Heat Treatment Of Metals.pdf

Heat Treatment of Metals

Heat Treatment • Why do we heat treat? – To relieve stresses (i.e. from cold working) – To increase softness, ductility and toughness – and/or to produce a specific microstructure

• Why is it important to understand thermal processing in metal alloys? – Because it can produce mechanical properties required for specific applications. – Because the mechanical properties of an alloy that has been heat treated can be altered significantly if it is reheated (i.e. welding). 2

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Annealing Processes •

Annealing refers to a heat treatment with the following stages: 1. Heat to the desired temperature. 2. Hold or “soak” at that temperature. •

3.

•

to allow for any necessary transformation reactions to occur.

Cool, usually to room temperature.

Heating and cooling time is important – –

Poor control can lead to temperature gradients throughout part. temperature gradients can induce internal stresses that may lead to to warping and cracking.

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Annealing of Ferrous Alloys • •

Several different annealing procedures for steels Normalizing – –

•

Cooled in air. Gives a good combination of strength and ductility

Full Annealing – – –

•

Used for low or medium carbon steels that will be machined or plastically deformed. Furnace turned off, both steel and furnace cool together. Result: Course Pearlite, Soft and ductile.

Spheroidizing – – –

Used for medium and high carbon steels that have coarse pearlite that may still be too hard to machine or deform. Heated just below eutectoid (700°C) for 15 – 25 hours. Coalescence of Fe3C to form spheroid particles

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Heat Treatment of Steel – Quench and Temper 1.

Austenitize –

–

2.

Heat to a temperature region where 100% Austenite is formed Hold to allow complete transformation

727°C

Quench (normally use water or oil) 23°C

3.

Temper – –

Reheat to 200 - 550°C Decrease hardness, regain ductility (i.e. martensite).

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Tempering of Martensite • Heat for 1 hour 200-550 oC • During tempering, two things occur: – BCT (α´) phase transforms to BCC (α) phase (i.e. a =c) – A very fine distribution of Fe3C particles (precipitates) are formed in an α matrix

• If we hold at temperature for too long the steel will become “soft” and very ductile, equivalent to spheroidized steel 6

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Tempering of Martensite

Microstructure Changes

Property Changes

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Martensite

24.6 µm

•

Quench

2 µm

•

Tempered – – –

564°C small particles cementite 8 matrix is α ferrite

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Effect of Carbon Content on Hardness (Martensite)

Precipitation Hardening • Small uniformly dispersed second phase particles within the original matrix phase can enhance strength and hardness. • This can be achieved by an appropriate heat treatment. • Precipitation hardening is also called “Age hardening” because the strength develops over time • Major strengthening mechanism for alloys of Al, Fe, Ni, Cu • Need appreciable solid solubility of one alloying component (several %) in the other. • Also require a large decrease in solid solubility at T ↓, e.g. Cu in Al

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Precipitation Hardening • Precipitation hardening will only occur in alloys that: 1.

2.

show appreciable solid solubility of one alloying component (several %) in the other.

1.

2.

show a solubility limit that rapidly decreases in concentration of one major component with temperature reduction

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Heat Treatment of Precipitation Hardened Alloys 1.

Solution Treatment –

2.

to form a single phase solid solution.

Quench –

3.

to obtain supersaturated solid solution at room temp.

“Age” –

to form a fine distribution of precipitates. 12

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Microstructural Changes During Ageing • •

Aged at T2 Formation of very small β precipitates –

– –

• •

1 – 10 nm

The character of β precipitates and strength and hardness depend on

•

T2 Time at T2

Some alloys age at room temperature over a period of time (Natural Ageing) Some need elevated temperature (Artificial Ageing)

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Microstructural Changes During Ageing •

• •

Yield Strength increases as zones or precipitates form Strength reaches a peak value And then decreases (overageing)

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Precipitation Hardening in Al-Cu •

•

•

Precipitation hardening most widely studied in Al-Cu alloys α phase is a substitutional solid solution of Cu in Al θ phase is an intermetallic compound CuAl2 15

Precipitation Hardening in Al-Cu • • •

Solution treat at 550 oC Water quench Age at 120260 oC

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Mechanism of Hardening during Artificial Ageing (Stages for Al-Cu system)

Supersaturated α solid solution

A transition, θ”, precipitate phase Interface is coherent 1 atom in ppt for each atom in matrix

The equilibrium θ phase, within the α matrix phase (max strength) (Actual particle sizes are much larger than shown here incoherent

Strengthening Curve

Overaged Coarsening of preciptates

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Ageing Curves (2014 Al Alloy, 4 different temperatures) •

•

Strengthening Process is accelerated if temperature is increased Ideally the temperature and time for precipitation heat treatment should be designed to produce a hardness or strength in the vicinity of the maximum. 19

Mechanism of Strengthening • During plastic deformation: – Zones or precipitates act as obstacles to dislocation motion – Stress must be increased to “push” the dislocation through the distribution of precipitates.

• Consequently the alloy becomes harder and stronger.

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Precipitation hardening versus Quenching and Tempering Al-Cu Alloy • Solution treat to obtain a single phase α (θ phase dissolves) – FCC (No change)

• Quench to prevent formation of precipitates (Supersaturated at R.T.)

Steel • Ausenitise to form single phase γ – Fe3C dissolves – BCC ⇒ FCC

• Quench to prevent Fe3C formation and transform γ to martensite

– FCC – Low strength

– FCC ⇒ BCT – high strength, very brittle

• Age to form zones or precipitates

• Temper to precipitate very fine Fe3C

– Strength increases – Peaks and then decreases

– strength decreases – ductility increases

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Example: The replacement of steel sheet panels on automobiles by aluminum sheet products • e.g. hood and trunk panel for the Ford Taurus.

• Steel – Typically a 0.05 wt% carbon, ρ = 7.8g/cm3, σy = 250 MPa

• Aluminum (Al-Mg-Si) Alloy – ρ = 2.8g/cm3 – Precipitates of Mg2Si – Strength

Problems: 1. Relatively poor formability of Al Alloys 2. Would like higher strength

• as quenched - σy = 60 MPa • after natural ageing - σy = 150 MPa (still too soft) • after a 1 hour, 180°C paint bake cycle – 250 MPa

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Strengthening Mechanism Review 1. Grain Size reduction

σ y = σ o + kyd

−1

2

2. Cold Working –

Increase in yield stress as dislocation density increases

3. Solid Solution Hardening 4. Hardening due to Pearlite

σ y = σ o + KS −1

5. Quenching and Tempering (martensite) 6. Precipitation Hardening

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