Sigma Phase Specification?

[bmoorthy]

Hello All

Now the situation is this

I have a pressure vessel that has ASTM 321 materail claded.

This vessel is required to be heat treated. User has expressed fear (Verbally)of Sigma Phase formation, but does not prohibit PWHT.

Just to quell his fear and boost the confidence of the customer, i wanted to perform MICRO Examination on a PQRT Piece (The spec calls was to Qualify PQRT using a clad plate-One to One as per the spec) in the CRA Layer (Plate) and the CRA weld.

Our format for test request calls for the specification to be filled up. If there is no test spec and spec for acceptance criteria it will be classified as developmental work, and our project manager will not permit development work. The problem is the acceptance criteria and the analysis method reference standard (Determination of Sigma phase for SS 321 grade material).

On the weld should i check in the undiluted portion (Top 1.5mm). Is presence of Sigma Phase in the area closer to weld (In CRA) a defenite occurance?
In which case i will avoid testing in this area

The filler wire used is 347 grade.

 

[metengr]

bmoorthy;
Typically, for titanium stabilized grade of stainless like TP321, the tendency to sensitize in service is obviously significantly reduced because titanium has a greater tendency to combine with carbon than chromium.

Assuming the TP321 was properly solution annealed to assure the formation of TiC, I believe the risk of sigma phase formation from carbide precipitation is minimal. According to time-temperature-precipitation diagrams for TP 321, you can get M(23)C(6) carbide precipitation after a 20 hour aging time between approximately 1200-1350 deg F (peak). Above or below this peak temperature range, the precipitation of this carbide drops off significantly to between 20 and 200 hour aging time. This conclusion is based on the time-temperature-precipitation plots for solution annealed TP321 in The Handbook of Stainless Steels by Peckner and Bernstein.

 

[arunmrao]

I agree with metengr's observation.