Examination of Cracked Type 321 Stainless Steel Pipe

A cracked 2-inch, schedule 80 pipe with a welded-on flange was examined. The pipe was reportedly in service carrying hydrogen, water, and hydrogen sulfide (H2S). The pipe contained a through-wall crack that resulted in a leak. The objective of the lab analysis was to locate and determine the cause of the crack and the leak in the pipe. Visual examination, two types of penetrant examination, metallography, scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDS) analysis, and optical emissions spectroscopy (OES) chemical analysis were performed on the pipe. The results indicated that the pipe cracked because of chloride stress corrosion cracking that originated on the internal surface of the pipe adjacent to the pipe-to-flange weld. The pipe and flange were constructed of type 321 stainless steel while the weld was made of type 347 stainless steel. No manufacturing or welding defects or anomalies were associated with the crack.

By Scott Harding and Sudhakar Mahajanam, Stress Engineering Services, Inc.

Stainless steels represent an important group of iron-based alloys that are utilized in a wide variety of industries. They derive their ‘stainless’ nature from the addition of chromium to steels, which was first shown by Frenchman Berthier in 1821. To be classified as stainless, the steel must contain at least 11% chromium. This amount of chromium prevents rust formation in most environments. There are nearly 200 grades of stainless steels produced today. The stainless steels can be categorized into five groups based on their metallurgical structure: austenitic, ferritic, martensitic, precipitation-hardened and duplex.

This article presents a failure analysis of a cracked type 321 stainless steel pipe. The pipe was 2 inches in diameter, schedule 80 with a welded-on flange, reportedly carrying hydrogen, water, and hydrogen sulfide (H2S) in service. The pipe contained a through-wall crack that resulted in the leak. It was requested to locate and determine the cause of the crack and leak in the pipe.

Laboratory Examination

Visual Examination

Figure 1 shows the leak in the 2-inch, type 321 stainless steel line adjacent to a pipe- to-flange weld both while leaking and after the deposit buildup had been removed from the pipe to expose the leak site. After the pipe and flange were sent for analysis, as-received photographs were taken and are shown in Figure 2. Visually, no apparent cracking was visible on the surface of the pipe, even when examined using a stereo microscope at magnifications up to 40x, indicating the leak was a result of a small, tight crack.

Figure 1: The 2-inch, Type 321 stainless steel pipe that was found leaking near a pipe-to-flange weld.

Penetrant Examination

The area of the leak site was subjected to several rounds of standard, dye penetrant examination. The crack was only faintly visible after the outside surface of the pipe had been lightly filed to remove a small surface layer. Figure 3 displays the resulting faint, penetrant indication of the crack and the small, tight crack observed on the pipe surface adjacent to the weld.

Table 1: Chemical Analysis Results (weight percent).

The pipe was split axially to expose the inner surface of the pipe and leak site, and the leak site was subjected to a fluorescent penetrant exam. Fluorescent penetrant generally has higher sensitivity as it can be used without the obscuring effects of a developer. Figure 4 shows the resulting fluorescent penetrant indication of the crack on the inside surface and the same surface viewed with normal white light. A slight amount of undercut was observed at the pipe-side weld toe, but the undercut was not associated with the crack.

Figure 2: The 2-inch pipe and flange shown as-received at SES for analysis. Approximate location of the leak is indicated by an arrow in the lower photograph.
Figure 3: After several attempts, penetrant examination of the outside surface of the pipe revealed a faint indication in the pipe adjacent to the weld. The lower photograph shows a crack-like, linear feature at the location of the penetrant indication. The faint presentation of the indication indicates the “crack” was tight and absorbed little penetrant.
Figure 4: Fluorescent penetrant examination of the inside surface of the pipe revealed a small, approximately 0.4 inch long crack adjacent to the weld. Slight undercut was observed along the weld toe but was not associated with the crack.
Figure 5: An unetched metallographic cross section of the crack responsible for the leak. Crack is branched and transgranular, characteristic of stress corrosion cracking.

Metallographic Examination

Once the precise location of the crack was determined, a metallographic cross-section of the crack was prepared at one end of the crack. Figures 5 and 6 show unetched and etched views of the crack, respectively. The crack is highly branched and transgranular (running across the grains of the microstructure), characteristic of stress corrosion cracking in austenitic stainless steels. Figure 7 shows a close-up at higher magnification of the branched, transgranular morphology of the crack.

The general microstructure of the pipe material was documented and is provided in Figure 8. The general microstructure of the pipe consisted of equiaxed austenite grains with bands of residual ferrite, the dark etching, approximately linear features in the microstructure. The residual ferrite bands are typical of formed stain- less steels.

Figure 6: An etched metallographic cross section of the crack responsible for the leak. Crack is branched and transgranular, characteristic of stress corrosion cracking.

Scanning Electron Microscope (SEM) Examination

The crack that resulted in the leak was broken open to expose the two-mating surface of the crack surfaces, shown in Figure 9. The crack surfaces were analyzed using an SEM in conjunction with energy dispersive x-ray spectroscopy (EDS) to determine the cause of the transgranular stress corrosion cracking observed in the pipe (the leak). Austenitic stainless steels typically suffer stress corrosion cracking as a result of either chlorides (chlorine) or caustic (sodium hydroxide) exposure. EDS analysis of the crack surface indicated the presence of oxygen, vanadium, silicon, iron, chromium with traces of nickel, sulfur and chlorine. The presence of chlorine and not sodium indicates the branched, transgranular crack was a result of chloride stress corrosion cracking and not caustic cracking.

Chemical Analysis

 To confirm the materials of construction of the pipe, weld and flange, optical emission spectroscopy was performed on each to determine their chemical compositions. Table 1 provides the results of the chemical analyses along with the chemical requirements for types 321 and 347 stainless steels for comparison.

The pipe satisfied the compositional requirements of type 321 stainless steel. The weld was potentially slightly low on niobium concentration since the tantalum concentration is unknown; however, there was some minor amount of titanium, another carbide-forming element. The flange was slightly low in chromium concentration, but slightly high in nickel concentration.

Conclusion

Based on the analyses of the leaking 2-inch, type 321 stainless steel pipe, the pipe leaked as a result of chloride stress corrosion cracking. The leak consisted of a tight, through-wall crack that was high- ly branched with a transgranular cracking mode, consistent with stress corrosion cracking in austenitic stainless steels. EDS analysis of the crack revealed the presence of significant amounts of chlorine with no sodium detected indicating the cracking resulted from chloride stress corrosion cracking. No defects or anomalies were observed in the pipe, weld or flange that contributed to the cracking. The chemistry of the pipe met the compositional requirements of type 321 stainless steel. The weld met the compositional requirements of type 347 stainless steel except for being possibly low on niobium as the tantalum content was not analyzed. The chemistry of the flange was low on chromium concentration, but slightly high on nickel concentration.

Figure 7: Etched micrograph showing the branched, transgranular nature of the crack at higher magnification.
Figure 8: General microstructure of the pipe consisted of equiaxed austenite grains with bands of ferrite (dark, somewhat linear features).
Figure 9: The through-leak/crack in the pipe broken open to expose the mating surfaces of the crack. Mul- tiple secondary cracking visible on the crack surfaces is consistent with the branched cracking seen in Figure 7.

References:

  1. JC. Lippold and D.J. Kotecki, “Welding Metallurgy and Weldability of Stainless Steels”, John Wiley & Sons, Inc., 2005.
  2. A. J. Sedriks, “Corrosion of Stainless Steels”, John Wiley & Sons, Inc., 1996.
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