Definition of "weld sensitization" for nickel alloys 12

[BRT549]

While trying to decide a suitable filler metal, I came across the following statement in a tech bulletin:

"Inconel 625 has columbium, which stabilizes the alloy against sensitization during welding."

What is weld sensitization?

By the way, I'm trying to choose between Inconel 625 (N06625)and Nicrofer 5923-alloy 59 ((N06059) to join Nicrofer 3127-alloy 31 (N08031) seamless tubes to a Hastelloy G-30 ((N06030) clad tubesheet for wet process phosphoric acid service. Any thoughts would be appreciated.

 

[Metalguy]

Sensitization involves the creating of Cr carbides in the heat-affected-zones (HAZ) of the base metal, but it can also occur in the weld metal too.

The Cr carbides cause a depletion of Cr in the areas surrounding them-each C atom attaches itself to ~4 Cr atoms.

Now, if your environment really needs the Cr, you may have problems. Sens. is "usually" more of a concern with aust. SS than Ni-base alloys, for reasons that we probably don't need to get into here.

Nb is preferrentially attracted to C instead of Cr (leaving the Cr alone).

Sorry, can't give specific recos on which material to use.

 

[sayee1]

Just a few points in addition to what metalguy already said,
sensitization occurs by cooling/heating slowly in a specific range of temperature(s) specific to the type of alloy being welded. For austenitic stainless steels this range would be typically 425-870 Degrees C. The C precipitates from the solid solution mainly at the grain boundaries, and thus the Cr in the adjacent matrix is depleted. As we know the Cr is responsible for making the steel "stainless" by forming a thin uniform oxide layer on the surface. The loss of Cr means localised intergranular attack when exposed to corrosive service conditions. Note that the explanation above is for aust ss.When Metalguy says that sensitization can also occur in the weld, I think he means to say that it can happen in the previously deposited weld bead. Note that the amount of precipitation of carbides depends on the percentage of C (C Content) in the base metal/weld metal.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
End of all knowledge is the attainment of immortality!

 

[Metalguy]

Sayeeprasadr,

Thanks for completing what I started late last night. Since it appears that there is a real shortage of red stars around here, here's one for you.

Now, to complete something else, I think (haven't seen any reports on this) that the reason why aust. SS is usually affected so much by sensit. is that the grain-boundaries in effect become carbon steel, so there is a large voltage (relatively) between the CS and the surrounding unaffected metal, whereas with nickel-base alloys the voltage is much less (Ni minus Cr vs. the alloy itself).

 

[sayee1]

Thanks for the star and the interesting thought on Inconels. I havent seen any technical paper stating that but would be worthwhile to look up for the same. Will do it and post if I find anything interesting.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
End of all knowledge is the attainment of immortality!

 

[BRT549]

Thanks for the great responses to my original question.

Could you confirm another concept that we have been discussing concerning moly during welding procedures?

I have been told by weld shops that moly will be depleted in the weld process, so always use a filler with more moly than the base metal. Is this a major loss (over 2% Mo) or only a minor concern? I ask this because moly is a major contributor to the PRE, which seems to be a good predictor of corrosion resistance for wet process phos acid service.

 

[mcguire]

Molybdenum isn't "lost" in the welding. That idea, I believe, comes from duplex stainless steels having a single phase ( the ferrite) with less than equilibrium and less than designed amounts of moly in it due to rapid solidification. With a single phase alloy, you're okay.
Metalguy is correct about the chrome depletion. Besides setting up a anodic area, the chrome-depleted region has less corrosion resistance because its chrome IS lower. Not only that, the possibility of other phases popping up exists because the alloy really is lower chrome there. This can result in transformation of otherwise stable austenite to martensite under certain conditions.

 

[CalvinKelly]

I have heard that Inco-Weld 686 CPT (ERNiCrMo-14) makes a bead with somewhat better chloride pitting corrosion than the various materials you mentioned.
The reason for using a higher Mo in weld than in base metal is that during solidification the Mo, being very high melting, tends to segregate. Same is true to an extent of Cr, but it is Mo segragation that can be a problem. The low Mo areas are less resistant to chloride pitting corrosion. Once a pit begins it can continue to grow, including into the high-Mo segraeated areas. Because conditions in a pit, as in a crevice, become progressivly more concentrated in chlorides and lower in pH.

