J Fail. Anal. and Preven. (2008)8: 41-47 Table 3 Short-time tensile properties of virgin and service-exposed materials at high temperature SI No and direction Tensile strength. MPa EA (A),% RA(2), %o Toughness(Akv), J 445 48 AHAHAHAAA I15 442 l13 101 136 465 ENI >250 A, axial; H, hoop Microstructure creep process. A significant reduction of dislocation density is observed and few dislocation-free regions can be seen Scanning electron microscopy(SEM) was used Extensive carbide precipitates can be seen at prior aus the fracture surfaces of the test samples. Figure 1 vs tenite and martensite lath boundaries with the finer the fracture surfaces of tensile test from an exposed sample precipitates in martensitic laths. Large coarsening carbides at room temperature. This sample displays almost complete in irregular spheroid formed along the boundaries. Com- intergranular fracture on a macroscopic level, although the pared with virgin material, the carbide morpholog fracture mode is ductile rupture. Subcracks propagate along coarsened distinctly grain boundaries, and wedge cracks appear at triple points. The observations indicate that the matrix of the tem- The fracture facets are quite flat, even though a process of pered martensite has undergone a deterioration durin tensile necking had taken place. Such fracture implies a long-term creep. The dislocations climbed or glided and weakness of the grain boundaries, which is caused by the terminated at boundaries. As the number of dislocations at segregation of phosphorus and the presence of coarsened the boundaries increased, networks formed and substruc carbides in the grain boundaries during long-term exposed tures developed. The carbides morphology in boundaries service at high temperature [7]. Figure 1(b) shows the coarsened distinctly, and most of the strengthening phase fracture surfaces of tensile test from the same exposed have dissolved or coarsened. However, the tiny V-rich sample tested at 550C. It shows that the fracture was precipitates that formed during service increased the transgranular ductile rupture; however, a few subinter- hardness and are responsible for the hardness maintenance granular cracks can be seen in the fracture facets. These in the material [9] observations are consistent with the trend toward weak ening of the grain boundaries with exposed time The general TEM microstructure of the sample is shown Stress Analysis in Fig. 1(c). Obviously, the main structure is a typical tempered martensite containing laths and many coarsened In the absence of discernible cavitation and flaws, stress carbide precipitates along grain boundaries. The ferrite rupture tests were selected to assess the condition of the regions appear very clean in the TEM image, and most main pipe. One of the most widely used techniques for life regions contain some finer precipitates. No form of inter- assessment of components involves the removal of samples granular cavities was observed in the foil sample, but some and conducting accelerated tests at service temperature. An evidence of creep damage can be seen in TEM. The shapes estimate of the residual life is then made by extrapolatio of the laths are changed; in particular, the lath boundaries of the results to the service conditions look like bamboo knots called cell structure. which is a Table 4 list the results of accelerated test at 550 oC typical microstructure morphology caused by creep [8]. Stress rupture data have been plotted in term of log stress Many low dislocation density regions appeared in the lath with rupture time in Fig. 2. The curves show that stresses structure, and some typical substructures can be seen in are linear with rupture times; thus, the threshold strength of Fig. I(d). The substructure seems to develop as subgrains the main steam pipe at 550C was determined by extrap- boundaries are formed by dislocation movement during the olation method to be 2 SpringMicrostructure Scanning electron microscopy (SEM) was used to inspect the fracture surfaces of the test samples. Figure 1 (a) shows the fracture surfaces of tensile test from an exposed sample at room temperature. This sample displays almost complete intergranular fracture on a macroscopic level, although the fracture mode is ductile rupture. Subcracks propagate along grain boundaries, and wedge cracks appear at triple points. The fracture facets are quite flat, even though a process of tensile necking had taken place. Such fracture implies a weakness of the grain boundaries, which is caused by the segregation of phosphorus and the presence of coarsened carbides in the grain boundaries during long-term exposed service at high temperature [7]. Figure 1(b) shows the fracture surfaces of tensile test from the same exposed sample tested at 550 C. It shows that the fracture was transgranular ductile rupture; however, a few subinter￾granular cracks can be seen in the fracture facets. These observations are consistent with the trend toward weak￾ening of the grain boundaries with exposed time. The general TEM microstructure of the sample is shown in Fig. 1(c). Obviously, the main structure is a typical tempered martensite containing laths and many coarsened carbide precipitates along grain boundaries. The ferrite regions appear very clean in the TEM image, and most regions contain some finer precipitates. No form of inter￾granular cavities was observed in the foil sample, but some evidence of creep damage can be seen in TEM. The shapes of the laths are changed; in particular, the lath boundaries look like bamboo knots, called cell structure, which is a typical microstructure morphology caused by creep [8]. Many low dislocation density regions appeared in the lath structure, and some typical substructures can be seen in Fig. 1(d). The substructure seems to develop as subgrains boundaries are formed by dislocation movement during the creep process. A significant reduction of dislocation density is observed, and few dislocation-free regions can be seen. Extensive carbide precipitates can be seen at prior aus￾tenite and martensite lath boundaries, with the finer precipitates in martensitic laths. Large coarsening carbides in irregular spheroid formed along the boundaries. Com￾pared with virgin material, the carbide morphology coarsened distinctly. The observations indicate that the matrix of the tem￾pered martensite has undergone a deterioration during long-term creep. The dislocations climbed or glided and terminated at boundaries. As the number of dislocations at the boundaries increased, networks formed and substruc￾tures developed. The carbides morphology in boundaries coarsened distinctly, and most of the strengthening phase have dissolved or coarsened. However, the tiny V-rich precipitates that formed during service increased the hardness and are responsible for the hardness maintenance in the material [9]. Stress Analysis In the absence of discernible cavitation and flaws, stress rupture tests were selected to assess the condition of the main pipe. One of the most widely used techniques for life assessment of components involves the removal of samples and conducting accelerated tests at service temperature. An estimate of the residual life is then made by extrapolation of the results to the service conditions. Table 4 list the results of accelerated test at 550 C. Stress rupture data have been plotted in term of log stress with rupture time in Fig. 2. The curves show that stresses are linear with rupture times; thus, the threshold strength of the main steam pipe at 550 C was determined by extrap￾olation method to be: Table 3 Short-time tensile properties of virgin and service-exposed materials at high temperature Material SI No. and direction Tensile strength, MPa EA (A), % RA (Z), % Toughness (Akv), J Rp0.2 Rm Bend 1 A 357 444 18 50 109 H 360 445 20 48 100 2 A 365 447 18 51 115 H 358 442 19 54 102 3 A 378 466 21 73 113 H 361 441 19 50 101 Virgin 1 A 379 472 24 71 136 2 A 369 465 22 72 142 3 A 378 466 21 73 135 EN10216-2 [250 A, axial; H, hoop J Fail. Anal. and Preven. (2008) 8:41–47 43 123