2. High Temperature Atmospheric Reactions
For high temperature applications, materials must be resistant to oxygen.
In this process, a clean metal surface reacts with oxygen in the atmosphere to form an oxide scale on the surface.
Further reaction depends on the properties of the scale, which separates the two reactants.

3. High Temperature Atmospheric Reactions
Steels having the BCC structure tend to form an oxide scale.
Steels (and FCC Aluminum) having the FCC structure tend to form pits, if and when they corrode.
How is a scale different than a pit?
How do these types of corrosion affect the performance of the steel?
scale oxide reduces the thickness of the steel
pits form surface cracks, which can reach a critical crack size
(recall your fracture mechanics!)

4. High Temperature Atmospheric Reactions
Why might there be this difference in oxide morphology that depends on crystal structure?
Dislocations! Dislocations! Dislocations!
Dislocations form grain boundaries as a network and act as pipes for diffusion, which is required for corrosion.
Dislocations in BCC materials, having Burgers vectors, B = <111>{101}, do not react with each other to form extended networks so they remain localized at the surface creating a scale.
Dislocations in FCC materials, having Burgers vectors, B = <110>{111}, react with each other to form extended networks that can extend deep into the material from the surface enabling the formation of pits. (see next slide)

5. Dislocation-Dislocation Interactions
Dislocations in a FCC material where the dislocation having B = <110> can interact to form another dislocation of a similar Burgers vector of equal energy.
Edis ~ B2
Schematic of two dislocations interacting to form a third dislocation.

6. Aside: Work Hardening of Steels
How does work hardening depend on crystal structure?
Dislocations! Dislocations! Dislocations!
Just like alloying additions can strongly influence the corrosion properties of steel, so does alloying additions affect the work hardening properties of steel.
Dislocations in FCC materials, having Burgers vectors, B = <110>{111}, react with each other to form extended networks so they work harden naturally.
Dislocations in BCC materials, having Burgers vectors, B = <111>{101}, do not react with each other to form extended networks so, theoretically, they don’t work harden naturally but they do. Why?

7. Aside: Work Hardening of Steels
Work hardening not only depends on the steel’s crystal structure but also its alloying additions – mostly C.
Plastic deformation, which produces work hardening, increases the dislocation density, which for BCC materials, enables more dislocations to be pinned by C – Cottrell pinning.
Thus, BCC steels also work harden but not solely by the formation of dislocation networks.

8. Atomic Scale Oxidation Reactions
For irons alloys, in the FeO scale there is a defect structure containing Fe2+ ion vacancies, which permit the rapid outward diffusion of Fe2+ ions.
Electrons can also diffuse through the FeO structure by hopping from one Fe2+ ion to another Fe3+ ion, which makes FeO an electrical conductor. This enables steel to rust extremely rapidly in some corrosive environments.
Plain carbon steels thus have poor resistance to oxidation at high temperatures.
Dislocations and grain boundaries reduce the activation energy for corrosion through the oxide layer and iron metal.

9. Oxidation of Iron Carbide in Steels
Plain carbon steels are composed of a–iron (ferrite) and pearlite (laminar structure of ferrite + cementite).
In addition to the oxidation of Fe to form the FeO scale, the carbon in cementite (Fe3C) oxidizes to form CO, which evolves or evaporates as a gas.
Since carbon diffuses more rapidly in the steel than Fe, a carbide depleted region forms near the surface, which creates a decarburized zone.
The decarburized zone has a lower hardness than the interior of the sample.
Underneath the oxide scale, the steel is soft.
Surface
Ferrite + pearlite
Ferrite only

10. Protective Oxide Coatings
The following oxides have tightly bonded ceramic structures and form strongly adherent and non-porous films on various metal substrates.
Al2O3 – Alumina
TiO2 – Titania
ZrO2 – Zirconia
Cr2O3 – Chromia
Alumina forms a protective film on Al alloys at room temperature, which protects them from further oxidation and from aqueous corrosion.
However the use of Al alloys at high temperatures is ruled out by its relative low melting temperature of 660 oC. Al alloys are not used above 400 oC.

