Does ADI Microstructure contain bainite? 5

[construct2]

Hi all,

I read in some journals that bainite occurs within the second stage of austempering (unwanted), and that the wanted microstructure is austenite and ferrite (ausferrite).
In some other journals it said that adi contains bainite.
I thought it contains bainitic ferrite and austenite, and decomposes to ferrite and carbides when austempered too long.

Does anyone know what the the proper microstructure of austempered ductile iron should be?

Thanks in advance

 

[unclesyd]

ADI is 60% Bainitic Ferrite and retained, high carbon , Austenite. The retained Austenite is workhardenable and will transform to martensite under conditions of suitable stress, like shoot peening. This is one reason its good for gears.

 

[swall]

According to an article in the March 1990 issue of Modern Casting, ADI is not bainitic.The microstructure is described as "acicular ferite and high carbon stable austenite".

 

[TVP]

The following is from an article on the Ductile Iron website (

http://www.ductile.org/didata/Section4/4intro.htm)

by Chip Keough, one of the leading experts on ADI:

The preponderance of information on the austempering of steel and the superficial similarities between the austempering heat treatments applied to steels and ADI, have resulted in comparisons which are incorrect and damaging to the understanding of the structure and properties of ADI. ADI is sometimes referred to as "bainitic Ductile Iron", but correctly heat treated ADI contains little or no bainite. Bainite consists of a matrix of acicular (plate-like) ferrite and carbide. ADI’s ausferrite matrix is a mix of acicular ferrite and carbon stabilized austenite. This ausferrite may resemble bainite metallographically, however it is not because it contains few or none of the fine carbides characteristic in bainite. An ausferrite matrix will only convert to bainite if it is over tempered.

 

[arunmrao]

TVP I can't resist giving you a star for this reply though you have collected so many.

 

[TVP]

arunmrao,

Please don't let the number of stars that I have prevent you from giving me another. My ego loves to be complemented...

 

[construct2]

now it all makes sense. Thanx a lot to all of you. And the stars are being added to the helpers =)
thanx,

construct2

 

[unclesyd]

Hold off a little while until you check these sites.


DI Design Engineers Digest
Down load page 10 and 11

http://www.ductile.org/dimg/downloads.html

http://www.sorelmetal.com/en/ductile/main_family.htm

http://www.ductile.org/index.html

 

[TVP]

Just to add a little complexity, it really depends on how the ADI is processed whether or not carbides actually form. As Mr. Keough notes in the article that I referenced above, "correctly" heat-treated ADI does not contain carbides. What he means is that a carbide-free microstructure that features higher toughness and elongation is developed when ADI is austempered at higher temperatures (closer to 400 C), rather than the carbide-containing microstructure (harder, but more brittle) that develops at low temperatures (closer to 250 C). If anyone is interested in further reading, try the following:

Mat Sci Eng A 346 (2003) 273-286
Mat Sci Eng A 194 (1995) 87-98
Mat Sci Eng A 268 (1999) 15-31
Scripta Mat Vol 38, No. 8, pp. 1281-1287, 1998
Scripta Met Mat Vol 32, No. 9, pp. 1363-1367, 1995