Failure Analysis of Duplex Stainless Steel Tube Cracking after Hydraulic Expanding

In the petroleum, chemical, electric power and other industrial fields, heat exchangers are widely used as a common pressure vessel. The most commonly used is the shell-and-tube heat exchanger, which consists of a casing, a heat transfer tube bundle, a tube sheet, and a baffle. (Baffle) and tube box and other components.
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In the choice of heat exchanger materials, depending on the temperature and environment, metal, plastic, ceramics and other materials can be used. However, for some equipment operating under severe conditions such as high pressure and strong corrosion, 2205 type ferrite is used at home and abroad. Body-austenitic duplex stainless steel material, which has high strength, excellent toughness, good weldability and excellent resistance to Cl-corrosion compared with conventional common austenitic stainless steel, greatly improving the heat exchanger Service life.

The connection between the heat exchange tube and the tube sheet is one of the most critical technologies in the design and manufacture of the shell-and-tube heat exchanger, and it is also the part with the highest accident rate of the heat exchanger. Therefore, the connection between the heat exchange tube and the tube sheet may not be changed. The heater expires prematurely, causing production stoppages, casualties, etc.

1 corrosion investigation
In a project, 2205 duplex stainless steel was used to make the shell-and-tube heat exchanger. When the tube bundle was connected with the tube sheet by the expansion tube process, the tube wall of the heat exchange tube was found to be broken. The heat exchange tube used was a seamless steel tube and used before the expansion tube. Endoscopy did not reveal the presence of cracks. This investigation investigated the heat transfer tubes that failed after the expansion process, using metallographic microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) techniques. Analyze the axial and radial microstructures near the cracks, determine the cause of the rupture of the 2205 duplex stainless steel heat exchanger tubes, and eliminate the safety hazards of the failed heat exchangers into the market through targeted control to avoid economic losses.

2 the test process and results
2.1. Macroscopic observation of the rupture heat transfer tube is 2205 duplex stainless steel tube, outer diameter 20 mm, wall thickness 2 mm, wall cracking. The stainless steel nozzle near the crack end is defined as the 0 mm position of the tape measure and the long crack at 70 ~ 150 mm. The most obvious (Figure 1). Macroscopic observation of the crack is not a straight type, but a spiral type. Using a stereo microscope to check along the stainless steel tube, it is found that there is a small crack at the 0 mm position of the heat exchange tube and it is not penetrated, and the 650 mm position can still see tiny Crack, and the crack never stops from 0~ 650 mm, but the crack penetrates the pipe wall only at the position of 70 ~ 150 mm, and the other position is non-penetrating crack.

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Figure 1 Photograph of stainless steel heat exchange tube cracking

2.2. From the fracture analysis, the outer wall of the stainless steel pipe is about 365μm, and the fracture surface shape is relatively smooth, which may be caused by longer oxidation time. No obvious fracture characteristics are observed. The outer surface of the pipe is more than 365μm, and the area is obviously tough. The socket is ductile fracture, and the direction of the inner dimple of the tube wall is consistent downward, which is caused by the tear of the expansion tube. It can be inferred from the fracture structure of the stainless steel that the outer wall is oxidized to the area of the first crack, while the middle and the inner side are After the fracture area.

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Figure 2 Micro appearance of the fracture surface

2.3. From the results of metallographic structure and phase analysis, the microstructure near the surface crack is abnormal. On the one hand, the grain at the crack is coarse, and it does not become long with the cold rolling process in the production process of the steel pipe. On the other hand, according to the results of EBSD, it is known that the austenite structure is basically single phase near the crack, while the normal duplex steel microstructure ferrite and austenite are uniformly distributed. From the radial metallographic structure of the penetrating fracture It can be seen that although the grain is obviously deformed by the stretching effect on the inner wall portion, it can be seen that it is a normal two-phase structure.

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Figure 3 Radial and axial cross section microstructure of duplex stainless steel tube

It can be seen that the crack source of stainless steel is located on the surface austenite strip region. The middle and inner sides of the tube wall have good plasticity due to normal structure. When the tube is subjected to force, the yield is deformed and finally broken. From the analysis results of SEM and EDS, The crack penetrates the band-shaped coarse austenite crystal region. The semi-quantitative results of EDS show that the content of N element in the coarse austenite region near the crack is higher, 0.45%, while the content of N element in the two-phase region is 0. At 19%, the content of N in the transition zone showed a decreasing trend. The content of other elements changed little in each region. In duplex stainless steel, N is a strong austenitizing element and the austenitizing effect is 30 times that of Ni. With the increase of N content, the ferrite volume fraction gradually decreases, the austenite volume fraction increases gradually, and the size increases. The austenite morphology changes from a long strip to a near-spherical shape. The coarse austenite near the crack The plasticity and strength of the belt zone are worse than that of the normal duplex steel. The formation of Cr-rich hard and brittle phase at the grain boundary will inevitably further deteriorate the plasticity and strength of the material, resulting in microcracks during the cold working of the stainless steel tube.

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Figure 4 Micro appearance and EDS analysis in the crack

3. The conclusion and cause analysis
(1) The duplex stainless steel tube has microcracks before the expansion tube.
Analysis of the cause: Before the expansion of the stainless steel tube, the stainless steel tube may form a coarse grained austenite single-phase band in the smelting and casting process due to the uneven composition of the micro-region, and there is a hard and brittle Cr-rich sheet precipitated. Therefore, the abnormal microstructure of the surface of the steel pipe is very poor. During the processing of the steel pipe, the surface of the steel pipe is forced to cause microcracks. Since the abnormal structure is only in the region of the steel surface with the austenite band down to about 365 μm, the area below the normal structure is The plasticity is good, so that the penetrating crack is not formed after the cold processing of the stainless steel tube is completed. When the heat exchanger manufacturer uses the steel tube to make the heat exchanger, the hydraulic tube expanding process makes the originally uniformly stressed stainless steel tube in the micro crack. Concentration of stress at the tip causes the steel tube to rupture.
Inhibition measures: The production enterprise can eliminate some hidden dangers by infiltrating the surface micro-cracks. In addition, after the expansion joint connection process, the enterprise also uses the endoscope to check and pass the hydraulic test above the pressure for reliability. Test to avoid safety and economic problems caused by the inflow of failed heat exchangers into the market.
(2) The duplex stainless steel tube forms a coarse austenite band due to the segregation of N element, and a flaky Cr-rich phase precipitates at the grain boundary, which causes the local plasticity of the steel tube to deteriorate, resulting in micro-cracks in the steel tube during processing.
Inhibition measures: In the production process of the electrical connector, the degree of oxidation of the surface of the contact body, the degree of pickling and the residue of Cl – should be strictly controlled to ensure the dense, non-porous and cracked coating.

Source: China Duplex Stainless Steel Tubes Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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References:

  1. https://www.yaang.com

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