Macroscopic observation, chemical composition analysis, hardness testing, metallographic inspection, scanning electron microscopy, and energy spectrum analysis are used to analyze the cracking causes of a large-diameter flange. The results show that the flange heat-affected zone matrix organization sensitization, cross-section changes at the overlap with the heat-affected zone resulting in stress superposition; flange matrix organization of coarse grains, and contains a large number of strip inclusions; flange long-term operation in the range of sensitization temperature, which ultimately led to its occurrence of stress corrosion cracking. Finally, the corresponding improvement measures are proposed for the specific reasons for cracking.
A company dehydrogenation reactor receiver flange leakage in operation; the flange leakage is located in the junction of the reducer section and straight edge section. The flange is a solid solution annealed stainless steel forgings machined stereotypes of products. The flange is installed at the inlet of the dehydrogenation reactor; the working pressure is 0.13MPa, the working temperature is 608℃, the working medium is hydrogen (content is about 50%), isobutane, propane, and a small amount of hydrogen sulfide gas mixture, the pipeline outside the package rock wool insulation.
The flange and connecting pipe are connected by welding, and the welding method is tungsten argon arc welding for the bottom and electrode arc welding for the filler cover. The specification of the connecting pipe is 660mm × 18mm (OD × wall thickness), the material is F304H steel, and the flange was put into use in 2017.
1. Physical and chemical inspection of large-diameter flanges
1.1 Macroscopic observation of large-diameter flanges
The outer wall of the flange has no corrosion traces, and the inner wall is covered with a thin layer of black material; the flange section is flat, with no shell pattern, fiber area, or shear lip area, which is characterized by brittle fracture, and the section is covered with black corrosion products. The flange reducer section and straight edge section junction have a ring crack, a crack at the lack of obvious plastic deformation and thinning, and a brittle fracture characteristic. The crack is located in the heat-affected zone of the welded joint between the flange and the inlet pipe, about 8.0mm away from the weld toe, parallel to the weld seam, and distributed in a circular direction. The crack opening on the outer surface is large, with a maximum opening gap of 2.0mm. The crack on the inner wall is about one-third of the circumference, with a smaller opening, and the crack expands from the outer surface of the flange to the inner surface. The macroscopic appearance of the cracked flange is shown in Figure 1.
The welded joints of the flange and the inlet pipe have wrong edges, and the maximum amount of wrong edges on the inner surface is 1.4mm. The weld cover layer is welded by single welding, and the width of the weld channel is too wide, which is about 18-20mm, and there are no surface weld defects such as pores, inclusions, biting edges, and weld nodes in the welded area.
Figure.1 Macro morphology of cracked flange
1.2 Analysis of the chemical composition of large diameter flange
By the standard GB/T 11170-2008 “Determination of multi-element content of stainless steel spark discharge atomic emission spectrometry (conventional method)” on the flange and weld chemical composition analysis, the results are shown in Table 1, which can be seen in its chemical composition in line with the ASTM A182/A182M-2006 “high-temperature use of forged or Rolled alloy steel and stainless steel flanges for high temperatures, forged fittings, valves and components,” the requirements of F304H steel.
Table.1 Chemical composition analysis results of flanges and welds %
|Measured value of weld seam||0.055||0.78||0.84||0.035||0.008||19.18||9.2|
|Measured value of flange||0.065||0.52||1.31||0.039||0.016||18.27||8.27|
1.3 Hardness test at the crack of large diameter flange
By the standard GB/T 4340.1-2009 “Vickers hardness test of metal materials Part 1: Test Methods” the Vickers hardness test of the flange matrix and weld heat-affected zone at the crack. The average Vickers hardness of the flange matrix is 194HV, and the average Vickers hardness of the weld heat-affected zone is 225HV, which is much higher than that of the flange matrix, indicating that the weld heat-affected zone has a large welding residual stress.
