Materials Science Materials Science
Chapter 9 Chapter 9
Shaping, Strengthening and
Toughening Processes
outline
9 -1 Shaping Processes Casting
Plastic Forming
Sintering of Ceramics and Metals
9-2 Solution Hardening
9-3 Strain Hardening & Annealing Cold Work
Recrystallization
Recrystallization Temperatures & Rates Processing Strain-Hardenable Materials
9-4 Precipitation Hardening Age Hardening
Overaging
Combined Hardening
9-5 Second-phase Strengthening Carbide-Containing Steels
Plastic Constraint
9 -6 Heat Treatments of Steels Annealing Processes
Quenching and Tempering
9-7 Hardenability of Steels Hardenability Curves
Use of Hardenability Curves Tempered Hardness
9-8 Strong & Tough Ceramics Induced Compression
"High- Tech" Ceramics
Shaping Processes
There are numerous procedures for shaping of materials:
a) casting: the procedure starts with liquid or semiliquid solid which solidifies in a mold.
The metal crystallizes during solidification, however, it may be thermoplastic resin or suspension for clay-based ceramics.
Some polymers may start out as a fluid in which reaction between the two components produces solid plastics.
Molds are used in all casting processes.
Molds contains cavity with negative shape of product
Ductile materials can be shaped by plastic deformation.
The mechanism is through slip along crystal planes.
Typical Procedure
To cast an ingot, which is simply a solidified block of metal that can be deformed by mechanical working to produce a rod, wire, tube, plate, forging, etc.
• Injection molding is used in polymer industry.
• Die casting is used in aluminium and zinc alloys
• A die is a permanent mold, it is used for low melting point materials.
• Plaster molds are used for clay-based ceramics because it absorbs water from the slurry.
• Solidification is accompanied by volume changes,
• There are 4-6% solidification shrinkage + a considerable volume concentration.
• A means of supplying more liquid into the casting during solidification.
• The liquid is added through risers in the mold design.
Plastic forming
During mechanical working stresses are applied so as:
Stresses > Yield Strength
The mechanical processing on initial large ingots is
performed at high temperatures where the material is soft and ductile less energy is required and therefore, less fracture during processing.
Rolling, forging and extrusion are among the hot defromation processes.
Secondary Deformation
At ambient temperature small dimensions and less energy is required
Sintering of ceramics and metals
Sintering is the process of bonding particles by heat.
It is the main agglomeration process for almost all ceramics (except glass), for making powder metal products and bonding of certain polymers such as Teflon).
MgO produced by sintering of MgO powder.
Sintering involves not only the bonding of powder particles, but also the elimination of the initial porosity to give a more dense product.
Principle involved: in the absence of any liquid, sintering reduces surface and boundary energies via the minimization of boundary areas.
Solution Hardening Solution Hardening
Addition of zinc to copper increases the alloy strength.
Accordingly, engineer can design not only stronger alloy, but also more ductile one.
Sometimes no excess costs are included.
Other alloys show similar hardening and strengthening behaviours.
Unlike zinc, nickel can be added to copper in unlimited quantities due to the absence of solubility limits.
Strain Hardening & Annealing Strain Hardening & Annealing
• Ductile materials become stronger when deformed plastically. This increase in strength is called strain hardening
Cold work
% Cold work = index of plastic deformation = the amount of plastic strain introduced during processing expressed by percent decrease in cross-sectional area from deformation.
100
% x
A A CW A
o f
o
−
=
Ao and Af are the original and final areas, respectively.
Recrystallization
• Recrystallization: is the process of growing new crystals from previously deformed crystals.
• Plastically deformed materials have more energy than do unstrained materials due to the presence of dislocations and point imperfections.
• The atoms will move to form more perfect, unstrained array upon heating this process is called annealing
• The greater thermal vibrations of the lattice at higher
temperatures permit a re-ordering of the atoms into less disordered, softer grains.
Recrystallization
Recrystallization temperature, T temperature, T
RRFactors affecting the re
Factors affecting the re - - crystallization temperature crystallization temperature
1. TR lies between 0.3 and 0.6 of the absolute, K, melting temperature Tm because self diffusion is directly- related to the melting temperature.
2. Time, e.g. recrystallization of a commercially pure aluminum alloy which is cold-worked 75% may be completed in one minute at 350oC – but requires 60 minute at 300oC and 40 days@ 230oC.
3. The amount of strain hardening, see the following Figure
4. Purity of the materials = pure materials recrystallize at lower temperatures.
Recrystallization rates
•The recrystallization typically follow an S-shaped or sigmoidal curves.
