Analysis of a Ruptured Inergen Cylinder
Posted on July 3rd, 2019 by Met-Tech
Analysis of a Ruptured Inergen Cylinder
A ruptured Inergen cylinder was received for analysis to determine the cause of failure. Results indicate the rupture initiated at a longitudinal alignment of several pre-existing axial cracks on the interior of the cylinder. The pre-existing cracks were characterized by black oxide covered, arc-shaped crack fronts, initiating from the interior and penetrating approximately 50% – 80% through the cylinder wall. There was evidence of two stages of crack progression before the cylinder ruptured. The time between the first and second stage could not be determined. However, both regions exhibited black oxide and both exhibited significant corrosion along the crack lengths. The second stage appeared to have been either very old or have occurred over a long period of time.
The black oxide on the pre-crack surfaces indicates the cracks have been present for a long period of time and may have occurred at high temperature during manufacturing thermal processing. Black oxide may also be a result of corrosion along the pre-existing cracks in a low oxygen environment. High magnification scanning electron microscope (SEM) examination of the fracture surface in the pre-crack zone after cleaning, revealed a corroded surface which may have obscured the original crack morphology.
Numerous similar long-term pre-existing axial cracks were noted near the cylinder rupture site. The observed pre-existing internal cracks were limited to within approximately 18 inches (0.5 meters) from the tank bottom. Shallow pre-existing circumferential cracks were observed on the circumferential fracture surface along the bottom of the tank. This was a continuation of the axial rupture at the bottom of the cylinder (see Figure 1). There appears to have been active corrosion along the pre-existing cracks. The remainder of the fracture surface (beyond the pre-existing crack zones) exhibited features indicative of rapid ductile overload.
Other than the pre-existing cracks, no other unusual conditions were found in the ruptured cylinder microstructure. The cylinder material meets the Department of Transportation (DOT) 178.37 Specification 3AA and 3AAX seamless steel cylinders for chemical composition. The cylinder steel exhibited a yield strength of 74,700 psi and an ultimate tensile strength of 105,100 psi. The strength specification is unknown.
A ruptured Inergen cylinder from a fire suppression system was received for analysis to determine the cause of failure. The cylinder was stamped near the top: “DOT **********, *********, ******* , *******, MADE IN USA”. The bottom was stamped: “********”. The tank was filled with an Inergen (52% nitrogen, 40% argon, and 8% carbon dioxide) mixture at approximately 2300 psi.
Figure 1 is a photo of the cylinder in the as-received condition. A violent rupture had split the cylinder wide open. The two fracture halves are labeled “A” and “B” for examination identification purposes and are referenced in subsequent figures. The direction of rupture propagation is indicated in the photo, which was determined from visual and low magnification examinations.
The top regulator had apparently broken off after the rupture and is presented in Figure 2. The fracture surface of the regulator, presented in Figure 3, was examined using a stereo-microscope. The fracture surface exhibited ductile overload and appeared to be a secondary failure.
Figure 4 is another view of the cylinder in the as-received condition. Visual examinations of the fracture surfaces indicated the rupture initiated at the axial fracture approximately 6 to 8 inches (15 to 20 cm) from the tank bottom. The bottom section of the cylinder was cut off for closer examination, as presented in Figure 5.
Figure 6 presents a close-up photo of fracture face “A” in the area of rupture initiation. The axial fracture surface displays multiple jogs resembling a stair-step appearance. The internal surface of the cylinder also exhibited numerous longitudinal cracks, which are presented in more detail in Figure 7. The longitudinal cracks (Figure 7) opened during rupture.
Figure 8 is a closer view of the “A” side fracture surface at the location of rupture initiation, approximately 8 inches (20 cm) from the bottom of the cylinder. Black oxide covered arc shaped regions on the fracture surface indicate pre-existing cracks initiating from the internal surface, and penetrated approximately 50% to 80% through the cylinder wall. The black oxide on the crack surface indicated the cracks have been present for a long period of time and may have occurred at high temperature during manufacturing thermal processing. It was subsequently found that the pre-existing cracks may have advanced in two stages.
