Stainless steel and FN - impact on properties?

[robsalv]

Just doing a spot of self education on stainless steel welding and ferrite number for SS equipment in cryogenic service.

On line resources are a bit scant on this topic. There is a bit of information on ferrite number scattered around these forums, but mostly related to hot service.

Would greatly appreciate someone share some knowledge or pointing me towards a resource in regards to how FN impacts on stainess steel properties, particularly in cryogenic service.

Thanks in advance.

Cheers

Rob

 

[unclesyd]

In searching my low temperature literature it doesn't mention Ferrite (FN) in discussing the impact properties of SS at cryogenic temperatures. In welded plate it only discusses the effect of sensitization on the impact values. Only 302 SS is mentioned as being the most affected by sensitization.

My literature is quite old and at the time FN wasn't the analytical tool it is today. I'll scan the very long bibliography and see if there is a name to search on. May take sometime as the print is extremely small for an old man.

 

[Goahead]

Austenitic stainless steels are a class of materials suitable, in certain conditions, for cryogenic service.
Once the type of austenitic stainless steel is selected, if fusion welding is proposed for fabrication, then the filler metal should be chosen so that the weld metal (an alloy of base and filler) contain a certain amount of ferrite in order to avoid weld cracking.
The composition of the weld metal can be predicted by making use of diagrams such as Schaeffler's or WRC-1992, based on structures represented as points in a field of nickel equivalent vs. chromium equivalent.
The ferrite number as measured by certain instruments, is inherently inaccurate due to excessive variation. See on the subject the ASM Handbook, Volume 6, page 461.

http://www.welding-advisers.com/

 

[robsalv]

Thanks UncleSyd and Goahead.

**hands unclesyd a big magnifying glass! lol**

Goahead, I'm looking into getting myself the ASM books on CD.

In the mean time, the info I have found says:

FN 0-3 gives welds sensitive to hot cracking.

FN 3-12 is pretty typical and gives good resistance to hot cracking.

FN >12 can result in continuous ferrite networks, which may make the item prone to corrosion or with high temperature, have the ferrite transform into sigma phase, which has a negative affect on desired properties.

Since it's the austenitic structure that provides cold temperature toughness, surely you wouldn't want a high FN number in cryogenic service?

Thanks.

Rob

 

[metengr]

The following reference book titled "Welding Metallurgy and Weldability of Stainless Steels" by Lippold and Kotecki, Wiley Press, provides some extensive information on the mechanical properties of weldments for austenitic stainless steels. This book also provides an in-depth look at the welding metallurgy/weldability of austenitic, ferritic, martensitic and PH stainless steels.

For example, there is a Table that shows the effect of ferrite number on YS, UTS, elongation and reduction in area at various service temperatures for 308 weld metal. In addition, another Table shows the effect of fracture toughness at -492 deg F for austenitic stainless steel weld and base metals as a function of ferrite number.

For those interested in welding metallurgy, this is a must have reference book.

 

[metengr]

Also forgot to add that duplex ss are discussed, as well.

 

[robsalv]

Metengr, judging by the high 2nd hand value price of the book, it must be well worth the money!

Will it educate me in the area I'm currently pursuing though?

I've now come across a reference to an AWS Nov 1978 article on the topic at hand! It discusses low temp properties versus ferrite content of austenitic stainless steels... now to extract that from AWS...

Cheers

Rob

 

[metengr]

Well, I happened to receive an advance copy of this book as part of a deal to review it for the authors. If you are involved with welding or failure analysis of stainless steel and duplex alloys as part of your job, this reference book is well worth the cost. Yes, it will answer all of your questions pertaining to the influence of ferrite on the mechanical properties of welded SS.

 

[unclesyd]

robsalv,
Here is a good place to get a copy of the paper.

http://www.lindahall.org/