FERRITIC STAINLESS STEEL

Microstructure and Chemical Compositions of Ferritic Stainless Steel

Case study on .By- Gyanendra AwasthiRoll No. 13135043Mechanical Engineering Department

Iron-carbonphase diagram, showing the conditions under whichferrite() is stable.

Ferrite, also known as-ferrite(-Fe) oralpha iron, is amaterials scienceterm for pureiron, with abody-centered cubicB.C.Ccrystal structure. It is this crystalline structure which givessteelandcast irontheirmagneticproperties, and is the classic example of aferromagneticmaterial.

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.

In this steel iron and chromium atoms are arrangedon the corners of a cube and in the center of that cube.

-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-iron, which crystallizes in ahexagonal close-packed(hcp) structure

In pure iron, ferrite is stable below 910C (1,670F). Above this temperature theface-centred cubic form of iron,austenite(gamma-iron) is stable. Above 1,390C (2,530F), up to themelting pointat 1,539C (2,802F), the body-centred cubic crystal structure is again the more stable form ofdelta-ferrite(-Fe).

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

We can see ferrite is in the form of roughness.

Microstructure of roller at un-worked condition (2% nital agent).

Fig: Air cooled from 790 o C ferrite plus carbide x1000The 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.

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

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

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

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

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

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, 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 600C, but are more resistant to thermal shocks thanhigh temperatureaustenitic stainless steels

REFERENCES:-

1- from Wikipedia

2-Article on Microstructural Changes of High-Chromium Ferritic Stainless Steel Subjected to Cyclic Loading in 475C Embrittlement Region Available from: sciencedirect.com

3- Park, S. H., K. Y. Kim, et al.Investigation of Microstructureand 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