UNS S32760 F55 Super Duplex Stainless Steel F55 alloy [611796]

Chapter 3
UNS S32760 F55 – Super Duplex Stainless Steel F55 alloy
3.1 I ntroduction
F55 type is a super duplex stainless steel alloy with a 50:50 austenite, ferrite microstructure. The
material combines high mechanical strength and good ductility with excellent corrosion resistance in
a variety of environments. This material is generally supplied in the annealed condition giving yield
strengths in excess of 80 KSI (550Mpa), this material cannot be hardened by heat treatment but
stronger surface str engths can be achieved by cold working.
This alloy is generally supplied with a Pitting Resistance Equivalent (PREn) of greater than 40 which guarantees high resistance to pitting corrosion. In addition high resistance to crevice and stress
corrosion coupl ed with the increased strength give this alloy the advantage over the austenitic (300
series) and 22%Cr duplex stainless steels. Typical applications include many components for the chemical processing, marine, oil and gas, pollution control and food indus tries amongst others…

3.2 Typical Chemical composition

3.3 Mechanical Property Requirements – Annealed condition
3.4 Forging
Forging temperature for this material should be 1100 – 1250Oc .Reheat as often as necessary and
cool in still air.
3.5 Heat Treatment
Anneal – Heat to 1100 – 1140oC ensuring sufficient time is allowed for the center to achieve furnace
temperature and hold for a time commensurate with the ruling section, followed by water quenching.
3.6 Machining
Material in the annealed condition is readily machinable by all conventional methods
3.7 Welding
F55 is readily wieldable using many of the standard electric arc welding processes but oxyacetylene
welding is not recommended because carbon pickup in the weld metal may occur.

3.8 Ferrite phase
Ferrite, also known as α -ferrite (α -Fe) or alpha iron is a solid solution of limited amounts of carbon
in iron with a body -centered cubic (BCC) crystal structure. It is this crystalline structure which gives

steel and cast iron their magnetic properties, and is the classic exam ple of a ferromagnetic material
[45].
It has a strength of 280 N/mm2 and a hardn ess of approximately 80 Brinell [46]. Mild steel (carbon
steel with up to about 0.2 wt% C) consist mostly of ferrite, with increasing amount s of pearlite (a fine
lamellar structure of ferrite and cementite) as the carbon content is increased. Since bainite (shown
as ledeburite on the diagram at the bottom of this page) and pearlite each have ferrite as a component,
any iron -carbon alloy will c ontain some amount of ferrite if it is allowed to reach equilibrium at room
temperature. The exact amount of ferrite will depend on the cooling processes the iron- carbon alloy
undergoes as it cools from liquid state. In pure iron, ferrite is stable below 910 °C (1,670 °F). Above
this temperature the face- centred cubic form of iron, austenite (gamma -iron) is stable. Above
1,390 °C (2,530 °F), up to the melting point at 1,539 °C (2,802 °F), the body -centred cubic crystal
structure is again the more stable form, as delta -ferrite (δ -Fe).
Ferrite above the critical temperature A 2 (Curie temperature) of 771 °C (1,044 K; 1,420 °F), where
it is paramagnetic rather than ferromagnetic. The term is beta ferrite or beta iron (β -Fe). The term
beta iron is not any longer used because it is crystallographically identical to, and its phase f ield
contiguous with, α -Fe.
Only a very small amount of carbon can be dissolved in ferrite [47]; the maximum solubility is about
0.02 wt% at 723 °C (1,333 °F) and 0.005% carbon at 0 °C (32 °F) [48]. This is because carbon
dissolves in iron interstitially, with the carbon atoms being about twice the diameter of the interstitial
"holes", so that each carbon atom is surrounded by a strong local strain field. Hence the enthalpy of
mixing is positive (unfavorable), but the contribution of entropy to the free ene rgy of solution
stabilizes the structure for low carbon content. 723 °C (1,333 °F) also is the minimum temperature at which iron- carbon austenite (0.8 wt% C) is stable; at this temperature there is a eutectoid reaction
between ferrite, austenite and cement ite.

