Thread Content
1 Structure of the heat exchanger: The cold exchanger in the synthesis section of nitrogen fertilizer plants is used for the second ammonia separation of the gas that has been cooled with ammonia. Its cylinder is formed by winding flat steel strips; it is welded to the end flange at the top and to the bottom head at the bottom. The internal component is divided into two parts: the upper part is a tube bundle heat exchanger, and the lower part is a separator. To ensure effective heat transfer, a 3 mm thick inner sleeve is welded to the flange on the inner wall end face of the outer cylinder, thereby maintaining a small gap between it and the annular baffle of the heat exchanger. The heat exchangers in the same section have different functions, but their structure is basically the same; they all feature an inner sleeve 3 mm thick, welded to the end flange, as shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0703141525519999.jpg Figure 1: Schematic diagram of the connection between the inner sleeve and the shell of a high-pressure heat exchanger. 1 – Flange on the end face of the shell ； 2—Heat exchange internals ； 3—Inner sleeve ； 4—Connection flange 2 Issues: In accordance with the regulations for pressure vessels, the user entity regularly hires qualified inspection agencies to carry out various inspections on the vessels. The inner sleeve of such containers must be removed during internal inspections and comprehensive inspections. At the time of manufacture, the cylinder flange and the inner cylinder wall are welded together. The flange material of this cylinder is 20MnMo, while the inner sleeve material is Q—235; it involves welding of dissimilar steels. When the manufacturer restores the inner sleeve by welding after inspection, the poor weldability of the 20MnMo material often leads to cracks in the container flanges, resulting in the container being scrapped. During the pressure vessel inspection of a factory in 1995, after removing the inner sleeve, multiple longitudinal and transverse cracks were found in the heat-affected zone of the fillet welds; the longitudinal cracks were 60 mm long, while the shorter ones were 10 mm long ； The longest transverse crack is 30 mm, and the shortest is 10 mm ； The deepest is 4.5 mm. After routine treatment, an infiltration test was conducted, cracks were eliminated, and it was put into use after welding the inner sleeve. During the inspection in 1999, penetrant testing revealed that the welded area was still filled with weld cracks. The reason analyzed is the cracks formed during the installation and welding of the inner sleeve after each inspection. For this assembly, learning from past experiences, after grinding, PT inspection, strength verification, and welding, an approach was adopted that used an acetylene-oxygen flame to maintain a certain temperature, along with aluminum silicate cotton felt for insulation. Nevertheless, within a few dozen minutes after the installation of the inner sleeve welding joint, a slight tearing roar could be heard. (It was late at night, the entire plant was shut down, and the environment was quiet.) It was put into use without being addressed for various reasons. At the end of 2000, the entire plant was shut down for a major overhaul, during which a penetrant inspection was carried out on this vessel. Numerous cracks were found in the original welds; the area affected was ground to a depth of 10 mm. As a result of multiple welding and grinding operations on the same area, the flange neck became thinner. The inspection agency issued a notice stating that there were serious problems, requiring replacement within a specified time frame, with operational monitoring to be carried out during that period. Due to repeated crack removal grinding at the connection area, a groove 6–10 mm wide formed inside the upper flange neck, resulting in a change in the structure that made it impossible to install the original sleeve. By redesigning the sealing mechanism and avoiding welding connections to prevent further damage to the forged flange, it was possible to keep this container in operation until the end of the monitoring period. At the end of 2001, when the monitoring period came to an end, a final inspection revealed a surface crack 160 mm long on the inner surface of the circumferential groove in the forging of the upper flange neck; as a result, this pressure vessel rated for 32 MPa was taken out of service. The same problem has occurred in several factories. During the processing, the author analyzed the reasons. Appropriate actions were taken depending on the circumstances. Several years of operational experience have shown that it is simple and effective. 3 Cause analysis: 20MnMo is a common low-alloy steel used in medium-temperature and high-pressure vessels. The chemical composition of 20MnMo steel forgings is shown in Table 1, and their mechanical properties are shown in Table 2. Table 1 Chemical composition of 20MnMo steel forgings, in %: http://www.nmtech.com.cn/jishuwang/upload/0703141530532961.jpg Table 2 Mechanical properties of 20MnMo steel forgings: http://www.nmtech.com.cn/jishuwang/upload/0703141535507957.jpg As can be seen from Table 1, the mass fraction of C in 20MnMo is 0.21%. Theory and practice in the field of melting and welding metallurgy show that the carbon content directly affects the width of the crystallization temperature range, thereby determining the level of crack sensitivity. 3.1 Determining the weldability of 20 MnMo steel based on the weld carbon equivalent Among the components of the weld microstructure, the carbon content has the greatest influence on the hardening tendency. Moreover, the mass fraction of Mn in the base metal is as high as 1.27%, which inevitably has an impact on the microstructure of the welded joint. Its effect on the hardening tendency of the weld and heat-affected zone materials is converted into an equivalent effect of carbon, that is, the carbon equivalent calculation method recommended by the International Welding Institute. The formula for calculating the carbon equivalent is Ceq (%) = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14. For 20MnMo steel, this calculation yields a Ceq value of 0.50 %. Generally, when the carbon equivalent Ceq is less than 0.4%, the welding properties are excellent ； 0.4 % < Ceq < 0.6 %, resulting in poor weldability. The Ceq value of 20MnMo steel is 0.50%, indicating poor weldability. 