MATSE 259 EXAM 3 REVIEW SESSION 1. Exam Structure: Multiple choice (same as previous exams), hours, 25 questions 2. Please write your student I.D. on the answer sheet (NOT SSN) 3. Please mark your answers clearly 4. Do not leave a question unanswered – there is no negative marking 5. Hand in both question and answer sheets at the end of the exam – the questions will be posted on the website after the last conflict exam

TOPICS  CASE HARDENING  JOMINY TEST (HARDENABILITY)  CAST IRONS  CORROSION  POLYMERS

CASE HARDENING  Steel alloy is exposed to a carbonaceous (usually CO) or nitrogenous (usually NH 3 ) atmosphere at elevated temperature so that the “case” becomes carbon or nitrogen rich. - In nitriding, Fe 2 N forms which is very hard. In carburizing, the alloy is then quenched and tempered to get a hard casing.  In induction hardening, a high frequency AC is used to heat the alloy surface past its austenitizing temperature and the alloy is then quenched and tempered.

JOMINY TEST  hardenability: ability to form martensite! (not hardness)  specimen is fully austenitized  max. cooling at the quench end, decreases with distance from quenched end.  hardness is measured as a function of distance from quenched end.

JOMINY TEST Cooling rate, “severity of quench”, is dependent on:  quenching medium: » Water --- most severe » Oil » Air --- least severe  specimen size and geometry: » as surface area / mass increases, more rapid cooling rates are achieved (rounded shapes)

JOMINY TEST Hardness at the center of a 2” diameter bar quenched in water with a little agitation for 1040, 3140 and 4340 steels?

CAST IRONS  Ferrous alloys with 2.5 < % wt C < 5.5 and other alloying elements.  For slow cooling rates,  Fe-C (as opposed to Fe-Fe 3 C) phase diagram:  + GRAPHITE  + GRAPHITE

CAST IRONS  Types: » Gray cas t iron  graphite flakes in an  or P matrix, depending on the cooling rate and % Si - Weak and brittle in tension, stronger and more ductile under compression - Effective in damping vibrations - High fluidity at casting temperature, low casting shrinkage

CAST IRONS » Nodular (ductile) cast iron - flakes become spheres, with addition of small amounts of Mg and/or Ce before casting - Several times stronger than gray cast irons - High ductility: % EL up to 40 » White cast iron - almost all C is as CM in P matrix - Extremely hard: wear resistant - Very brittle: unmachinable » Heating white cast irons to ~850  C for ~30 hrs gives Malleable Cast Irons - Graphite clusters in an  or P matrix - Mechanical properties: Relatively high strength and malleability

CAST IRONS Gray cast iron Nodular (ductile) cast iron White cast iron Malleable cast iron

CORROSION  Requirements: » Anode » Cathode » Electrolyte » Electrical connection Anode: oxidation  CORROSION Cathode: reduction

CORROSION  Classes of electro-chemical corrosion: - Uniform attack - Galvanic corrosion - Crevice corrosion - Pitting - Intergranular corrosion - Selective leaching - Erosion-corrosion - Stress corrosion

CORROSION  MACRO ~ Dissimilar metals - (Galvanic series); A galvanic series determines which metal will act as the anode – the lower the metal in the series, the more anodic it is. ~ Same metal in different electrolytes - Crevice / pitting corrosion ~ Deformation - Non-homogeneous residual stress ~ non-passivated vs. passivated metal couple

CORROSION  MICRO (single material) ~ grain boundary – anodic wrt bulk (high energy regions) ~ orientation difference in grains ~ presence of different phases ~ segregation

CORROSION  WELDING

CORROSION  Remedies: ~ Environmental alteration ~ Protective coating - anodizing (Al coating) - galvanizing (Zn coating) ~ Material selection (i.e. passivated material usage) ~ Design (i.e. keep the ratio [anode area / cathode area] large ~ Cathodic protection: -Electrically connect the metal to be protected to another metal (sacrificial anode) that is more reactive in the particular environment. Ex: Zn, Mg. - Connect the negative terminal of an external DC source to the metal to be protected and the positive terminal to an inert anode like graphite.

POLYMERS  Polymers: long chains of covalently bonded small, successively repeating units called monomers.  Classification of polymers: - Based on monomer/s: homopolymer, copolymer, blend. O O || || -NH-(CH 2 ) 6 -NH-C-(CH 2 ) 4 -C- PolyethyleneNylon (6,6) Copolymers: random, alternating, block, graft. – A – A – B – A – B – B – B – A – – A – A – A – A – B – B – B – B – B – B – B – B – A – A – A – A – A – – A – B – A – B – A – B – A – B – – A – A – A – A – A – A – A – A – A – A – A – A – A – A – A – A – A – A | | B B | | B B | | B B

POLYMERS - Based on molecular structure: linear polymer, branched polymer, crosslinked polymer, network polymer. CH 3 CH 3 CH 3 | | | – C – CH 2 – CH – CH 2 – CH – CH 2 – | CH 3 – CH | CH 2 | CH 3 CH 3 CH 3 | | | – C – CH 2 – CH – CH 2 – CH – CH 2 – Polyethylene | – C – CH 2 – CH – CH 2 – CH – CH 2 – | | | CH 3 CH 3 CH 3

POLYMERS - Based on tacticity: isotactic, syndiotactic, atactic. Polypropylene - Based on monomer geometry: cis-polymer, trans-polymer. H CH3 H | | | – CH 2 – C = C – CH 2 –– CH 2 – C = C – CH 2 – | CH 3 Polyisoprene - Based on properties: thermoplasts, thermosets.

POLYMERS  The molecular weight of polymers is determined in two ways: - Number-average molecular weight - Weight-average molecular weight  Degree of polymerization: