Which Hot Cracking Index is preferred?

Date 09-09-2009 12:51

The only way to evaluate accurately these types of indices is to go back to the original research work and determine what the representative samples were that generated the data in the first place. Barring that it is just opinion that may or may not be based upon an empirical assessment.
Its the same exact problem as with ferrite evaluations, carbon equivalents, Cr/Ni equivalence, pitting resistance equivelents, etc.

By OBEWAN

Date 09-09-2009 13:22

Thanks. I was afraid of that. I think I will go with equation 2 for UCS then since the original research was on ferritic stainless and that is what the focus of our evaluation is on. We are not even sure we have a hot crack issue, but we have to go to every node in the customers fault tree to cross off non-issues. It looks more like a stress corrosion crack at this point, but some mill runs have good welding results and other mill runs have bad welding results and they are all within AMS spec limits!

By Stephan (***)

Date 09-09-2009 13:45

OBEWAN,

great topic! By the way, the 'Avoiding Cracks with 312 Stainless' post either!

Please forgive me my ignorance, but I have never occupied myself with these indices before.

Hence, just for general interest.

Doesn't the material analysis matter when using these Hot Cracking Indices?

In other words. Is it feasible to indicate both e.g. Ni-Base Alloys and/or 'conventional' stainless grades with one and the same calculation?

It's just that I have remembered the 'Shaeffler-Diagram' or later on the 'DeLong-Diagram'. Even though certainly not comparable to your issue, it was not suitable to accurately predict the microstructure constituents when e.g. using higher Mn containing steel grades.

Like I say. Just for my personal interest.

Thanks!
Stephan

By js55

Date 09-09-2009 14:09

Stephen,
I would think that only an empirical investigation could determine how broad an application an index would have. The fact that Ni based alloys are invariably fully austenitic (not the case with SS) and that a Ni driven grain structure (devoid of Fe or predominantly so) would be different, may very well influence results.
And though I am not familiar with the indices Obewan mentions the very fact of their variance would argue so.

By Stephan (***)

Date 09-09-2009 17:19

Jeff,

"...I would think that only an empirical investigation could determine how broad an application an index would have..."

That's a most valuable hint towards the complexity, which I have supposed this tricky topic might be based on.

You remember? We have discussed a time ago when I had the pleasure to [quote: 'js55' 06-20-08] "...truly rubbing elbows with much of the brilliance of the industry..." by visiting the EPRI Conference in Florida at that time.

By following the really extraterrestric explanations of the world-famous Professors John C. Lippold (from THE Ohio State University) and John N. DuPont from the famous Lehigh University, my head was spinning around and I was not sure subsequently, if all that what I have ever learned or heard before was true.

Those ingenious gentlemen talked about 'Hot Cracking' phenomena - in particular Ductility Dip Cracking - as I had never heard anything similar before.

And honestly spoken, reminding even their lectures or presentations respectively, I felt tempted to ask my humble question to OBEWAN'S topic.

In my humble understanding, OBEWAN has begun to scratch on something very, very interesting.

Not only from the theoretical but also from the practical standpoint.

It would be marvellous to be furthermore updated with this!

Thanks!
Stephan

By OBEWAN

Date 09-09-2009 14:17

I am only comfortable going with the material groups listed in the original research. The AWS text does not mention that except in the titles of the papers listed in the equation footnotes. One is for SMAW on low alloy steel, and the other is for SAW on ferritic stainless.

They are mostly for filler metals, but we are trying to use them for base metals since we make autogenous welds.

It is even more complicated by the fact that we are making dissimilar welds - Inconel 600 to 430SS and have the issues like pickup and dillution to contend with. The best we can do is make assumptions and extrapolations like a 50-50 ratio for chemistry, and even then the output is not a direct probability, only a relative scale that might flag a "bad" mill run on chemistry.

By ravi theCobra (**)

Date 09-09-2009 17:12

A LOT has to do with trace elements , both required to be reported by spec and others that are in there -

Look at some of the reported elements Arsenic ,boron , tin , vanadium - and you can see why we get anomalous behavior that

sometimes we don't have any answer for -

By OBEWAN

Date 09-09-2009 19:21

The interesting thing is that the hot cracking equation for the low alloy steels lists vanadium in the denominator, whereas the equation from the ferritic stainless research does not address vanadium.

The AMS spec for 430 SS does not require vanadium. Here is my main concern. Some of the millruns have vanadium, and some do not. It appears at this point that the millruns without vanadium are the ones with a cracking problem. It may just be coincidence though.

I would post up the equations from the text I am referring to, but it might be a copyright problem since AWS owns it and I had to buy the book.

By Stephan (***)

Date 09-09-2009 17:22

OBEWAN,

thanks for the reply!

Please read my response to js55.

I admire you for having the opportunity to get a hand on these mind-blowing tricky little things! :-)

Thanks!
Stephan

By ssbn727 (*****)

Date 09-09-2009 17:27 Edited 09-09-2009 17:48

Hi OBEWAN!