Because of its significance for planetary cores, the physical properties of iron at high pressures and
temperatures have also been studied extensively. α- ferrite, which is the form of iron that is stable
under standard conditions, can be subjected t o pressures up to 15 GPa before transforming into a
high-pressure form termed ε- iron, which crystallizes in a hexagonal close- packed (hcp) structure.
Used in many types of electronic devices. Ferrite is used in:

• Permanent magnets
• Ferrite cores for transfor mers and toroidal inductors
• Computer memory elements
• Solid -state devices
3.9 Austenite phase
Austenite , also known as gamma -phase iron (γ -Fe), is a metallic, non -magnetic allotrope of iron or
a solid solution of iron, with an alloying element [49]. In plain -carbon steel , austenite exists above
the critical eutectoid temperature of 1,000 K (1,340 °F; 730 °C); other alloys of steel have different
eutectoid temperatures. It is named after Sir William Chandler Roberts -Austen (1843–1902) [50].
Table 3.4
3.9.1 Allotrope of iron
From 912 to 1,394 °C (1,674 to 2,541 °F) alpha iron undergoes a phase transition from body -centred
cubic (BCC) to the face -centred cubic (FCC) configuration of gamma iron see figure 3.3 , also called
austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as
2.03% by mass at 1,146 °C (2,095 °F)). This gamma form of iron is exhibited by the most commonly
used type of stainless steel for making hospital and food- service equipment.

3.9.2 Aust enitization
Austenitization means to heat the iron, iron- based metal, or steel to a temperature at which it changes
crystal structure from ferrite to austenite [51]. An incomplete initial austenitization can leave
undissolved carbides in the matrix [52].
For some irons, iron- based metals, and steels, the presence of carbides may occur during the
austenitization step. The term commonly used for this is two-phase austenitization [53].
3.9.3 Austempering
Austempering is a hardening process that is used on iron- based metals to promote better mechanical
properties. The metal is heated into the austenite region of the iron- cementite phase diagram and then
quenched in a salt bath or other heat extraction medium that is between temperatures of 300–375 °C
(572–707 °F). The metal is annealed in this temperature range until the austenite turns to bainite or
ausferrite (bainitic ferrite + high -carbon austenite) [54] .
By changing the temperature for austenitization, the austempering process can yield different and
desired microstructures [55]. A higher austenitization temperature can produce a higher carbon
content in austenite, whereas a lower tem perature produces a more uniform distribution of

austempered structure [55]. The carbon content in austenite as a function of austempering time has
been established [56] .
3.9.4 Behavior in plain carbon -steel
As austenite cools, it often transforms into a mixture of ferrite and cementite as the carbon diffuses.
Depending on alloy composition and rate of cooling, pearlite may form. If the rate of cooling is very
swift, the alloy may experience a large lattice distortion known as transformation in which it
transforms into a BCT -structure instead of into cubic latticed ferrite and cementite. In industry, this
is a very important case, as the carbon is not able to diffuse due to the cooling speed, which results in
the formation of hard martensite. The rate of co oling determines the relative proportions of
martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting
steel, such as hardness and tensile strength. Quenching (to induce martensitic transformation),
followed by tempering will transform some of the brittle martensite into tempered martensite. If low –
hardenability steel is quenched, a significant amount of austenite will be retained in the
microstructure.

