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2018
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06
Control measures and causes of cracks in heat exchanger tube openings
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A stainless steel heat exchanger from a certain factory. After more than a year of use, through cracks appeared at the pipe openings, affecting the factory's daily production and posing serious safety hazards. The cracks started at the expansion joint of the pipe opening and gradually extended to another pipe hole, with rust-like substances wrapped around the junction of the pipe openings. After removing the wrapping, multiple cracks were visible around the pipe openings with a total length of over 600mm. To find the cause of the cracks and propose corresponding improvement measures, analyses were conducted on the chemical composition of the tube sheet material, fracture morphology, medium environment, corrosion products, metallographic structure, etc., to comprehensively analyze the causes of the cracks. 1. Inspection and analysis.
A stainless steel heat exchanger from a certain factory. After more than a year of use, through cracks appeared at the pipe ends, affecting the factory's daily production and brewing serious safety hazards. The cracks started from the expansion joint at the pipe end and gradually extended to another pipe hole, with rust-like substances wrapped around the junction of the pipe ends. After removing the wrapping, multiple cracks were visible around the pipe hole with a total length of over 600mm. To find the cause of the cracks and propose corresponding improvement measures, analyses were conducted on the chemical composition of the pipe plate material, fracture morphology, medium environment, corrosion products, and metallographic structure, comprehensively analyzing the causes of the cracks.
1. Inspection and Analysis
1. Analysis of Pipe End Material Composition
Samples were taken from the pipe ends for chemical composition analysis and compared with the standard GB4237 "Stainless Steel Hot-Rolled Steel Plates" for 1Cr18Ni9Ti. The analysis results indicated that the actual material was significantly deficient in Ti and slightly high in C, with the pipe end material actually closer to 1Cr18Ni9 rather than 1Cr18Ni8.
2. Medium Environment Analysis
The steam used in the heat exchanger comes from the factory's circulating industrial water. On-site sampling of the medium, which is the condensate of the heating steam and machine oil, was conducted. The analysis results showed that the condensate contained a considerable amount of Cr, which may be related to the introduction of Cl- during the industrial water circulation process.
3. Overall Analysis
A macro inspection of the pipe ends revealed the presence of through cracks of varying widths, with the widest crack reaching 1.5mm. Fracture samples were prepared (without damaging the fracture morphology), and most of the metal at the pipe ends had lost its luster, showing a multi-step, uneven fracture characteristic, indicative of brittle fracture. Micro inspection of the polished surface of the samples revealed multiple intergranular cracks, gradually expanding from coarse to fine from the outside in; at the coarser end, there were obvious non-metallic inclusions; some areas also showed non-metallic inclusions distributed along the grains. After electrochemical etching, the microstructural characteristics showed that the matrix was austenitic, with numerous carbide particles distributed, and a network structure of carbides along the grains. The microcracks expanded along the grain boundaries where carbides precipitated, and the crack characteristics were consistent with intergranular corrosion (network) and stress corrosion (along the grain).
4. Corrosion Product Analysis
Samples of the corrosion products from the gaps at the pipe ends were taken and analyzed using Energy Dispersive X-ray Spectroscopy (EDS). The results indicated that the main components of the corrosion products were Fe and Cr, with a localized enrichment of Cr.
2. Corrosion Mechanism Analysis
1. Material Influence
The corrosion resistance of stainless steel is mainly due to a passive film on the surface, which raises the electrode potential and can prevent corrosion reactions. However, poor material quality of the pipe plate, with insufficient Ti and high C content,
can easily lead to chromium-depleted zones at the grain boundaries, creating active-passive microcells. Generally, the Ti content in 1Cr18Ni9 is 5 to 10 times that of the C content to achieve stabilization. However, the Ti content in this material is too low, and the C content is too high. If the material passes through the sensitization temperature range during manufacturing or use, the unstable carbon can consume a large amount of chromium at the grain boundaries, forming chromium carbides. The internal carbon diffuses to the grain boundaries faster than chromium, leading to chromium depletion at the boundaries (i.e., sensitization), preventing the formation of a passive film, damaging the passive state of the grain boundaries, and lowering the electrode potential (anodic, active), while the grains themselves maintain a passive state with a higher potential (cathodic). The grains and grain boundaries form active-passive microcells, with a large cathode to small anode area ratio, which under the influence of this coupling acceleration effect, leads to intergranular corrosion of the material, while the ability to resist intergranular stress corrosion also decreases.
2. Medium Environment and Stress Factors
Stress corrosion in stainless steel is relatively common. For stress corrosion to occur in stainless steel, three basic conditions must be met: a sensitive alloy (material factors), static tensile stress (mechanical factors), and characteristic medium (environmental factors).
Sensitive alloys, such as 1Cr18Ni9, can cause stress corrosion in the presence of chlorides, hydroxides, and polysulfides in the medium. Static tensile stress—if there is no static tensile stress, even with a sensitive alloy and specific medium, stress corrosion will not occur. The main role in stainless steel stress corrosion is played by macrostress, specifically residual tensile stress, rather than microstress. After the heat exchange tubes are expanded and joined with the tube plate, there exists expansion stress (residual tensile stress).
