Cause analysis on cracking of SUS304 austenitic stainless steel flange

During the pressure test of a new process pipeline in a chemical plant, the leakage of an austenitic stainless steel flange on the pipeline led to the interruption of the pressure test. The main material of the pipeline is austenitic stainless steel SUS304. According to the site macroscopic inspection, the pipeline installation position is qualified, the support layout is reasonable, and the pipeline structure is normal, which meets the design and technical requirements.

The design pressure of the pipeline is 1.6Mpa, and the test pressure is 2.4Mpa. When the pressure is slowly raised to 0.6MPa, a flange leaks and the pressure test is interrupted. Macroscopic examination shows that the crack is located in the neck of flange, which is a circumferential crack almost parallel to the weld. The fracture surface of the crack is basically vertical to the surface, and the fracture is even. The metal adjacent to the fracture has no obvious plastic deformation, which can eliminate the flange material failure caused by overpressure. The phenomenon shows that the flange leakage is not caused by improper test operation.
Austenitic stainless steel is a common metal material in petrochemical plant, but cracking of austenitic stainless steel flange during pressure test is relatively rare. In order to find out the causes of cracks and evaluate the performance of other austenitic stainless steel flanges of the same batch as the cracked flange, it is necessary to conduct a comprehensive analysis of this problem and propose solutions to such problems.

Physical and chemical analysis

Firstly, the chemical composition of the cracked flange was analyzed, and the results are shown in Table 1.

Table.1 chemical composition analysis results of cracked flange

Name

C

Si

Mn

S

P

Cr

Ni

Flange

0.058

0.60

1.61

0.020

0.023

17.9

9.0

Weld metal

0.016

0.58

1.70

0.015

0.014

23.3

13.1

Standard value of SUS304

0.07

1.0

2.0

0.03

0.035

17

19

8.0

11.0

The analysis results show that the chemical composition of the flange and welding materials meet the requirements of relevant standards. Brinell hardness test was carried out on the outer surface and sealing surface of the flange neck, and the Brinell hardness test value met the requirements of jb4728-2000 standard. On the premise that the chemical composition of the material is qualified, the hardness test is qualified, the pipeline installation meets the technical requirements and the pressure test operation specification, the pipeline flange cracks during the pressure test, which indicates that the defects are related to the internal structure of the flange or other unknown factors.

Analysis of crack properties

Surface crack morphology

The leakage flange was cut off from the pipeline, and a comprehensive macro inspection was carried out. Cracks were found on the inner surface of the flange, with a maximum length of 3 mm and a width of 3 mm 0.5mm, and rust stains have been produced in and around the crack; many cracks are also found on the inner surface of the flange bolt hole; many non-metallic inclusions are found on the flange sealing surface and other positions, and some non-metallic inclusions fall off and form small pits on the surface of the flange sealing surface, with the maximum pit of 2.0mm and the minimum of 0.8mm. There are micro cracks visible to the naked eye at the edge of the pit.
On site macro inspection was carried out on other austenitic stainless steel flanges of the same manufacturer and the same batch. It was found that there were various degrees of defects in many flanges, and obvious cracks were found on the outer surface of some flanges. The macro examination results further show that the flange leakage is related to the factors of the flange itself.

Profile crack morphology

In order to further explore the real cause of flange leakage, a flat welding flange of the same manufacturer and batch was selected randomly, and the small pit was formed on the sealing surface due to the non-metallic inclusion falling off. The flange was divided into three parts with uneven size. After careful observation of its profile, it was found that 4 out of the 6 sections of the flange had 4 There are visible cracks in three sections, and the cracks almost run through the whole section.

In one of the sections, the cracks are more obvious after coarse grinding, fine grinding and corrosion, and almost run through the whole section. The crack has a certain width. Most of the cracks contain black non-metallic inclusions. One of the inclusions is about 3.0 mm away from the sealing surface of the flange with a diameter of about 2.5 mm. There are obvious repair welding traces from the upper part of the inclusion to the sealing surface. The repair welding area is about 380 mm 2 and the repair welding depth is 3.0 mm (see Fig.1).
20200710234236 89921 - Cause analysis on cracking of SUS304 austenitic stainless steel flange

Fig.1 crack, black inclusion and repair weld trace on section I

But for the newly purchased product, many cracks containing non-metallic inclusions were found on the flange section. It can be preliminarily determined that the cracks are related to a large number of non-metallic inclusions. But the repair welding trace on the section is a kind of puzzling and abnormal phenomenon.

