Analysis of the causes of cracking and countermeasures of 304 stainless steel pipe fittings

During the inspection of a newly-built atmospheric and vacuum distillation unit (about six years idle), many cracks were found on 304 stainless steel (06Cr19Ni10) pipe fittings (flanges, elbows, tees). Two flanges with through cracks were dissected and sampled for inspection and analysis, and other pipe fittings were subjected to surface flaw detection, metallographic inspection and hardness test. Based on the comprehensive analysis of various inspection results, it is believed that the main reasons for the occurrence and propagation of cracks are that the solution heat treatment of pipe fittings does not meet the requirements, the metallographic structure has uneven austenite structure, the precipitated phase of chromium carbide, and there are suspected cracks around the precipitated phase; The solution heat treatment quality of 304 stainless steel pipe fittings must be strictly controlled, and the products must be metallographically inspected.

304 stainless steel (06Cr19Ni10) has good heat resistance, corrosion resistance, hot and cold processing properties and welding properties, widely used in high temperature corrosion parts of oil refining and chemical devices, is essential to ensure the safe operation of the device. In a new normal pressure reducing device inspection, found 304 stainless steel (06Cr19Ni10) pipe fittings (flange, elbow, tee) in many places there are cracks, seriously affecting the safe operation of the device. In order to analyze the causes and mechanisms of cracks in 304 stainless steel pipe fittings, completely eliminate hidden dangers, the presence of cracks through the two pieces of flange anatomical sampling test analysis, other parts of the surface flaw detection, metallographic inspection and hardness testing. Comprehensive analysis that the solid solution heat treatment of the fittings did not meet the requirements, the metallographic organization of austenite organization is not uniform, chromium carbide precipitation phase, precipitation phase is granular, striped, chain distribution, precipitation phase around the existence of suspected cracks, etc., resulting in the generation and expansion of cracks. The outer surface inspection did not find cracks in the pipe fittings, there is also the possibility of cracks on the inner surface or cracking again during operation, need to be handled carefully.

1. Stainless steel pipe fittings for surface inspection

1.1 External surface penetration inspection

A total of 27 pipelines were tested, with a total of 311 pieces of flanges testing, according to NB T47013.5 “Nondestructive Testing. Penetration Inspection”, a total of 14 pieces of pipe fittings were detected, 9 pieces of flanges, 3 pieces reducers, 1 piece of tees, 1 piece of elbows. Example: No. P-10714, flange size DN150 found surface cracks in the whole circle, flange side, at the base material and near the weld seam (see Figure 1).

1.2 Metallographic inspection

A total of 11 pipeline flanges and elbows, numbered 1-11 #, according to GB T17455-2008 “non-destructive surface testing of metallographic replication techniques” requirements for on-site metallographic inspection, test results are shown in Table 1 and Figure 2-4. flange metallographic organization for austenite + precipitation phase, precipitation phase is granular, striped, chain-like distribution, of which 11 # There are suspected cracks around the precipitation phase in the point organization.

20221212025539 28584 - Analysis of the causes of cracking and countermeasures of 304 stainless steel pipe fittings
Figure.1 P-107

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Figure.2 1# flange metallographic organization (400×)

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Figure.3 6# flange metallographic organization (400×)

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Figure.4 11# elbow metallographic organization (400×)

1.3 Hardness test

According to the standard GB/T 20878-2007, the Brinell hardness value of 304 stainless steel ≤ 201. The test results are within the qualified range. Hardness test results are shown in Table 1.

1.4 Other inspection

All 304 stainless steel flanges with a magnet for ferromagnetic inspection, are showing strong ferromagnetism, manifested as non-normal austenite organization.

