2 Mechanical Properties of SS 316L

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(2) Mechanical Properties of SS 316L
FYI Table 11.4

• SS316L is austenitic
• ?-Fe, FCC
• Stabilized by Ni
• non-magnetic, generally stronger
• Ferritic SS
• ?-Fe, BCC

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Pure Fe undergoes 2 changes in crystal structure
before melting
Ferrite
Alloy w/ C

• can dissolve 0.02 wt C _at_ 727 ?C
• Shape, size of BCC interstitials make it
  difficult to hold C

Heat to 912 ?C
Alloy w/ C
These all have ?-ferrite and cementite
Austenite

• can dissolve 2.14 wt C _at_ 1147 ?C

Heat to 1394 ?C
Different Microstructures
Alloy w/ C
Austentic Steel
Ferrite
Marstenite
Pearlite Microstructure
Heat to 1538 ?C
Bainite Microstructure
Tempered Marstenite
MELT
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AUSTENITE
Austenite is a solid solution of carbon and iron
that exists in steel above the critical T
723C. It contains up to 2.14 wt C at 1147?C. As
it cools, this structure either breaks down into
a mixture of ferrite and cementite (usually in
the structural forms pearlite or bainite), or
undergoes a slight lattice distortion known as
marstentic transformation. The rate of cooling
determines the relative proportions of these
materials and therefore the mechanical properties
(e.g. hardness, tensile strength) of the steel.
Quenching (to induce martensitic transformation),
followed by tempering (to break down some
martensite and retained austenite), is the most
common heat treatment for high-performance
steels. The addition of certain other metals,
such as manganese and nickel, can stabilize the
austenitic structure, facilitating heat-treatment
of low-alloy steels. In the extreme case of
austenitic stainless steel, much higher alloy
content makes this structure stable even at room
temperature. On the other hand, such elements as
silicon, molybdenum, and chromium tend to
de-stabilize austenite. Thus, above the critical
temperature, all of the carbon contained in
ferrite and cementite (for a steel of 0.8 C) is
dissolved in the austenite.
Fig. 9.25b
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?-FERRITE
Ferrite is a body-centered cubic (BCC) form of
iron, in which a very small amount (a maximum of
0.02 wt at 723C) of carbon is dissolved. Most
"mild" steels (plain carbon steels with up to
about 0.2 wt C) consist mostly of ferrite, with
increasing amounts of cementite as the carbon
content is increased. Any iron-carbon alloy will
contain some amount of ferrite if it is allowed
to reach equilibrium at room temperature.
Fig. 9.25a
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CEMENTITE

• iron carbide with the formula ___________
• orthorhombic crystal structure
• hard, brittle material, essentially a
  _______________ in its pure form.
• If can be formed from austenitic steel during
  cooling or from matensite during tempering.
• Cementite contains ________C by weight thus
  above that carbon content, the alloy is no longer
  steel or cast iron, as all of the available iron
  is contained in cementite.
• Cementite mixes with _____________, the other
  product of austenite, to form microstructures
  pearlite, bainite, and marstenite.

Photomicrograph of pearlite structure dark bands
are cementite
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Formation of Cementite
Eutectic Reaction For Fe-Fe3C System
Eutectic Reaction a reaction, where upon
cooling, __________________ transforms
isothermally reversibly to _____________________
__ that are intimately mixed. For Fe-Fe3C at
4.3 wt C Temp 1147 ?C
cool
L (liq.)
Austenite g-Fe
Cementite Fe3C

heat
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Formation of Cementite
Eutectoid Reaction For Fe-Fe3C System
Eutectoid Reaction A reaction, wherein, upon
cooling, ________________ transforms isothermally
reversibly into ________________________ that
are intimately mixed.
Cool to Teutectoid ? 727 ?C
Cementite Fe3C 6.7 wt C high C
Ferrite a-Fe 0.022 wt C low C
Austenite g-Fe 0.76 wt C Eutectoid composition

heat
This leads to microstructural changes
cool
heat
below 727 ?C
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FORMATION OF PEARLITE FROM AUSTENITE

• Pearlite
• 2-phase microstructure consisting of
  ________________________________________ of
  ferrite (?-Fe) and cementite (Fe3C)
• Due to transformation of austenite of eutectoid
  composition

Photomicrograph of euctectoid steel showing
pearlite microstructure consisting of alternating
layers of ? ferrite (the light phase) and Fe3C
(thin layers most of which appear dark) (500X)
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COURSE and FINE PEARLITE

• Fine pearlite (thin layers T 540) and course
  pearlite (thick layers T just or ltTeuc)

