1. How does microstructure affect tube performance?
Fine grains improve room temperature strength
Coarse grains better resist creep at high temperatures
Carbide distribution determines creep resistance
Oxide layer composition affects corrosion resistance
Phase stability critical for long-term service
2. What causes sensitization in stainless steels?
Chromium carbide precipitation at grain boundaries (450-850°C)
Depletes adjacent areas of chromium (below 12%)
Creates susceptibility to intergranular corrosion
Prevented by using stabilized grades (321, 347) or low-carbon (316L)
Detected by ASTM A262 practice tests
3. How do microalloying elements improve properties?
Niobium/titanium form carbonitrides that pin grain boundaries
Vanadium enhances precipitation strengthening
Rare earth elements improve oxidation resistance
Typically added in 0.01-0.1% ranges
Allow thinner walls or higher temperature service
4. What are the stages of creep deformation?
Primary creep: decreasing strain rate
Secondary creep: steady-state (design basis)
Tertiary creep: accelerating strain leading to rupture
Microstructural changes include cavity formation
Monitored via strain gauges or replicas
5. How does cold working affect tube properties?
Increases yield strength but reduces ductility
Introduces residual stresses
Affects corrosion resistance
May require stress relief heat treatment
Considered in forming and installation
