harikrishsuu;
There is a large amount of information on the internet regarding Grade 91 material. This material is a creep strength enhanced ferritic steel that requires very tight controls for both fabrication and post weld heat treatment. Why is PWHT required? Because this material is an air- hardening, high strength alloy steel and must be thermally treated to ensure correct properties before using in service.
This is not a social website,you must title your thread appropriately and not say Hello. Before attempting to place your question, check for information on the net, local libraries and your peers. You can then ask specific question, which can help you,rather than asking open questions.
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"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year