To esp Jeff & Pradip
I hereby express my sincere gratitude to both of you for the interest you have shown to my query.
I will go through the attachement and the answers sent by you and revert back to you if found any query.
One more question in the discussion rows is whether the Austenitic Stainless Steels are magnetic or nonmagnetic in liquid state?? If you have any article or doc regarding the topic please post it. It will be helpful for me as well as others.
Hope for your reply.
Regards
Jignesh
On Fri, Jul 23, 2010 at 10:11 PM, Jeff <jwsowards@gmail.com> wrote:
Jignesh,
The WiW paper Pradip posted should answer a lot of your questions - or
at least give you some additional references to find more answers.
Regarding your first question, you are on the right track I think.
Consideration of the APF is a simplified way to look at diffusivity.
The BCC lattice has more 'empty room' than the FCC lattice resulting
in BCC 'impurity' diffusion coefficients that are often an order of
magnitude (or more) greater than coefficients in FCC. The ferrite
phase also has a much larger solubility of P+S and does austenite.
This results in a greater "back-diffusion" of impurities during
solidification under the FA and F modes compared to the A or AF
solidification modes. That is to say, the ferrite rejects a smaller
concentration of impurities into the liquid than does the austenite.
Therefore when two solidifying grains impinge on one another at the
back of the weld pool, there will be lower amounts of impurities
(typically P+S) if the grains are ferrite than if they were
austenite. This reduces formation of low melting point eutectics
(typically FeS). It is these low melting point liquids that widen the
freezing temperature range and increase solidification cracking.
Your second question about film wetting has to do with the surface
tension of the eutectic liquids and the shape of the interface between
the liquid and solid phases. The worst case would be that you have a
straight smooth interface where two austenite grains intersect with a
low melting point liquid between them:
austenite || FeS || austenite
austenite || FeS || austenite
austenite || FeS || austenite
austenite || FeS || austenite
In this case the surface tension of the FeS liquid is low and easily
wets the boundary. If you apply some strain the grains will separate,
i.e. a solidification crack will form.
<--- austenite || FeS CRACK FeS || austenite --->
<--- austenite || FeS CRACK FeS || austenite --->
<--- austenite || FeS CRACK FeS || austenite --->
<--- austenite || FeS CRACK FeS || austenite --->
Note that liquids with higher surface tension will not easily wet
these boundaries and the packets of liquid can remain isolated. Now
the case where you have a ferrite/austenite boundary is not generally
a continuous coating of liquid between the grains because the boundary
path is very tortuous. Application of strain to this case will not
result in cracking since there are only isolated packets of liquid.
F \\ A
F \\ \\ A
F// FeS // A
F// // A
F\\ \\ A
F // // A
F // A
F \\ A
F \\ \\ A
F \\ FeS \\ A
F // // A
F // A
Your third question had to do with sigma phase formation and cryogenic
toughness.
Since chromium is a ferrite stabilizer, ferrite contains higher levels
of Cr than austenite does. This higher concentration of Cr greatly
accelerates the formation of the intermetallic sigma phase since it is
a Cr-rich phase (FeCr stoichiometry), resulting in embrittlement of
welds containing higher ferrite number. This phase forms rapidly in
ferrite containing welds between 600-900°C. The sigma phase reaction
is much slower in austenite and can take thousands of hours to form.
If the weld has any amount of ferrite, the sigma phase can form
readily. However, if FN is maintained between 3-8, a "continuous
network" of sigma phase will not form. Such a network is necessary to
create easy fracture paths.
Low temperature toughness is reduced as the FN increases for the
following reason: Ferrite has the BCC structure which has the
characteristic ductile to brittle transition temperature (DBTT).
Therefore as FN increases, the continuous fracture path increases
accordingly. A fully austenitic weld will not have such a DBTT,
although austenite can partially transform to martensite at liquid
helium (4K) temperatures. This can also degrade toughness.
Hope this helps answer your questions.
Regards,
Jeffrey Sowards, Ph.D.
> On Thu, Jul 22, 2010 at 12:39 PM, manpreet <manpreetsi...@rediffmail.com>wrote:
On Jul 22, 10:27 pm, jignesh makwana <jkmakwa...@gmail.com> wrote:
> Dear Friends,
> Sincere thanks esp to Jeff, Manprit & Vaidehi for their "Valuable
> Suggestions"
> Although some questions are still raising as following.
