Hur påverkar termisk cykling mellan 20 grader och 120 grader utmattningslivslängden för uppvärmningsrör av titan grad 2 svetsade till en tubplåt i brineservice?

During normal operation of titanium Grade 2 heating tubes welded to tube sheets in brine heaters, seawater heat exchangers and evaporators, repeated thermal cycling takes place. Each start-up heats the tube from ambient (20°C) to the operational temperature (typically 100-120°C) and each shut-down cools it back down. The weld joint experiences cyclic strain caused by the difference in thermal expansion between the titanium tube and the tube sheet, which is often fabricated from carbon steel, stainless steel or titanium. This strain may cause thermal fatigue cracking and corrosion after thousands of cycles. This work calculates the fatigue life decrease due to heat cycling in saline service and establishes design limits for welded tube-to-tubesheet junctions. Cyclic Loading and Thermal Expansion Mismatch CTE of Titanium grade 2 is about 8.6 x 10-6 /°C. The CTE of carbon steel is roughly 12.0×10 -6 / ° C . A 1-m length of titanium tube expands 0.86 mm for a 100°C temperature rise (20°C to 120°C), whereas the same length of carbon steel tube sheet expands 1.20 mm. This disparity of 0.34 mm/m is accommodated elastically in the weld and the adjoining tube wall, which produces cyclic stress. The maximal cyclic strain at the weld toe for a 100°C cycle for a typical 25 mm diameter tube welded to a steel tube sheet is estimated to be roughly 0.15-0.30%. The estimation is based on the theory of thick-walled cylinders and numerical finite element analysis. This strain is below the yield strain of titanium (around 0.5-0.7%) hence the deformation is elastic. But fatigue failure by cumulative damage does still occur in repetitive elastic cycling. Fatigue Life Data Base for Welded Titanium Joints S-N curves are established by rotating beam fatigue testing of welded titanium Grade 2 specimens. For a fully reversed stress amplitude of 150 MPa, typical of thermal cycling stresses in tube-to-tubesheet welds, the fatigue life of a butt-welded titanium joint with a smooth weld profile is on the order of 1 × 10 6 cycles. Due to stress concentration, the life of a fillet welded tube-to-tubesheet joint with sharp weld toe is 2×10^5 cycles. Thermal cycles and fatigue life: A typical brine heater experiences 2-3 thermal cycles per day (one start-up, one shutdown). That's between 7,300 and 11,000 cycles over ten years. This is well below the fatigue limit of 1 x 10^6 cycles even for a smooth weld profile, which implies that thermal fatigue should not be the cause of failure. Failures are actually caused by the combination of weld defects, residual loads and corrosion limiting the fatigue life. Corrosion fatigue interaction in brine service The chloride ions introduced by brine service enhance the fracture growth via corrosion-fatigue processes. In a weld which has been thermally cycled, the passive coating at the crack tip is continually destroyed by cyclic strain, exposing fresh titanium to the brine. This is called corrosion fatigue. The rate of anodic dissolution at the crack tip is increased by cyclic loading. The fatigue life of welded titanium Grade 2 in 3.5% NaCl brine at 80°C is decreased by 3-5 times at a similar strain amplitude in comparison to air testing. For example a weld that would endure for 1 x 10^6 cycles in air could disintegrate at 2-3 x 10^5 cycles in brine. For a heater used twice a day 2 x 10^5 cycles is 274 years. Long over its expiration date. The difference indicates that field failures are driven by residual stresses and weld defects, not by purely thermal fatigue. Important Factors that Shorten Fatigue Life Thermal fatigue life below the theoretical estimates is substantially limited by the following factors: Weld undercut or absence of fusion has a stress concentration factor (Kt) of 3-5, and lowers fatigue life by two orders of magnitude. Cyclic stresses superimposed on residual tensile stresses in the weld heat affected zone (usually 150-250 MPa) push the joint closer to yield. Partial penetration welds, cause strain localization at the weld root and the initiation of cracks that propagate through the tube wall. Tube sheet stiffness: Using a stiff tube sheet with little elasticity puts more stress on the weld than a flexible design. Welded Titanium Tubes in Brine Service Application Matrix Service Condition Thermal Cycle MagnitudesEstimated Cycles / Year Weld Quality SpecificationEstimated Life (Corrosion Fatigue Limited) Continuous brine heater (minimal cycles) 100-200°C 20-100°C GTFull penetration > 20 years AW standard Batch evaporator (cycles/day) 300-400 20-120°C Smooth weld profile, 10-15 years stress relieve. Intermittent desalination (2 cycles per day) 20-110°C 700-800 Post weld heat treatment High grade welding 8-12 year olds Backup heater, frequent cycling (4 cycles/day) 20-120°C 1400-1500 As-welded, toe polished 5-8 years Any service with weld flaws (undercut>0,1 mm) Alla reparationer som behövs<2 years Life Extension (Fatigue) Design Guidelines To maximize the thermal fatigue life of Grade 2 titanium tubes welded to tube sheets in saline service, engineers should specify full penetration welds with a smooth, convex profile (not concave). Post-weld heat treatment at 540-650oC for 1 h reduces the remaining stresses by 50-70%. The tube sheet should be made somewhat flexible to reduce limitation, i.e. 20-25 mm thick instead of 40 mm. A radius of at least 1 mm at the weld toe, obtained by grinding, reduces stress concentration. If the applications are more than 500 cycles per year consider Grade 12 titanium for the tube. Grade 12 titanium is characterized by increased fatigue strength (about 20% more than Grade 2) and better corrosion-fatigue resistance. Properly welded brine heaters and evaporators with daily thermal cycling of 20°C to 120°C will have a 10-15 year service life Good weld profiles and stress relief on titanium tubes grade 2. Bad welds will fail in 1 to 2 years regardless of how thick the tube wall is. Radiographic or ultrasonic inspection of all tube-to-tubesheet welds. Documentation of weld technique qualification, including fatigue testing if cyclic service exceeds 500 cycles/year.