Failure Analysis of Header Flange Cracking in Propane Dehydrogenation Unit

The boiler fault of propane dehydrogenation unit is investigated by macroscopic inspections, mechanical properties testing, chemical composition analysis, metallographic microstructure, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The experiments confirm the reason for cracks occurrence of SA182-F347H boiler that the stress corrosion cracking occurs. The sulfur injected into the device reacts with the flange metal to produce sulfide corrosion products. The improper control of flange manufacturing process and the operating temperature in the sensitized range lead to the sensitization of flange materials and reduce the corrosion resistance. During the equipment shutdown, the sulfide corrosion products on the surface of the flange reacted with air and water to form polysulfuric acid, and causing stress corrosion cracking and leakage of the sensitized F347H stainless steel flange.


Introduction
Under the joint action of tensile stress and specific chemical medium, the phenomenon of low stress brittle fracture produced after a period of time is called stress corrosion fracture [1][2][3][4].Stress corrosion fracture is not caused by the superposition of the mechanical failure of the metal under the action of stress and the corrosive failure under the action of chemical media, but by the joint action of stress and chemical media.According to the holding mechanism, its fracture resistance is much lower than the sum of the single factors acting separately [5][6][7][8].The low stress brittle fracture caused by the combined action of tensile stress and corrosive medium is called stress corrosion.Stress corrosion fracture may occur in both ductile and brittle materials [9][10][11].
In this paper, the stress corrosion cracking behavior of stainless steel is discussed by taking the flange of the collecting pipe of a petrochemical propane dehydrogenation unit as an example.It is understood that the boiler of the 600,000-ton/year propane dehydrogenation unit of the first phase of the project of a petrochemical Co., Ltd.It was stopped for maintenance, and when the air tightness test was conducted before it was put into use, it was found that the flange of the outlet manifold of the radiation section was cracked, and resulting in gas leakage.The flange size is φ355×12.7mmand the material is SA182-F347H.After consulting the data, we found that the operation time of the equipment was 2 years and 5 months and the operating temperature was 620℃.The system injects trace liquid sulfur into the front-end reactor and the sulfur content in the medium is designed to be .The actual online monitoring value is 50ppm.In order to analyze the cause of cracking of the manifold flange, the failure analysis is carried out.

Macro Detection
The crack is located at the junction of the inclined side of the neck of the flange and the straight end, and the crack length is about 567mm.Cracks can be observed in both inner and outer walls and the flange has been cracked.There are black corrosion products attached to the inner wall of the flange and some black corrosion products peel off to reveal yellow corrosion products, as shown in figure 1.

Fracture
The fracture is analyzed by SEM and EDS.The fracture morphology is shown in figure 2(a).The surface is covered with thick corrosion products and the original morphology cannot be observed.The rock sugar shape covered by corrosion products can be seen locally at the crack source, which is intergranular brittle fracture, as shown in figure 2(b).The selected positions of the energy spectrum of corrosion products at the crack source are shown in figure 2 and the results are shown in table 1, where S content is higher.Scanning electron microscopy and energy spectrum analysis are carried out on the black and yellow corrosion products on the inner wall respectively.The morphology of corrosion products and the selection positions of energy spectrum analysis are shown in figure 3. The results of energy spectrum analysis are shown in table 2, in which the S content is very high.The inner wall corrosion products are scraped off and the phase composition is analyzed by X-ray diffractometer.The results are shown in figure 4. The main components are Cr, Fe-S, Fe-C, iron oxide, etc.

Microscopic Structure Analyzing
The microstructure under metallographic microscope is shown in figure 5.There are micro cracks on the fracture, which are intergranular cracking.The matrix structure is austenite and the grain size is 2.5.There are large structures ① and non-metallic inclusions ② at the grain boundaries, and there are massive structures ③ in the grains, and the precipitates ④ are distributed in a network on the grain boundaries.The inner grain is corroded.The results of energy spectrum analysis show that there is "S" element in the surface attachment before and after fracture cleaning, which should be the reaction of asphalt vapor carrying with the material to form a compound.