3.9.5 Behavior in cast iron
Heating white hypereutectic cast iron above 727 °C (1,341 °F) causes the formation of austenite in
crystals of primary cementite [57]. This austeni sation of white iron occurs in primary cementite at the
interphase boundary with ferrite [57].When the grains of austenite form in cementite, they occur as
lamellar clusters oriented along the cementite crystal layer surface [57]. Austenite is formed by
withdrawal of carbon atoms from cementite into ferrite [57][58].
3.9.6 Stabilization
The addition of certain alloy ing elements, 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 tempe rature. On the other
hand, such elements as silicon , molybdenum , and chromium tend to de -stabilize austenite, raising the
eutectoid temperature.
Austenite is only stable above 910 °C (1,670 °F) in bulk metal form. However, the use of a face –
centered cubic (FCC) or diamond cubic substrate allows the epitaxial growth of FCC transition
metals [59]. The epitaxial growth of austenite on the diamond (100) face is feasible because of the
close lattice match and the symmetry of the diamond (100) face is FCC. More t han a monolayer of γ –

iron can be grown because the critical thickness for the strained multilayer is greater than a monolayer
[59]. The determined critical thickness is in close agreement with theoretical prediction [59] .
3.9.7 Austenite transformation an d Curie point
In many magnetic alloys, the Curie point , the temperature at which magnetic materials cease to
behave magnetically, occurs at nearly the same temperature as the austenite transformation. This
behavior is attributed to the paramagnetic nature of austenite, while both martensite and ferrite are
strongly ferromagnetic .
3.9.8 T hermo -optical emission
During heat treating , a blacksmith causes phase changes in the iron -carbon system in order to control
the material's mechanical properties, often using the annealing, quenching, and tempering processes.
In this context, the color of light, or " blackbody radiation," emitted by the work piece is an
approximate gauge of temperature. Temperature is often gauged by watching the color temperature of
the w ork, with the transition from a deep cherry -red to orange -red (815 °C (1,499 °F) to 871 °C
(1,600 °F)) corresponding to the formation of austenite in medium and high- carbon steel. In the
visible spectrum, this glow increases in brightness as temperature in creases, and when cherry -red the
glow is near its lowest intensity and may not be visible in ambient light. Therefore, blacksmiths
usually austenite steel in low -light conditions, to help accurately judge the color of the glow.
Maximum carbon solubility in austenite is 2.03% C at 1,420 K (1,150 °C).

3.10 Sigma phase
A sigma phase refers to a non -magnetic stage composed of predominantly iron and calcium in ferritic
and austenitic stainless steels. It is the intermolecular stage that causes metals to lose ductility,
toughness, stability and corrosion resistance.
The sigma phase occurs during metallic exposure at 560șC to 980șC (1,050șF to 1,800șF). It is
generally strain intolerant at temperatures under 120șC to 150șC (250șF to 300șF).
The sigma phase forms at ferrite/austenite interfaces. However, its formation is delayed with lower
energy ferrite or austenite surfaces that can be formed during welding processes. An X -ray diffraction
highlights the various mechanisms of sigma phase formation between 700șC a nd 850șC (1,292șF and
1,562șF). These mechanisms show changes from discontinuous precipitation to the growth of the
existing sigma after nucleation site saturation [60] .

3.11 C hi Phase

Chi phase is a Fe- Cr-Mo compound, chemically and structurally related to sigma phase, which was
first detected by Andrews and Brookes in thermally aged steels containing Mo. [62]. This phase was
subsequently shown to possess an ordered B.C.C., γ ~mn A12 structure having 58 atoms per unit cell
and with a 0 0.88 nm [63]. It is b elieved to be a carbon- dissolving compound. Its formation in AISI
316 stainless steel has been reviewed by Weiss and Stickler [61]. In their study, chi phase was
generally found in the same temperature range as sigma, and frequently preceded the formation of
sigma. The composition in wtZ was found to be 22Mo- 21Cr -52Fe -5Ni. Our own United observation.*
of chi phase are summarized in Figure 12. Minor quantities of this phase were observed in AISI 316
aged at temperatures from 700° to S50°C and also in a 316 + Ti alloy aged 750°C. A limited amount
of chi phase was observed in both AISI 316 and 316 + Ti irradiated in HFIR at 650°C.