11. Protective Oxide Coatings (cont’d)
Titania and Zirconia protect their respective metals from corrosion at room temperature and also provide oxidation resistance at temperatures up to about 600 oC.
Titania and Zirconia coatings are also very effective for protecting ferrous alloys and superalloys at temperatures above 1000 oC.
While Chromia effectively protects metallic Chromium from oxidation, however, due to the high cost of Cr metal, it is mainly used as chromium plating and not as the bulk material.
Chromia is mostly used for protecting stainless steels from both room temperature corrosion and high temperature oxidation.

12. Stainless Steels
The essential property of stainless steels is their resistance to corrosion, especially in saline solutions, under oxidizing conditions.
These properties are the result of a thin adhesive film of Cr2O3, which forms on the surface at room temperature and which is self healing when scratched or otherwise damaged.
In general a minimum concentration of 12% Cr is required to obtain a film that completely covers the exposed surface of a sample, so we’ll start this study with an examination of the composition of the alloys and then the Fe-Cr phase diagram.
The Cr2O3 in the steel is very stable against attack by a number of chemicals and electrolytic corrosion actions.

13. “Stainless Steels”
In general, there are four types of stainless steels based on their crystal structure and strengthening mechanisms. They are:
Ferritic stainless steels
Martensitic stainless steels
Austenitic stainless steels
Precipitation-hardened stainless steels
We shall see some examples in the Tables of the next few slides.

14. Note high Ni concentration
L – refers to low carbon, which offers more protection against corrosion. – Why?

15. Note low Ni concentration
A,B,C – refers to carbon content with C representing high carbon, which enhances hardenability giving a higher strength.
Martensitic stainless steals are special so do not have a high production.

16. “PH” stands for “precipitation hardened”. *
Note 
“PH” stands for “precipitation hardened”.
Cb is an old symbol for Niobium, which used to be called Columbium.
* - contains delta ferrite
(What is the difference between delta ferrite and alpha ferrite?)

17. Fe-Cr Phase Diagram Cr is a ferritic stabilizer.
The austenite phase is thus condensed into a small “ g-loop”, which extends out to 16% Cr over the range of temperature 900 – 1400 oC.

18. Fe-Cr Phase Diagram
At concentrations greater than 16% Cr, the a–Fe and d-Fe phases are not distinguishable and a common a–phase extends all the way to 100% Cr.
The 50/50 composition orders at temperatures below ~900 oC to form the s –phase, which causes embrittlement in stainless steels.
What does ‘orders’ mean? Is s stochiometric or non-stochiometric?

19. Carbide Phases in Stainless Steels
There are three Fe-Cr carbides phases formed in slowly cooled stainless steels that are shown below as a function of carbon and chromium content.
Up to 15%, Cr can enter cementite without changing its structure, to form (FeCr)3C, which is the carbide present in low alloy steels.

20. Carbide Phases in Stainless Steels
The next carbide is (FeCr)7C3, which contains a minimum of 35% Cr. This is the carbide formed in high-carbon high-chromium tool steels.
(FeCr)4C, which contains > 70% Cr, is the carbide normally found in stainless steels.

21. (Fe-18%Cr-8%Ni)-C Phase Diagram
Nickel stabilizes the austenite, g–phase in stainles steels (SS).
When 8% Ni is added to an 18% Cr steel – 18/8 SS – the g-phase is stable down to room temperature at very low C – the three phase (a + g + carbide) eutectoid region is at lower temperatures.
The high temperature
d-ferrite is also very restricted.
(a + g + carbide) eutectoid

22. The 300 Series of Austenitic Stainless Steels
These alloys are based on a minimum of 18% Cr – 8% Ni with a maximum of 0.15C.
Minimum grades are least expensive for bulk applications such as kitchen sinks.
20% Cr – 10% Ni have better properties for higher specifications such as very low carbon grade (L), eg., < 0.03% C is prevents the formation of (CrFe)4C at grain boundaries, which depletes the Cr below 12% in the bulk.