1.4 Metallographic examination of large diameter flanges at the cracking place
According to GB/T 13298-2015 “metal microstructure test method” on the flange crack microstructure test. The microstructure morphology of the cracked flange is shown in Figure 2. From Figure 2, it can be seen that the flange weld zone is a uniform dendritic casting organization with no cracks, pores, or other welding defects; the heat-affected zone organization is austenitic, coarse grain, the presence of intracrystalline inclusions, the presence of several grain boundaries widen the presence of carbide particles; the flange matrix organization is austenitic, some of the grain boundaries of carbide particles, the presence of a large number of stripe inclusions, by the GB/T 10561- 2005 “Determination of the content of non-metallic inclusions in steel standard rating chart microscopic examination method” rated as 2.5 level. Uncracked parts of the crack along the grain extension, the existence of secondary cracks, grain, and grain boundaries are distributed with diffuse carbide particles, the existence of sensitization phenomenon.
Figure.2 Microstructure morphology of the flange cracking place
1.5 Scanning electron microscope (SEM) analysis of large diameter flange section
SEM analysis of the flange section, the results are shown in Figure 3. Corrosion products cover the flange section, and no bare fracture is seen; after removing the corrosion products, the fracture has an icing-sugar-like pattern typical of the fracture morphology along the crystal, and several secondary cracks can be seen along the crystal. A large number of fine precipitates exist at the grain boundaries.
The use of a scanning electron microscope on the matrix metallographic specimen (observation surface perpendicular to the flange end face) for observation the results are shown in Figure 4. Figure 4 shows that the matrix’s polished state can be seen on the strip non-metallic inclusions, with a large number of carbide particles on the grain boundary.
Figure.3 SEM morphology of flange section
Figure 4 Metallographic specimen SEM morphology
Figure.5 SEM morphology of heat-affected zone and the distribution of Cr elements
1.6 Energy spectrum analysis of large diameter flange section
The corrosion products of the flange section were analyzed by energy spectrum, in which the elemental composition of the corrosion products in two positions is shown in Table 2. As can be seen from Table 2, the fracture surface corrosion products contain S elements, no Cl elements, of which the S element mass fraction is up to 11.31%.
Table.2 Energy spectrum analysis results of fracture surface corrosion products
The metallographic specimens were analyzed by energy spectrum, and the results are shown in Fig. 5 and Table 3. As seen from Fig.5, the mass fraction of the C element in the precipitates at the grain boundary in the heat-affected zone organization is 4.6%, and the mass fraction of the Cr element is 35.09%, larger than that of the Cr element in the grains. As can be seen from Table 3, the grain boundary precipitates are mainly alloying element carbides, which come from the alloying elements produced by sensitization of the matrix in the heat-affected zone.
Table.3 Metallographic specimen energy spectrum analysis results
|Measured values of grain boundary precipitates||4.6||1.37||0.45||35.09||2.03||49.67||6.4|
|Measured value of matrix||3.87||–||0.76||19.28||1.35||66.92||7.82|
2. Comprehensive analysis
2.1 Environmental reasons
Dehydrogenation reactor internal reaction medium containing H2S gas, the large diameter flange is located in the chemical park production workshop. Its operation process environment is complex, and in the process of equipment start, stop, and maintenance, the flange contact with a variety of media; the pipeline insulation layer easily has corrosive media containing S elements and water enrichment for austenitic stainless steel stress corrosion to provide corrosive media.
