1. What are stainless steels?
Alloys of iron, chromium and other elements that resist corrosion in many environments
A minimum of ~12% Cr is required for it to be a stainless steel

2. Classification of stainless steels
Ferritic stainless steels
Martensitic stainless steels
Austenitic stainless steels
Precipitation-hardening stainless steels
Duplex stainless steels

3. Fe-Cr Phase Diagram

4. Iron nickel phase diagram

5. Ferritic stainless steels
Have a BCC ferrite microstructure
Low carbon contents (<0.2%)
Cr in the range 12-20%
They remain ferritic all the way up to melting point
Poor weldability
Extra low interstitial grades developed with improved weldability

6. Some standard grades of ferritic stainless steels
AISI grade C Mn Si Cr Mo P S
Comments/Applications
405
0.08
1.0 -
0.04
0.03 Al
409
0.045
(6xC) Ti min
429
0.12
430
446
0.20
1.5
0.25 N
Ferritic stainless steels: contain typically more chromium and/or less carbon than the martensitic grades. Both changes act towards stabilisation of ferrite against austenite so that ferrite is stable at all temperatures. Therefore, ferritic stainless steels cannot be hardened by heat-treatments as is the case of martensitic ones. They exhibit lower strength but higher ductility/toughness. Typical application may include appliances, automotive and architectural trim (i.e. decorative purposes), as the cheapest stainless steels are found in this family (aisi 409).
Some standard grades of ferritic stainless steels

7. Austenitic stainless steels
Three major alloying elements – Fe, Cr and Ni
Carbon as low as commercially feasible to obtain
Cr 16-26% and Ni >8%
Ni is to promote a completely austenitic structure
At room temperature they have a completely austenitic structure

8. Standard grades of austenitic stainless steels
AISI grade
C max.
Si max.
Mn max. Cr Ni Mo Ti Nb
301
0.15
1.00
2.00
16-18
6-8
302
17-19
8-10
304
0.08
8-10.5
310
0.25
1.50
24-26
19-22
316
10-14
321
9-12
5 x %C min.
347
9-13
10 x %C min.
Standard grades of austenitic stainless steels

9. Grain structure of austenitic type 316L, solution annealed at 954 °C and etched with
waterless Kalling's
Microstructure of annealed 26Cr-1Mo E-Brite ferritic stainless steel, revealed using aqueous 60% HNO3 at 1.2 V dc for 120 s

10. Fe-Cr Phase Diagram

11. Without carbon, the limit beyond which austenite no longer forms is about 13 wt%. However, additions of carbon stabilise the austenite and therefore increase this limit.

12. Martensitic stainless steels
Contain 12-18%Cr and %C
Amenable to strengthening by heat treatment, unlike the ferritic SSs.
The heat treatment comprises of quenching from austenitic phase field to produce martensite and tempering the martensite.

13. Standard grades of martensitic stainless steels
AISI grade C Mn Si Cr Ni Mo P S
Comments/Applications
410
0.15
1.0
0.5 -
0.04
0.03
The basic composition. Used for
cutlery, steam and gas turbine blades
and buckets, bushings..
416
1.2
0.60
Addition of sulphur for machinability,
used for screws, gears etc. 416 Se
replaces suplhur by selenium.
420
Dental and surgical instruments,
cutlery.
431
0.20
Enhanced corrosion resistance,
high strength.
440A
0.75
Ball bearings and races, gage blocks,
molds and dies, cutlery,
440B
As 440A, higher hardness
440C
As 440B, higher hardness
Standard grades of martensitic stainless steels

14. Super 12%Cr Martensitic Stainless Steels
In additions to the standard grades, a large number of alloyed martensitic stainless steels have been developed for moderately high temperature applications.
Most common additions include Mo, V and Nb.
These lead to a complex precipitation sequence.
The 12Cr-Mo-V-Nb steels are used in the power generation industry, for steam turbine blades operating at temperatures around 600 C.

