Failure Analysis and Prevention of Hastelloy C-276 Heat Exchange Tube
Hastelloy C-276 heat exchanger tube in service corrosion failure occurred, the weld area significantly thinned, the weld localized small holes. Through metallographic, scanning electron microscopy, spectral analysis and other means of analysis of the failure of the C-276 welded tube analysis. The analysis results show that only the weld was cold drawn, improper welding process and subsequent imperfect solution treatment is the main reason for the reduction in corrosion resistance of the weld area. Finally, the corresponding preventive measures were proposed.
Hastelloy C-276 material has good corrosion resistance, so it is widely used in chemical pressure vessels . A chemical company 300,000 tons / year of acetic acid plant after 4 years of service found an acetic acid water separation tower condenser (shell and tube heat exchanger) column tube leakage failure. Leakage of the heat pipe material for the Hastelloy C-276, specifications ∅ 19mm × 2mm, the failure of the heat exchanger tube for the imported plate made in the domestic welded tube. The failure mechanism of the failed C-276 heat exchanger tube was analyzed, and preventive measures were proposed.
1. Macroscopic inspection
The heat exchanger tube has obvious uniform wall thickness thinning and hole corrosion phenomenon, and in the place where hole corrosion occurs, the most obvious thinning. Through macroscopic inspection found that the failure site are in the welding area, and in the weld area there is a small hole through the wall of the tube. Heat exchanger tube corrosion failure and heat exchanger working conditions have a great relationship, so the need to determine the working parameters of the heat exchanger.
Heat exchanger service process conditions to obtain the technical parameters shown in Table 1.
Table.1 Heat exchanger technical parameters
|Project||Tube side||Shell pass|
|Texture of material||C-276||Carbon steel lining C-276|
Organic matter (including H2O: 27.06%;
Heat exchanger tube macro-corrosion profile as shown in Figure 1, 2. From Figure 1 can be seen, the heat exchanger tube 3 location of the diameter of the size than the 1 location reduced by 1.8 mm, the 3 location of the thinning area is precisely to produce perforation area. Figure 2 (a) is a local enlargement of Figure 1 in the area of No. 3, you can see that the width of the weld is 4mm, and corrosion generated by the small hole is located near the center of the weld.
Figure.1 Pipe uniform thinning morphology
In order to further observe the impact of corrosion on the size of the weld near the small hole cut specimens (sampling area as shown in the box in the figure) to observe the changes in the cross-section of the heat exchanger tube, from Figure 2 (b) can be seen in the weld at the tube wall thinning significantly, the weld is flatter without circular transition, after local cold processing. In addition, the tube 3 position wall thickness size measurement, the measurement results are: No. 1 position wall thickness of 0.80mm, No. 2 position wall thickness of 0.44mm, the weld at the wall thickness of only 0.2mm.
2. Testing analysis
2.1 CHEMICAL COMPOSITION TESTING
The substrate near the small hole of the pipe was cut and sampled for chemical composition analysis. Table 2 lists the measured chemical element content of the base material and welding wire, after comparison with the element content of the relevant standards found that, in addition to the high W content, other chemical element content are in line with the standard requirements, excluding the material composition of unqualified factors.
Figure.2 Weld corrosion morphology
2.2 METALLOGRAPHIC ANALYSIS
In order to observe the tube matrix and weld organization and its corrosion, the specimen shown in Figure 2 (b) for the preparation of metallographic, the tube matrix inside and outside the wall for organizational observation, A area of the parent material metallographic organization as shown in Figure 3, the solid solution state of the parent material is good, the inner and outer walls have not occurred in serious corrosion. As the weld is a serious thinning area, so the weld (corresponding to Figure 2 (b) in the B region) and the fusion line (corresponding to Figure 2 (b) in the C region) under the metallurgical microscope for detailed observation and analysis of the metallurgical organization, as shown in Figure 4. The weld was found to have a dendritic structure, indicating that the post-weld solution annealing is not perfect , and the presence of this dendritic structure will inevitably reduce the corrosion resistance of the material .
3. Scanning electron microscope analysis
In order to further analyze the influence of the existence of weld dendrite structure on the corrosion performance, scanning electron microscopy analysis was performed on the inner `wall and outer wall of the corrosion hole, as shown in Figure 5.
Table.2 Tube matrix and weld chemical composition and standard comparison %
|ASTM B575 standard||≤0.010||≤0.03||≤0.08||≤1.0||≤0.04||4.0-7.0||14.5-16.5||15.0-17.0||≤0.35||3.0-4.5|
|ERNi CrMo-4 welding wire standard||≤0.02||≤0.03||≤0.2||≤1.0||≤0.04||4.0-7.0||14.5-16.5||15.0-17.0||≤0.35||3.0-4.0|
Figure.3 Heat exchanger tube base material metallographic organization
Figure.4 Welding seam and fusion line metallographic organization
From Figure 5 (a) near the small hole strip corrosion morphology, corrosion cracking is along the grain boundary gradually expanded, can be concluded that the corrosion cracking along the crystal. In addition, along the direction marked by the arrow in Figure 5 (c) can clearly see the dendritic structure, the outer wall dendritic structure is more obvious than the inner wall of Figure 5 (a).
From Figure 5 can be seen in addition to the dendrite structure, you can also see that the weld matrix has a lot of adhesion. These attachments are heat exchangers in the long-term work of the shell process of organic residues, its chemical composition and its possible impact on the corrosion performance of the weld needs to be further studied through energy spectral analysis.
