corrosion resisitance of Inconel and some related queries 6

[sayee1]

The theories of Cr oxides giving the stainless steel is ok. Is the corrosion resistance of inconels also due to the same reasons? Do these alloys also have a thin Cr oxide layer giving the corrosion resistance? And if that is the case why do we not see any sensitization problems with Inconel welds?
Inconel is fully austenitic as I beleive, any guidance on the Microstructure?
If Inconel is fully austenitic, is the role of ferrite not importanat as in case of stainless steels(normally specify 5~15 FN for welds made with aust ss.
Very basic things to be known for a metallurgist and I am unhappy that a metallurgist is posing these queries.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!

 

[Metalguy]

Sayeeprasadr,

Don't feel bad-no one can be familiar with details of all metals. But having worked for years on "Alloy 600's" problems in nuke steam generator tubing applications, I can shed some light on what we have learned-slowly.

Some of this is very basic. When we lose Cr along the GB because of carbide precip., we end up with tiny anodic areas of mostly Fe, vs. an 18% Cr alloy. Fairly large voltage. But when carbides precip. on the GB of Alloy 600 (in the nuke industry we don't mention the "I" word), we have a mostly Ni vs. Ni/Cr alloy--low voltage.

BTW, it is possible thru HT to make the carbides form either intra or inter granul. Since we don't have a big corr. concern, we take advantage of the mech. "keying" of GB carbides to resist IGSCC.

More later-have to run.

 

[ijzer]

If you plot the chemical composition of Incoloys/Inconels into the Schaeffler diagram , you will see that it is not possible to have a ferritic phase. The 5-15 ferrite in SS welds is adviced to avoid hot cracking. Since Inconel/Incoloy are fully austenitic they are more prone to hot cracking than SS. For that reason the impurity level shall be very low (to avoid low melting phase) and the heat input / interpass temperature during welding shall be low to avoid long cooling times.

 

[sayee1]

Metalguy/Ijzer
That reconfirms what I thought I knew about Inconels, Metalguy, remember seeing a post by you on similar lines in some other query. A star to each of you.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!

 

[TVP]

Sayee,

I'm not sure by reading your post if you understand that Inconel (which is a tradename for a range of alloys from Special Metals) alloys are nickel based, not iron based. Nickel alloys are generally split into two categories-- solid solution alloys and superalloys. Solid solution alloys are primarily strengthened by cold working, whereas superalloys contain multiple phases due to precipitation heat treating. You can obtain a great deal of information on properties, metallurgy, etc. by visiting the websites of the producers of these alloys:

Special Metals (Inconel 625, etc.)

http://www.specialmetals.com/products/index.html

Carpenter (Pyromet 625, etc.)

http://www.cartech.com

Haynes (Haynes 625 alloy, etc.)

http://www.haynesintl.com/625alloy/625.htm

Allvac (Nickelvac 625, etc.)

http://www.allvac.com/allvac/pages/Nickel/Default.htm

Allegheny Ludlum (AL 625 HP, etc.)

http://www.alleghenyludlum.com/Ludlum/pages/products/xq/asp/P.104/qx/product.html#

click on Technical Data Bluesheet

ThyssenKrupp VDM (Nicrofer 6020 hMo - alloy 625, etc.)

http://www.kruppvdm.com/Info_Dienst_pdf_01_eng.html

 

[sayee1]

TVP
I do understand that inconels are Ni Based alloys and that is why my post as to whether the corrosion resistance mechanism in Inconel is the same as in stainless steels? i.e. by formation of a Cr oxide layer or is it that the entire alloy itself is corrosion resistant!


Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!

 

[TVP]

Cr oxide is not responsible for the corrosion protection of nickel alloys. Nickel forms a stable, passive oxide film under most conditions. Some of the more highly alloyed grades used for very high temperature applications contain Y & Al, which promote the formation of oxide scales (Y₂O₃ and Al₂O₃) that confer high temperature oxidation resistance.

 

[rolledalloy]

The chromium in the inconels is important for its corrosion resistance, especially at high temperatures. Whereas yttrium, aluminum, and other additions enhance the scaling resistance by promoting thinner more adherent protective oxide layers to form.

Regarding welding, the post discussing weld solidification cracking is correct. These alloys are fully austenitic and do not form ferrite in their welds. As a result, they are more prone to cracking. Restricting the levels of residual elements such as phosphorus and sulfur helps to reduce the likelihood of cracking. In addition, many weld fillers add high levels of manganese or columbium to reduce the tendency to crack. As an example, the filler used commonly on Inconel 600 is Inconel 82. This weld filler contains 3% Mn and 2.7% Cb. Inconel 600 on the other hand has 1% Mn max and no Cb added.