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Effects of Long-Time Service on the Material of a Steam Boiler Fire Tube
Effects of Long-Time Service on the Material of a Steam Boiler Fire Tube
D. T. Levcovici,* V. Munteanu,* S. M. Levcovici,† and M. Ursu‡ *The Research and Design Institute for Steel Flat Products and Metal Coatings ICPPAM S.A., 2 Smârdan, 6200 Galatzi; †Department of Metallurgy and Materials Science, Lower Danube University of Galatzi, 47 Domneasca, 6200 Galatzi; and ‡The Integrated Iron and Steel Complex SIDEX-S.A., 1 Smârdan, 6200 Galatzi, Romania After 23 years of operation, the fire tube of a 2000 kg steam/h boiler broke down in a waterfilling operation. Because of the intense vaporization processes, the boiler body blew up at about 250 m. The study of metallographic features, of mechanical properties in various areas of the fire tube pointed out microstructural transformations that caused the material’s embrittlement and damage. © Elsevier Science Inc., 1998
INTRODUCTION
The fire tube was made of 11mm heavy plate of ASTM A 515 grade 60 steel and contained longitudinal and circular welds. The generatrix of the cylindrical surface was directed along the rolling direction of the heavy plate. The laboratory analyses and tests were aimed at defining:
On 1 January 1994, after 23 years of operation, the fire (flame) tube of a 2000kg steam/h MANOTEHNIKO-type boiler broke down in a water-filling operation. The intense vaporization process that followed pushed the upper part of the tube inside and threw the 9000kg boiler about 250m away after destroying the building in which it was operated and killing the service operator (Figs. 1 and 2). The fracture occurred along 75% of the circumference length near the weld joint to the posterior tube plate (see the diagram in Fig. 3). Subsequently, analysis of the fire-tube material was required to ascertain the cause of the failure. To this end, samples were taken from various areas (Fig. 3):
• chemical composition using spectroscopy; • metallographic features through microstructural, microhardness, and diffractometrical analyses with the use of, respectively, an OLYMPUS BX60M light optical microscope, a PMT 3 microhardness tester, and a DRON 3 X-ray diffractometer; • mechanical properties through tension tests, impact bending, and hardness measurements with, respectively, a FPZ 100⁄1 tensile testing machine, a PSW 30 impact testing machine, and a HPO 10 Vickers hardness tester.
• along the upper generatrix of the cylindrical surface on the front area (location A) and posterior area (location B); • at the level of the lower generatrix of the cylindrical surface in the weld area between the fire tube and the posterior tube plate (location C) where the fracture did not propagate (this location would have undergone temperature conditions similar to those at location B).
RESULTS The chemical composition of the fire-tube material, shown in Table 1, was found to be in conformance with the requirements of 43
MATERIALS CHARACTERIZATION 40:43–48 (1998) © Elsevier Science Inc., 1998 655 Avenue of the Americas, New York, NY 10010
1044-5803/98/$19.00 PII S1044-5803(97)00108-3
D. T. Levcovici et al.
44
FIG. 1. The steam boiler after failure.
ASTM A 515 grade 60. Examination under a light microscope indicated that the material was of a satisfactory cleanliness for the intended application (Fig. 4). Macroscopic inspection showed that the crack started in the upper generatrix area of
the fire tube near its weld to the posterior tube plate. The crack remained in the firetube material for the entire length of propagation and did not cross into the weld filler metal. The inspection also revealed local reductions in the thickness of the fire-tube
FIG. 2. The upper part of the fire tube pushed inside.
