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 ( H 2

S ). 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 . 1 To be classified as stainless , the steel must contain at least 11 % chromium . This amount of chromium prevents rust formation in most environments . 2 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 ,

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

reportedly carrying hydrogen , water , and hydrogen sulfide ( H 2

S ) 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 pipeto-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 .

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 .

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 .

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

Element Pipe Weld Flange Type 321 SS Type 347 SS Carbon 0.056 0.054 0.056 0.08 max 0.08 max

Manganese 1.64 1.59 1.93 2.0 max 2.0 max Silicon 0.33 0.38 0.63 1.0 max 1.0 max

Phosphorous 0.037 0.027 0.020 0.045 max 0.045 max Sulfur < 0.003 0.004 < 0.003 0.030 max 0.030 max

Chromium 18.13 18.37 15.94 17.0 – 19.0 17.0 – 19.0 Nickel 10.21 10.49 12.23 9.0 – 12.0 9.0 – 13.0

Molybdenum 0.29 0.27 0.34 - -

Titanium 0.469 0.089 0.491

5 x C min ( 0.28 min )

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12 Stainless Steel World Americas - August 2023 | www . ssw-americas . com