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FORMATION OF BAINITE FROM AUSTENITE

• Bainite
• 2-phase microstructure consisting of ferrite
  (?-Fe) and ________________________
• __________________cementite (Fe3C)
• Forms as needles of plates
• Due to transformation of austenite of eutectoid
  composition

Grain of bainite
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FORMATION OF MARTENSITE FROM AUSTENITE
When austenite rapidly cooled ? C cannot diffuse
out (diffusionless process)

• Martensite is more or less _______________________
  ______________________
• Martensite is a body-centered tetragonal (BTC)
  form of iron in which some carbon is dissolved
• FCC lattice of austenite is distorted into the
  body centered tetragonal (BCT) structure without
  the loss of its contained carbon atoms into
  cementite and ferrite. The carbon is retained in
  the iron crystal structure, which is stretched
  slightly so that it is no longer cubic.

Body Centered Tetragonal (BTC) Unit Cell
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FORMATION OF MARTENSITE FROM AUSTENITE
Fig. 10.33
Tempered Martensite
Martensite
(Small particles of cementite, matrix is
?-ferrite)
White phase austenite (didnt change during
quenching retained austenite) Black phase
marstenite
single phase
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Table 10.2
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Categories of Metallic Biomaterials

• Stainless Steel
• Cobalt-based Alloys
• Titanium Alloys
• Specialty Metallic Alloys

2. Cobalt Based Alloys

• a. Composition
• 2 basic components Co and Cr
• - Co and Cr form solid solutions up to 65 wt Co
• - Cr increases corrosion resistance
• - Mo produces fine grains ? increases strength
• _____________________________________
• - Co-Cr-Mo
• - Composition wt Co (58.9-69.5), Cr (27 -30),
  Mo (5-7), and minor amounts of Mn, Si, Ni,
  Fe, C
• Other terms start to show up cast, hot forged,
  etc

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Fig. 11.7

• Forming Operations
• Shape change by plastic deformation
• Hot working deformation at T gt Trecrys ?
  increase ductility (decrease strength)
• Cold working deformation at low T ? increase
  strength (decrease ductility)
• Forging, rolling, extrusion, drawing

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FORMING OPERATIONS
Adapted from Fig. 11.8, Callister 7e.
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Fig. 11.7

• 2. Casting
• Totally molten metal poured into a mold cavity of
  desired shape ? cool ? some shrinkage
• Sand casting, die casting, investment casting,
  lost foam casting, continuous casting.

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CASTING TECHNIQUES
Sand Casting

• 2-piece mold formed by packing sand around a
  pattern of desired shape
• Pour molten metal into mold
• Sand will withstand T gt1600ºC and is cheap, easy
  to mold
• Used to make large parts Engine blocks, fire
  hydrants, large pipe fittings

Die Casting

• Liquid metal forced into a mold under pressure
  and at relatively high velocity solidified under
  pressure
• Fast, inexpensive, good for small pieces and
  alloys with low Tmelt

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CASTING TECHNIQUES

• Mold prepared by fabricating a wax pattern to
  near-final dimensions ? coating (or investing)
  the pattern with ceramic (sometimes plaster of
  paris) ? heat mold so that pattern melts and is
  burned out ?mold cavity
• Pour molten metal into mold ? solidifies ? remove
  ceramic mold (cracked away)
• Excellent dimensional accuracy
• Used for ASTM F75 for joint replacement also for
  dental crowns

Investment Casting
ceramic formed around wax prototype (pattern)
Fill mold cavity with molten metal
Burn out wax
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Categories of Metallic Biomaterials

• Stainless Steel
• Cobalt-based Alloys
• Titanium Alloys
• Specialty Metallic Alloys

2. Cobalt Based Alloys
c. _____________________________________ -
Co-Cr-W-Ni - Composition wt Co
(45.5-56.2), Cr (19-21), W (14-16), Ni (9-11),
and minor amounts of Fe, Mn, C, P, Si, S -
sometimes referred to as wrought Co-Cr-W-Ni
alloy
- Annealed vs. cold worked (see Table)
Cast alloys Wrought alloys
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Categories of Metallic Biomaterials
2. Cobalt Based Alloys
d. ____________________________________ -
Co-Ni-Cr-Mo-Ti - Composition wt Co
(29-38.8), Ni (33-37), Cr (19-21), Mo (9-10.5),
Ti (1) and minor amounts of Fe, Mn, Si, S -
sometimes referred to as wrought Co-Ni-Cro-Mo-Ti
alloy