Metallurgy Fundamentals: Steels, Phase Diagrams, and Mechanical Properties
Steels & Cast Iron
Cast Iron: >2.1% low melting temperature, good for casting, engine blocks, gears, machines, pans. Steel: <1.4% add=”” elements=”” to=”” change=”” properties,=”” good=”” general=”” properties,=”” bad=”” corrosion=”” res.=”” low=”” carbon=”” steel:=””>1.4%><.3% good=”” formability,=”” weldability,=”” strength,=”” and=”” fracture=”” res. =””>%><.15% used=”” for=”” deep=”” drawing=”” app,=””>.15% used for structures (^strength less form)
Medium Carbon .3-.6, forms martensite when heat treated, machined components that are heat treated (gears, shafts) low hardenability, can distort when quenched a lot, Cr Ni Mo to add hardenability, strong but less tough, less ductile than low C steel. Eg. Railway, gear, crankshaft. High Carbon Steels .6-1.4, heat treatable. High strength high wear res. Hard strong poor form and weld. Cr V W Mo add for wear res. Cutting edges, dies, punches. Ni Cr ad strength, harden. Impact str, fatigue res. Mo increase all properties minimize temper embrittlement. Tool Steel used to form, machine, cut or mold other metals, polys, cerams. Designed for high hardness, wear shock and impact res. Usually a high C steel with alot of alloys. Easily shaped then treated for hardness and wear res.^str low alloy – 1.5 Mn <.1 nb=”” ti=”” or=”” v=”” better=”” mechanical=”” properties=”” than=”” plain=”” carbon=”” steel.=”” nb=”” ti=”” and=”” v=”” minimize=”” grain=”” growth=”” during=”” hot=”” rolling.=”” bridges=”” transmission=”” towers,=”” chassis.=”” stainless=”” steel=”” -=”” alloy=”” of=”” iron=”” and=”” chromium=”” +=”” other=”” alloys=”” ^=”” corrosion=”” +=”” oxidation=”” res.=”” cr=”” reduce=”” size=”” of=”” austenitic=”” region,=”” c=”” increases=”” carbide=”” formation=”” ni=”” increases=”” size=”” of=”” austentitic=”” region.=”” ferritic=”” ss=”” .12-.3%=”” cr=””><.12 c=”” less=”” expensive,=”” used=”” for=”” general=”” construction,=”” bad=”” ductility=”” &=”” weldability,=”” exhaust=”” pipe=”” dishwasher=”” decor.=”” martensitic=”” ss=”” .1-.18=”” cr=””><1.2 c=”” ^=”” c=”” allows=”” for=”” ^=”” austenite=”^” martensite=”” good=”” wear=”” &=”” corrosion=”” res=”” use=”” for=”” cutting=”” edge.=”” austentic=”” ss=”” .16=”” -.25=”” cr=”” .07-.2=”” ni=”” low=”” c=”” to=”” prevent=”” sensitization.=”” ni=”” stabilize=”” austenite.=”” not=”” heat=”” treatable,=”” cold=”” worked=”” ^=”” corrosion=”” res=”” and=”” form.=”” sinks,=”” appliances.=”” precipitation=”” hardened=”” ss=”” -=”” martensite=”” or=”” austenite=”” matrix.=”” small=”” ti=”” al=”” nb=”” v=”” to=”” form=”” precipitates=”” after=”” age=”” harden=”” ^=”” str=”” corrosion/oxidation=”” res.=”” duplex=”” ss=”” -=”” combo=”” of=”” austenitic=”” and=”” ferritic=”” ss=”” with=”” properties=”” between=”” each.=”” cast=”” iron=”” -=”” ternary=”” fe-c-s=”” alloy=”” low=”” c=”” good=”” casting,=”” ^=”” wear=”” and=”” corrosion=”” res.=”” less=”” dense=”” than=”” steel.=”” ^=”” graphitization.=”” low=”” cost,=”” easily,=”” melted.=”” white=”” ci=”” -=”” produced=”” with=”” fast=”” solidification,=”” cementite=”” forms=”” rather=”” than=”” graphite.=”” low=”” si,=”” hard=”” brittle,=”” rolling=”” mills=”” grinding=”” plates,=”” cement=”” mixers.=”” gray=”” ci=”” -=”” common,=”” low=”” solidification=”” rate=”” -=”” large=”” graphite=”” flakes=”” vice=”” versa.=”” ^=”” machinability=”” and=”” wear=”” res.=”” ductile=”” ci,=”” like=”” gray=”” but=”” stronger=”” &=”” tougher.=”” malleable=”” ci,=”” annealed=”” white=”” iron.=”” compact=”” graphite=”” ci,=”” worm=”” like=”” graphite=”” between=”” gray=”” and=”” ductile=”” properties.=”” good=”” for=”” ^=”” temp.=””>1.2>%>
Atomic & Ionic Arrangements
No Order – no orderly arrangement, atoms are not connected occupy random space, eg. Monatomic gas, atoms, ions. Short Range Order – arrange of atoms extends only to nearest neighbours, molecules occupy space randomly, eg steam, glass, some polymers, directional covalent bonds. Long Range Order – Crystalline structures with long atomic arrangements forming repetitive 3d patterns, atoms occupy well defined positions eg solids. Solids may be made from a singe crystal or many crystals (polycrystalline) a single crystal (eg garnet) is an extended arrangement of atoms LRO, properties depend on the crystal direction. Polycrystalline are made of multiple crystals, since each crystal is randomly oriented, poly crystalline materials have more uniform properties. Small crystals are called grains and the borders between the grains are called grain boundaries. Some solids are not crystalline (only SRO) and are called amorphous. Form when kinetics do not allow for formation of crystals. Eg glass and thermoplastic polymers. When a material exhibits LRO it is possible to represent its crystal structure with a unit cell. Unit cell is a single repeat unit that reproduces the entire crystal structure. Lattice parameters & interaxial angles are used to define the size and shape of the unit cell. Lattice parameters are axial lengths (a,b,c) interaxial angles are the angles between the lattice parameters (axial lengths) (alpha beta gamma) the dimensions are in (nm). The number of atoms in the unit cell are required to fully describe the unit cell. Simple Cubic (SC) rare due to low packing density, close packed directions are cube edges. Coordination # (# nearest neighbours) = 6, 1 atom/unit cell, 8 corners (1/8). BCC atoms touch each other along cube diagonals 2 atoms/unit cell 1 center + 8 corners, 8 neighbours. FCC atoms touch each other along face diagonals, 4 atoms/unit cell 6 face(1/2) + 8 corners (1/8), 12 neighbours. The stacking of atoms within the crystal determines the size of the UC, the dimensions is related to the number of atomic radii along the direction the atoms are touching. The directions where atoms are touch are called close-packed directions. (important in deformation of metals). Hexagonal close packed, 2 atoms/unit cell, 12 neighbours. Material’s may have more than one crystal structure – Allotropic (for pure elements) or Polymorphic (for compounds). Transformations typically occur with changes in temp or pressure. Changes material properties. Gives steel the ability to be heat treated for ^ str. May result in large volume changes that cause fracture. Some properties of a crystal are dependant on directions and planes of atoms. Miller indices can define directions and planes. Coordinates of atoms are given as the number of lattice parameters moved in the xyz directions. For a unit cell coordinates range from 000 to 111. A Crystallographic direction is a line between two points within a crystal (eg. A vector), a crystallographic plane is a planar area within a crystal. Close packed directions are directions in which atoms are in contact, close packed planes are planes of atoms in contact. HCP has 2 parallel close packed planes. FCC has 4 non-parallel close packed planes BCC only has closely packed planes.
Solid Solutions and Phase Equilibrium Dispersion strengthening
Phase is a structurally homogenous (thoroughly mixed) portion of a system. Phases – have the same structure or atomic arrangement throughout, roughly the same composition and properties throughout, definite interface between the phase and any surrounding adjoining phases. Solubility is the limit of how much material can be mixed into another material without producing and additional phase. Unlimited solubility – only 1 phase is produced when materials are mixed in any ration. Limited solubility – small amount of material may be added, if solubility limit is exceeded more phases exits. Immiscible – no solubility. Solid solution strengthening – only one phase, dissolved atoms introduce point defects and distort the lattice, greater atom size differences increase strength but reduce ductility. If solubility limit is exceeded dispersion strengthening happens – Matric: continuous solid phase in a complex microstructure, dispersed phase: solid phase that forms from the original phase after s.Limit is exceeded. Requirement for dispersion strengthening: 1. Matrix is soft and ductile, 2. Dispersed state is small, numerous, discontinuous, round, hard, 3. Large amounts of precipitates. Phase diagrams show the phases and their composition at any combinations of temperature and alloy composition. Liquidus – temperature above which the material is 100% liquid, gives composition of liquid in 2phase region. Solidus – temperature below which a material is 100% solid, gives composition of solid in 2phase region. Solvus – line representing the solid solubility limit of a phase. Solidification requires nucleation – formation of small solid particles, and growth – once nucleated solid particles can grow. Under slow cooling, solidification begins when the liquid temperature reaches the liquidus line. First solid to form will have a composition given by the solidus line. Cooling must continue to completely solidify, complete when cooled below the solidus. FOr solid to grow during solidification latent heat of fusion must be removed and diffusion must occur for compositions to follow the liquidus and solidus lines. High melting point material will concentrate to form first solid. As solidifcation progresses during cooling, Ni atoms in the liquid will diffuse to the solid Cu concentrations in the liquid. Ni atoms in the first solid diffuse to new solid. Last liquid to freeze contains the least Ni. At solidus, solid has uniform concentration of Ni, same as the bulk concentration of the alloy. Non-equilibrium – diffusion in solids at low temperatures is slow, fast cooling limits diffusion results in non-equilibrium structures, diffusion in liquids is fast resulting in composition that follows the liquidus line, diffusion in the solid is slow results in composition that doesn’t follow the solidus line. Noe equilibrium solidification results in micro segregations or coring. Microsegration is a non-uniform composition produced from non-equilibrium solidification, aka interdendritic segregation and coring. Dendrites will have a higher concentration of the high melting point material since they are the first solid to form, last solid to form will be rich in low melting temperature material as it is the last liquid to solidify. Compositional variation results in variation of mechanical properties. During non-equilibrium cooling the composition of the solid will vary over short distances, this non-uniform compsotion is called coring. Middle is richer in hihger melting point compenent and outside is richer in lower melting point stuff (because it solidifes last) Homogenization can reduce microsegregation, heat llows for diffusion and reduction of composition variations in solid.
