heat affected zone

Date 12-22-2013 14:54

There is little the inspector can say or do unless there are restrictions invoked by the applicable welding code or project specifications. Most welding standards permit the reuse of the fitting if it meets the original manufacturing specification, but the typical material of construction is carbon steel. ASME piping codes do not address corrosion mechanisms. That must be taken into consideration by the designer. The designer may base decisions about corrosion on information available from NACE.

Your concerns are not without merit. The designer most likely selected AISI 347 material to mitigate the probability of sensitizing the HAZ while welding. The two typical methods of reducing sensitization are the use of low carbon stainless steel alloys or adding stabilizing elements such as Titanium or Niobium. Both of the alloy additions, Titanium and Niobium, tie up the carbon as carbides (M23C6). This reaction prevents the formation of Chromium Carbides which in turn reduces the corrosion resistance along the grain boundaries in the HAZ. Once the Carbon has chemically combined with the stabilizing elements, the Carbon is no longer available to combine with the Chrome and sensitization is averted.

The process of sensitization involves diffusion which is time and temperature dependent. The longer the material is held within the critical temperature range, the greater the extent of diffusion. Maintaining low interpass temperature reduces the time the base metal is subject to the critical temperature range (11000 to 1700 F) where sensitization occurs. However, every bead deposited adds to the total time the weld and HAZ passes through the critical temperature range, increasing the extent of sensitization.

If the contractor disassembles the existing pipe spools and removes all the material included in the existing sensitized HAZ, the remaining material is essentially "virgin material" once again. All the "sensitized material" is removed, thus it is no different than starting with new fittings.

If I were in your position, I would refer the matter to the designer/engineer responsible for the project.

Best regards - Al

By Metarinka (****)

Date 12-24-2013 16:33

Al you're right on the money.

Diffusion and segragation in SS at the HAZ is due to time at critical temperature. The question the engineer will have to ask himself is "how much HAZ is allowed" If the original weld procedure was qualified for X pass or X amount of rework than perhaps rewelding is tolerated for corrosion purposes. If this is riding the ragged edge of what SS can offer for corrosion protection than perhaps it will corrode out at the haz.

It is impossible to call without knowing the service conditions. Could thing is that solid state diffusion is relatively shallow and slow. Grinding back 1/8 of an inch or so from the original joint would probably bring it back to material that never entered the critical temperature range for an appreciable amount of time

By WeldinFool (**)

Date 12-23-2013 20:53

The thing I never liked about re-using SS pipe or fittings is dealing with the shrinkage! You will get some pretty bad hi-lo on your fit-up when tacking them back up to new material.

By G.S.Crisi (****)

Date 12-27-2013 13:55 Edited 01-02-2014 14:27

A word of warning!
If you decide to follow Al's and Metarinka's suggestion to remove the heat affected zone and start a new weld, you may do it in the lathe, for example, or by grinding.
In the first case, you should not use a high speed (i.e., high carbon) steel nor a tungsten carbide cutting tool because they'll leave traces of free carbon in the stainless steel, thus promoting intergranular corrosion. In the second case, you should not use a silicon carbide grinding wheel, because of the same reason. In this case, you should use an alumina (i.e., aluminum oxyde) grinding wheel.

Happy New Year!
Giovanni S. Crisi
Sao Paulo - Brazil

By Metarinka (****)

Date 12-27-2013 23:41

Thanks, that's a good reminder. Same goes for carbon steel wire brushes or wheels.

It is good shop practice to keep grinding media for SS and carbon steel separate and of the right material.

By Stringer (***)

Date 12-28-2013 00:18

As mentioned by Weldinfool, it's my experience that even moderately resized fittings put additional difficulty into the fit making critical root passes a real concern, not to mention trying to make the cap look nice a pain. Additionally, in my work, flow transitions are important and resized elbows work against that issue as well.

By G.S.Crisi (****)

Date 01-02-2014 14:48

Stainless steel being an expensive material, it's an usual piping engineering practice to use small schedule numbers (i.e., 10, 20) when at all possible. In this case, wall thicknesses are small and the pipe and fittings are subject to deformation and warping, as Stringer and Weldingfool pointed out.
In Weldinccaw's case, however, schedule is 120, which means a good wall thickness. In my opinion, this will make deformation or warpage negligible, or at least acceptable.
Nevertheless, taking into account Stringer and Weldinfool's warning, I suggest Weldinccaw to run a test and see what happens before making the final welds.

Giovanni S. Crisi
Sao Paulo - Brazil

By Tommyjoking (****)

Date 01-05-2014 03:59

By 803056

Date 01-05-2014 15:28

?

Al