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Analysis and Discussion on short term leakage failure of duplex stainless steel 2205 heat exchange tube

Duplex stainless steel is widely used in some aggressive working conditions in petrochemical enterprises. However,after short t-erm operation of the oil /gas heat exchanger of crude oil and atmospheric tower overhead system of a petrochemical enterprise,there appeared several leaking points at the location of heat exchanger tubesheet. In this paper,the failure causes of this heat exchanger tube due to its short- term leakage was comprehensively analyzed by detection methods of corrosion morphology,metallography, hardness,etc. And in combination with the corrosion environment of the heat exchanger tube,which pro- vide technical support for the subsequent corrosion protection of duplex stainless steel.

Duplex stainless steel (DSS) is a kind of stainless steel with ferrite and austenite accounting for 50% and 30% respectively. This kind of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferritic stainless steel, this kind of steel has higher plasticity and toughness, no room temperature brittleness, and its intergranular corrosion resistance and welding performance are significantly improved; compared with austenitic stainless steel, it has high strength, intergranular corrosion resistance and chloride stress corrosion resistance. Based on the above characteristics, duplex stainless steel is widely used in some harsh working conditions of petrochemical enterprises. Although duplex stainless steel has good corrosion resistance, failure accidents due to corrosion of duplex stainless steel or other reasons still occur frequently in some special environments. After a short-term (2 months) operation of overhead crude oil-gas heat exchanger of atmospheric tower of a petrochemical distillation unit, several leakage points occurred in the expansion joint area of several pipe heads. In order to understand the cause of short-term leakage failure of heat exchange tubes, a leaky heat exchange tube and a group of non leakage heat exchange tubes were cut out from the extraction tube bundle of the heat exchanger. The failure causes were comprehensively analyzed through corrosion morphology analysis, metallographic analysis, hardness test and energy spectrum analysis, so as to take effective protective measures and avoid similar accidents.

Process analysis

The tube sheet and tube material of the heat exchanger are 2205 duplex stainless steel (0cr22ni5mo3n), and the medium is normal top oil and gas, and the temperature is 80 ℃. There are a lot of H2S, Cl – and H2O in oil and gas, which belong to the low-temperature corrosion of HCl + H2S + H2O in complex medium environment. When pH > 7, the dominant damage mechanism is acid water corrosion; when pH < 7, the dominant damage mechanism is hydrochloric acid corrosion. In addition, the presence of H2S will cause stress corrosion cracking of carbon steel and low alloy steel in wet hydrogen sulfide environment, and chloride ion will cause chloride stress corrosion cracking of austenitic stainless steel. Because the process of “three injection and one removal” is adopted in the normal roof, ammonia injection may also produce corrosion under scale of NH4Cl.

Analysis of appearance and material properties of leakage failure tube bundle

Macroscopic morphology

The heat exchanger is a U-shaped tube heat exchanger. The bent pipe is smooth tube, and the solid solution treatment is carried out according to the design requirements. The straight pipe is the spiral tube processed by rolling. Figure 1 shows the damage morphology of the groove on the outer wall of the heat exchange tube, and Figure 2 shows the local pitting damage morphology on the inner surface of the heat exchange tube.

20200716012641 74838 - Analysis and Discussion on short term leakage failure of duplex stainless steel 2205 heat exchange tube

Fig.1 spinning damage morphology of groove on outer wall of pipe section

20200716013151 53274 - Analysis and Discussion on short term leakage failure of duplex stainless steel 2205 heat exchange tube

Fig.2 local pitting damage morphology of inner surface of pipe section

The macroscopic features of the tubesheet and the intercepted tube of the leaky heat exchanger are as follows.
(1) Quantity and distribution characteristics of leakage tubes on tube sheet of heat exchanger

  • 1) There are more than 30 leakage tubes on the tubesheet;
  • 2) The location of the leakage pipe is irregular.