Summary Analysis of Results
(1) The content of S in corrosion products is very high, and the main components are Cr, Fe-S, FeCr2S4, Fe-C, iron oxide and so on.FeCr2S4 is a black with water-insoluble spinel structure.When FeS meets water and oxygen in the air, it forms H2SxO6.
(2) The macroscopic test results show that the crack originated from the inner wall of the flange, both the fracture and the inner wall have thicker corrosion products.The results of fracture analysis show that the rock sugar morphology can be seen locally at the crack source, which is an intergranular brittle fracture.The content of S in corrosion products at crack source is higher.The results of corrosion products analysis show that the content of S in corrosion products is very high.
(3) The microstructure analysis results show that there are Cr-poor, Ni-poor and Nb-poor structures with many holes in the austenite grain boundaries at the fracture site.The improper control of flange manufacturing process is large grain size, serious carbide precipitation and chromium deficiency phenomenon occurs when the operating temperature is in the sensitized range, which leads to the sensitization of flange materials and reduces the corrosion resistance.The massive niobium carbide is distributed in the grain boundaries and the carbides of Cr, Ni and Nb are distributed in the grain boundaries.The material has been sensitized and the inner grain is corroded.
(4) There are lumps of niobium carbide in austenite grains and on the grain boundaries at the end bolt holes, and carbides of Cr, Ni and Nb are distributed in a network on the grain boundaries.The Cr is one of the most important alloying elements of stainless steel.Its content determines the corrosion resistance of stainless steel.The Ni can improve the corrosion resistance and weldability of stainless steel.The Nb can improve the strength and can improve the resistance to corrosion and hydrogen.The presence of Cr-poor, Ni-poor, and Nb-poor structures will lead to the sensitization of flange materials and reduces the corrosion resistance.The material has been sensitized and no structure is found.The grain size at the fracture and near the end bolt hole is large and the size is uneven.The carbide precipitates in weld and heat affected zone (HAZ) are less than those at flange fracture and bolt hole and the sensitization degree is lighter and the grain is finer.In this device, a small amount of liquid sulfur is injected into the front-end reactor to react with the flange metal and produce sulfide corrosion products and resulting in higher S content in the corrosion products at both the fracture and the inner wall.
(5) The grain size of the flange body is coarse which indicates that the manufacturing process is not well controlled.The fracture, weld and HAZ are in direct contact with the medium and the temperature is close.The bolt hole is not in direct contact with the medium, and the temperature is low.The carbide precipitated in weld and HAZ is less, and it is more at fracture and bolt hole.The carbide precipitated in flange body is produced in manufacturing, rather than in operation.The temperature 620℃ is in the sensitization temperature zone of austenitic stainless steel, and poor-Cr, poor-Ni, poor-Nb structure appears in the fracture and HAZ, but it is not in the bolt hole, indicating that it is produced during operation.A large amount of carbides precipitated on the grain boundary and Cr, Ni and Nb poor regions appear, indicating that the flange material has been sensitized.In summary, the flange crack of the collector tube originated from the inner wall is intergranular brittle fracture.The improper control of flange manufacturing process and the operating temperature in the sensitized range lead to the sensitization of flange materials and reduce the corrosion resistance.The crack originates from the inner wall and spreads along the grain boundary to the outer wall, and finally caused the flange to crack through and leak.

Suggestion
Equipment needs to be flushed with lye or baking soda solution after downtime to neutralize polysulfuric acid, or protected with dry nitrogen or a mixture of nitrogen and ammonia to prevent contact with air and water during downtime.It is necessary to keep the workpiece temperature above the dew point temperature during shutdown to prevent the formation of polysulfuric acid.The X-ray detection and metallography should be carried out on other pipe orifices of the same kind and the cracked ones should be replaced.It is necessary to strengthen the procurement experience and the 300 series stainless steel material is used to replace the material with sulfuric acid corrosion that has the possibility of cracking, and the metallographic inspection is carried out at the time of procurement to control the carbide and grain size.

Figure 3 .
Figure 3. Morphology of inner wall corrosion products and location of energy spectrum selection.(a) Black area; (b) Yellow area.

Figure 5 .
Figure 5. Metallographic microstructure (a) fracture 200×; (b) Fracture 1000×; (c) Fracture 1000×; (d) Inner wall 500 ×.The microstructure under scanning electron microscope is shown in figure 6.There are more holes in the tissue.The length of nonmetallic inclusions ② is 57μm.The results of energy spectrum analysis of austenite matrix and various tissues are shown in table 3. The results show that the tissues are poor in Cr, Ni and Nb; Non-metallic inclusion ② is silicate, according to GB/T10561-2005 "Determination of non-metallic inclusion content in steel standard rating diagram microscopic inspection method", the non-metallic inclusion grade is C0.5, relatively mild.The main component of tissue ③ is niobium carbide and the main components of precipitated phase ④ are carbides of Cr, Ni and Nb.

Table 1 .
Energy spectrum results of corrosion products at crack source (wt%).

Table 2 .
Energy spectrum results of inner wall corrosion products (wt%).

Table 3 .
Results of energy spectrum analysis of each tissue (Wt%).