1. Physical Metallurgy
EBB222
Stainless steel

2. Topics to be covered Stainless steel and cast iron
Non-ferrous metals and alloys
Heat treatment of non-ferrous metals and alloys
Alloys for high temperature and low temperature
applications
Steelmaking
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

3. Coursework Individual assignment Test
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

4. CLASSIFICATION OF FERROUS ALLOYS
Steel (< 2 wt% C)
Cast Iron (> 2wt% C)
Low alloy (<10 wt.% alloying elements)
Micro-alloyed steel
High alloy (> 10wt.% alloying elements)
Grey Iron (1-3 wt.% Si)
White Iron (<1 wt.% Si)
Tool
Stainless steel (≥ 11 wt.% Cr)
Low C (<0.25 wt.% C)
Medium C (0.25 – 0.6 wt.% C)
High C (0.6 – 1.4 wt.% C)
Plain
Plain
Plain
Tool
High strength, low alloy
Heat treatable

5. Stainless steels High corrosion resistance due to the presence of
chromium.
Chromium forms a thin film of chromium oxide (Cr203)
on the steel surface protects the steel from further
oxidation, making it stainless.
Stainless steels contain 12% - 18% of Cr
Other alloying elements of the stainless steels are nickel, molybdenum, titanium and manganese.

6. General Properties of Stainless Steels
Electrical Resistivity
Surface & bulk resistance is higher than that for plain-carbon steels
Thermal Conductivity
About 40 to 50 percent that of plain-carbon steel
Melting Temperature
Plain-carbon : °C
Martensitic : °C
Ferritic : °C
Austenitic : °C
Coefficient of Thermal Expansion
Greater coefficient than plain-carbon steels
High Strength
Exhibit high strength at room and elevated temperatures
Several parameters are consistent with all grades of stainless steels. Let us first examine electrical resistance in more detail. The surface and the bulk resistance is high tan for plain carbon steel. The bulk resistance because of the higher alloy content, and the surface resistance because of the tenacious chromium oxide layer which is high resistance and is the factor that makes the stainless steel corrosion resistant.

7. Types of stainless steel
Major alloy addition
AISI
Ferritic -α
Fe-Cr
4xx
Austenitic - γ
Fe-Ni-Cr
2xx, 3xx
Martensitic
Fe-Cr-C
4xx, 5xx
Duplex (α + γ)
Fe-Cr-Ni
Precipitation
hardened
Super ferrites &
austenitics
Higher Mo & Ni

8. Alloying elements and their purposes
Amount (%)
Purpose Cr
12-30
Cr2O3 – gives stainless character, stabilises α phase Ni
0-25
Stabilises γ phase (FCC) Mo
0-9
Reduces pitting and crevice corrosion, stabilises γ-phase N
<0.5
Reduces pitting/crevice corrosion,
stabilizes γ-phase
Ti, Nb
<1
Strong carbide formers, reduce sensitization C
<0.15 except
martensitic grades
Hard martensite for cutting edges, stabilizes γ -phase Mn
0-12
Ni replacement; stabilizes γ -phase

9. Fe-Cr phase diagram Duplex loop Austenic loop
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

10. Effect of 12%Cr on Fe-C phase diagram

11. Effect of 18%Cr on Fe-C phase diagram

12. Range of austenite in Fe-C-Cr
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

13. Range of austenite in Fe-C-Cr
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

14. Ferritic stainless steel
low carbon content (< 0.12 wt.%)
main alloying element Cr : wt.%,
composed of α - ferrite (BCC) phase at all temperature
non heat treatable alloy, properties can influenced
by cold working
low cost

15. Microstructure of ferritic stainless steel

16. Properties of ferritic stainless steel
Soft
Magnetic
Good machinability
Good strength
Moderate ductility and formabality
Excellent corrosion resistance

17. Application of ferritic stainless steel
decorative and architectural parts, automotive trims and
exhausting systems, computer floppy disc hubs, hot water tanks.
Exhaust

19. Austenitic stainless steel
main alloying elements are chromium (16-25 %), nickel
(8-25%) and manganese (4-10%)
Containing Cr + Ni series ; Cr+Ni+Mn series
composed of austenitic (fcc) phase
non heat treatable alloy, strengthening can be accomplished
by cold work or by solid-solution

20. Microstructure of austenitic stainless steel

21. Austenitic stainless steel
If austenitic stainless steel is heated at temperatures 500 – 800 °C, chromium
carbides form along the austenite grains boundaries.
This causes depletion of chromium from the grains resulting in decreasing the
corrosion protective passive film.
This effect is called sensitization. It is particularly important in welding of
austenitic stainless steels to use low heat input and restrict the maximum
interpass temperature to around 175 °C.
Small quantities of either titanium or niobium added to stabilise the material
will inhibit the formation of chromium carbides

22. Properties of austenitic stainless steel
non magnetic
high weldability
high ductility
high toughness
high formability
high corrosion resistance
high creep resistance
resistant to oxidizing acids, alkalis

23. Application of austenitic stainless steel
screws, bolts and implants, low temperature applications,
vessels and pipes, pharmaceutical and food industries, kitchen
utensils, chemical equipment, food equipment, kitchen sinks,
medical devices, heat exchangers, parts of furnaces and ovens.
Screw
Kitchen sink
Pipes

26. Martensitic stainless steel
12 – 18 % Cr and low nickel content (2 – 4 % Ni)
Have austenitic structure (FCC) at high temperature,
which transforms to martensitic structure (BCT) as a
result of quenching
Heat treatable

27. Microstructure of martensitic stainless steel

28. Properties of martensitic stainless steel
Poor weldability
Poor ductility
Moderate corrosion resistance

29. Martensitic stainless steel
scalpels, knives, hooks and tweezers in medical applications,
drive systems and high performance parts for airplanes, turbine
blades, knife blades, surgical instruments, shafts, pins, springs.
knives

31. Austenitic-ferritic (Duplex) stainless steels
low carbon
higher chromiun (21-24%), lower nickel (4-6%)
and 2-3% molybdenum
mixture: Ferrite {bcc} + Austenite {fcc}
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

32. Microstructure of duplex stainless steel

33. Properties of duplex stainless steel
fatigue resistance in corrosive medium
good resistance against stress corrosion.
higher tensile and yield strength than austenitic and ferritic
stainless steels
good weldability and formability.
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

34. Application of duplex stainless steels
equipment for chemical, environmental and offshore industries,
architecture, marine equipment, petrochemical plants, heat
exchangers
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

36. Precipitation hardening (PH) stainless steel
The precipitation-hardening stainless steels are iron-nickel-chromium
alloys
Containing one or more precipitation hardening elements such as
aluminum, titanium, copper, niobium, and molybdenum.
The precipitation hardening is achieved by aging either austenitic or
martensitic stainless steel
The precipitation hardening process involves the formation fine
intermetallic phases such as Ni3Al, Ni3Ti, Ni3(Al,Ti), NiAl, Ni3Nb, Ni3Cu
and carbides phases.
Prolonged aging causes the coarsening of these intermetallic phases,
which in turn causes the decline in strength, due to the fact that
dislocations can bypass coarse intermetallic phases.
Types of PH stainless steel :
* Martensitic Precipitation-Hardening Stainless Steel
* Austenitic Precipitation-Hardening Stainless Steel
* Semiaustenitic Precipitation-Hardening Stainless Steel

37. Properties of PH stainless steel
Magnetic
High strength
High toughness
Good weldability
High corrosion resistance.

38. Application of PH stainless steel
pump shafts and valves, turbine blades, paper
industry equipment, aerospace equipment