This document discusses the microstructure and chemical compositions of ferritic stainless steel. It begins by defining ferrite as the body-centered cubic crystal structure of pure iron that gives steel and cast iron their magnetic properties. It then discusses how adding nickel changes the crystal structure from body-centered cubic to face-centered cubic. The document also examines the different groups of ferritic stainless steels based on their chromium content, from 10-14% chromium to those with over 18% chromium. It notes that ferritic stainless steels have lower strength at temperatures over 600°C but greater resistance to thermal shocks than austenitic stainless steels.

1. Microstructure and Chemical
Compositions of Ferritic Stainless Steel
Case study on ….
By- Gyanendra Awasthi
Roll No. – 13135043
Mechanical Engineering
Department

2. Iron-carbon phase diagram, showing the conditions under
which ferrite (α) is stable.

3. Ferrite, also known as α-ferrite (α-Fe) or alpha iron, is
a materials science term for pure iron, with a body-centered
cubic B.C.Ccrystal structure. It is this crystalline structure which
gives steel and cast iron their magnetic properties, and is the
classic example of a ferromagnetic material.

4. The structure of ferrititc stainless steel is Body Centered Cubic(
BCC) . By adding nickel to this stainless steel the structure
changes from bcc to Face Centered Cubic ( FCC) , which is
called austenitic.

5. In this steel iron and chromium atoms are arranged on the
corners of a cube and in the center of that cube.
α-ferrite, which is the form of iron that is stable under
standard conditions, can be subjected to pressures up to ca.
15 GPa before transforming into a high-pressure form
termed ε-iron, which crystallizes in a hexagonal close-
packed (hcp) structure
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 of delta-ferrite (δ-Fe).

6. Time-temperature relationships to produce 25 and 100°C
DBTTs for a 29Cr-4Mo ferritic stainless steel as a function of
aging times that cover both the 475°C embrittlement range
and the σ phase embrittlement range

7. We can see ferrite is in the form of roughness.
Microstructure of roller at un-worked condition (2% nital
agent).

8. Fig: Air cooled from 790 o C ferrite plus carbide
x1000
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.

9. TEM analysis of microstructural changes in ferretic stainless
suggest that the increase in hardness by the aging at around
793 K has occurred due to 748 K embrittlement resulting
from the spionidal decomposition.
Fig: Micrograph showing microstructure in
unaged specimen

10. Fig: Micrographs showing microstructures in type 447 at
793K -

11. Chemical Composition
According to chemical composition Ferratic grades are
classified in to five groups- three families of standard grades
and two of “special” grades. . By far the greatest current
use of ferritics, both in terms of tonnage and number of
applications, is centered around the standard grades

12. Various types of ferretic stainless steel according to their
Cr content -

13. Following table presents the chemical composition
of the most relevant ferritic stainless steels

14. Group 1: 10-14% Cr (type 409/410L)
-It has the lowest chromium content with 13% Cr, no Ni and
extra low interstitial elements (C/N) may present a fully ferritic
structure at all temperatures. ferritic 12-14% Cr grades with
sufficient ductility can only be produced by an optimum heat
treatment and a stringent control of chemistry including
interstitial elements (carbon/nitrogen) or in the fully annealed
condition.
Group 2: 14-18 Cr % (type 430)
-
It is the most widely used ferritic stainless steel. Its typical
composition, by weight, is 16-18% Cr, <0.08% C. .
Nitrogen is generally of the order of 0.030%

15. Group 3: 14-18% Cr + stabilization elements (Ti,
Nb, Zr...)
It includes types 430Ti, 439, 441. During solidification and
cooling, Ti, Nb, Zr additions in steels tie up carbon and/or
nitrogen in the form of highly stable compounds. Carbides
and nitrides are precipitated leaving the ferritic structure with
much lower carbon / nitrogen contents in solid-solution. As a
result, the 16-18% Cr stabilized grade often has a fully ferritic
microstructure at all temperatures.
Group 4: 10-18% Cr -
It has 10-18% Cr and Mo content higher than 0.5%
includes types 434, 436, 444, etc. These grades are
molybdenum alloyed.

16. Group 5: (Cr > 18% ) -
It contains most often 25-29% Cr and 3% Mo, these
grades are superior to type 316 with respect to this property.
They are very sensitive to embrittlement due to intermetallic
phase precipitations and are very difficult to weld.
Due to their ferritic structure, the ferritic steels
show lower strength at temperatures exceeding 600°C, but
are more resistant to thermal shocks than high
temperature austenitic stainless steels

17. REFERENCES:-
1- from Wikipedia
2-Article on Microstructural Changes of High-Chromium Ferritic
Stainless Steel Subjected to Cyclic Loading in 475°C
Embrittlement Region Available from: sciencedirect.com
3- Park, S. H., K. Y. Kim, et al.“Investigation of Microstructure
and Texture Evolution in Ferritic Stainless Steels,
ISIJ International Vol.42, No.1 (2002): 100.
4-From google
5-Materials Science and Engineering by M Kato, E Werner