23. The 300 Series of Austenitic Stainless Steels
Addition of 2-3% Mo enhances corrosion protection in neutral salt solutions. As well, very low carbon grade < 0.03% C is required for welded components
Addition of Ti (5xC) or Nb (10 x C), enables carbon to be increased to 0.08% for welded products by forming TiC or NbC instead of (FeCr)4C.
Microstructures are shown next slide.

24. The 300 Series of Austenitic Stainless Steels
Microstructures of 302 Stainless Steel containing 18Cr – 8Ni – 0.11C
Quenched from 985 oC
Austenite + annealing twins
(boundaries are lines) plus undissolved carbides
(mag – 1000x)
Quenched from 1205 oC
Course Austenite + annealing
twins and no undissolved carbides
(mag – 1000x)

25. Carbide Precipitation at Grain Boundaries
The precipitation of (CrFe)4C, which contains 70 % Cr, at grain boundaries causes the concentration of Cr in the adjacent austenite to fall below 12%, which degrades the corrosion resistance properties of the steel.
The optimum temperature for precipitation of (CrFe)4C is around 650 oC, which is attained in the heat affected zone adjacent to a fusion weld.
Stainless steels with carbon as low as 0.15% can thus suffer “weld decay”.
It can be eliminated by 1) lowering carbon to 0.03%, or 2) use Ti or Nb to remove the carbon as TiC or NbC, without lowering the Cr content of the austenite. (see micrographs next page).

26. Precipitation of (CrFe)4C at Grain Boundaries
Quenched from 1150 oC
Reheated 24 h at 650 oC
Carbides at grain boundaries
(high mag – 1000x)
Quenched from 1150 oC
Reheated 24 h at 650 oC
Carbides at grain boudaries
(low mag – 240x)

27. Precipitation of (CrFe)4C at Grain Boundaries
The concentration profile of Cr in the matrix adjacent to a precipitate of (CrFe)4C is given below.
The Cr level falls from 18% to 7-8%, which is well below the 12% limit for effective corrosion protection.

28. Oxidation Resistant Stainless Steels
In order to maintain stability of the austenite phase the Cr was increased to 22 – 26% Cr with Ni of 12 – 22%. The addition of Ni gives increased resistance to oxidation at high temperatures.
These steels are very expensive and only used for special applications.
(see micrograph next page)

29. Oxidation Resistant Stainless Steels
Micrograph of a welded joint in 20Cr – 12Ni Stainless Steel, x50
The structure of the original metal is shown on the left.
The fine-grained dark structure on the right is the weld material (filler).
In the centre where the metal has been heated close to its melting point the structure is largely austenitic with some darker alloyed ferrite.
In the heat affected zone, the austenite shows pronounced grain growth and is thus weaker than the original fine grained structure.

30. Pseudo-binary (Fe – 12%Cr)-C Phase Diagram
Carbon is soluble in Fe-Cr austenite and increases the Cr limit of the g–loop.
Hardenable cutlery steels, which contain the minimum 12% Cr, are described in terms of a pseudo-binary (Fe + 12%Cr)-C phase diagram.
The g-field is severely constricted compared to the Fe-C diagram.
The maximum solubility of C is 0.7% and the eutectoid is at 0.35% C.
In addition, the eutectoid temperature (range) is raised to >800 oC.
Two forms of carbide are in equilibrium with the
g–phase, ie., the (CrFe)4C and (CrFe)7C3, depending on the carbon content.
eutectoid temperature (range)
Note 12%Cr