4. Energy spectrum analysis
Through the energy spectrum analysis of the adhesion shown in Figure 6, the content of each element was obtained as shown in Table 3. From the energy spectrum test in Table 3, it can be seen that the residue contains a small amount of iodine ions.
Figure.5 Microscopic morphology of the inner and outer walls of the tube (SEM)
5. Analysis and discussion
5.1 CORROSION MEDIUM
Acetic acid is a weak oxidizing organic acid, its corrosive properties at room temperature is not much, in some mixed media environment with oxygen, the corrosion characteristics of acetic acid is very similar to oxidizing acid, when the acetic acid contains more halogen ions, its corrosion is stronger, because the halogen ions can strongly adsorbed on the metal surface, so that the metal surface passivation film local damage . And with the increase of temperature, its corrosiveness gradually increases , and the tendency of pore corrosion is serious, and its corrosiveness increases sharply when the temperature approaches or exceeds the boiling point (117.9°C) . In this case, the external medium of the failed heat exchanger tube acetic acid content mass fraction of 47.6%, and heat exchanger tube working temperature at about 110 ℃, many of the above factors can indicate that the working medium has a strong corrosive, C-276 welded tube corrosion resistance to put forward higher requirements.
Figure.6 Attachment energy spectrum analysis (SEM)
Table.3 Energy spectrum test analysis
|Element||Mass fraction/%||Atomic fraction/%|
5.2 WELDING AND COLD WORKING TREATMENT
ASTM B626 “welded nickel and nickel-cobalt alloy tube (Tube)” standard mentioned in the supply of C-276 welded pipe fittings a total of five levels, for the text of the service in high-temperature acetic acid solution heat exchanger tube, the highest level should be used (that is, the manufacturing process should be strictly in accordance with the welding, cold drawing, solution annealing, nondestructive testing in order to perform) after acceptance into service. The welding in this case used filler wire welding, welding process does not meet the standard provisions.
In addition, the cold drawing deformation process has not been enough attention to foreign countries in 1989, there was only cold drawing deformation treatment of the weld and eventually lead to heat exchanger tube failure of similar cases, Figure 7 shows the Hastelloy welded tube only 8 months of service, the weld corrosion failure . From Figure 2 can be seen, the weld is relatively straight without circular transition, it can be seen that the pipe only on the weld after local rolling treatment, but not the overall cold drawing deformation treatment. In order to obtain better corrosion performance should be cold-drawn weld and base material at the same time , the overall cold-drawn deformation treatment can not only form a small amount of fiber tissue (fiber tissue lattice distortion is serious), to facilitate recrystallization to form the core , and the formation of carbide core inside the deformed grain, so that the steel in the sensitization process along the grain boundaries of the precipitation of carbide to a minimum .
Figure.7 Corrosion failure of the welded Hastelloy tube in service for only 8 months 
Although the overall cold deformation treatment of the metal material to obtain better corrosion resistance , but in order to ensure that the material can still maintain excellent corrosion resistance after processing and forming and welding, the manufacturing is required to complete the solid solution treatment. In the literature , it is mentioned that solution treatment can ensure corrosion resistance, and when the elongation of the outer fibers is less than 7%, solution annealing treatment is generally not required. However, if the holding temperature does not meet the requirements of the heat treatment process, resulting in forming parts in the sensitization temperature zone for too long will reduce the corrosion performance of the weld [10,11], the best solution treatment curve is shown in Figure 8 [9,10,12].
In the corrosion of small holes near the weld seam samples were taken for solid solution treatment (heated to 1100 ℃, holding 30min after water cooling), solid solution treatment of the weld metallurgical organization as shown in Figure 9, solid solution of the weld organization than Figure 4 (a) dendritic organization is significantly reduced, indicating that the original failure of the heat exchanger tube solid solution treatment is not perfect.
Figure.8 Solid solution treatment process 
Figure.9 Solid solution after the weld metallurgical organization
Analysis of the C-276 heat exchanger tube corrosion failure can be seen:
Heat exchanger working medium CH3I content of 3.4% (indicating the presence of halogens), dissolved in water will exist very corrosive HI and I- ions, the heat exchanger tube has a very strong corrosive. According to JB/T4756-2006 “nickel and nickel alloy pressure vessel”, the welding of C-276 heat exchanger tube should be performed according to ASTM B626 “welded nickel and nickel-cobalt alloy tube (Tube)”, which stipulates that the welding of the welded tube should be automatic welding without filling material, and the welding in this case is filled with wire welding, and the welding process does not meet the standard; ASTM B626 of the five levels of welded tubes are ultimately required to solid solution treatment, if not solid solution treatment, or insufficient solid solution treatment are not in line with the provisions of the standard, the case of the weld and the base material without simultaneous cold drawing treatment and after welding of the formed parts solution annealing treatment is not appropriate. Therefore, the manufacturing process of the failed C-276 Hastelloy welded pipe does not meet the requirements of JB/T4756-2006 “nickel and nickel alloy pressure vessel”, which reduces the corrosion resistance of the welded seam.
7. Preventive measures
Should be welded according to the ASTM B626 standard for automatic welding process without filler material; after welding should be simultaneously cold drawn on the base material and the weld, and in accordance with ASTM B626 standard for cold drawn formed parts for reasonable solution annealing treatment.
When ordering C-276 heat exchanger tubes used in strong corrosive media environment, additional corrosion resistance inspection requirements should be attached to eliminate the occurrence of such failure.
Authors: Sun Haisheng, Chen Ji, Guan Kaishu, Wang Zhiwen
Source: China Hastelloy C-276 Heat Exchange Tube Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
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