> 1) Why BCC structure allows more rapid diffusion of impure elemetns??
> I think the packing factor of FCC is 0.72 and that of BCC is 0.68
> (Packing Factor= V of Sphere of atom / V of Unit Cell ). So, Austenite is
> already more denser than ferrite. So, diffusion i.e. movement of atoms will
> be towards the new site for our case it is ferrite having BCC structure.
> 2) Jeff, as you mentioned that Interfaces reduces /increases the wetting of
> the liquid films along the grain boundries. How it takes place?? & How
> Austenite/Ferrite interfaces reduces the wetting of the liquid films along
> the grain boundry.??
> 3) Why & How FN of the order of 15 FN reduces the low temperature tough ness
> in cryogenic applications. Also, How it can promote the Sigma-Phase
> Formation at elevated temp/prolonged heat treatments??
>
> Dear Jeff,
> Collegue working with me are from the Metallurgy background and they are
> really happy and are appreciating your reply.
>
> Hope for your Valuable Suggestions.
>
> Regards.
>
> Jignesh
> > visit this group's bolg athttp://materials-welding.blogspot.com/>
> > Hi
>
> > A minimum ferrite content is necessary to avoid hot cracking in stainless
> > steel welds. The amount of ferrite in the weld
> > metal also controls the micro structural evolution during high temperature
> > service. Moreover the amount of ferrite
> > controls the corrosion and stress corrosion resistance. The low temperature
> > toughness of the weld metal is also related to
> > the weld metal ferrite content
>
> > The exact amount of it can't be specified, depends on different materials.
> > for eg-
>
> > -When joining ASME P-8, Group 1 austenitic stainless steels, the filler
> > metal must contain at least 1 FN or 1 percent ferrite. However, 347
> > electrodes shall contain at least 4 FN.
>
> > -When joining 310 or 330 stainless steels and for cryogenic and special
> > corrosive service, 0 FN filler metal may be used.
>
> > -For service temperatures exceeding 430°C (800°F), the deposited weld metal
> > shall not exceed 10 FN.
>
> > -For cryogenic service with temperatures of –100°C (–150°F) and lower, the
> > ferrite content of all austenitic stainless steel welding material shall be
> > in the range of 2 to 5 percent.
>
> > Hope Mr. Jeff has solved your quarry, this is just an example to
> > demonstrate that ferrite content can't be specified.
>
> > Regards
> > Manpreet Singh
> > Welding Engineer
> > Spiecapag
>
> > On Wed, 21 Jul 2010 12:58:22 +0530 wrote
>
> > Hi,
> > We have come across, with very low delta ferrite (
> > 0.5- 1.5%) in the weld, leading to cracking & failure of components. With
> > high % of delta ferrite also (10%), cracking is observed (even during
> > storage).
> > Minimum 3% is required to avoid fissuring. Low and high FN-both are not
> > advisable. I can explain afterwards, how it prevents.
> > Regards,
> > Vaidehi Ganesan.
>
> > -----------------------------------------------------------------------------------------------------------------------------
>
> > ----- Original Message -----
> > From:
> > jignesh
> > makwana
> > To: materials-welding@googlegroups.com
>
> > Sent: Wed, Jul 21, 10 9:43
> > Subject: [MW:0] How Delta- Ferrite
> > prevents Solidifacation Cracking/ Hot Cracking in SS??
>
> > Dear All,
> > Good Morning
> > I have query regarding the solidification Cracking.
> > Generallly, FeS the compound responsible for solidification cracking.
> > Also Arsenic, Sulphur shows same phenomenon.
> > Now Delta- Ferrite in 3-8 FN proportion prevents hot cracking.
> > But how ?? In most of the books, it is written that, " Delta- Ferrite has
> > tendency to lock the impurities".
> > But how Delta- Ferrite can be helpful for the prevention of hot cracking
> > by locking the impurities??
> > Also, what happens if Delta- Ferrite is very low of theorder of 0.5
> > FN or so or what happens if Delta-Ferrite is high of the order of 15 FN
> > or
> > so.
>
> > Dear All,
> > Please help me to find the answers.
> > Thanks in advance.
>
> > Jignesh
>
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