Energy dispersive x-ray spectrographic (EDS) micro-analysis in general accordance with ASTM E1508-98 was performed on the fracture surface deposits. This was done to verify the composition of the black oxide covering the pre-existing crack zones and to detect any detrimental constituents.
Figures 9 and 10 are the resulting EDS analyses spectrums of the deposits on the pre-existing crack face near the inner diameter (first stage cracking along the ID) and mid-wall (second stage cracking near mid-wall), respectively. Mostly iron oxide with some phosphorous is detected. Traces of sulfur, calcium, silicon and chlorine were also noted. The chromium content was attributed to the cylinder steel. The second stage region exhibits a higher concentration of oxygen. Figure 11 is the EDS spectrum of the crack overload area identifying mostly iron. Small amounts of phosphorous, chromium, and oxygen, with traces of aluminum, silicon, sulfur, and calcium were present. The source of the trace elements was unknown.
Figure 12 is a close-up view of the axial fracture on side “A” approximately 6 inches (15 cm) from the bottom of the cylinder. Additional black oxide covered pre-crack regions penetrating from the interior surface were observed on the fracture surface.
Figure 13 is a view of the “B” side of the fracture near the tank bottom. Arrows identify the area of rupture initiation. The cleaned fracture surface at this location is shown in a close-up view in Figure 14. The black-oxide covered pre-existing crack zones along the internal surface were observed. Variations in the black oxide deposit indicated the pre-existing crack advanced in two stages. The pre-existing crack region extended to 80% through the cylinder wall.
A view of the circumferential fracture surface on the cut-away bottom of the cylinder is displayed in Figure 15. Corrosion on the bottom cylinder surface is noted. A closer view of the circumferential fracture surface (Figure 16) revealed chevron marks, fanning out in each direction from the center area. This indicated the crack progressed downward from the axial fracture and fanned out in each direction after reaching the bottom of the cylinder. Shallow circumferential pre-existing crack zones were noted along the internal surface.
A section from the “B” side fracture initiation site (previously shown in Figure 14) was cut out and cleaned in an inhibited hydrochloric acid (HCl) in an ultra-sonic bath for 10 seconds to remove the oxide deposits. The cleaned fracture surface was then examined using a scanning electron microscope (SEM).
Figure 17 is a low magnification SEM image of the cleaned fracture surface. Two stages of cracking were indicated by the variations in the surface contrast. The long narrow slits in the pre-crack zone on the fracture surface are areas of preferential corrosion of sulfide stringer inclusions, indicating active corrosion along the pre-existing cracks. Increased magnification SEM images of the pre-crack zone on the fracture surface in the initiation region along the ID surface are presented in Figures 18 and 19. A corroded surface, that may obscure the original crack morphology, was noted. Other pre-crack areas in the fracture surface exhibited similar features.
One of the tighter secondary axial cracks near the rupture site on the “A” side of the fracture was opened. The opened crack also exhibited a black oxide covered pre-existing crack zone. The opened secondary crack surface was cleaned with an industrial detergent (Alconox) and prepared for SEM examination.
Figure 20 is a low magnification SEM image of the opened secondary crack fracture surface. Much of the oxide remained on the surface. Two stages of the pre-existing crack propagation were observed. Increased magnification SEM images of the opened crack region along the ID edge in the first stage of the pre-crack are presented in Figures 21 through 23. These images revealed a corroded surface. The corrosion may be obscuring the original crack morphology.
High magnification SEM images of the second stage pre-crack region at the mid-wall (Figure 24) and the second stage pre-crack tip area (Figure 25) also exhibited a corroded surface. However, some intergranular features were resolved.
Figure 26 reveals a dimpled morphology (microvoid coalescence) typical of ductile overload fracture in the fresh fracture region, resulting from laboratory opening of the secondary crack.
Cross-section samples were taken across the fracture initiation site and through secondary cracks from the “A” side of the fracture and prepared for metallographic analysis (ASTM E3-01). Etching (ASTM E407-99) revealed the microstructure that was examined using an optical microscope (ASTM E883-02).
