Causes of cracks on 00Cr17Ni14Mo2 steel flange
Cracks were found on a 00Cr17Ni14Mo2 steel flange when it was ready for welding and reuse after removal. The causes of cracks on the flange were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, fracture analysis and mechanical property test. The results show that the cracking property of the flange is stress corrosion cracking, and the cracking mainly occurs at the position with the largest tensile stress; The carbon content of the flange is too high, which reduces the corrosion resistance of the material, and finally leads to cracks on the flange.
0. What is 00Cr17Ni14Mo2 steel?
Table of Contents
00Cr17Ni14Mo2 steel is an ultra-low carbon austenitic stainless steel with good welding performance and corrosion resistance to sulfite, phosphoric acid and sulfite. It is generally used for manufacturing chemical and industrial equipment. A company purchased a finished flange for the connecting pipe of phosphoric acid+ammonium phosphate unit. The flange is made of 00Cr17Ni14Mo2 steel. Cracks were found on the flange when welding and preparing for secondary use. The author analyzed the causes of cracks on the flange by means of macroscopic observation, chemical composition analysis, metallographic examination, fracture analysis and mechanical property test, so as to avoid the recurrence of such problems.
Chemical composition (mass fraction) (wt.%) of the 00Cr17Ni14Mo2
| C(％) | Si(％) | Mn(％) | P(％) | S(％) | Cr(％) | Ni(％) | Mo(％) |
|---|---|---|---|---|---|---|---|
| 0.03 | 1.0 | 2.0 | 0.035 | 0.03 | 16.0-18.0 | 12.0-15.0 | 2.0-3.0 |
Mechanical Properties of steel grade 00Cr17Ni14Mo2
|
Yield Rp0.2(MPa) |
Tensile Rm(MPa) |
Impact KV/Ku (J) |
Elongation A (%) |
Reduction in cross section on fracture Z (%) |
As-Heat-Treated Condition | Brinell hardness (HBW) |
|---|---|---|---|---|---|---|
| 987 (≥) | 172 (≥) | 22 | 43 | 13 | Solution and Aging, Annealing, Ausaging, Q+T,etc | 141 |
Physical Properties of steel grade 00Cr17Ni14Mo2
|
Temperature (°C) |
Modulus of elasticity (GPa) |
Mean coefficient of thermal expansion 10-6/(°C) between 20(°C) and |
Thermal conductivity (W/m·°C) |
Specific thermal capacity (J/kg·°C) |
Specific electrical resistivity (Ω mm²/m) |
Density (kg/dm³) |
Poisson’s coefficient, ν |
|---|---|---|---|---|---|---|---|
| 24 | – | – | 0.21 | – | |||
| 577 | 572 | – | 14.3 | 141 | – | ||
| 261 | – | 44 | 22.2 | 132 | 211 |
Heat Treatment of steel grade 00Cr17Ni14Mo2
- Heat treated : 1186°C – 1172°C
1. Physical and chemical inspection
1.1 Macro observation
The macro morphology of the cracked flange is shown in Figure 1. It can be seen from Figure 1 that the cracks are located at the inner side of the connecting hole and distributed in a cross line along the radial direction. The opening on the flange surface is wide and the weld seam side is thin. The cracks extend from the flange side to the weld seam side and have penetrated the flange thickness; There are cracks on the outside of two connecting holes at one side of the weld, and the crack propagation direction is consistent; There is a branch at the end of the crack outside the connecting hole, which is in a “herringbone” shape.
Manually open the flange along the crack and observe the fracture morphology, as shown in Fig. 2. It can be seen from Figure 2 that the crack section is relatively flat, covered by light green attachments, and no macroscopic plastic deformation trace is found near the fracture; The manually torn area is metallic.
1.2 Chemical composition analysis
The chemical composition analysis of the cracked flange samples is shown in Table 1. It can be seen from Table 1 that the carbon element content of the flange exceeds the requirements for 00Cr17Ni14Mo2 steel in Stainless Steel Bars (GB/T1220-2007), and the content of other elements meets the requirements of GB/T1220-2007.
1.3 Fracture analysis
Use Quanta 250 scanning electron microscope (SEM) to observe the fracture, and the results are shown in Figure 3. It can be seen from Fig. 3 that the fracture surface is mostly covered by attachments, and the morphology cannot be distinguished. The intergranular fracture surface and secondary cracks distributed along the grain boundary can be seen at the end of the crack growth; The fracture surface of the artificial tear area with very small area is dimple morphology.
