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Phase Transformations / Hardenability (Jominy End-Quench)

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Phase Transformations / Hardenability (Jominy End-Quench) Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Experiment #7 Phase Transformations & Hardenability of Steels (Jominy End-Quench Test) Jominy End Quench Test ASTM Standard A255 Concept Non-equilibrium phase transformations Continuous cooling transformation diagram & Critical cooling rates Concept of Hardenability Effect of %C & alloying on Hardenability Objective Compare hardenability of 1045 & 4340 steels (very similar wt% C) 2 Review: Eutectoid Reaction in Steels γ (Austenite , FCC) → α (Ferrite , BCC) + Fe 3C (Cementite , FC Orthorhombic) No time element; temperature change assumed slow enough for quasi-equilibrium at all points γ (Austenite) → α (Ferrite) + Fe 3C (Cementite) 0.76 %C → 0.022 %C + 6.70 %C Schematic representations of the microstructures for an iron-carbon alloy of eutectoid composition (0.76 wt% C) above and below the eutectoid temperature. 3 Fig. 9.26 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 MECE-306 Materials Science Apps Lab 1 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Isothermal Transformations of Eutectoid Steel – Pearlite Isothermal transformation diagram for a eutctoid Austenite (unstable) iron-carbon alloy, with superimposed isothermal heat treatment curve (ABCD ). Microstructures before, during, and after the austenite-to-pearlite transformation are shown. Notice fast transformation creates “fine” pearlite and slow transform. This figure is nicknamed the “T-T-T” plot (for creates time-temperature-transformation) “coarse” pearlite! 4 Fig. 10.14 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 Isothermal Transformations of Hyper-Eutectoid Carbon Steel Isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy: A, austenite; C, proeutectoid cementite; P, pearlite. • Note differences from eutectoid TTT diagram • Primary (or pro-eutectoid) cementite transformation start line (A+C) above & left of pearlite transformation start line (A+P) • (A+C) start line extends above eutectoid temperature! • Pearlite start line (A+P) shifted to the left 5 Fig. 10.16 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 MECE-306 Materials Science Apps Lab 2 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Isothermal Transformations of Eutectoid Steels – Bainite Isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, including austenite-to-pearlite (A–P) and austenite-to-bainite (A–B) transformations. “Nose” of T-T-T curve between pearlite and bainite transformations means bainite cannot be formed in slow-cooled steels (temperature must drop at least 200 °C in less than 1 s). 6 Fig. 10.18 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 Isothermal Transformations of Eutectoid Steels – Martensite The complete isothermal Martensite transformation diagram for can only be an iron-carbon alloy of formed with eutectoid composition: very rapid A, austenite; cooling rates B, bainite; M, martensite; P, pearlite. 7 Fig. 10.22 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 MECE-306 Materials Science Apps Lab 3 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Microstructures of Carbon Steels 20 µ m Coarse Fine Bainite grain (bottom left Partially transformed Pearilte grains to top right) in Martensite Martensite grains (3000 ×) (~19,000 ×) in Austenite* (~ 1200 ×) Pearlite Bainite Martensite Increasing hardness * Martensite transformation “frozen” partially completed (Dark martensite “needles” in light austenite matrix) 8 Crystal Structures Austenite, Ferrite & Martensite Slow – carbon diffuses out of BCC Crystal smaller a (α – Ferrite , FCC Crystal a a≈2.9nm) (γ – Austenite , interstitial a≈3.6nm) spaces a a a a Fast – crystal structure “warps”; carbon trapped by low diffusion rate (only “metastable”) BCT Crystal c (Martensite , a≈2.8nm, c>2.8nm) a a 9 MECE-306 Materials Science Apps Lab 4 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Non-Isothermal Transformations of Eutectoid Steels (Continuous Cooling) Superimposition of Transformations isothermal and continuous start later and/or at cooling transformation lower temperature diagrams for a eutectoid during continuous iron-carbon alloy. cooling Pearlite / bainite “nose” moves down & is not horizontal 10 Fig. 10.25 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 Critical Cooling Rates for Eutectoid Steel Cooling rates Continuous cooling transformation diagram for a faster than the eutectoid iron-carbon alloy critical cooling and superimposed cooling rate form only curves… martensite (no time for Cooling rates slower diffusion-based than this cooling rate transformation form only pearlite (the to occur) diffusion-based transformation has Intermediate time to complete cooling rates before the martensite form some start temperature is pearlite, but the reached) reaction stops before finishing 11 Fig. 10.27 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 MECE-306 Materials Science Apps Lab 5 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Microstructures & Hardness Cooling Relative Microstructure Rate Hardness Fast Martensite Very Hard Martensite + Medium- Medium Pearlite Hard Slow Pearlite Soft 12 Hardenability of Steels Hardenability – ability of steel to be hardened by formation of martensite Low Hardenability – on quenching austenite, martensite forms to a shallow depth only “Shallow Hardening” Steel High critical cooling rate only allows martensite formation near surface of part High Hardenability – on quenching austenite, martensite forms at surface and deep in interior “Through Hardening” Steel (or also “Deep Hardening”) Requires lower critical cooling rate to allow martensite formation deeper in interior of part 13 MECE-306 Materials Science Apps Lab 6 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Carbon Content & Hardenability Among plain carbon steels, eutectoid steel (0.76 %C) has highest hardenability If |0.76 - %C| increases, the nose of pearlite transformation shifts to the left If |0.76 - %C| increases, hardenability of steel decreases 14 Alloying Elements & Hardenability Alloying elements such as Cr, Ni, Mn, Mo, V cause significant change in positions and shapes of transformation curves, such as: Shift the nose of pearlite transformation curve to the right Cause formation of a separate bainite Isothermal transformation diagram for an alloy steel (type 4340)… nose 15 Fig. 10.23 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 MECE-306 Materials Science Apps Lab 7 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) 4340 Alloy Steel – Continuous Cooling Transformation Diagram Cooling rate in this Cooling rate range allows time above critical for primary phase cooling rate (α) to form but not forms only enough for Pearlite Martensite. transform. to get started. Cooling rate in this range begins forming Bainite Cooling rate in this but transform. range allows Pearlite doesn’t finish transform. to get before Martensite started but not to transform. begins. finish. Then Bainite transform. starts but Continuous cooling transformation diagram for 4340 doesn’t finish either. alloy steel and several Some Martensite still superimposed cooling curves forms. 16 Fig. 10.28 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 Hardenability Test – Jominy End Quench Water jet cools quenched end quickly; opposite end air-cools more slowly Schematic diagram of Jominy end-quench specimen ( a) mounted during quenching and ( b) after hardness testing from the quenched end along a ground flat. 17 Fig. 11.11 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 MECE-306 Materials Science Apps Lab 8 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Jominy Test on Eutectoid Steel Cooling rate decreases with distance from quenched end Microstructure changes with cooling rate Hardness depends on microstructure Variation of hardness with distance from quenched end reveals info. about continuous cooling transformation diagram Correlation of hardenability and continuous cooling information for an iron-carbon alloy of eutectoid composition. 18 Fig. 11.13 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010 Microstructure vs. Hardenability Curves 100% martensite 100% pearlite (coarse) Mix of martensite 100% pearlite and pearlite (fine) 19 MECE-306 Materials Science Apps Lab 9 Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151) Hardenability Curves by Alloy 4340 alloy steel: high hardenability (small change in hardness w/ distance) 1040 plain-carbon steel: low hardenability (large change in hardness w/ distance) Hardenability curves for five different steel alloys, each containing 0.4 wt% C… 20 Fig.
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