UPPER BAINITE - LOWER BAINITE 5

[ibf]

What is the most appropriate method for differentiating upper bainite from lower bainite ? Is light microscopy able to do it via special etching such as SMB (sodium metabisulfite) and / or picral ? Is the use of polarizing filters and sensitive-tint plate going to help ? Material to be checked is 4340 liquid quenched and tempered.

Thank you.

 

[NickE]

IIRC - (if i recall correctly) -

lower and upper bainite are distinguishable by the size of the structure. Use a good brightfield scope, a 550nm (green) filter might help bring the contrast out, Nital etch.

nick

 

[ibf]

THANK YOU VERY MUCH NickE.

 

[NickE]

Oh and to add to the above:

I also think that the lower bainite will be the finer structure.

nick

 

[NickE]

And for a PPS:

Picral is also a reccomended etch from Metals handbook 9th edition.
nick

 

[GRoberts]

George Linnert in "Welding metallurgy Vol 2" says "Studies of the kinetic features of the bainite transformation have indicated that upper and lower bainite can be differentiated metallographically. Lower bainite is characterized by precipitation of carbides at about a 60 degree angle to the major axis of the bainitic-ferrite plate, whereas upper bainite may be identified by carbide precipitation parallel to this plate axis."

 

[unclesyd]

Check this site out and look under defects. Once there they have a java measuring program for doing a little measuring. The have an annealed structure with 10% Banite, a lot.

http://www.metallograf.de/start-eng.htm

 

[metengr]

unclesyd;
Interesting web site - I better brush up on my German.

 

[unclesyd]

metengr,
The webmaster appears to translating more everytime I've visited the site.
They respond in English so there is hope.

unclesyd

 

[Salvatio]

One best way to differentiate between upper bainite and lower bainite is to know your specimen’s history and background. Another way would be using electron microscope, say to 15000x will do. You can check out those photos taken from the internet. Usually upper bainite will show bainite plates which composed of a volume of ferrite with carbide embedded in it. Basically upper bainite would form at a temp around 400-500 deg C, where you may observe dark patches of ferrite and lighter areas in these darker patches are carbides.

As for lower bainite, it would be more regular needlelike shape. The temp for lower bainite to form is around 200-300 deg C.