31. The 400 Series of Heat Treatable Stainless Steels (Martensitic Stainless Steels)
These steels are based on Martensite, 12-16% Cr with various amounts of Carbon.
Low carbon grades containing up to 0.2 C containing up to 12-13% Cr are hardenable by air quenching to form a low-carbon martensite (lath type) and are used for cutlery.
High carbon grades contain C and Cr form much harder high-carbon martensite (lenticular type) on quenching and are used for surgical instruments.
Air cooled from 955 oC. Low carbon martensite x1000

32. The 400 Series of Heat Treatable Stainless Steels (Ferritic Stainless Steels)
Low carbon grades with up to 0.2 C and 14-18% Cr are ferritic and can only be hardened by 1) cold work or 2) precipitation of carbide.
Recall Cr is a ferrite stabilizer.
Air cooled from 790 oC ferrite plus carbide x1000

33. Precipitation Hardened Ferritic Stainless Steels
Ferritic stainless steels with ~17% Cr have very low carbon of 0.04 – 0.07 C, which give good corrosion resistance and high strength.
The 17 – 4 PH* (with 4% Ni) alloy is transformed to low carbon martensite (lath martensite) on cooling from austenite and is hardened by ageing at 480 oC due to the precipitation of Al-Ti and a Nb-Cu compound. (cont’d)
* - PH stands for “precipitation hardened”.

34. Precipitation Hardened Ferritic Stainless Steels
The 17-7PH ( with 7% Ni) alloy is semi-austenitic and requires a more complicated series of treatments to produce a precipitation-hardened martensite.
5% - 20% d-ferrite is present after this steel is quenched from the solution annealing temperature of 1065 oC as Al is a strong ferrite former.
It is easily worked in this condition but it rapidly “work hardens”* because of its low Ni content.
It is also hardened by ageing at 565 oC when an Al-based compound is precipitated.
An ageing treatment at 510 oC gives a higher strength at the expense of lower ductility.
* - Recall the concept of work hardening in bcc steels by dislocation pinning by carbon.

35. Mechanical Properties of Wrought Stainless Steels
Work hardened
Work hardened
austenitized +
quenched
Same as 50/50 M/P in carbon eutectoid steels

36. General Thermomechanical Treatments for Steels
These treatments increase the strength of a steel by deformation before, during, or after the austenite transformation to martensite or to a ferrite + carbide aggregate.
We will discuss each of these 8 processes in turn.

37. Thermomechanical Treatments for Steels
Deformation “before” Austenite transformation
Type 1a refers to normal hot working to save energy and to permit relief of deformation stresses by recovery and recrystallization processes.
Do you recall recovery and recrystallization?

38. Thermomechanical Treatments for Steels
Deformation “before” Austenite transformation
Type 1b, ie., deformation before transformation to martensite, called ausforming, can only be applied if the composition of the steel results in a bay between the pearlite and bainite reactions, which gives unlimited time for the process.
After deformation, the steel is quenched to room temperature to form martensite.
This treatment gives a greater improvement in both yield and ultimate strengths compared to any other heat treatment while retaining a reasonable ductility.
The results for steel H11 show that a deformation of 94% causes a 30% increase in strength with relatively little loss of ductility.

39. Thermomechanical Treatments for Steels
The 1) austenitization temperature, the 2) grain size and the 3) exact temperature of deformation do not appear to be significant variables for this process.
The carbon content linearly increases the strength and lowers the ductility, as would be expected for a martensitic steel.
It is thought that carbon diffuses to dislocations generated in the deformed austenite and that this causes additional deformation of the martensite resulting in increased hardness without decreased ductility.

40. Thermomechanical Treatments
ausforming
Increase strength with
Increase ductility

41. Thermomechanical Treatments for Steels
Type 1c, ie., deformation before transformation to ferrite-carbides.
This causes a fine austenite grain size by recrystallization before transformation resulting in smaller colonies of pearlite or bainite with a consequent increase in notch toughness.
This process is regularly applied to plate and pipe steels.

