it was a very elaborate answer. frankly , it refreshed my fundamentals
in Metallurgy.
Regards
Mayank
On 7/23/10, 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 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
>> On Thu, Jul 22, 2010 at 12:39 PM, manpreet
>> <manpreetsi...@rediffmail.com>wrote:
>>
>> > 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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