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A cracked nozzle sleeve stub from XXXX Vessel of the proprietary vaporizer system was analyzed to determine the cause of cracking. Results indicate axial and circumferential cracks initiated at shallow intergranular corrosion penetrations on the inner diameter (ID) surface and propagated by corrosion assisted fatigue. Metallographic examination revealed the presence of shallow intergranular corrosion penetrations in the ID surface of the nozzle sleeve stub. These penetrations were filled with corrosion product and were observed throughout the nozzle ID surface. The intergranular penetrations are attributed to general corrosive attack rather than stress corrosion cracking (SCC).
Some of the shallow intergranular penetrations were driven to significant depths due to corrosion fatigue. Circumferential corrosion fatigue cracking was mainly driven by thermal stress cycles and corrosion. Axial corrosion fatigue was mainly driven by hoop stresses from internal operating pressures and corrosion.
Energy dispersive x-ray spectrometer (EDS) elemental analysis detected mainly organic residue (carbon and oxygen) stainless steel oxidation corrosion products (chromium and iron oxides) and a trace of chlorine on the opened crack surface and within the corrosion deposits.
Chemical analysis indicated that the nozzle sleeve was fabricated from 316L, low carbon stainless steel. There was no evidence of sensitization in the base material, which exhibited a properly annealed microstructure. No material defects or deficiencies were observed.
A more corrosion resistant and possibly higher strength steel is needed for this application. Increasing the wall thickness is another method of reducing the stress in the component.
A cracked nozzle sleeve was received to determine the cause of corrosion and cracking. The nozzle, presented in Figure 1, was from vessel xxxx in the xxxxxxxxx vaporizer system. Cracking was observed in the ID surface therefore, the stub was axially cross-sectioned to examine the internal surface.
A close-up view of the ID surface is presented in Figure 2. Multiple axial and circumferential cracks are revealed. The longest, and presumably the deepest, axial crack was manually opened to examine the exposed crack surface. Figure 3 displays a close-up view of the opened axial crack fracture surfaces. Red and black deposits are noted on the opened crack surfaces. Curved features on the opened crack surface indicated crack progression by fatigue.
EDS micro-analysis in general accordance to ASTM E1508-98 was performed on the opened crack surface deposits to determine if any detrimental inorganic constituents (e.g. chlorine, sulfur, sodium, etc.) were present. Figure 4 is an EDS spectrum of the red deposits comprised primarily of iron (Fe), chromium (Cr) and oxygen (O), indicating stainless steel oxidation corrosion products. Other stainless steel alloying elements of nickel (Ni), molybdenum (Mo), and silicon (Si) are present. The high carbon (C) is likely from the PTA deposits.
Similar results were observed in the black deposits on the opened crack surface. The EDS spectrum provided in Figure 5 identifies the black deposits as chromium rich oxides with other stainless steel alloying elements of iron, silicon, molybdenum, and nickel. A trace of chlorine (Cl) is also noted.
The opened axial crack surface was cleaned and prepared for examination at increased magnifications using a scanning electron microscope (SEM). A low magnification SEM image of the opened crack fracture surface is presented in Figure 6. (A circumferential crack is noted in the opened axial crack surface). Ratchet marks are noted along the ID surface indicating multiple fine crack initiation sites. Ratchet marks are formed when multiple cracks, progressing on slightly different planes, meet to form one main crack front. Fatigue arrest marks are also observed near the OD on the fracture surface. Selected regions of the opened crack surface are shown at increased magnifications in subsequent figures.
The increased magnification SEM images in Figures 7 and 8 reveal an intergranular corrosion morphology along the ID surface edge. Intergranular etching of the opened crack surface is also noted (Figure 9) in the fatigue progression zone. The SEM image in Figure 10 clearly resolves the fatigue arrest marks near the OD surface. A characteristic of fatigue, arrest marks verify the cyclic nature of crack growth.
A transverse cross-section through the center of one side of the opened axial crack was prepared for metallographic examination to determine the crack path and morphology. The cross-section was prepared for metallographic examination in accordance with ASTM E3-01. Etching techniques per ASTM E407-99 revealed the microstructure that was evaluated in accordance with ASTM E883-02.
A low magnification optical photomicrograph of the cross-section is presented in Figure 11. Numerous additional axial cracks along the ID surface are noted. These are primarily corrosion fatigue cracks, with one that extends through almost 2/3 of the wall thickness. Additionally, fine intergranular corrosion penetrations are observed along the ID surface.
Figure 12 is an increased magnification optical microscopic view of one of the largest cracks at the ID surface (from the boxed in area in Figure 11). Several intergranular corrosion penetrations are also revealed along the ID surface. The fatigue cracks have propagated from these ID intergranular corrosion penetrations. The major crack plane is transgranular, straight, and lacks branching, all characteristics of fatigue.
Figures 13 and 14 detail an incipient corrosion fatigue crack at a region of intergranular corrosion along the ID surface at increased magnifications. Prior to etching (Figure 13), the corrosion penetrations are observed to be filled with corrosion product. No extensive crack network, typically observed in SCC cracking is observed. The optical photomicrograph in Figure 14 reveals the etched microstructure, a normal annealed austenitic stainless steel microstructure.
A separate axial cross-section across a circumferential crack was also prepared for metallographic examination. A low magnification optical microscopic view of the circumferential corrosion fatigue crack is depicted in Figure 15. The crack is also straight and transgranular, indicating fatigue progression. However multiple incipient intergranular penetrations are noted along the crack as shown at increased magnification in Figure 16.
The high magnification optical microscopic views in Figures 17 and 18 detail the corrosion fatigue crack tip. Note the competing mechanisms of propagation (intergranular corrosion vs. transgranular fatigue). No oxide filled, wedge-shaped crack morphology is present, which would typically be associated with high temperature thermal fatigue cracking. However, thermal expansion stresses are generally considered a driving stress mechanism for circumferentially oriented high temperature corrosion fatigue. No evidence of sensitization (carbide precipitation along the grain boundaries) or any other material deficiencies are noted in the microstructure.
Chemical analysis was performed on the "N" nozzle sleeve using an optical emission spectrometer in accordance with ASTM E1086-00. The nozzle sleeve stub was fabricated from Type 316L, low carbon austenitic stainless steel. Chemical analysis results are presented below.
No unusual conditions were noted in the 316L stainless steel composition.
Rockwell hardness testing of the nozzle sleeve stub was performed under ASTM E18-02 guidelines. Hardness results indicated an average hardness value of 78 HRB (W) (W indicates tungsten ball used in Rockwell B testing). The hardness is typical for annealed austenitic 316L stainless steel pipe material.
The nozzle sleeve cracking is due to axial and circumferential interior corrosion fatigue that initiated at shallow intergranular corrosion penetrations. The intergranular penetrations are attributed to general corrosion. Corrosion fatigue initiated from the bottom of the shallow intergranular penetrations due to the corrosive environment and thermal and cyclic operational stresses.
A more corrosion resistant material is needed for the nozzles. A higher strength and or thicker wall material may also benefit in this application.
Metallurgical Technologies, Inc. is a Metallurgical Engineering Service Laboratory specializing in Forensic Failure Analysis of Metallurgical Components. They also offer testing, analysis, and consulting services.
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