Materials Science Exam 3 Review Session Summary

1. MATSE 259 EXAM 3 REVIEW SESSION • Exam Structure: Multiple choice (same as previous exams) , hours , 25 questions • Please write your student I.D. on the answer sheet (NOT SSN) • Please mark your answers clearly • Do not leave a question unanswered – there is no negative marking • 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

2. TOPICS • CASE HARDENING • JOMINY TEST (HARDENABILITY) • CAST IRONS • CORROSION • POLYMERS

3. CASE HARDENING • Steel alloy is exposed to a carbonaceous (usually CO) or nitrogenous (usually NH3) atmosphere at elevated temperature so that the “case” becomes carbon or nitrogen rich. - In nitriding, Fe2N 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.

4. 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.

5. 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)

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

7. 1153 g +GRAPHITE 740 a + GRAPHITE 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-Fe3C) phase diagram:

8. CAST IRONS • Types: • Gray cast iron  graphite flakes in an a 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

9. 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 a or P matrix - Mechanical properties: Relatively high strength and malleability

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

11. CORROSION • Requirements: • Anode • Cathode • Electrolyte • Electrical connection • Anode: oxidation  CORROSION • Cathode: reduction

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

13. 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

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

15. CORROSION • WELDING

16. 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.

17. 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-(CH2)6-NH-C-(CH2)4-C- Polyethylene Nylon (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

18. POLYMERS - Based on molecular structure: linear polymer, branched polymer, crosslinked polymer, network polymer. CH3 CH3 CH3 | | | – C – CH2 – CH – CH2 – CH – CH2 – | CH3 – CH | CH2 | CH3 CH3 CH3 | | | – C – CH2 – CH – CH2 – CH – CH2 – Polyethylene | – C – CH2 – CH – CH2 – CH – CH2 – | | | CH3 CH3 CH3

19. POLYMERS - Based on tacticity: isotactic, syndiotactic, atactic. Polypropylene - Based on monomer geometry: cis-polymer, trans-polymer. H CH3 H | | | – CH2 – C = C – CH2 – – CH2 – C = C – CH2 – | CH3 Polyisoprene - Based on properties: thermoplasts, thermosets.

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