James Kelly

http://www.rolledalloys.com

 

[BRT549]

Thanks for the info on 686. We are going to use 625 mainly because it's a known performer in the phos acid service. Sorta out of my hands at this point.

 

[sayee1]

Some facts about inconel, sensitization, queries and replies



Sensitisation is a process by which alloy carbide particles (generally chromium carbides) precipitate out from the metal onto the grain boundaries. This leaves the regions adjacent to the grain boundaries depleted in alloy content, and hence more susceptible to corrosion.


Certain grades of stainless steel and nickel alloy contain additions of Ti or Nb (Cb) so avoid the problem of sensitisation; these are known as "stabilised" grades. This is because Ti and Nb are strong getters of carbon, hence they remove the carbon from solution and prevent alloy additions (such as chromium) from forming carbides.



To ensure that the Nb or Ti have consumed the carbon, a stabilisation heat treatment is often specified. This heat treatment is designed to give the Ti or Nb additions time and energy to remove the carbon from solution. These heat treatments will depend upon the chemistry and thermal properties of the alloy to be heat treated, hence standards applicable to any one grade may not be applicable to others.



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ASM Metals handbook Volume 4 (Ninth Edition): Page 624/Stainless steels and heat resisting alloys, paragraph 4 states that
"When maximum corrosion resistance of stabilized austenitic grades is required, it may be necessary to employ a heat treatment known as stabilized annealing. The heat treatment involves heating to the range 1550-1650 degrees F and maintaining this temperature for about 5 hours depending on the thickness prior to or in course of fabrication followed by short time stress annealing if required at 1300 F without danger of harmful carbide precipitation." Refer to note E, Table 1, ASTM B444, Standard specification for Ni-Cr-Mo-Cb alloys (UNS 6625) Pipe and tube. The spec calls for solution annealing at 1093 Deg C minimum followed by stabilized annealing (Not mandatory) at 982 Degrees C minimum, whereas for grade UNS 8825 as in ASTM B564, B366 and B423 stabilized annealing is not specified.

B366-99 Standard Specification for Factory-Made Wrought Nickel and Nickel Alloy Fittings Vol. 02.04
B444-94 Standard Specification for Nickel-Chromium-Molybdenum-Columbium Alloys (UNS N06625)* Pipe and Tube Vol. 02.04
B564-99a Standard Specification for Nickel Alloy Forgings Vol. 02.04
B423-99 Standard Specification for Nickel-Iron-Chromium-Molybdenum-Copper Alloy (UNS N08825 and N08221)* Seamless Pipe and Tube Vol. 02.04


Please clarify:
1. Stabilized annealing: please specify the recommended cycle(s) for the heat treatment
2.
a. Environment that may cause IGC: What typically are such environments?
b. In case of the welding procedures for one of the abovementioned two projects, there is a requirement for IGC A262 practice B on the welds including the root in single sided butt welds and weld overlays. I do know that IGC A 262 is not the best of tests for Inconel materials since the A262 was designed for austenitic stainless steels. But since it is a part of the Contract we are just complying. Your thoughts on the applicability of A262 for Inconel materials!

3. Does Incoloy 625 also require stabilized annealing?
4. Is the stabilized annealing for Incoloy 825 required because of the~0.6-1.2 Ti?

Answers:

1. The stabilisation heat treatment specified by you supplier (927 to 980°C) is within the recommended range for this material. The temperature specified in ASTM B444 is for Ni-Cr-Mo-Cb alloys and not Ni-Fe-Cr alloys such as alloy 825.



2a. Materials that are heavily sensitised may corrode in environments as benign as steam. High temperature materials such as alloy 825 should, therefore, be considered at risk of corrosion in any service environment if they are sensitised.



2b. ASTM test A262 is not specified to be applied to nickel alloys. However, in the absence of a more suitable test, A262 Method B, (commonly called a Huey test) is probably the best way forward; it is specified titanium stabilised stainless steel (type 321).



3. Alloy 625 is not as susceptible to sensitisation as Alloy 825, however, if the possibility of sensitisation exists during service a stabilisation treatment may be considered. The stabilisation heat treatment specified in B444 (982°C) is suitable for this purpose.



4. As per comments above, the stabilisation heat treatment is required to enable the titanium present in Alloy 825 to consume the carbon thus preventing future chromium carbide precipitation and hence sensitisation.
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What role Cr carbides play in reducing the corrosion resistance of Inconel materials. In austenitic ss, the Cr carbide reduces the Cr content of the surrounding areas, thus reducing the Cr oxides which give the stainless/corr resistance properties. Is the mechanism of corrosion resistance or the loss of resistance thereof, for Inconels, also the same as for austenitic stainless steels?

If the resistance for inconels is also by the formation of Cr oxide layer(s), is it a good practice to do pickling and passivation of clad pipes and or Inconel welds after the maqnufacture of the pipes to restore/form the uniform oxide layer?
Answers

1. Typically materials leaving the mill have been processed to provide good corrosion resistance. To maintain these properties subsequent working temperatures should not exceed the temperature range 926°C-982°C. Working operations at temperatures above 980°C could compromise these properties, particularly if the cooling rates involved are slow. A stabilisation anneal will restore resistance to corrosion. Obviously, if the material is to be welded or subjected to thermal treatments and exposed to an environment that might cause intergranular corrosion a stabilising treatment should be performed to ensure minimal risk. Although quenching might not be necessary for small cross section parts, sufficiently rapid cooling is advised to avoid sensitisation in thicker sections.

2. Cr imparts oxidation resistance to nickel alloys such as Incoloy 825 by formation of protective Chromium Oxide in a similar manner to that in stainless steels. Other elements such as Aluminium complement the effect of Cr with formation of aluminium oxide and, therefore, the mechanisms of protection are more complex compared to those in stainless steels and depend on the species of the alloying elements and their amounts which are carefully controlled in the case of nickel alloys. The nickel content of the material imparts sufficient resistance to chloride stress corrosion cracking while the combination of Molybdenum and Copper additions increase the resistance to pitting and to corrosion in reducing acids.

Inconel clads such as those made with Inconel 625 filler metal have very good resistance to pitting and intergranular corrosion. Passivation treatment via acid pickling e.g.HNO3 is optional and might be used to increase the oxidation resistance of nickel alloys and welds. The oxidising nature of the nitric acid promotes formation of the protective passive layer while it removes many surface contaminants through oxidation. However, care must be taken in time for acid pickling as overexposure can result to excessive metal loss or grain boundary attack which can be much more severe if the clad contains small fabrication cracks or other crevices that could trap the pickling solution. Cleaning of the pickled areas with alcohol or clean water should follow acidic pickling to remove all traces of the acid solution and minimise the risks mentioned above.


ASM Metals handbook Volume 4 (Ninth Edition): Page 624/Stainless steels and heat resisting alloys, paragraph 4 states that
"When maximum corrosion resistance of stabilized austenitic grades is required, it may be necessary to employ a heat treatment known as stabilized annealing. The heat treatment involves heating to the range 1550-1650 degrees F and maintaining this temperature for about 5 hours depending on the thickness prior to or in course of fabrication followed by short time stress annealing if required at 1300 F without danger of harmful carbide precipitation." Refer to note E, Table 1, ASTM B444, Standard specification for Ni-Cr-Mo-Cb alloys (UNS 6625) Pipe and tube. The spec calls for solution annealing at 1093 Deg C minimum followed by stabilized annealing (Not mandatory) at 982 Degrees C minimum, whereas for grade UNS 8825 as in ASTM B564, B366 and B423 stabilized annealing is not specified.

B366-99 Standard Specification for Factory-Made Wrought Nickel and Nickel Alloy Fittings Vol. 02.04
B444-94 Standard Specification for Nickel-Chromium-Molybdenum-Columbium Alloys (UNS N06625)* Pipe and Tube Vol. 02.04
B564-99a Standard Specification for Nickel Alloy Forgings Vol. 02.04
B423-99 Standard Specification for Nickel-Iron-Chromium-Molybdenum-Copper Alloy (UNS N08825 and N08221)* Seamless Pipe and Tube Vol. 02.04
============================================================

Please clarify:
1. Stabilized annealing: please specify the recommended cycle(s) for the heat treatment
2.
a. Environment that may cause IGC: What typically are such environments?
b. In case of the welding procedures for one of the abovementioned two projects, there is a requirement for IGC A262 practice B on the welds including the root in single sided butt welds and weld overlays. I do know that IGC A 262 is not the best of tests for Inconel materials since the A262 was designed for austenitic stainless steels. But since it is a part of the Contract we are just complying. Your thoughts on the applicability of A262 for Inconel materials!

