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
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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
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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
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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)
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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. 11.14 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010
Hardenability Bands
Hardness vs. distance from quenched end along a Jominy specimen can vary due to slight fluctuations in composition and cooling rate, giving a range of possible values
The hardenability band for an 8640 steel indicating maximum and minimum limits.
21 Fig. 11.16 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8 th ed., J. Wiley & Sons, 2010
MECE 306 Materials Science Apps Lab 10 Phase Transformations / Hardenability (Jominy End Quench) Presentation Fall '15 (2151)
Today’s Samples
Austenitized @ 850 °C for ~40 minutes
1045 Steel: Plain Carbon, 0.45 wt% C
4340 Steel: “Low Alloy”, 0.40 wt% C, 1.85 wt% Ni, 0.80 wt% Cr, 0.70 wt% Mn, 0.25 wt% Mo
End quenched until near room temperature
Quenching apparatus configuration specified by ASTM A255
Cooling rate varies with distance from quenched end
Flat surface machined along length
Rockwell C hardness measurements
Distances from quenched end similar to those specified by ASTM A255
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Hardenability Test Report
Cover Page & Abstract Cover page & abstract on own page (remember abstract stands alone!) Objective Procedure Outline format Making a Rockwell hardness measurement can be just a step in the End Quench procedure (How to make indent needs no detail, but use of micrometer stage does) Analysis Plot hardness vs. distance on a single plot to compare materials Observations Compare results to Fig. 11.14 from Callister textbook (on slide #20) Indicate which steel has higher hardenability & why (what feature of the continuous cooling transformation diagram is changed by adding alloying elements in 4340 steel?). Conclusion References & Appendix Reference “Callister” textbook for figures from presentation, not PowerPoint file 23
MECE 306 Materials Science Apps Lab 11 Phase Transformations / Hardenability (Jominy End Quench) Presentation Fall '15 (2151)
Due by midnight…
tonight
Micro hardness Test Report
Micro hardness Quiz & Survey
next week
Hardenability (Jominy End Quench) Test Report
Hardenability Quiz & Survey
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MECE 306 Materials Science Apps Lab 12