Figure 27 is a low magnification optical photomicrograph of the transverse cross-section through the fracture initiation site on side “A”. The pre-crack zone was indicated. Increased magnification (in Figure 28) revealed slight deformation at the initiation site. The exterior side exhibited yielding. The ID surface was slightly carburized. Figures 29 and 30 reveal slight corrosion penetrations along the fracture surface at high magnification, indicative of active corrosion along the pre-existing crack.
Figure 31 is a low magnification optical photomicrograph of the cross-section through several secondary pre-existing cracks in the interior surface. The cracks likely opened during rupture. Higher magnification views of one of the smaller ID cracks, presented in Figures 32 and 33, revealed corrosion along the crack. Figures 34 and 35 revealed corrosion penetrations at manganese sulfide stringers along the crack at high magnification. This indicated active corrosion.
Figure 36 presents a high magnification optical photomicrograph of a secondary crack tip. Notice the contours of the mating crack faces do not align with each other, which also indicated active corrosion. The over-exposed low magnification optical photomicrograph in Figure 37 revealed oxide lining the entire crack length in another shallow pre-existing secondary crack. Other than the pre-existing cracks, no other unusual conditions are noted in the cross-section microstructures of pearlite and ferrite.
Figures 38 and 39 are photomicrographs of a longitudinal cross-section from the “B” side of the fracture in the non-etched and etched conditions, respectively. A normal number of manganese sulfide stringers were observed running in the axial direction. The steel was not unusually dirty (did not exhibit a high number of inclusions). The pearlite and ferrite microstructure appeared normal.
Rockwell hardness testing of the cylinder steel was performed in accordance with ASTM E18-02. Four separate readings were taken at random locations on the cylinder transverse cross-section for an average hardness value of 97 Rockwell B (HRBW). (W indicates tungsten ball used in Rockwell B testing). The hardness agrees with the tensile strength values.
An axial sample from the cylinder material was subjected to tensile testing per ASTM E8-04. Results are as follows:
|*Yield Strength PSI||Ultimate Strength PSI||% Elongation 2 in. gage|
*Yield Strength at 0.2% offset.
The specified strength level is unknown.
The tank ruptured at pre-existing cracks in the internal surface of the cylinder. The pre-existing cracks exhibited two apparent stages of advancement with active corrosion and appeared to have been present for a long period of time. Numerous pre-existing cracks were found in the fracture surface penetrating approximately 50% – 80% through the cylinder wall. The black oxide covering the pre-crack surfaces indicated they may have formed during manufacturing thermal processing. Corrosion in a low oxygen environment with some moisture present may also result in black oxide.
The cause of the pre-existing cracks could not be determined, due to the corrosion along the cracks, which likely obscured and obliterated the original crack features. Other than the pre-existing cracks, no other unusual conditions were found in the cylinder steel. The composition of the cylinder steel met DOT specifications.
Similar cylinders at the plant site should be inspected using shear wave ultrasonic inspection techniques. Any detected crack should be cause for replacement.
Spectrographic chemical analysis was performed using an optical emission spectrometer in accordance with ASTM E415-99a. Results are provided as follows:
Chemical Composition of Components (wt. %)
Spec. (wt. %)
Results indicate the cylinder material met the Department of Transportation (DOT) 178.37 Specification 3AA and 3AAX seamless steel cylinders.
Figure 1: View of the ruptured cylinder in the as-received condition. A mainly axial rupture split the cylinder wide open. The axial fracture turned circumferential in opposite directions at the tank bottom. The two fracture halves are labeled “A” and “B” for examination identification purposes. The green arrows indicate the rupture initiation site. The smaller black arrows indicate the direction of fracture propagation.
Figure 3: The fracture surface of the regulator appears to be a secondary overload fracture and is not considered a part of the initial rupture. The fracture surface exhibits ductile shearing overload. A heavy strike mark is noted by the arrow.
Figure 5: View of the cut-off bottom section of the cylinder with the two fracture halves labeled “A” and “B”. The boxed region is displayed in a closer view in Figure 6.
Figure 7: View of the interior of the cylinder along the rupture on side “A” exhibits numerous longitudinal cracks. The multiple ID cracks opened during rupture. All of the cracks are within approximately 18 inches (46 cm) of the cylinder bottom.
