A106 Gr.B is typically used for applications upto 350-400 deg.c in practice. Beyond which designers would go for 1.25 Cr 0.5 Mo material or higher grade alloys for reasons known to you. In case you need to change the piping, you may suggest P11 or P22 Gr. material for this application.

Now, for the present problem, you need to go through the drawing and find out the pipe scedule used during construction, that is the original thickness. Carry out the thickness measurement survey throughout the piping with the help of a calibrated ultrasonic thickness measurement machine. Take the minimum thickness into your calculations and carry out strength calculations considering all steam pressure fluctuations. If you still have enough material as corrosion allowance for one year based on the measured corrosion rate, the choice is yours, else change the line immediately.

You are correct in the upper limit (426 C). Yours is not a question of whether or not it is feasible to use this line for another year. It is a question of legality. You should issue a non-conformance report outlining the situation to your engineering department, with a copy to the manager of the plant. In my opinion, immediate action should be taken to replace the line.

There will always be "pressure" to allow the installation (until the next shutdown). Odds are it would last that long. You have to ask yourself "what if", followed by "what do I say when I'm on the witness stand"?

Good luck,
Charles.

ps, I'm not intending to argue, and perhaps one of the metallurgical wizards of the forum will chime in; but the issue is not corrosion, it is creep. Creep failure, due to high pressure and temperature, may occur even if the material is at full thickness.

I'll try to explian my answer step by step.
1. Who states which is the maximum allowable working pressure for a piping material is the ASME/ANSI Code for Pressure Piping, not ASTM. ASTM states the hydraulic test pressure, which CAN NOT be taken as a basis for working pressure, which depends from a number of circumstances. In this case, as the piping belongs to a petrochemical plant, the applicable Code is B 31.3
2. Asarkar is right in saying that, apart from tensile stress, carbon steel at this high temperature is subjected also to creep and graphitization.
Charles Hall is also right in saying that creep will occur REGARDLESS whether the material is still at its full thickness.
3. I've just taken a look at Pedro Silva Telles' book "Materials for Process Plants". Pedro Silva Telles is the most respected material engineer in Brazil. He's a consultant to Petrobras (the Brazilian state oil company), a university professor and the author of many books and articles.
4. In the mentioned book, which is actually written in Portuguese language, Silva Telles says that, although theoretically carbon steel could be used at up to 450 ºC, creep and graphitization can not be overlooked.
Creep, he says, MAY start at 400 ºC, depending on the grain size. The word MAY means that the grain size has a great importance: the larger the grain the higher the temperature.
So, carbon steel working at temperatures higher than 400 ºC should be inspected for creep once a year.
5. Also in the mentioned book, Silva Telles says that graphitization MAY start at 440 ºC. The word MAY means that the aluminum content of the steel has a great importance: the higher the aluminum the lower the temperature. Small amounts of aluminum are present in aluminum killed steel, which is not the case of A-106, which is killed with silicon.
So, carbon steel working at temperatures higher than 440 ºC should be inspected for graphitization once a year.
6. So, Asarkar can keep on operating his steam line provided he runs those tests once a year. The first one should made at once. A reputed metallurgical lab should know how to make them.

Giovanni S. Crisi
Sao Paulo - Brazil