Ferrite Test of SS Vessels

[Imran.Idris]

When is it necessary to perform ferrite test on SS vessel after carrying out repair by welding?

[ashfaqanwer]

Ferrite test is normally done on those piping or vessels weld joints for which process licenser mentions it as mandatory mainly due to handling of highly corrosive fluids. If some part of a SS vessel has been repaired and it has to handle highly corrosive fluid like ammonium carbamate solution or urea, it is good to conduct a ferrite test. Ferrite test authenticates that the final structure of the repaired area is austenitic having greater resistance against corrosion. Lower ferrite number is better for corrosion resistance.
For new pipelines weld joints intended to handle highly corrosive fluids, ferrite test is normally done at three locations that is root pass, intermediate and on capping in order to have a stringent control over welding and to have a uniform and acceptable ferrite content in the weld joints. There are many variables which may change the ferrite content like welding technique, heat input etc. ; same welding filler wire used by different welders, techniques, current input may result in different ferrite number.

Hope this answers your question.

[Imran.Idris]

Thanks ashfaq,
What tools and techniques are needed to carryout the ferrite test? Are the tools easily available? How much time is required to perform the test?

[ashfaqanwer]

Well we need a Ferrite Meter to estimate the relative ferrite number using different reference blocks available with the meter. The meter operates on eddy current principal and relies on the change in magnetic permeability. Resultant ferrite number can then be converted into ferrite content (percentage). The meter is easily available, no problems. Regarding time, may be some 3-4 minutes per test.

Hope this answers your question.

[Nabeel Ahmed Qureshi]

What is the acceptable limit of Ferrite Number? Is it mendatory to redo a weld joint if it has a high Ferrite content & it has to handle highly corrosive service?

[ashfaqanwer]

Acceptable limits varies depending upon type of SS base metal or welding material. Some stainless steels are manufactured for highly corrosive services not allowing FN greater than 0.2. Special material 25Cr-22Ni-2Mo and its welding metal E/ER-310Mo are of the same type and specially manufactured by materials research group Sandvick.
Yes, if FN is out of acceptable limit, weld joint shall be cut.