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FATIGUE CRACKING OF TITANIUM TUBES IN AN INDUSTRIAL HEAT EXCHANGERABSTRACTThe fatigue fracture of commercially pure (cp) titanium tubes at relatively low temperatures120(250)in a shell-and-tube heat exchanger was studied. The cp titanium tubes were subjected to flow-induced vibration and compression axial loading in service. The combined stresses resulted in numerous intergranular cracks in circumferential direction.Some cracks reached a size that resulted in complete rupture of tubes.Fractographic analyses indicated that the fracture was caused by high-cycle fatigue. A large portion of the tube wall indicated a lack of characteristic ductility of cp titanium. The reduced ductility was a result of second phase particle inclusion and hydrogen absorption in tube material. The second-phase particles provided easy sites for initiation and coalescence of microvoids. Hydrogen absorption of titanium resulted in an increase in hydride content and thus a decrease in ductility. INTRODUCTIONOne of the problems heat exchanger designers have to face is the vibration and subsequent damage of tubes, baffle plates, or tubesheets. Since tubes are more flexible, their damage is more commonly countered. As the shell-side fluid flows past the tubes at a large flow rate, the tubes vibrate at their natural frequencies when the flow velocity becomes sufficiently large 1. Increased flow velocity is, in practice, intended in heat exchangers to improve heat transfer and to reduce buildup of undesirable condensate on tube surfaces. The frequency of tube vibration depends primarily on tube cross-sectional geometry, means of support, and tube material. Since natural frequency varies as the reciprocal of tube length squared, between two rigid supports, the frequency can be increased with more intermediate supports by transverse baffles or support plates along the length. In many constructions, the tubes are not all supported at the same support plates or baffles. Therefore, a spectrum of vibration frequencies is expected. As the flow velocity varies, tubes vibrate at discrete frequencies.In recent years, reactive materials have been widely adopted in industrial processing equipment where the material of construction must be highly resistant to corrosion. Titanium of commercial purity with excellent corrosion resistance, thermal conductivity, and other engineering properties is ideal for use in severely corrosive service. To reduce costs, thin-walled tubes, 18 or 20 Brown wire gauge (BWG), are frequently specified. Because of the small tube wall thickness, titanium tubes are relatively flexible and are susceptible to destructive flow-induced vibration. Most tube failures are caused by thinned tube wall due to rubbing against neighboring tubes or abrasion with baffle plates 2. This paper presents the fatigue cracking of cp titanium tubes as a result of flow-induced vibration and axial loading in a shell-and-tube heat exchanger.FAILED TITANIUM TUBESThe construction of the heat exchanger was a steel shell equipped with stationary tubesheets and 25.4mm outside diameter (O.D.18BWG (1in.O.D.18BWG) cp titanium tubes. The transverse baffles were made of steel. The span length between the first or the last baffle and tubesheet, 0.6 m (1.97ft), is greater than that of intermediate spans in order to accommodate shell nozzles. The tube-side fluid entered at an average temperature of 116(240) and came out at 77(170). The shell-side fluid is pretreated cooling water with an inlet temperature about 18(65) and outlet temperature 50(120) at a nominal flow velocity of 1.5m/s(4.9 ft/sec).After four years in service, cracks were found in several tubes near tubesheets. The majority of cracks were either within the span between tubesheets and their nearest baffles, or right under the baffles next to the tubesheets. After the tubes were pulled out, cracks along the circumferential direction were visible on tubes. The visible cracks were through-cracks. Although most cracks did not grow to a size which might result in the complete breaking of tubes, several tubes did rupture between tubesheets and the nearest baffles. Tubes were also found to be slightly bowed with no sign of rubbing with surrounding tubes observed. Tubes that contained fracture surface or visible cracks were cut and prepared for examination.FRACTURE OF TUBES NEAR TUBESHEETSThe complete rupture of tubes occurred mainly between tubesheets and nearest baffles. Metallographic examination performed near fracture surface on the completely ruptured tubes revealed that the fracture was caused by intergranular cracking, Figure 1.Numerous cracks were found to have initiated at both surfaces of tube wall and propagated in radial direction. The cracks appeared to be intergranular, with the number of cracks per unit length of tube decreased with increasing distance from fracture