JNTU B.Tech IV HIGH TEMPERATURE MATERIALS Previous Question Paper (September - 2022 - R18)

Creep resistant steels are specialized alloys designed to maintain mechanical strength, dimensional stability, and resistance to deformation when subjected to constant mechanical stress at elevated temperatures over extended periods. These steels typically contain alloying elements such as chromium, molybdenum, vanadium, and tungsten, which form stable carbides to pin grain boundaries and prevent dislocation movement, thereby inhibiting creep mechanisms like diffusion and grain boundary sliding.

Applications include:

• Steam turbine rotors, blades, and casings in power generation plants.
• Superheater and reheater tubes in high-pressure boilers.
• Pressure vessels and piping systems in petrochemical refineries.
• Components in jet engines and gas turbines, such as combustion chambers and exhaust systems.
• Nuclear reactor structural components operating at high temperatures.

The influence of material ductility on creep is significant, as creep behavior is fundamentally linked to the ability of a material to undergo plastic deformation. Ductile materials generally exhibit higher creep rates because their crystal structures allow for easier dislocation glide and climb, which are primary mechanisms of creep at high temperatures. In ductile alloys, the continuous accumulation of strain leads to a distinct tertiary creep stage characterized by necking and micro-void formation, eventually resulting in a ductile fracture.

Conversely, brittle materials possess limited capacity for plastic deformation. In these materials, creep is often dominated by grain boundary sliding or diffusion-controlled processes rather than dislocation movement. Because brittle materials cannot accommodate stress concentrations through local deformation, they are highly susceptible to intergranular cracking. Creep in brittle materials typically proceeds with minimal elongation and culminates in sudden, catastrophic failure without the significant warning signs of necking observed in ductile materials. Consequently, while ductile materials may fail due to excessive strain, brittle materials are more prone to premature rupture due to the propagation of cracks along grain boundaries.