Characteristic medium—especially the presence of certain impurities in the solution, known as characteristic ions, is the most dangerous. In water at 200°C, even a concentration of 2ppm Cr can cause stress corrosion in austenitic stainless steel, often originating from pitting and crevice corrosion.
The steam report from the heat exchanger indicates that its Cr content is sufficient to cause intergranular stress corrosion in stainless steel at temperatures between 200 and 300°C. EDS analysis results also show significant localized enrichment of Cr on the crack surfaces.
3. Formation of a Crevice Cell
Due to the stagnant flow state of the solution in the crevice at the expansion joint, oxygen can only diffuse into the crevice. This makes it difficult for oxygen consumption in the crevice to be replenished, causing the cathodic reaction in the crevice to cease. However, the anodic reaction in the crevice continues, forming a cavity with a high concentration of metal cations. To maintain neutrality in the solution, negatively charged anions (Cr) migrate into the cavity, and the generated metal chlorides undergo hydrolysis, increasing the acidity in the cavity. This leads to the breakdown of the passive film, lowering the electrode potential in the cavity, forming an anode, while the entire outer surface becomes a cathode. Particularly, Cr can react with H+ to form hydrochloric acid, which is more corrosive and accelerates the corrosion rate in the cavity. Due to insufficient Ti content in the material, it is also prone to activation of the passive state. As corrosion progresses and corrosion products accumulate, a crevice cell corrosion forms in the crevice, and under the combined action of stress and corrosion, it expands into cracks that develop deeper.
4. Metallurgical and Heat Treatment Factors
The metallographic structure shows austenite with a large amount of undissolved carbides. There are also network carbides distributed along the grain boundaries. This indicates that the material has not undergone solution treatment, or the solution treatment temperature was not high enough and the time was too short, resulting in insufficient dissolution of carbides and poor alloying degree. Moreover, the low heat treatment temperature slows down the diffusion rate of chromium, accelerates the precipitation of chromium carbides, and further promotes the formation of chromium-depleted zones, reducing the electrode potential. At the same time, the chromium-containing carbides distributed along the grain boundaries can exacerbate chromium depletion at the grain boundaries, leading to stress corrosion along the grain boundaries.
The presence of non-metallic inclusions along the grain boundaries indicates that the material has coarse grains. Coarse grains have a greater tendency for intergranular corrosion. This is because the grain boundary area per unit volume of coarse grains is small, while the precipitation of carbides generated under given sensitization conditions is constant. This results in a higher density of carbides at the grain boundaries of coarse grains compared to fine grains. On the other hand, the large coarse grains promote the accelerated precipitation of M23C6, while the non-metallic inclusions distributed along the grain boundaries severely disrupt the continuity of the matrix, reducing the strength of the material. The tips of non-metallic inclusions are prone to cause stress concentration.
3. Conclusion
The material analysis and metallographic analysis of the pipe ends indicate that the material does not meet the requirements, leading to a decline in the resistance of the matrix and grain boundaries to intergranular corrosion and intergranular stress corrosion. The chromium in the medium, under the combined action of grain boundary sensitization and residual stress, causes intergranular stress corrosion, and chromium stagnates in the gaps between the pipe and the tube sheet. Under residual stress and acidic autocatalytic action, this leads to gap corrosion; meanwhile, the non-metallic inclusions along the grain boundaries cause stress concentration and coarsening of the grain structure, which further exacerbates the occurrence and development of corrosion. To prevent crack formation, improvements can be made in the following four areas:
1. Material Control
Strictly control the quality of the material by adding stabilizing elements or reducing carbon content, controlling grain boundary adsorption and inhibiting grain boundary precipitation. The heat treatment temperature, time, and rate of temperature change must meet the requirements to reduce the precipitation of chromium carbides, control grain size, and refine grains, thereby reducing the likelihood of corrosion.
2. Structural Improvement
Avoid forming enclosed spaces as much as possible. During design, consider sealing the gaps between the tube sheet and the tubes. It is recommended to use sealed welding and perform stress relief treatment after welding.
3. Control of Working Medium
Strictly control the impurities in the medium, especially the content of Cl. It can be controlled below the concentration that causes stress corrosion through process means.
4. Material Selection Considerations
Due to the high carbon content in 18CrNiMo7-6, it will increase the sensitivity of stainless steel to stress corrosion. Additionally, during the manufacturing process, after thermal processing or welding thermal cycles, the stabilizing effect of Ti may decrease or even disappear, easily causing grain boundary sensitization and reducing resistance to intergranular corrosion. Therefore, in the pressure vessel industry, this steel grade is currently not recommended. It is advisable to choose chromium-nickel austenitic stainless steels with lower carbon content, such as 00Cr18Ni10Ti steel, 0Cr18Ni10Nb steel, or ultra-low carbon stainless steel 00Cr9Ni10 steel, to reduce the hazards of grain boundary sensitization. Consideration can also be given to using duplex stainless steel to reduce the sensitivity of stainless steel to stress corrosion cracking caused by chloride ions.
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