I would read the conclusions on page 327 in this book I posted below because it does give some insight, and definitely validates Jeff's argument. The book is titled: "Hot cracking phenomena in welds" By Thomas Böllinghaus, Horst Herold

http://books.google.com/books?id=OveDQzH5fPwC&pg=PA347&source=gbs_toc_r&cad=8#v=onepage&q=&f=false

Here are a few good articles:

http://nels.nii.ac.jp/els/110003380327.pdf?id=ART0003861463&type=pdf&lang=en&host=cinii&order_no=&ppv_type=0&lang_sw=&no=1252514979&cp=

http://www.journalarchive.jst.go.jp/jnlpdf.php?cdjournal=isijinternational1989&cdvol=42&noissue=7&startpage=708&lang=en&from=jnlabstract

http://www.msm.cam.ac.uk/phase-trans/2001/Ferrite_number.pdf

http://www.msm.cam.ac.uk/phase-trans/abstracts/neural.review.pdf

http://bunter.msm.cam.ac.uk/phase-trans/2002/solidification.stainless.steel.1991.pdf

Here are a whole list of articles from the University of Cambridge in the UK:

http://www.googlesyndicatedsearch.com/u/cammsm?hl=en&domains=msm.cam.ac.uk&ie=ISO-8859-1&q=hot+cracking+in+welding&btnG=Search&sitesearch=msm.cam.ac.uk

Check this out to see if this may guide you in any way:

http://www.msm.cam.ac.uk/map/data/materials/weldhotmat-b.html

http://www.msm.cam.ac.uk/map/data/materials/austenitic.data.html

http://www.msm.cam.ac.uk/map/data/materials/fatnimat-b.html

EDIT: A Note on Ferritic SS:

Ferritic stainless steels comprise approximately half of the 400 series stainless steels. These steels contain from 10.5 to 30 weight percent chromium along with other alloying elements, particularly molybdenum. Ferritic stainless steels are noted for their stress-corrosion cracking (SCC) resistance and good resistance to pitting and crevice corrosion in chloride environments, but have poor toughness, especially in the welded condition.

Ideally, ferritic stainless steels have the body-centered cubic (bcc) crystal structure known as ferrite at all temperatures below their melting temperatures. Many of these alloys are subject to the precipitation of undesirable intermetallic phases when exposed to certain temperature ranges. The higher-chromium alloys can be embrittled by precipitation of the tetragonal sigma phase, which is based on the compound FeCr.

Molybdenum promotes formation of the complex cubic chi phase, which has a nominal composition of Fe36Cr12Mo10. Embrittlement increases with increasing chromium plus molybdenum contents. It is generally agreed that the severe embrittlement which occurs upon long-term exposure is due to the decomposition of the iron-chromium ferrite phase into a mixture of iron-rich alpha and chromium-rich alpha-prime phases. This embrittlement is often called "alpha-prime embrittlement." Additional reactions such as chromium carbide and nitride precipitation may play a significant role in the more rapid, early stage 885 °F embrittlement.

The ferritic stainless steels have higher yield strengths and lower ductilities than austenitic stainless steels. Like carbon steels, and unlike austenitic stainless steels, the ferritic stainless alloys exhibit a transition from ductile-to-brittle behavior as the temperature is reduced, especially in notched impact tests. The ductile-to-brittle transition temperature (DBTT) for the ultrahigh-purity ferritic stainless steels is lower than that for standard ferritic stainless steels. It is typically below room temperature for the ultrahigh-purity ferritic stainless steels. Nickel additions lower the DBTT and there by slightly increase the thicknesses associated with high toughness. Nevertheless, with or without nickel, the ferritic stainless steels would need engineering review for anything other than thin walled applications as they are prone to brittle failure.

Anywho, that's enough out of me for today because I've got things to do people to see and places to go!!! ;) ;) ;) Hope this is helpful! :)

Respectfully,
Henry

By Stephan (***)

Date 09-09-2009 18:04

Henry!

"Hope this is helpful!" --> ???

Sir, you ARE the Information Czar!!! :-):-):-)

All the best to you,
Stephan

By OBEWAN

Date 09-09-2009 18:39 Edited 09-09-2009 18:58

Thanks for the info! Yes, we are looking into the 885 embrittlement thing too and that is one of the questions that our customer will be asking. The weld itself is the main thing that would contribute to embrittlement if that is in fact a problem, but it also looks like we have heat to heat variation in the 430SS that effects "weldability". We have seen intergrainular fractures in welds made on ""bad" but NOT "good" heats of material that were all within spec. The other potential problem to address is corrosion induced cracking due to chrome depletion. We have some of the top experts in the world working on this problem and still do not have a root cause identified - SEM photos the whole works. I think there is a standard ASTM test for sigma phase.