Transcript Ti and its Alloys & their Heat TreatmentsPresented by

Ti and its Alloys & their Heat Treatments
Presented by
Professor Ali H. ATAIWI
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Ti and its Alloys
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Classification of Ti Alloys
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Heat Treating of Titanium and Titanium Alloys
 i and Ti alloys are heat treated for the following purposes:
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●To reduce residual stresses developed during fabrication
(stress relieving )
●To produce an optimum combination of ductility, machinability, and
dimensional and structural stability
(annealing)
●To increase strength (solution treating and aging )
●To optimize special properties, such as fracture toughness, fatigue
strength, and high creep strength.
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*Various types of annealing treatments (single, duplex,
beta, and recrystallization annealing, for example)
and solution-treating- and-aging treatments are
imposed to achieve selected property combinations.
*Stress relieving and annealing may be employed to
prevent preferential chemical attack in some corrosive
environments, to prevent distortion (a stabilization
treatment), and to condition the metal for subsequent
forming and fabricating operations.
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Stress Relieving
Titanium and titanium alloys may be stress relieved without
adversely affecting strength or ductility, depending on the alloy and
specific heat treatment.
*A stress relief of mill- or beta annealed Ti-6Al-4 V would not affect
the properties, whereas a full stress relief solution treated and
aged Ti-6Al-4V would result in a strength reduction.
*On the other hand, beta alloys, such as Ti-10V-2Fe-3Al can be stress
relieved at lower temperatures, and the aging temperature may be
sufficient to also impart a stress relief
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* Stress-relieving treatments decrease the undesirable
residual stresses that result from no uniform hot
forging deformation from cold forming and
straightening, forging, welding, and asymmetric cooling
following heat treatment (more severe with faster
cooling rates).
* Removal of such stresses helps maintain shape stability
and eliminate unfavorable conditions, such as the loss
of compressive yield strength commonly known as the
Bauschinger effect ( The phenomenon by which plastic
deformation increases yield strength in the direction of
plastic flow and decreases it in other directions).
* Table 18 presents combinations of time and mperature
that are used for stress relieving titanium and titanium
alloys.,
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Annealing
*The annealing of titanium and titanium alloys serves
primarily to increase:
- fracture toughness.
- ductility at room temperature.
-dimensional and thermal stability.
- and creep resistance.
* Many titanium alloys are placed in service in the
annealed state.
* Common annealing treatments include:
- mill annealing.
- duplex annealing.
- recrystallization annealing.
- and beta annealing.
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* Recommended annealing treatments for several alloys
arc given in Table 19.
* Either air or furnace cooling may be used, but the two
methods may result in different levels of tensile
properties.
* If distortion is a problem, the cooling rate should be
uniform down to 315°C (600 oF). It may be difficult to
prevent distortion of close-tolerance thin sections
during annealing.
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A* Mill annealing is a general-purpose treatment given to all
mill products. It is not a full anneal and may leave traces
of cold or warm working in the microstructures of heavily
worked products, particularly sheet .
* Duplex annealing alters the shapes, sizes, and
distributions of phases to those required for improved
creep resistance or fracture toughness.
* Recrystallization annealing and beta annealing are used to
improve fracture toughness.
* In recrystallization annealing, the alloy is heated into the
upper end of the alpha-beta , held for a time, and then
cooled very slowly.
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* Like recrystallization annealing, beta annealing improves
fracture toughness. Beta annealing is done at
temperatures above the beta transus of the alloy being
annealed. To prevent excessive grain growth, the
temperature for beta annealing should be only slightly
higher than the beta transus.
* Annealing times are dependent on section thickness and
should be sufficient for complete transformation. Time at
temperature after transformation should be held to a
minimum to control beta grain growth.
* Larger sections should be fan cooled or water quenched to
prevent the formation of alpha phase at the beta grain
boundaries
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Solution Treating and Aging
* Time/temperature combinations for solution treating are
given in Table 20.
* A load may be charged directly into a furnace operating at
the solution-treating temperature. Although preheating is
not essential, it may be used to minimize distortion of
complex parts
Quenching
* The rate of cooling from the solution-treating temperature
has an important effect on strength. If the rate is too low,
appreciable diffusion may occur during cooling, and
decomposition of the altered beta phase during aging may
not provide effective strengthening .
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Aging
* The final step in heat treating titanium alloys to high
strength consists of reheating to an aging temperature
between 425 and 650 oC(800 and 1200 oF).
* Aging causes decomposition of the supersaturated beta
phase retained on quenching.
* A summary of aging times and temperatures is
presented
in Table 20. The time/temperature combination selected
depends on required strength.
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