2.2 Flange welding reasons
Austenitic stainless steel working temperature of 400-850 ℃, due to the small size of carbon atoms, in the metal diffusion ability, easy to diffuse to the grain boundary, with the grain boundary and its neighboring regions of the chromium atom combined to form Cr23C6 and precipitation in the grain boundary, so that the grain boundary in addition to the carbide part of the chromium content decreased; at the same time, chromium within the crystal will also be diffused to the grain boundary to replenish, but chromium atoms diffusion ability is much smaller than the carbon atom, it is difficult to supplement the loss on the grain boundary. However, the diffusion ability of chromium atoms is much smaller than that of carbon atoms, so it is difficult to supplement the loss on the grain boundary. So, with the continuous precipitation of chromium carbide on the grain boundary, the formation of Cr element content is greatly reduced in the grain boundary of the Cr-poor area. When the chromium content is lower than the amount required for passivation (mass fraction of about 12%), the passivation state is destroyed, and the potential declines. At the same time, the crystal still maintains the passivation state, thus constituting a large cathode (crystal) and small anode (grain boundary area of the chromium-poor area) of the microcouple batteries, reducing the strength of the grain boundary and corrosion resistance. To shorten the residence time of the welded joint at the sensitization temperature, the austenitic stainless steel welding process must be used in the small current, fast welding, and strict control of the width of the weld in a single molding to avoid too much heat input.
Cracked flange weld cover weld for single welding, weld channel is too wide, up to 18-20mm, the welding rate is too slow, the welding heat input is too large, the welding process of welded joints in the sensitization temperature interval stays too long, the residual stress is large, the alloying elements in the heat-affected zone in the grain boundaries precipitation, the grain boundaries of the strength has decreased significantly, the cracking at the state of sensitization, coupled with the flange for a long time in the stainless steel sensitization temperature range, resulting in severe sensitization of heat-affected zone. The material is prone to stress corrosion cracking along the crystal, resulting in serious sensitization of the heat-affected zone.
2.3 The structure of the flange
Flange cracks are located at the junction of the flange reducer section and straight edge section. Crack at the cross-section changes, welding, the residual stress is large; operation, the pipe system constraints and internal pressure caused by the axial tensile stress, the occurrence of stress concentration, greatly increasing the actual force of the place.
2.4 Organization of the flange
Flange cracks at the matrix organization grain coarse. In contrast, the matrix contains many strip inclusions, improving the material’s plasticity and toughness and increasing the possibility of brittle cracks under stress.
2.5 Flange material reasons
The material of the flange is F304H stainless steel, which has a high content of carbon elements and is susceptible to sensitization, reducing the corrosion resistance of the flange.
3. Conclusions and Recommendations
The cause of flange cracking is sulfide stress corrosion. In addition, sensitization occurs in the heat-affected zone; the flange cross-section changes at the overlap with the heat-affected zone, leading to stress superposition, and the flange matrix organization grain is coarse. It contains many strip inclusions, the flange is operated for a long time in the range of sensitization temperatures, and the flange material with high carbon content and so on to the stress corrosion cracking has a promotional effect.
It is recommended that the atmospheric environment and the environment under the insulation layer of the reactor should be monitored regularly to control the content of corrosive elements; it is recommended that the flange material should be replaced by F304 steel or F304L steel with low carbon content. In accepting the flange into the factory, based on the flange material meeting the requirements, the microstructure of the flange should also be reviewed to eliminate the unqualified microstructure materials in the warehouse. It is recommended to adjust the flange neck structure, shorten the length of the straight section of the flange, or reduce the slope of the inclined section of the flange neck to avoid the overlap of the welded heat-affected zone and the structural stress concentration part, and reduce the degree of stress concentration at the cross-section change; the welding process parameters must strictly control the welding process. The use of a low welding rate and low line energy output to prevent the occurrence of alloying elements of the phenomenon of burning after welding to rapid cooling, to avoid the weld in the sensitized zone staying too long.
It is recommended that the existing similar large-diameter stainless steel flange and thick-walled stainless steel pipe weld for inspection, for the welding channel, is too wide weld; in addition to macro-observation, it should also use penetration testing and ultrasonic testing methods of its welded joints heat-affected zone detection, will be the existence of defective welded joints for rework; in the process of equipment overhaul, the state of the equipment under the state of open and shut down to take the necessary measures to protect the equipment, to prevent the austenitic stainless steel Corrosive element aggregation phenomenon.
Author: Zhang Naixin