surface. Metallographic examination at higher magnification also revealed a large number of finer transgranular cracks. These fine cracks generally were short and parallel to the major cracks, Figure 2. All the cracks (including fracture surface) were normal to the tube axis.Scanning electron microscope examination at various portions of rack surface indicated that the initial cracking was of brittle nature. Striations observed on fracture surface suggested that the crack propagation was under cyclic stressing. Some ductility was observed near the location of final separation. Fig1Fracture along grain boundaries with intergranular cracks near fracture surface(100)Fig. 2. Transgranular cracks. ( 550)TUBE CRACKS UNDER BAFFLESThe cracks found under end baffles were similar to those found between tubesheets and baffles. The cracks also extended in the circumferential direction. Nevertheless, none of the cracks resulted in complete rupture of tubes under baffles. The distribution and orientation of fine transgranular cracks were also pretty much the same as those of cracks found between tubesheets and baffles. Scanning electron microscope (SEM) examination also indicated a lack of ductility along the path of crack propagation.DISCUSSIONIt is considered that the titanium tubes were subjected to a complex state of stress in service. The major stresses include bending stress arising from tube deflection at midspan between two rigid supports during vibration, and axial stress imposed by tubesheets.The stress arising from tube vibration varied with the lateral movement of tube. Treated as a simple beam, the stress acting on the tube cross section is the largest (regardless of sign) at the external surface on concave or convex side near the location of maximum deflection, and only slightly smaller at the inner surface. As a result, cracks initiated at both surfaces and propagated in the radial direction in response to the cyclic stress as the tube vibrated. Since the largest lateral movement of a tube occurs at midspan between the tubesheet and the nearest support plate, the major cracks are expected to appear near that location.The tubesheets in the heat exchanger were stationary, which probably had allowed axial loading on tubes 3. The axial loading could be tension or compression, depending on the operating temperatures and pressures. In general, higher tube-side pressure and larger shell expansion (relative to tube bundle expansion) in lengthwise direction result in tension axial loading. But judging from the slight bowing of several tubes, the tubes were likely to have been under compressive loading. This is the opposite of the prediction based on operating conditions and linear expansion coefficients of steel and titanium. Improper manufacturing procedure particularly that for tube-to-tubesheet joints, probably had set tubes under compression and caused the reversed effect. A compressive axial loading would decrease the natural frequencies of tubes and increase the possibility of tube vibration. The bowed tubes had to be under stress, which also increased the possibility of fatigue fracture.Photomicrographs of material samples showed that the tube material contained a significant amount of second-phase particles on grain boundaries and along twins, Figure 3. The particles must have reduced the ductility of titanium by providing easy sites for initiation and coalescence of microvoids. Figure 4 shows the presence of second-phase particles on the fracture surface. CP titanium with reduced ductility would be expected to undergo brittle cracking as evidenced by the semi-cleavage fracture surface, Figure 5. Fatigue cycling at low temperatures also may have caused the decrease in ductility, but it is not considered to be a major cause.Fig. 3. Second-phase particles on grain boundaries and twins. (x 100) Fig. 4. Second-phase particles on fracture surface. (x550)Fig. 5. Semi-cleavage fracture surface. (450)It was also suspected that titanium in contact with steel baffles might have been embrittled due to hydrogen absorption. Titanium is known to have good resistance against galvanic corrosion. In a galvanic couple consisting of titanium and another metal, titanium becomes cathode and the other metal anode 4. The coupling of titanium with steel baffle could induce galvanic corrosion. The hydrogen evolved in the corrosion reaction of anodic steel could be absorbed by titanium, especially when the temperature was sufficiently high. Consequently, the ductility of titanium decreased as the formation of hydride became significant. Hydriding of titanium can be significant when the hydrogen content increases above approximately 205. Metallographic examination confirmed that titanium in contact with steel baffle did contain a certain amount of hydride. Figure 6 shows the distribution of hydride near the tube surface. The hydriding of titanium was n