Long-Time Service Effects on Steam Boiler Fire Tube
FIG. 3. Diagram of the fire (flame)-tube damage and sampling areas.
wall due to the penetration corrosion. Examination of the microstructure near the fracture surface (Fig. 5) showed the crystalline grains in this region to be heavily distorted. Etching with 2% nital revealed that material from location A, which had not been exposed to flames, had retained the original microstructure (ferrite and lamellar pearlite) of the normalized plate out of which the fire tube was made (Fig. 6). However, material taken from location B, which had been exposed to flames, had a microstructure that consisted of ferrite, globular pearlite, and cementite precipitates on the ferrite grain boundaries (Fig. 7). The microstructure of samples taken from location C consisted of ferrite and pearlite in the process of spheroidizing (Fig. 8). Microhardness measurements indicated that there had been significant changes in
45
FIG. 4. Unetched cross section showing the cleanliness of the material.
the ferrite grain substructure. Ferrite in location A had an HV0.196 of 1290MPa, whereas the ferrite hardness in location B was HV0.196 1580MPa and that in location C 1360MPa. X-ray diffraction of a sample from location B revealed a greater broadening of the (110) and (220) lines compared with those from a sample from location A, thus indi-
Table 1 Chemical Composition of Fire-Tube Material Element
C
Mn
Si
P
S
Al
Wt.%
0.18
0.47
0.27
0.010
0.015
0.007
FIG. 5. Etched microstructure near the fracture surface.
D. T. Levcovici et al.
46
FIG. 6. Microstructure of material from location A, showing ferrite and lamellar pearlite.
FIG. 8. Microstructure of material from location C, showing ferrite and pearlite in the course of spheroidizing.
cating an increase in the dislocation density and the forming of subboundaries in crystalline grains. Macrostress measurements based on the X-ray data established that a tensile stress of 45 6 5MPa existed along the fire-tube wall thickness of the sample from location A, but a compressive stress of
105 6 5MPa existed in the sample from location B. This change in the macrostress state in the location B sample probably occurred in the accident when a strong additional plastic deformation was imposed on the material. To define the effect of microstructural changes on the material mechanical properties, a mechanical test program was carried out on specimens processed from locations A, B, and C. All specimens were prepared in parallel with the longitudinal axis of the flame tube. Tension tests were performed on specimens machined on all faces to remove the corrosion effects. The results of these tests and the initial values (IV) are shown in Ta-
Table 2 Tensile Properties of Fire-Tube Material
Location
FIG. 7. Microstructure of material from location B, showing ferrite, globular pearlite, and cementite precipitates.
A B C IV
Tensile strength (MPa)
Yield strength (MPa)
Total elongation (%)
428 445 423 461
282 314 264 —
32.4 28.3 30.6 33
Long-Time Service Effects on Steam Boiler Fire Tube Table 3 Notch-Toughness Values of Fire-Tube Material Location
A B C IV
Notch toughness (single values)
J/cm2 (average)
169, 165, 158 157, 149, 168 167, 159, 160 —
164 158 162 160
47 Table 5 Hardness Variation with Material Zone Zone
Hardness, HV (MPa)
MBP
ZITP
MA
ZITT
MBT
1460 1710 2050 1830 1720
Abbreviations: MBP, plate base material; ZITP, heat-affected zone in the plate; MA, metal added (weld zone); ZITT, heataffected zone in the tube; MBT, tube base material.
DISCUSSION AND CONCLUSION ble 2. All values meet the requirements specified in ASTM A 515 grade 60. However, the sample from location B appeared to have higher strength and lower ductility than did those from either location A or location C. The material’s behavior under impact loading at ambient temperature was investigated by using specimens of size 7.5mm 3 8mm with U and V notches. The results of the notch-toughness tests performed on specimens with the U notch and the initial values are shown in Table 3; those obtained from the V-notch specimens are listed in Table 4. Again, these values meet the requirements of ASTM A 515 grade 60 material but, as with the tension-test results, the samples from location B appeared less ductile than did those from the other two locations. However, the V-notch data from location C, although slightly higher than those from location B, are much lower than those measured at location A. This difference is due to the long-time aging process that had taken place at this location. To define the welded joint between the fire tube (T) and posterior tube plate (P), hardness measurements were done across the joint. The results are given in Table 5.
