece from the edge of a workpiece

Bending

• Creates a permanent bend of a metal sheet

Drawing

• Forming a sheet into a cup-shaped, box-shaped, curve-shaped part (e.g., soda can)

Metal Forming (Special Case): Powder Metallurgy (P/M)

• This process produces near net shape and small-size parts (e.g., hand-watch gears), with few materials
• It is good for metals with high Tm, e.g., W filament, had to cast metals has high costs (tooling and powders)
• Sintering heat treatment to bond metallic particles

Machining

• A material removal process to shape material parts

Machining: Advantages

• High dimensional accuracy and good surface finish and a variety of part shapes

Machining: Disadvantages

• Wasteful of materials and is time consuming

Principle of Metal Cutting

• Cutting action involves shear deformation of work material to form a chip and as the chip is removed, a new surface is exposed

Machinability of Metals

• Ease with which metals can be machined by considering the following criteria: Surface finish and perfection of the machined metal, Tool life of the cutting tools used to cut the metal
• Cutting force and power required for machining the metal
• Additionally chip removal process should have chips that are easily broken into small pieces for easy handling
• The machinability of steels are improved by alloying with Lead (Pb) and Sulfur (S) → Free machining steel, Chips can be easily broken into small pieces

Metal Cutting Tools and Materials

• They must be harder than the cut workpiece
• They must also have:
  • Toughness – avoid fracture failure
  • Hot hardness – retain hardness at high T
  • Wear resistance – resist abrasive wear
  • Chemical stability
• High-Speed Steel: Solid tool-steel bar
• High-alloyed tool steels with high hardness at high temp, e.g., W or Mo alloyed steels
• Carbide Tip: An insert made of carbides, e.g., WC

Machining Classifications

• Rotational: Cylindrical or disk-like shape
• Nonrotational (also called prismatic): Block-like or plate-like shapes

Machining Operations

• Turning: Single-point cutting edge, Rotating cutpiece. Generates a cylindrical shape
• Milling: Multiple cutting, Rotating cutting tool. Generates a planar surface
• Broaching: Provides Multiple-tooth cutting tool, A good surface finish and close tolerance and variety of worked shape (e.g., internal surface of a hole) possible
• Drilling: Rotating cutting tool which creates a round hole in a workpiece

Welding

• Joining when one large part is impractical
• A filler is the same as the base metal and both melt
• A heat affected zone (HAZ) is the region in which the microstructure has been changed
• Metal pieces must be similar; e.g., cannot weld Cu to steel
• Avoid cracking or fractures in the weld by pre-heating the workpiece before the welding.

Welding Metallurgy

• Heat during welding recrystallizes the elongated grains of cold-worked metal
• Grains away from the weld area → recrystallize into fine grains → strong
• Grains close to the weld metal → grain growth → weak

Welding Classifications

• Liquid state is melting two parts to be joined. In some cases, filler metals with lower Tm than base metals may be used to assist melting and welding process
• Solid state is when heat and/or pressure are used to achieve welding but no melting of base materials occur
• Consumable electrode is consumed during the welding process and the electrode is the source of filler metal
• Nonconsumable electrode is not consumed welding process and filler metals must be added separately and is typically made of tungsten to resist melting

Gas Welding

• Oxygenfuel gas welding (OFW) fuel gas + oxygen to produce flame to melt metals
• The common fuel gas is Acetylene (C2H2)

Brazing

• A joining process where a filler metal is placed along a narrow channel between the two surfaces to be joined and the temperature is raised sufficiently to melt the filler metal (but not the base metal)