Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 1

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 Steelsγ (Austenite, FCC) →

α (Ferrite, BCC) + Fe3C (Cementite, FC Orthorhombic)

γ (Austenite) → α (Ferrite) + Fe3C (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.

No time element; temperature change assumed slow enough for quasi-equilibrium at all points

Fig. 9.26 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 20103

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 2

Isothermal Transformations of Eutectoid Steel – Pearlite

Isothermal transformation

diagram for a eutctoid

iron-carbon alloy, with

superimposed isothermal

heat treatment curve

(ABCD). Microstructures

before, during, and after

the austenite-to-pearlite

transformation are shown.

Austenite (unstable)

This figure is nicknamed the “T-T-T” plot (for time-temperature-transformation)

Notice fast transformationcreates “fine” pearlite andslow transform. creates “coarse” pearlite!

Fig. 10.14 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 20104

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

Fig. 10.16 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 20105

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 3

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).

Fig. 10.18 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 20106

Isothermal Transformations of Eutectoid Steels – Martensite

The complete isothermal

transformation diagram for

an iron-carbon alloy of

eutectoid composition:

A, austenite;

B, bainite;

M, martensite;

P, pearlite.

Martensite can only be formed with very rapid

cooling rates

Fig. 10.22 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 20107

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 4

Microstructures of Carbon Steels

Fine Bainite grain (bottom left

to top right) in Martensite(~19,000×)

Coarse

Pearilte grains(3000×)

Partially transformedMartensite grains

in Austenite* (~1200×)

20µm

Increasing hardness* Martensite transformation “frozen” partially completed

(Dark martensite “needles” in light austenite matrix)

Pearlite Bainite Martensite

8

aa

c

Slow – carbon diffuses out of

smaller

interstitial spaces

Crystal StructuresAustenite, Ferrite & Martensite

FCC Crystal(γ – Austenite,

a≈3.6nm)

BCC Crystal(α – Ferrite,

a≈2.9nm)

BCT Crystal(Martensite,

a≈2.8nm, c>2.8nm)

Fast – crystal structure “warps”; carbon trapped by low diffusion rate (only “metastable”)

9

a

a

a

aa

a

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 5

Non-Isothermal Transformations of Eutectoid Steels (Continuous Cooling)

Superimposition of

isothermal and continuous

cooling transformation

diagrams for a eutectoid

iron-carbon alloy.

Transformations start later and/or at lower temperature during continuous

cooling

Pearlite / bainite “nose” moves down & is not horizontal

Fig. 10.25 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201010

Critical Cooling Rates for Eutectoid Steel

Continuous cooling

transformation diagram for a

eutectoid iron-carbon alloy

and superimposed cooling

curves…

Cooling rates slower than this cooling rate form only pearlite (the

diffusion-based transformation has time to complete

before the martensite start temperature is

reached)

Cooling rates faster than the critical cooling rate form only

martensite (no time for

diffusion-based transformation

to occur)

Fig. 10.27 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201011

Intermediate cooling rates form some

pearlite, but the reaction stops

before finishing

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 6

Microstructures & Hardness

Cooling Rate

MicrostructureRelative

Hardness

Fast Martensite Very Hard

MediumMartensite +

PearliteMedium-

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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Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 7

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 changein positions and shapes of transformation curves, such as:

� Shift the nose of pearlite transformation curve to the right

� Cause formation of a separate bainite nose

Isothermal transformation diagram

for an alloy steel (type 4340)…

Fig. 10.23 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201015

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 8

4340 Alloy Steel – Continuous Cooling Transformation Diagram

Cooling rate above critical cooling rate forms only Martensite.

Cooling rate in this range begins forming Bainite but transform. doesn’t finish before Martensite transform. begins.

Cooling rate in this range allows time for primary phase (α) to form but not

enough for Pearlite transform. to get started.

Cooling rate in this range allows Pearlite transform. to get started but not to finish. Then Bainite transform. starts but doesn’t finish either. Some Martensite still forms.

Fig. 10.28 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 2010

Continuous cooling

transformation diagram for 4340

alloy steel and several

superimposed cooling curves16

Hardenability Test –Jominy End Quench

Schematic diagram of Jominy end-quench specimen (a) mounted during quenching

and (b) after hardness testing from the quenched end along a ground flat.

Water jet cools quenched end quickly; opposite end air-cools more slowly

Fig. 11.11 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201017

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 9

Jominy Test onEutectoid Steel

Correlation of hardenability and continuous cooling

information for an iron-carbon alloy of eutectoid composition.

� 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

Fig. 11.13 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201018

Microstructure vs. Hardenability Curves

100% martensite

Mix of martensite and pearlite

100% pearlite(fine)

100% pearlite(coarse)

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Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 10

Hardenability Curves by Alloy

Hardenability curves for five

different steel alloys, each

containing 0.4 wt% C…

4340 alloy steel: high hardenability

(small change in hardness w/ distance)

1040 plain-carbon steel: low hardenability

(large change in hardness w/ distance)

Fig. 11.14 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201020

Hardenability Bands

The hardenability band for an 8640

steel indicating maximum and

minimum limits.

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

Fig. 11.16 from Callister & Rethwisch, Materials Science & Engineering, An Introduction, 8th ed., J. Wiley & Sons, 201021

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 11

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 file23

Phase Transformations / Hardenability (Jominy End-Quench) Presentation Fall '15 (2151)

MECE-306 Materials Science Apps Lab 12

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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