3. Does Incoloy 625 also require stabilized annealing?
4. Is the stabilized annealing for Incoloy 825 required because of the~0.6-1.2 Ti?



INCOLOY alloy 825 is stabilize annealed at 940-980C for 5 minutes (thin
sheet) to 1 hour depending on thickness. This is not a solution anneal, but a stabilizing anneal which ties up the carbon as titanium carbides. The 0.6 - 1.2% titanium is added to the alloy for this reason. A solution anneal would dissolve (solution) any carbides present. A solution anneal is not required prior to the stabilizing anneal. With the material in this condition, welding etc. / exposure to temperatures which might otherwise cause sensitisation, should not cause a problem. If however, material from your supplier had not been stabilised prior to such exposure then sensitisation is likely to occur.

The mentioned 1550F - 1650F for 5 hour plus 1300F heat treatment has been used, with limited success, for unstabilized alloys. This heat treatment is not applicable to stabilized INCOLOY alloys 825 and INCONEL alloy 625.

ASTM A262-B is applicable to INCOLOY alloy 825 and is intended to be applied to material which has received a stabilizing anneal. The procedure requires a sensitizing heat treatment of 650-675C for 1 hour to be applied to the test sample prior to testing ( "before loading the sample in the corrosive media"). This test verifies that the previous stabilizing anneal has effectively prevented sensitization when the sample was subjected to a sensitizing heat treatment. ASTM A262-B is the same as ASTM G28-A.

ASTM A262-B and G28-A is also used for INCONEL alloy 625, though a sensitizing heat treatment is not specified for this alloy in G28-A.

INCONEL alloy 625 is stabilized by annealing at up to 1040C, where carbon is tied up as niobium carbides. Higher temperature "solution" anneals are used when softer, but unstabilized, material is desired.

Environments which cause IGC are highly oxidizing acids such as nitric acid or sulphuric acid + an oxidizer such as ferric sulphate (ASTM A262-B). Also polythionic acid, which is sometimes found in the petrochemical industry, can cause SCC of sensitized materials.

Welds are difficult to evaluate in an IGC test. The best approach is to use ASTM A262-B or G28-A to verify that the base metal is properly stabilized. If the base metal is properly stabilized then there should be no adverse effects of welding.

2. Yes heat treatments described for INCOLOY alloy 825 are different to that for INCONEL alloy 625 - the two alloys are very different in composition (see above).

3. Exposure of INCOLOY alloy 825 to 930 to 980°C is not a solution anneal. It results in stabilization of the alloy by formation of titanium carbides. To solution anneal this structure (for whatever desired reason), exposure to a higher temperature (1150°C) would be required. Were an alloy 825 component to be so heat treated, it should be given the stabilization anneal afterward. However, our standard recommendation as stated in our technical bulletin is to not expose the alloy to such a high temperature. Limit exposures to 980°C max.

4. If everything meets the acceptance criteria (after the 1 hr sensitising heat treatment has been applied to the test sample) it would seem likely that there would be no need for additional heat treatments.

INCONEL alloy 625:-

Note E in Table 1 of ASTM B444 refers to grade 2 alloy 625. This is the solution annealed grade that is designed for service at elevated temperatures. We would expect grade 1 alloy 625 to be used for your application. That is normally annealed in the 1800 to 1900 F range which results in stabilization of the product (as described above - the use of Niobium in the case of INCONEL alloy 625 to tie up carbon). The definitions and intended uses of grade 1 and 2 alloy 625 are clearly stated in the Scope of B 444 in paragraph 1.1.1.

2) The logic of the statement is as stated in (1) above. Grade 1 material is annealed at a temperature that results in stabilization of the alloy (1040 C) so a two-stage heat treatment is not required. INCONEL alloy 625 is of a very different chemical composition to INCOLOY alloy 825.

Thanks and regards
Sayee Prasad R CEng MWeldI MIOMMM


If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[SMF1964]

A recommended reference for SCC of Nickel-base Alloys is "Stress-Corrosion Cracking", Russell H. Jones, ed., published 1992 by ASM International (ISBN 0-87170-441-2). They have a detailed discussion of the carbide precipitation mechanism in there, written by Narasi Sridhar & Gustavo Cragnolino of SWRI (at the time).