42. Thermomechanical Treatments for Steels
Deformation “during” Austenite Transformation
Type 2a, ie., deformation during transformation to martensite.
This is most often applied to austenitic stainless steels and results in increase strengths due both to 1) the transformation to martensite (i.e., phase hardening) and 2) the strain hardening of the austenite that occurs at the lower temperature of working.
Type 2b, ie., deformation during transformation to ferrite + carbide.
This results in improved strength through microstructural refinement.
The deformation increases the number of nuclei for the carbide transformation resulting in a fine dispersion of the carbide phases.

43. Thermomechanical Treatments for Steels
Deformation “after” Austenite Transformation
Types 3a - Deformation of martensite followed by tempering
Type 3b – Deformation of tempered martensite
These treatments induce strengthening by deforming the martensite and thus can only be applied to low carbon steels.
Further strengthening is obtained by precipitation of carbide from the martensite and by the formation of compounds from other alloy additions.
Type 3c Deformation of ferrite-carbide aggregates
This strengthening occurs primarily by dispersion of the carbide phases.
What is the difference between precipitation and dispersion?

44. Ultrahigh-Strength Steels
These steels have strengths in the range Mpa with elongation greater than 7.0% to provide adequate ductility.
These steels contain %C to give a relatively hard martensite without excessive brittleness.
Further strength is obtained by secondary hardening during tempering.

45. Composition of Ultrahigh-Strength Steels
Note:
Ultrahigh-strength steels have been developed for demanding tasks at temperatures near the ambient, i.e., from –100 to +200 oC.

46. Mechanical Properties of Ultrahigh-Strength Steels
Why is the endurance limit given in cycles and not as an endurance strength in MPa or psi?

47. Ultrahigh-Strength Steels
The first six of these steels that contain %C give a relatively hard martensite without excessive brittleness with further strengthening obtained by secondary hardening during tempering.
AISI 4340 is a medium carbon steel alloyed with Ni and Cr so that martensite is formed by oil quenching and toughness is obtained by tempering.
4330V is a steel of similar composition with additional V, which raises the coarsening temperature for grain growth so that the heat treated steel has a finer grain size.

48. Ultrahigh-Strength Steels
AISI H11 is a hot worked tool steel with a low enough carbon content of 0.4% to give acceptable ductility.
The Cr and Mo cause it to air harden and induce secondary hardening during tempering.
This steel requires preheating before welding to reduce cracking due to martensite formation and thermal stresses.

49. Ultrahigh-Strength Steels
DCA is another 0.4% carbon steel with the addition of Cr and Mo to enable oil quenching and induce some secondary hardenng during tempering.
It has lower ultimate strength than H11, but it has a higher yield point and a slightly greater ductility.

50. Ultrahigh-Strength Steels
HP is a 0.3 %C Ni-Co steel with a slightly lower strength, but superior ductility than H11 and DCA in the quenched and tempered condition.
The Co increases the amount of martensite formed at room temperature and also strengthens the tempered martensite by solid solution.

51. Ultrahigh-Strength Steels
The second group of steels, ie., HP 18 Ni, 17-4PH, and 17-7PH, having 0.03 – 0.09%C form low carbon martensite, which are less hard but inherently more tough than the 0.4 %C martensites.
These tempered martensites are hardened either by cold work and/or precipitation hardening.

52. Ultrahigh-Strength Steels
18 Ni steels
Form a mixture of martensite and retained austenite on slow cooling from 815 oC.
The low-carbon, high-nickel martensite can be cold-rolled to 80-90% without cracking and subsequent ageing at 480 oC precipitates a hardening phase based on Co-Ti.
This material is readily weldable and has a high fracture toughness.
It is very suitable for high strength pressure vessels.

53. Ultrahigh-Strength Steels
17-4PH and 17-7PH steels
These steels are high strength precipitation hardened stainless steels that maintain good corrosion resistance as there is no Cr loss from the matrix during the precipitation process

54. The End of Ferrous Alloys
Congratulations!