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Understanding Steel Transformations: Equilibrium and Nonequilibrium Phases

This lecture focuses on the transformations in steels, primarily examining equilibrium transformations through slow cooling, which leads to phases like ferrite and pearlite. It also discusses nonequilibrium transformations related to fast cooling or quenching, resulting in phases such as bainite and martensite. Key concepts include the hardness and brittleness of martensite and the importance of heat treatments, including normalizing and tempering, in enhancing mechanical properties. The role of alloying elements in improving hardenability and corrosion resistance of steels is also explored.

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Understanding Steel Transformations: Equilibrium and Nonequilibrium Phases

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  1. 530.352 Materials Selection Lecture #22 Steels - part IITuesday October 30th, 2006

  2. Transformations : • Equilibrium transformations (slow cooling) ->  “ferrite” -> Fe3C “iron carbide” ->  + Fe3C “pearlite” • Nonequlibrium (fast cooling / quenching) ->  plates + other phases “bainite” -> ’ “martensite” http://www.azom.com/details.asp?ArticleID=399

  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. TTT - curves Time-Temperature Transformation http://www.key-to-steel.com/Articles/Art17.htm

  8. TTT - curves Time-Temperature Transformation

  9. 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 .

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

  11. 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

  12. 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

  13. 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)

  14. 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.

  15. 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

  16. 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 http://www.key-to-steel.com/

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