(2) The machining damage characteristics of heat exchange tube outer surface were investigated.
There are a lot of rolling defects on the outer surface of the spinning groove of the leaked heat exchange tube. Although there are many defects, the depth of the defects is relatively shallow, which is obviously not the cause of the cracking and leakage of the heat exchange tube in short-term operation, but it can not be ruled out that it will continue to expand in the long-term operation of the device.
(3) The corrosion damage characteristics of the inner surface of the heat exchange tube were investigated.
The pitting pits on the inner surface of the tube near the tubesheet are the most. With the increase of the distance from the tubesheet, the pitting corrosion decreases gradually. The shallow pitting depth on the inner surface of the heat exchange tube should not be the cause of the short-term cracking and leakage of the heat exchange tube.
Metallographic analysis
The local structure of the cross-section and longitudinal section of the pipe section was observed by metallographic microscope, as shown in Fig.3 and Fig.4. The matrix structure is composed of ferrite and banded austenite precipitates, the number of two phases is approximately the same, and the austenite precipitates are distributed in strips along the rolling direction of the steel pipe.

20200716013804 14857 - Analysis and Discussion on short term leakage failure of duplex stainless steel 2205 heat exchange tube
Fig.3 local microstructure and microcrack morphology of cross section of pipe section
20200716013956 68868 - Analysis and Discussion on short term leakage failure of duplex stainless steel 2205 heat exchange tube

Fig.4 local microstructure and microcrack morphology of longitudinal section of pipe section

There are longitudinal microcracks on the inner surface of the heat pipe. The radial length of these microcracks is short (within 150 μ m), but the number is large. In addition, it is found that the microstructure of the microcracks in the inner wall is different from that of the normal duplex stainless steel pipe. There is a thin layer (less than 100μm) of banded structure similar to columnar crystal layer along the tube wall in the microcrack region. The microcracks in the inner wall are basically distributed between the banded boundaries of this thin layer similar to columnar crystal layer. The existence of longitudinal microcracks reduces the load-carrying capacity, especially the plastic deformation capacity and welding performance of heat exchanger tubes.
There is an obvious deformation band near the spiral groove on the outer surface of the pipe section, and the direction of the deformation band completely surrounds the spiral groove processed by rolling. In addition, it is found that there are rolling cracks about 100 μ m in length at the bottom of some spiral grooves, which indicates that the spinning pressure is very high; the existence of spinning deformation zone indicates that the spiral heat exchange tube is not subject to solution stress relief annealing treatment after spinning.

Chemical composition analysis

Chemical composition analysis was carried out by sampling the elbow part of the heat exchanger,
The specific data are listed in Table 1. According to the analysis results, except for the difference of nitrogen data, other components meet the 2205 material standard.

Table.1 analysis results of chemical composition of heat exchange tube                                            

Element

C

Si

Mn

S

P

Cr

Ni

Mo

N

Heat exchange tube

0.02

0.57

1.17

0.005

0.016

22.32

5.25

3.13

0.247

2205 standard requirements

0.030

1.00

2.00

0.020

0.030

21.023.0

4.506.50

2.503.50

0.080.20

Hardness test

The hardness detection positions are selected on the inner surface of the spiral groove bottom of the spiral heat exchange tube and the inner surface of the pipe with the non machined spiral groove. The test results are shown in Table.2.
Table.2 hardness test results of 2205 heat exchange tube