Metallographic structure analysis

In order to further explore the causes of cracks and find out whether the newly purchased flange has undergone repair welding before being assembled and welded to the pipeline, metallographic examination was carried out on the crack position of flange section.
20200711004047 57657 - Cause analysis on cracking of SUS304 austenitic stainless steel flange

Fig.2 strip non-metallic inclusions in crack (× 100)

20200711004347 73281 - Cause analysis on cracking of SUS304 austenitic stainless steel flange

Fig.3 scattered non-metallic inclusions (× 100) on the profile

20200711004535 89907 - Cause analysis on cracking of SUS304 austenitic stainless steel flange

Fig.4 crack tip and inclusion morphology of flange section (× 100)

20200711004624 54949 - Cause analysis on cracking of SUS304 austenitic stainless steel flange

Fig.5 structure of welded joint on flange section (× 100)

Take the test piece described in Fig.1, grind and polish the position with repair welding trace on the flange section, observe under 100 times microscope, the crack is more clear, the long crack contains large volume of strip non-metallic inclusions (see Fig.2) and scattered point non-metallic inclusions on the profile (see Fig.3); the crack tip is blunt, and no obvious abnormal phenomenon is found on both sides of the crack (see Fig.4) The metallographic structure contains dendrite austenite, which is a typical structure of austenitic stainless steel welding joint (see Fig.5).

Crack property judgment

Based on the above analysis results of crack morphology, fracture characteristics, crack morphology and metallographic structure, it is shown that the cracks were produced in the process of flange production and belong to forging cracks along the tip of inclusions. In order to eliminate the obvious cracks on the flange surface, the flange was indeed repaired before delivery.

Cause analysis of cracks

A large number of non-metallic inclusions are the source of cracks

Because there are a lot of non-metallic inclusions in the forging blank for manufacturing flange, in the forging process, the volume type inclusion is forged repeatedly and becomes the area type inclusion, which causes the forging crack along the inclusion tip in the material. The larger inclusions become larger cracks after forging, and the smaller inclusions become smaller microcracks. Most of these cracks are buried cracks.
After machining, some embedded cracks become surface cracks, which is the reason why there are many micro cracks on the inner and outer surface of flange and bolt hole. For the large cracks on the flange surface, in order to make the products leave the factory, the manufacturer adopts the means of grinding, eliminating the surface cracks, repairing welding and re machining to make the appearance of the flange free of obvious defects. This is the reason why the flange repair welding area is 380mm2 and the repair welding depth is 3mm.

The forging cracks further expand under the test pressure, resulting in the final leakage of the flange

A large number of non-metallic inclusions will reduce the plasticity and toughness of the material. The slag inclusion in the crack will cause stress concentration at the crack tip, and the flange can not bear the external load. As a result, during the pressure test, the test pressure just reaches 0.6MPa, and the flange will crack and leak.

Measures taken

  • (1) Replace all problem flanges.
  • (2) Continue to search for other pipe fittings of the same manufacturer and batch used in the device, adopt corresponding inspection methods to confirm whether it is qualified.
  • (3) For the newly replaced flange, in addition to rechecking its chemical composition and mechanical properties, macroscopic inspection should be carried out to evaluate its geometric dimension and surface finish, and to check whether there is any discontinuity of abnormal geometric dimension.

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference:

  • [1] Xiao Jimei. Metallographic problems of stainless steel [M]. Beijing: Metallurgical Industry Press
  • [2] Lu Shiying. Stainless steel [M]. Beijing: Atomic Energy Press, 1995
  • [3] Pan Yong. Forging process of stainless steel flange [J]. Guangzhou shipbuilding technology, 2004 (3): 45-46
  • [4] The editorial board of this book. Examples of metallographic inspection of metal material defects and defect metallographic chart
  • [M] Beijing: China Knowledge Press, 2006
  • [5] Hao Ruiqin, Pei Tingzhen, Yang Shuju. Genesis analysis of inclusions in type 18-8 stainless steel and Study on the way to reduce the inclusion content [J]. Shanxi metallurgy, 2000 (2): 31-32, 96
  • [6] Fan Guangwei, Wang Guiping, Li Zhibin. Sources of nonmetallic inclusions in 0Cr18Ni9 stainless steel [J]. Journal of Beijing University of science and technology, 2007,29 (8): 776-780

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