Figure.1 Macro appearance of cracked flange
Figure.2 Macro morphology of fracture surface
Table.1 Chemical composition analysis results of cracked flange
| Project | Quality Score | |||||||
| C | S | P | Si | Mn | Cr | Ni | Mo | |
| Measured value | 0.17 | 0.028 | 0.031 | 0.5 | 1.12 | 15.4 | 9.15 | 1.56 |
| Standard value | ≤0.030 | ≤0.030 | ≤0.045 | ≤1.00 | ≤2.00 | 16.00–18.00 | 10.00–14.00 | 2.00–3.00 |
Figure.3 SEM morphology of fracture surface
The energy spectrometer (EDS) is used to analyze the attachments on the fracture surface. The results are shown in Fig. 4. It can be seen that the attachments mainly contain oxygen, phosphorus and other elements in addition to metal matrix elements.
Figure.4 EDS Analysis Results of Adhesion on Fracture Surface
1.4 Metallographic inspection
The metallographic sample is cut longitudinally at the fracture surface of the cracked flange. After grinding and polishing, the non-metallic inclusion is graded according to GB/T10561-2005 Determination of the Content of Non metallic Inclusions in Steel by Micrographic Examination of Standard Rating Chart. The results are A1.5, B1, C1.5 and D1.
After the sample is eroded by ferric chloride+hydrochloric acid+ethanol solution, observe the microstructure of the sample, and the results are shown in Figure 5. It can be seen from Fig. 5 that the matrix structure of the sample is austenite+carbide distributed along the grain; There are dendritic cracks extending to the weld seam on the surface of the inner hole, and there are obvious bifurcations at the tail, showing typical stress corrosion cracking characteristics [1]. There are many secondary fine cracks distributed along the grain boundary on both sides of the main crack and the crack tail.
Figure.5 Microstructure and morphology of cracked flange
1.5 Mechanical property test
The mechanical properties of the cracked flange samples were tested, and the results are shown in Table 2. It can be seen from Table 2 that the hardness of the flange is higher than the requirements of GB/T1220-2007, and the tensile strength meets the requirements of GB/T1220-2007.
Table.2 Mechanical property test results of cracked flange
| Project | Hardness/HBW | Tensile strength/MPa |
| Measured value | 198 | 749 |
| Standard value | ≤187 | ≥480 |
2. Comprehensive analysis
According to the macro observation and fracture analysis results, the cracked flange has undergone stress corrosion cracking. The use environment of the flange is phosphoric acid+ammonium phosphate, and the fracture surface contains a lot of oxygen and phosphorus elements, so it is determined that the crack is a stress corrosion crack. Stress corrosion cracking is the cracking of metal parts caused by stress (applied external stress or residual stress) and corrosion medium [2].
According to the chemical analysis results, the carbon element content of the cracked flange seriously exceeded the standard. High carbon content reduces the corrosion resistance of the material, and the material is prone to stress corrosion cracking under the combined action of tensile stress and corrosion medium.
The connection method of flange and pipe is to insert the pipe into the proper position of the flange inner hole, and then conduct overlap welding. In this state, the flange is subject to the maximum tensile stress in the 90 ° crosshair direction, where local micro areas produce slip steps, which destroy the protective film on the surface and expose the fresh alloy surface. A large number of dislocations are accumulated on the slip band near the slip step, and a small amount of alloy elements and impurity atoms are precipitated on the slip band, which makes the metal near the slip step activated and become the anode of electrochemical corrosion. The unbroken area of the protective film becomes the cathode, and electrochemical corrosion occurs in a specific weak corrosion medium [3]. Stress corrosion cracking is caused by the mechanical action of small stress, slip steps and electrochemical anodes, and the essence of corrosion is chemical action. Therefore, the place where the maximum tensile stress of the flange meets the hypothesis of mechanical electrochemical reaction, which is very easy to produce stress corrosion cracks, and the main location of the flange cracks also corresponds to it. Phosphoric acid and ammonium phosphate are sensitive stress corrosion media, and the environmental conditions are easy to cause electrochemical corrosion of Fe Cr Ni austenitic stainless steel.
3. Conclusions and Suggestions
The cracking nature of the flange is stress corrosion cracking, which is caused by the high carbon content in the flange, which reduces the corrosion resistance of the material and promotes the stress corrosion cracking of the flange; There are assembly stress and residual stress in the service of materials, and the service environment meets the medium conditions for stress corrosion.
It is recommended to select appropriate materials and assembly measures to reduce the assembly stress of parts. Do not force assembly. During use, the flanges shall be sealed with gaskets to avoid direct contact with corrosive media.
Author: Zhou Baoyu
Source: China Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
Reference:
- [1] Hu Shiyan. Mechanical Failure Analysis Manual [M]. Chengdu: Sichuan Science and Technology Press, 1989
- [2] Chen Yu. Fracture Analysis of Natural Gas Drill Pipe Hanger [J]. Physical and Chemical Inspection (Physical Volume), 2012, 48 (7): 483-490
- [3] Tian Yongjiang. Fracture Failure Analysis of Metallic Materials [M]. Beijing: Beijing Institute of Aeronautics, 1982