Project

Serial number

Hardness HRC

Hardness HV

Bottom of spiral groove

1

2

36

35

355

340

3

34

330

Average value

35

341.7

The inner surface of pipe material is not machined

1

2

3

27.5

28.5

29.7

285

290

300

Average value

28.6

291.7

Standard of pipe material

20

260

Analysis of test results

Based on the above inspection results, the chemical composition and matrix structure (ferrite + banded austenite) of the heat exchange tube basically meet the requirements of 2205 duplex stainless steel standard. The main problems are as follows.
(1) There are a lot of microcracks in the inner wall of heat exchange tube.
Through metallographic analysis, it is found that there are a large number of microcracks in the parts near or far away from the tube head, which indicates that these cracks have been formed in the process of pipe rolling.
(2) There are a lot of machining defects in heat exchange tubes.
There is an obvious deformation band near the spiral groove on the outer surface of the heat exchange tube, and the direction of the deformation band completely surrounds the spiral groove processed by rolling. In addition, there are rolling cracks at the bottom of the spiral groove of some heat exchange tubes. The existence of rolling cracks indicates that the processing stress is very large; the existence of rolling deformation band indicates that the solution stress relief annealing is not carried out or the stress relief annealing is not sufficient.
(3) Local pitting damage of heat exchange tube.
After short-term operation, there are many but not very deep pitting pits (about 100 μ m) on the inner wall of the heat exchange tube. These corrosion pits show that the batch of heat exchange tubes have insufficient pitting corrosion resistance to working medium, which will obviously affect the service life of heat exchange tubes.
(4) Hardness of heat exchange tube.
The test results show that the hardness of the machined spiral groove is significantly higher than that of the untreated area, and the hardness of the heat exchange tube is higher, which will lead to the increase of the brittleness of the pipe and the decrease of the plastic toughness. The higher hardness should be caused by work hardening, which indicates that the deformation of heat exchange tube is too large when machining spiral groove, and the stress relief treatment is not carried out or the stress relief treatment is not sufficient.

Analysis of short term leakage of tube head on tube sheet of heat exchanger

Influence of pipe expansion construction after pipe head welding

According to the investigation, after the tubesheet welding, the pipe head was over expanded. Because the duplex stainless steel is an unstable austenite structure, austenite transforms into martensite after cold working, which makes the hardness and strength increase, and the brittleness sensitivity increases. At the same time, the stress of the expanded joint will also be very concentrated. BASF, Germany
Wh75-100 pipe to tubesheet welding standard stipulates that in the presence of stress corrosion environment, any type of expansion joint is not recommended

Effect of hardness

The hardness test results show that the average hardness of the failure part is higher than that of the unprocessed part. The main reason of high hardness is expansion joint construction and rolling processing. Hardness and stress level are two key parameters that affect the susceptibility to stress corrosion cracking (SCC). With the increase of hardness, the susceptibility of steel to SCC will also increase. In nace-mr0103-2012, NACE studied the hardness indexes of various materials for sulfide stress corrosion cracking (see Table 3). The hardness of dual phase steel for sulfide stress corrosion resistance is less than hrc28, while the average hardness of heat exchanger tubes failed this time reaches HRC The hardness value exceeding the standard means that strain martensite precipitates during cold working, which reduces the ability of dual phase steel to resist sulfide stress corrosion. The failure tube bundle is also in the wet hydrogen sulfide corrosion environment. If the hardness exceeds the standard, sulfide stress corrosion cracking will inevitably occur, which will accelerate the expansion of micro cracks existing in the pipe itself, which will lead to cracking and leakage.
Table 3 hardness index of steel against sulfide stress corrosion

Steel products

Carbon steel

1Cr-0.5Mo

1.25Cr-0.5Mo

2.25Cr-1Mo 5Cr-1Mo

9Cr-1Mo

Stainless steel (ferrite, martensite, austenite)

Duplex stainless steel

Hardness index

HB200

HB225

HB235

HB245

HRC22

HRC28

Epilogue

Based on the above analysis, the leakage failure of the heat exchange tube head is due to the wet hydrogen sulfide corrosion environment of the heat exchanger tube side, and the precipitation of martensite phase, high hardness and stress concentration caused by expansion joint and spinning processing will greatly increase the sensitivity of sulfide stress corrosion cracking. At the same time, the raw materials of heat exchange tubes do not meet the quality standards, and there are a large number of longitudinal microcracks on the inner wall. Although these microcracks are short in length, these microcracks will rapidly expand and form penetrating cracks under the promotion of sulfide stress corrosion during operation.
Due to the instability of duplex stainless steel structure, it is suggested that the expansion joint of duplex stainless steel tube and tube sheet should not be adopted. At the same time, rolling will also lead to strain martensite in the groove and increase the hardness. This structure is widely used. It is suggested that surface damage should be avoided during rolling and the whole pipe should be treated with solution after processing.

Author: Liu Min

Source: China 2205 Heat Exchange Tube Manufacturer – Yaang Pipe Industry (www.epowermetals.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@epowermetals.com

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