Power Engineering - 2A2 (Chapter 5) Metalurgy

What is metallurgy? - ANS The study of metals What are the three groups of metallurgy? - ANS Extractive Mechanical Physical What is extractive metallurgy? - ANS The study of the extraction and purification of metals from their ores. What are the 3 types of extractive metallurgy? - ANS Pyrometallurgy - Melting the ore in a furnace to release the metal. Hydrometallurgy - Dissolving the metal from the ore and recovering it as a powder. Electrometallurgy - Dissolving the metal from the ore with the use of electricity, plate the metal out of solution. What is mechanical metallurgy? - ANS The study of the techniques and mechanical forces that shape and make the finished forms of metal. What is physical metallurgy? - ANS The study of the structure of metals. the transition when atomic structure is heated to the point it becomes liquid - ANS Latent heat of Fusion What are the three basic structures of unit cells formed when metals solidify at their freezing point? - ANS Face-Centered Cubic (FCC) Structure Body-Centered Cubic (BCC) Structure Close Packed Hexagonal (CPH) Structure What are the characteristics of FCC structures? What are examples? - ANS There are 14 atoms High ductility Low shear Low tensile strength Good thermal and electrical conductivity Gold, aluminum, silver, lead, nickel, and gamma iron (910 to 1390) What are the characteristics of BCC structures? What are examples? - ANS There are 9 atoms High strength Low ductility Very resistant to shear deformities Chrome, tungsten, moly, vana, alpha iron (below 910) & Delta iron (above 1390) What are the characteristics of CPH structures? What are examples? - ANS There are 17 atoms Intermediate strength and ductility Zinc, magnesium, cadmium, titanium What can the structure of metals tell us? - ANS They are related to the properties of the metals How can the structure of metals be changed? - ANS By modifying the chemical composition, alloys and heat treatments. Metals with more desirable and useful properties can be created. How can metals be defined? - ANS They are good conductors of heat and electricity. They are generally malleable and ductile. What states can metals exist in? - ANS Solid, liquid, and gas. Metals are generally only worked with as solids and liquids What is it called when energy is added to transition a solid to a liquid? There is no increase in temperature during this phase. - ANS The latent heat of fusion What happens to the structure of solid metals at their freezing point? - ANS The atoms form themselves into fixed configurations known as unit cells. When molten metal cools and unit cells become crystals, what do they occupy? - ANS A space lattice What are the crystals that form in cooling metals called? - ANS Dendrites What do dendrites look like? - ANS Branches of an evergreen tree and extend through the metal matrix What happens to impurities in molten metal as it cools and solidifies? - ANS Impurities are pushed ahead of the growing crystal dendrites and get trapped in the grain boundaries between the limbs of the dendrites. What are the properties of smaller grain sizes in metals? - ANS Increase in tensile strength & ductility What are the properties of larger grain sizes in metals? - ANS Resistant to creep and deformation but prone to cracking What determines a metals shear strength? - ANS The type of unit cell structure What is the ability of a metal to change to a different unit cell structure depending on temperature? - ANS Polymorphism The change in unit cell structure dependent on temperature? - ANS Allotropy of Iron How much carbon is in carbon steel? - ANS 2% or less How much carbon is in cast iron? - ANS 2% to 6% What are the three grades of carbon steel and what is their carbon content? - ANS Low (max carbon content 0.30%) Medium (max carbon content 0.30 to 0.60%) High (max carbon content 0.60 to 1.00%) What are the three things that effect the physical properties of carbon steel? - ANS The carbon concentration The carbon's final resting place (inside the unit cell or on the exterior) The form that carbon assumes in the iron matrix (single atoms, graphite flakes, spheres, molecular combinations) What happens to steel as the carbon content increases? - ANS increases hardness and brittleness What happens to steel as the carbon content decreases? - ANS Increase in softness, ductility, and weldability What is the iron-carbon equilibrium diagram - ANS shows the phase changes that as the carbon content and temperature vary in steel -Solidus line, below are solid -Liquidus line above is molten What is austenite? - ANS - Iron in a unit cell of FCC form, AKA Gamma iron -Contains up to 2% iron -Phase where all quenching heat treatments start What is cementite? - ANS Iron-carbon in the form of molecular iron carbide Fe3C - C atoms fit interstitial between FE atoms What is the Eutectic Reaction Point? - ANS -When the liquid alloy changes directly into solid austenite and cementite, no pasty phase state - Only at 4% carbon & 1130 C What is ferrite? - ANS -Iron in the BCC cell structure -Dissolves Carb at 0.025% & 723 C What is the lower critical change line? - ANS -Temperature (723° C) where the iron alloy of carbon returns to a BCC unit cell What is pearlite? - ANS -layered ferrite and cementite -Forms below lower critical line (723 C) What is a peritectic reaction? - ANS -Liquid Delta iron in BCC changes directly into austenite with no pasty state change -occurs at (1942 C) What is the general purpose of heat treating a metal? - ANS -Force a physical and/or chemical transformation & cooled at a rate to retain desired properties For power engineers and inspectors, what is the purpose of heat treatment? - ANS Stress relieving What do Annealing heat treatments produce - ANS Ferrite, Pearline and cementite What is the purpose of annealing? (4 things) - ANS -Homogenize the microstructure -Increase ductility & machinability -Remove stress Describe the "Full Annealing" process - ANS - Heat steel just above the transition temp to produce austenite - Cooled very slowly at 38 C/h per hour to room temp in sand