Table 4 Impact-Testing Data on Fire-Tube Material Location
A B C
Input energy (single values)
J (average)
43, 42, 40 12, 19, 23 22, 24, 23
41.7 18.0 23.0
The chemical composition, the tensile-test properties, and impact-bending properties confirmed that the material met the specifications of ASTM A 515 grade 60 steel. After the microstructural features and the mechanical properties determined in the various sampling locations were compared, it was deduced that the fire-tube material near the posterior plate had undergone a series of degradation processes. Some of these processes were the result of long-time service under severe conditions, whereas others occurred during the failure itself. For example, the decrease in fire-tube wall thickness was the result of corrosion over the 23 years of operation. In addition, the thermal cycling that occurred, particularly while the fire tube was operating in transition conditions, contributed to spheroidization of the cementite in the pearlite, precipitation of cementite on the ferrite grain boundaries, and the formation of subboundaries in the ferrite grains. Such structural changes also affected the weld joint, leading to increased embrittlement of the heat-affected zone. The effects of plastic deformation of the fire-tube wall in location B added to the long-time degradation (aging) process observed in the samples from location C. As a result of the work-hardening process, the values of strength indicators increased in this zone, whereas the measures of ductility (total elongation and input energy) became lower. The decrease in material strength at the time of crack propagation, indicated by the properties of the samples from location C, was accentuated by the work-hardening due to plastic deformation of the wall at location B.
48
It was thus concluded that the structural changes induced by prolonged service of the fire tube under severe conditions, particularly thermal cycling in the transition region, led to material degradation and embrittlement. Oxidation and corrosion attack on the internal and external surfaces of the tube led to thinning of the tube wall and decreased load-carrying ability. As a result,
D. T. Levcovici et al.
during an operation under transition conditions, the fire tube finally fractured in the heat-affected zone of the weld joint to the posterior tube plate. Subsequently, water penetrated into the combustion chamber and the steam explosion occurred.
Received April 1997; accepted October 1997.
D. T. Levcovici,* V. Munteanu,* S. M. Levcovici,† and M. Ursu‡ *The Research and Design Institute for Steel Flat Products and Metal Coatings ICPPAM S.A., 2 Smârdan, 6200 Galatzi; †Department of Metallurgy and Materials Science, Lower Danube University of Galatzi, 47 Domneasca, 6200 Galatzi; and ‡The Integrated Iron and Steel Complex SIDEX-S.A., 1 Smârdan, 6200 Galatzi, Romania After 23 years of operation, the fire tube of a 2000 kg steam/h boiler broke down in a waterfilling operation. Because of the intense vaporization processes, the boiler body blew up at about 250 m. The study of metallographic features, of mechanical properties in various areas of the fire tube pointed out microstructural transformations that caused the material’s embrittlement and damage. © Elsevier Science Inc., 1998
INTRODUCTION
The fire tube was made of 11mm heavy plate of ASTM A 515 grade 60 steel and contained longitudinal and circular welds. The generatrix of the cylindrical surface was directed along the rolling direction of the heavy plate. The laboratory analyses and tests were aimed at defining:
On 1 January 1994, after 23 years of operation, the fire (flame) tube of a 2000kg steam/h MANOTEHNIKO-type boiler broke down in a water-filling operation. The intense vaporization process that followed pushed the upper part of the tube inside and threw the 9000kg boiler about 250m away after destroying the building in which it was operated and killing the service operator (Figs. 1 and 2). The fracture occurred along 75% of the circumference length near the weld joint to the posterior tube plate (see the diagram in Fig. 3). Subsequently, analysis of the fire-tube material was required to ascertain the cause of the failure. To this end, samples were taken from various areas (Fig. 3):
• chemical composition using spectroscopy; • metallographic features through microstructural, microhardness, and diffractometrical analyses with the use of, respectively, an OLYMPUS BX60M light optical microscope, a PMT 3 microhardness tester, and a DRON 3 X-ray diffractometer; • mechanical properties through tension tests, impact bending, and hardness measurements with, respectively, a FPZ 100⁄1 tensile testing machine, a PSW 30 impact testing machine, and a HPO 10 Vickers hardness tester.
