Iron Dilution?

[bmoorthy]

Hello All

The customer specification gives limitation on Iron for 625 alloy, where it restricts the max Iron (After dilution) to 10% in the weld.

My understanding is that generally if the dilution is given it is on the welds where the alloy (In the welds) are similar to the base material.

If the material that is being welded is Inconel 625 and if the electrode with Alloy 625 is used to weld then the Max iron Dilution is required to be restricted to max 10% (Since the base material Max Iron is in the range of 5 %)

Now the situation i have is like this

The vessel is constructed with CS + Incolloy 825 Clad Plate (Noting that the MIN iron for Incolloy is 22%).

(For SMAW Welding)In ASME II C there is no electrode that meets the Incolloy 825 chemistry and E Ni Cr Mo 3 is recommended electrode (Recommendations obtained from internet search and from Electrode manufacturer's broucher) and subtel references are also seen in API and ASME Sec II C- Although some suggest E Ni Cr Fe 3.

We selected E Ni Cr Mo 3. Now the customer suggests that even in this configuration, the Iron is to be restricted to 10%. Although this does not seem to be impossible, i would like to know whether it is logical to restrict Max Iron to 10% in Inconel 625 welds when in the adjacent material the MIN Iron is 22%.

What is so harmful about the Iron?
Is it logical to ask for the max Iro to be restricted in Inconel weld, when weding Incolloy material?

 

[SJones]

As to why iron is restricted, see

http://www.nace.org/nacestore/assets/ConferencePapers/1996/96427.pdf

As to whether your customer knows what he is dealing with, difficult to say. The effect of iron will be on the corrosion performance of the alloy and, therefore, the principal area of interest in a clad system will be within a certain (short) distance of the surface exposed to the corrosive environment. What joint design are you using: back gouge or strip back as this will affect dilution? If you purchase low iron consumables for the correct welding process, 10% Fe content is not unrealistic.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Carburize]

There does not appear to be great deal of logic however, I would think that you would have no problem meeting the 10% iron level in the 625 weld metal on top of 825. Typical dilution levels of base material into the weld deposit should be well less than 20% so that is only a maximum of 5.5% potential iron from the 825 into the 625 deposit.

 

[bmoorthy]

Thank You S Jones and Carburize

The NACE article is very disturbing for one the conclusions reached by the author (One of the conclusion, that Lower Iron retards the formation of Grain Boundry Precipitates) is not based on the expriment since the expriment was carried out only in one heat. Probably he compared with the result of previous works (As suggested indirectly)by others. Further it suggests that even 4.5% Fe is higher.

Secondly, traditionally on all vessels (Including claded-even if it is Incolly clad 825 or Inconel 625) Clad vessel in Sour Service (Whether wet or Dry) it is habitual on the part of the user to specify PWHT (Probably taking cue from NACE MR 0175).

I have even seen some spec that insists that the PWHT temp is required to be minimum 621 Deg C citing NACE MR 0175 as the basis. Incidentally NACE MR 0175 suggests PWHT if during welding the substrate temperature exceeds Lower critical temperature!!

I am at loss here, how could some one weld overlay the material with out having the temperature of the substrate exceeding the lower critical temperature. May be i got the meaning of substrate wrong.

Having said all this, NACE corrosion publication and NACE MR 0175 (Comparing both side by side) does not lead us any where.

On one hand we have the publication that suggests that Inconel material is unstable if it were to be heat treated at or above 600 Deg C (longer duration exposure increases risk) and on the other hand NACE MR 0175 dictates that PWHT is a must.

In case of critical vessels such as seperators where the thickness are in the ranges of 4 " +, under such circumstances it is common to heat treat for longer soaking time.

 

[SJones]

If you look at ISO 15156-2 (2005) you will note that if the stipulated hardness requirements are met, then there is no requirement for heat treatment. Clearly, this statement had to be added to remove the mythology to which you refer. So, a blanket PWHT simply for H2S containing service is not required by ISO (NACE). Also look at item b) of A.2.1.5 of ISO 15156-2; you will note that this item can override item a) referring to the exceedance of the subcritical temperature. Of course, the pressure vessel code may require a heat treatment and that is where the fun starts. Without delving into the codes to examine whether they would require a further PWHT after overlaying a previously PWHT substrate, at first sight you would have two options:

1 PWHT then overlay
2 Overlay, PWHT, and corrosion test to confirm retention of corrosion performance

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/