Macrosegregation is variation in composition over larger distance. Cannot be removed by homogenization (diffusion distance too great) hot working can break up solidifcation and can reduce macrosegregation. Euctectic : liquid -> 2solid, Peritectic: liquid +solid->solid, Montectic: liquid->liquid+solid, Eutectoid: solid->2solid, Peritectoid: 2solid-> solid. 123 solidification (liquid transformations) 45 solid state (solid transformations). Complext phase diagrams can occur, intermediate solid solutions amy exist. Intermetallic compounds (compounds with distince chemical formulae) can form. Pb – Sn Eutetic reaction, liquid transforms into two phases directly, forms characteristic lamellar structure, alternating layers of alpha and beta, 100% eutectic microconstituent. Eutectic alloys contain the euctectic composition of a particular system. Hypoeutectic contains less but exceed maximum soolubility( primary proeuctectic (alpha) forms upon intial solidifcation, liquid transforms into alpha and beta due to eutectic transformation just below 183 final microstructure consists of primary alpha and eutectic microstituent(liquid only phsae that transforms in eutectic reaction)), hypereutectic contain more but exceeds the maximum solubility(behave similar manner to hypoeutectic but with pramary beta). Strengthening can occur by controlling the eutectic structure through: 1 colony size – refining colony size improves str, enhancing nucleation of colonies by inoculation reduces colony size. 2 Interlamellar spacing – smaller spacing improves strength, reduce solidification time reduces interlamellar spacing. 3 Amount of eutectic – ^ eutectic increase strength. 4 Eutectic microstructure – shape depends on cooling rate, control cooling rate and alloys to improve str. Non-Equilibrium Freezing – rapid cooling can cause NE Cooling producing NE solidus, last liquid to solidify can have eutectic composition and transform into eutectic microconstituent, reheating above eutectic temperature can cause melting of eutectic. Eutectics have low MP easy to solder and cast.
Age Hardening
Produces a uniform dispersion of a fine, hard coherent preciptate in a ductile matrix, coherent precipates: planes of preciptate atoms are connected with matrix atom planes. Incoherent structures – poor alignment at matrix, very little matrix lattice strain, moderate strengthening effect. Coherent structures- matrix and ppt strutucres alighn (GP zone) generates greater lattice strain, increased strengthening effect. Age Hardening steps: 1. Solution treatement – heat above solvus temperature but below eutectic solution to avoid hot shortness (melting of lower melting point NE phase) 2. Quench – rapdily cool to room temp, forms super saturated solid solution, no time for precipatates of 2nd phase to form. 3. Age – rehate to below solvus temperature to form NE precipitates. Natural aging = room temp, artificial aging – at elevated temp. SSSS has hihg energy – intermediate precipitates form to reduce energy. Precipitates are choerent. Lower aging temperatures require longer times to reach max str, aritifical aging can lead to over aging. Requirements of AH – phase diagram, matrix is soft & ductile, precipates hard and brittle, alloy must be quenchable, coherent precipitates must form.
Impact Testing
The loads in a standard tensilte test are applied slowly, under impact conditions the loads are applied rapidly, less time for disloactions thus material may act in brittle manner. Impact energy is affected by: yield str and duct, notches, temp, strain rate, fracture mechanism. Charpy test mearsures the work done to fracture the specimen. The amount of energy absorbed by the sample related to the height difference between the pendulum before and after impact. Most of the work done is plastic deformation. Therefore ^ ductile = ^ impact energy. Ductile to brittle transition temperature – many materials undergo a transition between ductile and brittle behaviour. 3 methods to determing DBTT, average of upper and lower shelf energies, fracture surface (temperature where surface is half brittle and half ductile), fixed energy. FCC tend to remain ductile at low temperatures cause they have alot of slip systems and a low critical resolve shear stress, therefore easier for dislocations to move, more likely for the slip systems to be properly aligned and the close-packed planes remain active at low temps. BCC have higher critical resolve shear stress and slip systems that are inactive at low temps because they only have closely packed planes. Thus difficult for slips to occur, behave in brittle manner at low temp. At high temp more slip systems active and behave in ductile manner. Notches cause stress concentrations in a part, where stress experienced locally is much greater.
Basic temper designations