•Recrystallization time is related to the temperature through:
Ln t = C + B/T
C and B are constants. The reaction rate, R can be calculated as follows:
Ln R = Ln Ro – E/KT Therefore, C = Ln Ro and B = E/K
E = the energy required for recrystallization, k = Boltzamnn, s constant.
Precipitation hardening
•Hardness increases during initial stages of precipitation from supersaturated solution.
•Precipitation occurs throughout the interior of the parent grain.
•Hardening that accompanies precipitation is called age- hardening
• in age-hardening solubility decreases with decreasing temperature and a supersaturated solution is obtained.
•The process of age-hardening involves solution treatment followed by quenching to supersaturates the solid solution.
•After quenching the alloy is reheated to an intermediate temperature at which precipitation is initiated in a
reasonable length of time.
Interpretation of age-hardening
•The supersaturated atoms tend to accumulate along specific crystal planes in the manner indicated in Fig. 9 - 4.2(b).
•The concentration of the copper (solute) atoms in these positions lowers the concentrations in other locations,
producing less supersaturation and therefore a more stable crystal structure.
•At this stage, the copper atoms have not formed a phase that is wholly distinct; a coherency of atom spacing exists across the boundary of the two structures. Dislocation
movements proceed with difficulty across these distorted regions; consequently, the metal becomes harder and more resistant to deformation under high stresses.
Overaging
•A continuous local segregation process over long periods leads to true precipitation and to
overaging, or softening. e.g., the development of a truly stable structure in an alloy of 96% Al and 4%
Cu involves an almost complete separation of the copper from the fee aluminum at R.T.
Nearly all the copper forms CuAl
2because the
growth of the second phase provides larger areas
that have practically no means of slip resistance, a
marked softening occurs.
Overaging
•Overaging of a commercial aluminum alloy (2014). The initial hardening is followed by softening as the resulting precipitateis agglomerated.
•Two effects of the aging temperature may be observed:
(1) precipitation, and therefore hardening, starts very quickly at higher temperatures;
(2) overaging, and therefore softening, occurs more rapidly at higher temperatures.
•These two phenomena overlap to affect the maximum hardness that is attained.
•Lower temperatures permit greater increases in hardness, but longer times are required.
Annealing processes
• The term annealing originally was used by craftspeople who discovered the benefits of heating some materials to elevated temperatures, then cooling the materials slowly (as opposed to quenching them).
• Structural changes are not the same for all materials
•When glass is annealed, the glass transition temperature is exceeded slightly, so residual strain are relived, the
possibility of fracture decreases.
•
Annealing processes
•We anneal cold-worked brass to soften it and to re- establish its ductility. We now know that the brass is recrystallized during the annealing process.
• The same results are achieved when cold-drawn steel
wire or cold-rolled steel sheet is process annealed. The full anneal of a steel for the machining of a gear blank,
however, has different results. In this case, the ductility may be reduced, with the consequence that chips form more readily, facilitating machining.
Annealing processes
•Process Anneal Products formed by cold
working are inherently small in cross-section. * As a practical matter, this annealing process is limited to sub-eutectoid temperatures of steels.
If higher annealing temperatures were used, austenite
•would form. Because of the very large surface-to- volume ratio of wire and sheet products, cooling could be rapid enough to form the brittle
martensite.
Annealing processes
Full Anneal The full anneal process is used for products that are to be machined subsequently, such as
transmission gear blanks. Thus, the steels typically have medium to high carbon contents (0.35 to 0.65 % carbon).
The steel is first austenitized; then furnace cooled.
The temperature of austenization varies with the carbon content, margin above the so-called Ac3line is used to
ensure complete austenization in the center, as well as at the surface, of the product.
Furnace cooling is slow cooling; it produces very coarse pearlite that is as soft as possible, and still allow chips to form.
•Typically, the product receives additional heat treatments after machining to restore hardness and strength.
Annealing processes
•Normalizing: A third annealing process used for steels is normalizing. Its purpose is the homogenization of the alloy steels. Austenization is performed at approximately 50 to 60oC above the AC3 line (Fig. 9-6.1).
The purpose is to accelerate the diffusion required for the homogenization of the substitutional atoms (Ni, Cr, Mo, V, etc., for Fe), while avoiding the excessive grain-growth that would occur at still higher temperatures. The heating is
followed by air cooling.
•Stress Relief Quenched steels, and machined steels are subject to distortion and possible cracking from the
presence of residual stresses.
A stress relief is necessary to remove these stresses. the necessary temperature is lower,