• along the upper generatrix of the cylindrical surface on the front area (location A) and posterior area (location B); • at the level of the lower generatrix of the cylindrical surface in the weld area between the fire tube and the posterior tube plate (location C) where the fracture did not propagate (this location would have undergone temperature conditions similar to those at location B).
RESULTS The chemical composition of the fire-tube material, shown in Table 1, was found to be in conformance with the requirements of 43
MATERIALS CHARACTERIZATION 40:43–48 (1998) © Elsevier Science Inc., 1998 655 Avenue of the Americas, New York, NY 10010
1044-5803/98/$19.00 PII S1044-5803(97)00108-3
D. T. Levcovici et al.
44
FIG. 1. The steam boiler after failure.
ASTM A 515 grade 60. Examination under a light microscope indicated that the material was of a satisfactory cleanliness for the intended application (Fig. 4). Macroscopic inspection showed that the crack started in the upper generatrix area of
the fire tube near its weld to the posterior tube plate. The crack remained in the firetube material for the entire length of propagation and did not cross into the weld filler metal. The inspection also revealed local reductions in the thickness of the fire-tube
FIG. 2. The upper part of the fire tube pushed inside.
Long-Time Service Effects on Steam Boiler Fire Tube
FIG. 3. Diagram of the fire (flame)-tube damage and sampling areas.
wall due to the penetration corrosion. Examination of the microstructure near the fracture surface (Fig. 5) showed the crystalline grains in this region to be heavily distorted. Etching with 2% nital revealed that material from location A, which had not been exposed to flames, had retained the original microstructure (ferrite and lamellar pearlite) of the normalized plate out of which the fire tube was made (Fig. 6). However, material taken from location B, which had been exposed to flames, had a microstructure that consisted of ferrite, globular pearlite, and cementite precipitates on the ferrite grain boundaries (Fig. 7). The microstructure of samples taken from location C consisted of ferrite and pearlite in the process of spheroidizing (Fig. 8). Microhardness measurements indicated that there had been significant changes in
45
FIG. 4. Unetched cross section showing the cleanliness of the material.
the ferrite grain substructure. Ferrite in location A had an HV0.196 of 1290MPa, whereas the ferrite hardness in location B was HV0.196 1580MPa and that in location C 1360MPa. X-ray diffraction of a sample from location B revealed a greater broadening of the (110) and (220) lines compared with those from a sample from location A, thus indi-
Table 1 Chemical Composition of Fire-Tube Material Element
C
Mn
Si
P
S
Al
Wt.%
0.18
0.47
0.27
0.010
0.015
0.007
FIG. 5. Etched microstructure near the fracture surface.
D. T. Levcovici et al.
46
FIG. 6. Microstructure of material from location A, showing ferrite and lamellar pearlite.
FIG. 8. Microstructure of material from location C, showing ferrite and pearlite in the course of spheroidizing.
cating an increase in the dislocation density and the forming of subboundaries in crystalline grains. Macrostress measurements based on the X-ray data established that a tensile stress of 45 6 5MPa existed along the fire-tube wall thickness of the sample from location A, but a compressive stress of
105 6 5MPa existed in the sample from location B. This change in the macrostress state in the location B sample probably occurred in the accident when a strong additional plastic deformation was imposed on the material. To define the effect of microstructural changes on the material mechanical properties, a mechanical test program was carried out on specimens processed from locations A, B, and C. All specimens were prepared in parallel with the longitudinal axis of the flame tube. Tension tests were performed on specimens machined on all faces to remove the corrosion effects. The results of these tests and the initial values (IV) are shown in Ta-
Table 2 Tensile Properties of Fire-Tube Material
Location
FIG. 7. Microstructure of material from location B, showing ferrite, globular pearlite, and cementite precipitates.
