(a) The two steels, ASME SA-213 T9 and T91, appear to have almost the same chemical compositions, but the creep performance of T91 is superior. Explore this statement further and explain why T91 is so superior.
(b) Boiler tubes made of austenitic stainless steels (ASSs) are known to be able to withstand higher temperature service than steels with ferritic, pearlitic, bainitic, or martensitic microstructures. What are the reasons for this?
(c) From your answers to (a) and (b) above, what do you think would be the basic principles in the metallurgical design of alloys with creep strengths higher than the basic ASS?