1. What are the characteristic signs of short-term overheating failure?
Sudden rupture with thin-edged, fish-mouth opening showing minimal scale buildup. Typically exhibits ductile fracture with significant wall thinning near the burst. Metal temperature exceeds critical transformation points, causing grain growth. Often results from flow blockage or low water conditions. Immediate shutdown is required to prevent cascading damage to adjacent tubes.
2. How does hydrogen embrittlement manifest in boiler tubes?
Brittle fractures with minimal plastic deformation, often near welds or bends. Microscopic examination reveals intergranular cracking patterns. Associated with low pH water chemistry (below 7) and active corrosion. May show blistering on internal surfaces where atomic hydrogen penetrates. Prevention requires strict control of feedwater chemistry and oxygen levels.
3. What causes stress corrosion cracking in austenitic stainless steel tubes?
Chloride-induced cracking occurs at temperatures above 60°C with tensile stress. Cracks propagate perpendicular to applied stresses, often branching. Sensitized material (from welding) is particularly vulnerable. Requires both chlorides (>10 ppm) and oxygen to initiate. Mitigation includes controlling chloride levels and using stabilized grades (e.g., 321, 347).
4. How can you distinguish between erosion and corrosion damage?
Erosion produces smooth, tapered thinning patterns following flow direction. Corrosion creates more irregular, pitted surfaces with oxide layers. Erosion often shows directional grooves from particle impacts. Metallurgical analysis reveals different elemental signatures. Combined erosion-corrosion exhibits features of both mechanisms.
5. What are the stages of creep failure in boiler tubes?
Stage 1: Initial deformation with minimal creep rate. Stage 2: Steady-state creep with constant deformation rate. Stage 3: Accelerated creep leading to rupture. Macroscopic voids form at grain boundaries in final stages. Microstructural changes include carbide coarsening and subgrain formation.