Figure 9: EDS analysis of the black oxide on the pre-existing crack on the side “A” axial fracture surface along the interior surface. Mostly iron oxide with traces of carbon, phosphorous and sulfur are noted. The chromium content is attributed to the cylinder steel.
Figure 10: EDS analysis of the black oxide deposit near the tip of the pre-existing crack region (mid-wall) on the “A” side axial fracture surface. Mostly iron oxide with traces of carbon, phosphorous, sulfur, chlorine and calcium are present. The chromium and silicon content is attributed to the cylinder steel. The oxygen content is higher than in Figure 9.
Figure 11: EDS spectrum of the “A” side fracture surface near the exterior, beyond the pre-crack region. Iron with a smaller amount of oxygen and traces of carbon, phosphorous, sulfur and calcium are noted. The chromium and silicon are attributed to the cylinder steel.
Figure 13: View of the cut-off cylinder bottom on the “B” side. The black arrow identifies an area where the fracture surface is shown in a close-up view in Figure 14.
Figure 14: The “B” side axial fracture surface after cleaning reveals black oxide covered pre-existing crack patterns along the cylinder ID surface, extending across approximately 80% of the cylinder wall. A slight variation in the black deposit suggests a two-stage pre-crack (between dashed lines).
Figure 15: A view of the circumferential fracture at the cylinder bottom revealed after sectioning. The bottom internal surface displays some corrosion (brown staining). The fracture propagation direction is indicated by the arrows.
Figure 16: Closer view reveals chevron marks fanning out in each direction from the center of the sheared wall. This indicates the axial crack progressed downward then branched to the right and left at the tank bottom. Shallow circumferential pre-existing crack zones (black regions at arrows) are noted along the ID surface. The internal surface exhibits corrosion deposits.
Figure 17: Low magnification SEM image of the inhibited acid cleaned “B” side axial fracture surface. The narrow slits in the fracture surface are sulfide stringers that have been preferentially corroded. The discoloration suggests two stages of crack propagation before rupture. (SEM Photo, Mag. 15X)
Figure 19: Higher magnification SEM image of the “B” side axial fracture surface in the pre-crack region near the cylinder ID reveals a corroded surface that may be obscuring the original crack morphology. (SEM Photo, Mag. 1,000X)
Figure 21: Increased magnification SEM image of the opened “A” side crack surface along the ID reveals a corroded surface possibly obscuring the original crack morphology.(SEM Photo, Mag. 200X)
Figure 23: High magnification SEM image of the “A” side opened crack surface near the ID reveals a corroded surface. (SEM Photo, Mag. 2,000X)
Figure 25: High magnification SEM image of the “A” side opened crack in the second stage pre-crack tip region also reveals a corroded surface with hints of some intergranular features (arrows). (SEM Photo, Mag. 2,000X)
Figure 27: Low magnification optical photomicrograph of the “A” side transverse cross-section through the rupture initiation region. The ID initiation site is identified at the arrow. The pre-existing crack zone is indicated. The OD side has yielded. The boxed areas are displayed at increased magnification in subsequent figures as indicated. (Mag. 15X, Nital etch)
Figure 29: High magnification optical photomicrograph of the unetched cross-section along the fracture surface reveals axial corrosion penetrations, indicating active corrosion along the pre-existing crack. (Mag. 500X)
Figure 31: Low magnification optical photomicrograph of the transverse cross-section across several of the internal secondary cracks. The cracks have widened during rupture. The crack within the boxed area is shown at increased magnifications in subsequent figures. (Mag. 15X, Nital etch)
Figure 33: High magnification of one of the pre-existing “A” side ID cracks reveals corrosion along the crack (arrows). (Mag. 500X, Nital etch)
Figure 35: High magnification optical photomicrograph reveals axial corrosion along a manganese sulfide stringer (arrow) along a second crack in the transverse cross-section. (Mag. 1,000X, Nital etch)
Figure 37: Low magnification overexposed optical photomicrograph reveals corrosion lines (arrows) the entire crack length (gray lining the crack) on another pre-existing crack. (Mag. 100X, Nital etch)