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530.352 Materials Selection

530.352 Materials Selection. Lecture #21 Steels - part II Wednesday November 2 nd , 2005. Transformations :. Equilibrium transformations (slow cooling)  ->  “ferrite”  -> Fe 3 C “iron carbide”   ->  + Fe 3 C “pearlite”

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530.352 Materials Selection

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  1. 530.352 Materials Selection Lecture #21 Steels - part IIWednesday November 2nd, 2005

  2. Transformations : • Equilibrium transformations (slow cooling) ->  “ferrite” -> Fe3C “iron carbide” ->  + Fe3C “pearlite” • Nonequlibrium (fast cooling / quenching) -> ’ “martensite”

  3. Pearlite : • Two phases ( + Fe3C)

  4. Normalized (slow cooled) steels;mechanical properties : 2,500 50 Strength (MPa) Ductility (%) f UTS 500 YS 0 0 1% w/o Carbon

  5. Martensite : • Quenched in distortions : + C C Fe

  6. Martensite mechanical prop. : • very hard and • very brittle (too brittle !!!)

  7. Quench and Temper : • Temper : heat treat at intermediate T (300-600 C) • Mechanical properties: • regains toughness (critical) with only a moderate drop in hardness • Microstructurally : • C comes out of lattice a precipitates as Fe3C, and distortion decreases with decrease amounts of dissolved C. • Loss of distortion leads to bcc structure and ductility. • Fe3C precipitates - precipitation strengthen the .

  8. Quenched and tempered : 2,500 50 UTS f YS Strength (MPa) Ductility (%) f UTS • normalized • tempered 500 YS 0 0 1% w/o Carbon

  9. Heat treatments and cooling : • Normalizing (heating to form ) • T ~ 1,000 C • much easier to roll / forge / form at this temperature • Quench - or - Slow cooling • martensite • pearlite • Tempering (heating to “soften” martensite) • carbides form, distortions relax

  10. Quench rates : • To form martensite in pure Fe • Critical Cooling Rate (CCR) ~ 100,000 C/sec • To form martensite in mild steel : Fe - 0.8% C • CCR ~ 200 C/sec • To form martensite in alloy steels:Fe - 0.2-.6%C + 2-7% (Mo, Mn, Cr, Ni) • CCR < 1 C/sec

  11. Alloying elements are added to : • improvehardenabilityof the steel • aides nucleation of martensite • solution strengthen and precipitation hardening • MxCy carbides form • give corrosion resistance • especially Cr which forms Cr203 • stabilize FCC austenite at RT • especially Ni • tougher, more ductile and easier to form • non-magnetic and creep resistant (diffusion is slower in FCC)

  12. Alloying of steels : Type of steel:Fe + ...Typical uses: Low-alloy .2%C + pressure vessels, aircraft .8Mn,1Cr,2Ni parts, high  applications. High-alloy .1% C High T and anti-corrosion, Stainless-steels .5Mn,18Cr,8Ni silverware, medical, etc.

  13. Steel terminology (SAE-AISI) : • Plain carbon • 10xx • Manganese steels • 13xx • Nickel steels • 23xx • Ni-Cr-Mo steels • 43xx • HSLA • 9xx Note: xx indicates carbon content in hundredths of a percent

  14. Other steel standards : • SAE-AISI • Society of Automotive Engineers ; American Iron and Steel Institute • ASTM • American Standards for Testing and Materials • AMS • Aerospace Materials Specifications • DIN • Deutsches Institut fur Normung • JIS • Japanese Industrial Standards Committee • UNS • Unified Numbering System

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