2. Chemical composition recheck

According to the standard GB/T 20878-2007, 304 stainless steel components standard values as shown in Table 2.
Table.1 Metallographic test results hardness test results statistics

Sample No Line No Metallographic structure Drawing No Hardness value
1# P1603-4 Austenite+precipitated phase, the precipitated phase is granular, banded and chain distributed Figure 2 150154 154
2# P 10615-4 Austenite+precipitated phase, the precipitated phase is granular, banded and chain distributed 141144 145
3# P 10702-1 Austenite+precipitated phase, the precipitated phase is granular, banded and chain distributed 150 145 141
4# P 10705-2 Austenite+precipitated phase, the precipitated phase is granular 156158 154
5# P10413-2. Austenite+precipitated phase, the precipitated phase is granular 143 145 146
6# P10712-3 Austenite+precipitated phase, the precipitated phase is granular Figure 3 149150 160
7# P10712-2. Austenite+precipitated phase, the precipitated phase is granular 135 139 140
8# P10712(Elbow) Austenite+precipitated phase, the precipitated phase is granular 130125 1 37
9# P10611 Austenite+precipitated phase, the precipitated phase is granular 160 185 182
10# P20412-1 Austenite+precipitated phase, the precipitated phase is granular 141 145 150
11# P10611-1 Austenite+precipitated phase, the precipitated phase is in granular, strip and chain distribution, and there are suspected cracks around the precipitated phase Figure 4 141150 139

The presence of penetration cracks in the flange to take four samples, component testing, the results are shown in Table 2.
The average value of the test and the standard components for comparison, the components of the specimen and the standard value is similar, Mn lower than the standard value of about 0.57.
Table.2: Component test values of specimens

Project C Si Mn P S Cr Mo Ni Cu Fe
Mean value 0.081 0.85 1.43 0.036 0.0035 18.03 0.065 8.015 0.33 70.21
Standard value 0.08 1 2 0.05 0.03 18.8-20.0  / 8.0-11.0 / /

3. Metallographic examination of two pieces of flanges with penetration cracks

The organization of the base material is relatively small and uniform, but there is a large number of precipitated phases on the grain boundaries, and the precipitated phase is Cr-rich phase by EDS analysis, which may be Cr23C6 carbide. Optical microscope observation of the presence of some black stripes at the grain boundary of the base material, SEM analysis, such stripes for the microcrack along the grain boundary, the specimen weld and the base material metallographic photos shown in Figure 5.
The main crack and secondary cracks are along the grain cracks, crack at the metallographic photo in Figure 6.
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Figure.5 specimen metallographic picture
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Figure.6 flange crack at the metallographic picture

4. 304 stainless steel pipe fittings cracking cause analysis

  • 1) From the metallographic photo can be obtained, the parent material organization is a typical single-phase austenite organization, there are more thin stripes microcrack in the metallographic picture. There is a large number of precipitation phase on the grain boundary of the base material organization, and the precipitation phase is Cr-rich phase by EDS analysis, which may be Cr23C6 carbide.
  • 2) Overlay weld organization is columnar crystal austenite organization, near the fusion line austenite columnar crystal will have a certain degree of deformation.
  • 3) According to the crack cross-section metallographic photos, the preliminary judgment of the cracking form of cracking along the crystal. There is a large amount of carbon-rich material between the cracks (about 70%) and the precipitation phase at the grain boundaries.
  • 4) 304 stainless steel flange according to the current product standards, the supply state is solid solution treatment. Flange metallographic organization for austenite + precipitation phase, if the standard requirements for solid solution treatment, precipitation phase should be absorbed.
  • 5)Ferrite mainly exists in the precipitated phase, and the quantity is high, so the flange shows a strong ferromagnetism.
  • 6) chemical composition and hardness testing basically meet the standard requirements.

Comprehensive examination and analysis of the above, 304 stainless steel pipe fittings are not solid solution treatment according to the standard requirements, metallographic organization there is a large number of precipitation phase, precipitation phase is granular, striped, chain-like distribution, the presence of suspected cracks around the precipitation phase, etc., is the main factor in the generation and expansion of cracks.

5. Conclusions and recommendations

  • 1) 304 stainless steel pipe fittings after the installation is completed (not put into production), there is the possibility of cracking at room temperature.
  • 2) Solution treatment is not sufficient, austenite tissue grain boundaries on the existence of a large number of precipitation phase, is the main cause of 304 stainless steel pipe fittings crack generation and development.
  • 3) should be 304 stainless steel pipe fittings solution heat treatment for strict quality control, metallographic inspection, to ensure that the solution heat treatment to meet the requirements.
  • 4) For cylindrical pressurized equipment, its inner wall stress is higher than the outer wall stress, when defects appear on the outer wall, theoretically the possibility of defects on the inner wall is not lower than the outer wall, so the problem of defects on the inner wall of the pipe also requires special attention.

Author: Tao Tao

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

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