15. Precipitatioin hardening (PH) stainless steels
Martensitic type is the most common among PH steels
Carbon level low, <0.1%
13Cr8Ni,15Cr5Ni, 17Cr4Ni are some important grades
Precipitation hardening accomplished by adding elements like Ti, Al or Cu in small quantities

16. Important grades of martensitic PH Stainless steels
Carbon
Chromium
Nickel
Other elements
13-8
0.05
Mo, Al
15-5
0.07
Cu, Cb
17-4
3-5
Cb, 3-5Cu

17. Duplex stainless steels
Contain very low carbon content (<0.03%), 20-30%Cr, ~5%Ni
Ferrite and austenite phases are present in the range 40% to 60%
Yield strength twice that of all austenitic counterpart.
Improved resistance to stress corrosion cracking and improved weldability
Duplex stainless steels: duplex stainless steel typically contain 50% austenite and 50% ferrite. This confers them properties intermediate between the two types of steels:
a typical strength about twice that of austenitic grades, but lower than martensitic grades,
a better toughness than ferritic stainless steels, but lower than austenitics,
because of the high chromium content of the standard grades, the corrosion resistance is superior to that of the standard 304 and 316.
Only one duplex steel has an AISI designation (329) so that ASTM numbers are more currently used to reffer to different grades. The archetypal stainless steel, type 2205, contains 22-23Cr, Ni and 3-3.5Mo. This grade represents 80% of all duplex stainless steel use.
Duplex stainless steels suffer from the 475 C embrittlement described earlier for ferritic stainless steels and are therefore mostly confined to applications below 300 C.

18. Delta-ferrite in the martensitic matrix of solution-annealed and aged 17-4 PH stainless steel,
revealed using superpicral.
Duplex stainless steel IC381 (dark phase is ferrite).

19. Mechanism of sensitization in austenitic stainless steels.
Grain boundary carbides in sensitized 316 stainless steel

20. Various solutions can be implemented to avoid sensitisation:
The first one is obviously to reduce the carbon content of the material so as to limit the precipitation of M23C6. This approach defines the AISI L grades, such as 304L and 316L, which have lower carbon content than their standard counterparts. For both these steels, the maximum acceptable carbon content is reduced to 0.03 wt% (from 0.08 for the corresponding standard grades).
Another `similar' solution consists in introducing carbide formers which have an even greater affinity for carbon than chromium. These include Nb, Ti, V or Ta. Steels containing these elements (or a combination) are said to be stabilised (with regard to grain boundary precipitation of M23C6). Grades 321 (Ti stabilised) and 347 (Nb stabilised) represent the most common stabilised austenitic stainless steels. In welding applications, grade 321 is not used as a filler metal because titanium does not transfer well accross a high temperature arc. 347 is therefore used as a filler metal when joining components made out of 321 or 347 (the latter being seldom used as parent material). To obtain stabilisation, it is not sufficient to add Nb or Ti. A stabilisation heat-treatment must be performed to ensure formation of TiC or NbC. This is usually performed by maintaining the steel for 1 or more hours at temperatures around 900 C. At lower temperatures, M23C6 may form faster than TiC or NbC.
Effect of sensitization on corrosion of type 304 steel in inhibited boiling 10% H2SO4.

21. Precipitation Hardening Stainless Steels – Heat Treatment
Solution treatment at ~1050 oC
Aging in the temperature range 480 to 620 oC

22. Mechanical properties of 17-4PH SS as influenced by heat treatment
0.2%YS
(MPa)
UTS
%Elongation
%Reduction in area
Charpy impact value
Aged at 480C
1262
1365 15 52 21
Aged at 550C
1117
1158 16 58 54
Aged at 580C
1020
1131 17 59 61
Aged at 620C
869
993 20 60 75
Heat at 760C+aged at 620C
600
848 22 66
136

23. Optimum Heat Treatment to combat stress corrosion cracking
The PH SS 17-4 can be aged in the temperature range C
If there are potential risks of stress corrosion cracking (SCC), a higher aging temperature (550C) is to be used.
To achieve an acceptable level of sulfide stress cracking, a relatively high aging temperature is to be chosen
SCC failures were encountered in service, when lower temperatures were used for aging.

24. Optimum Heat Treatment to realize high degree of machinability
The PH SS 17-4 can be aged in the temperature range C
To realize the best machinability, special heat treatment is adopted – heating at 760C + aging at 620C
A final heat treatment is given if high strength is important for the intended application.