A B C IV
Tensile strength (MPa)
Yield strength (MPa)
Total elongation (%)
428 445 423 461
282 314 264 —
32.4 28.3 30.6 33
Long-Time Service Effects on Steam Boiler Fire Tube Table 3 Notch-Toughness Values of Fire-Tube Material Location
A B C IV
Notch toughness (single values)
J/cm2 (average)
169, 165, 158 157, 149, 168 167, 159, 160 —
164 158 162 160
47 Table 5 Hardness Variation with Material Zone Zone
Hardness, HV (MPa)
MBP
ZITP
MA
ZITT
MBT
1460 1710 2050 1830 1720
Abbreviations: MBP, plate base material; ZITP, heat-affected zone in the plate; MA, metal added (weld zone); ZITT, heataffected zone in the tube; MBT, tube base material.
DISCUSSION AND CONCLUSION ble 2. All values meet the requirements specified in ASTM A 515 grade 60. However, the sample from location B appeared to have higher strength and lower ductility than did those from either location A or location C. The material’s behavior under impact loading at ambient temperature was investigated by using specimens of size 7.5mm 3 8mm with U and V notches. The results of the notch-toughness tests performed on specimens with the U notch and the initial values are shown in Table 3; those obtained from the V-notch specimens are listed in Table 4. Again, these values meet the requirements of ASTM A 515 grade 60 material but, as with the tension-test results, the samples from location B appeared less ductile than did those from the other two locations. However, the V-notch data from location C, although slightly higher than those from location B, are much lower than those measured at location A. This difference is due to the long-time aging process that had taken place at this location. To define the welded joint between the fire tube (T) and posterior tube plate (P), hardness measurements were done across the joint. The results are given in Table 5.
Table 4 Impact-Testing Data on Fire-Tube Material Location
A B C
Input energy (single values)
J (average)
43, 42, 40 12, 19, 23 22, 24, 23
41.7 18.0 23.0
The chemical composition, the tensile-test properties, and impact-bending properties confirmed that the material met the specifications of ASTM A 515 grade 60 steel. After the microstructural features and the mechanical properties determined in the various sampling locations were compared, it was deduced that the fire-tube material near the posterior plate had undergone a series of degradation processes. Some of these processes were the result of long-time service under severe conditions, whereas others occurred during the failure itself. For example, the decrease in fire-tube wall thickness was the result of corrosion over the 23 years of operation. In addition, the thermal cycling that occurred, particularly while the fire tube was operating in transition conditions, contributed to spheroidization of the cementite in the pearlite, precipitation of cementite on the ferrite grain boundaries, and the formation of subboundaries in the ferrite grains. Such structural changes also affected the weld joint, leading to increased embrittlement of the heat-affected zone. The effects of plastic deformation of the fire-tube wall in location B added to the long-time degradation (aging) process observed in the samples from location C. As a result of the work-hardening process, the values of strength indicators increased in this zone, whereas the measures of ductility (total elongation and input energy) became lower. The decrease in material strength at the time of crack propagation, indicated by the properties of the samples from location C, was accentuated by the work-hardening due to plastic deformation of the wall at location B.
48
It was thus concluded that the structural changes induced by prolonged service of the fire tube under severe conditions, particularly thermal cycling in the transition region, led to material degradation and embrittlement. Oxidation and corrosion attack on the internal and external surfaces of the tube led to thinning of the tube wall and decreased load-carrying ability. As a result,
D. T. Levcovici et al.
during an operation under transition conditions, the fire tube finally fractured in the heat-affected zone of the weld joint to the posterior tube plate. Subsequently, water penetrated into the combustion chamber and the steam explosion occurred.
Received April 1997; accepted October 1997.