3.2 Inferring the weldability of 20 MnMo steel from weld hardness The highest hardness value in the heat-affected zone of the weld can also be determined using the carbon equivalent Ceq. During welding, the heat-affected zone is heated by the molten zone, reaching temperatures well above the Ac3 critical point; as a result, the grain size increases significantly. The heat-affected zone, which has been heated to high temperatures (above Ac1) by the end of welding, is then cooled rapidly by the unheated metal surrounding it. This causes the hardness of the weld to be greater than that of the original base material, with a marked increase in hardness and a decrease in plasticity, leading to a material that is more prone to cracking. The hardness of the heat-affected zone in 20MnMo welds can be determined using the formula HVmax = (666 C e q + 40) ± 40. Calculations show that HVmax = 413 ≥ 350, indicating that 20MnMo steel has low plasticity, a high tendency to weld cracking, and relatively poor weldability. 3.3 Inferring the weldability of 20 MnMo steel from weld crack sensitivity: The formation of cold cracks is, in essence, the result of a combination of factors such as the low plasticity of the heat-affected zone resulting from the welding process, the presence of hydrogen, and welding stresses. When the molten pool is rapidly heated to above 1350°C, all of the grains in the weld metal transform into austenite. As the heat source moves, the weld cools quickly, causing the austenite to become undercooled, and martensite or intermediate phase transformations occur at lower temperatures. This martensitic structure features large grains and low plasticity; moreover, the volume change associated with the martensitic transformation generates high transformation stresses. These stresses, combined with the welding residual stresses, occur at the interfaces of the martensitic structure, resulting in microscopic crystal defects. The welding cold crack sensitivity index Pc m for 20MnMo steel is calculated using the formula for crack sensitivity: Pc m (%) = C + Si / 30 + Mn / 20 + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B. The resulting value is Pc m = 0.30%, indicating that 20MnMo steel has a tendency to develop cold cracks. 3.4 Hydrogen embrittlement caused by high hydrogen exposure environments: The heat exchangers operate at pressures of up to 31 MPa, with media including hydrogen, nitrogen, ammonia, and methane; the H2 concentration can be as high as 60%, which facilitates hydrogen corrosion of the steel, leading to a significant reduction in its mechanical properties, especially a considerable decrease in plasticity. According to the research findings of G. Nelson, in a hydrogen-rich environment, when the temperature exceeds 221°C and the pressure is greater than 1.41 MPa, hydrogen penetrates into the steel, where it forms methane at the grain boundaries, leading to internal decarburization of the steel. The reaction equations are as follows: Fe3C + 4H → 3Fe + CH4 C (in α-Fe) + 4H (in α-Fe) → CH4. Especially at high pressures and low temperatures, the ability of carbon to diffuse decreases; internal hydrogen corrosion can occur without any obvious surface decarburization. The increasing amount of methane accumulates in the existing microscopic voids at the grain boundaries, creating localized pressure that results in numerous grain boundary cracks, and this process is irreversible. Research shows that the carbon content is related to the susceptibility to hydrogen corrosion; generally, it is believed that hydrogen corrosion is less likely to occur when C ≤ 0.15%. The carbon content of 20MnMo steel is 0.21%, which is higher than 0.15%. Fortunately, the operating temperature of the cold exchanger is low, and that of the hot exchanger does not exceed 160°C; as a result, the rate of hydrogen corrosion is extremely slow, and its incubation period exceeds the normal service life. Nevertheless, the impact of hydrogen corrosion on the material properties cannot be ignored. 3.5 Cracks caused by ambient hydrogen during welding: During welding, due to environmental factors, failure to dry the electrodes as required, oil, rust, or moisture on the surface of the weld area, or inappropriate welding procedures, hydrogen enters the molten pool in atomic form. As the temperature of the molten pool drops, the solubility of hydrogen in the metal decreases; when the metal undergoes phase transformation, this solubility changes abruptly. During welding, the cooling rate is very fast, and hydrogen does not have time to escape, remaining in the weld metal. Generally, the carbon equivalent of the weld metal is slightly lower than that of the base material; as a result, the temperature at which austenite transformation occurs in the weld metal is higher than that in the heat-affected zone of the base material. When austenite transformation takes place in the weld metal, its hydrogen solubility drops suddenly, and the supersaturated hydrogen diffuses into the heat-affected zone where austenite transformation has not occurred. More hydrogen accumulates in the heat-affected zone near the weld line. As the temperature drops further and austenite transformation occurs in the heat-affected zone as well, the solubility of hydrogen decreases and its diffusion capacity weakens; as a result, hydrogen remains in a supersaturated state within the martensitic structure of the heat-affected zone, contributing to the embrittlement of that martensitic structure. Some hydrogen atoms combine to form hydrogen molecules, which enter the tiny pores of the metal in gaseous form. This creates high pressure, resulting in significant stress in the metal and the formation of cold cracks. Experimental research results from both domestic and international sources show that the penetration of hydrogen into steel cannot be ignored either; under high pressure, hydrogen has a strong ability to penetrate steel, and it is capable of dissolving in steel. High hydrogen environments lead to high hydrogen content in steel. 3.6 20 MnMo has relatively weak plastic deformation capacity; its yield strength is high, reaching 415 MPa, and it is highly sensitive to notches, making stress concentration likely to occur. Due to its high rigidity and rapid cooling rate, thick plate welding leads to the formation of a quenched microstructure. After welding thick forgings, significant restraint forces arise, resulting in high welding stresses and an increased tendency to develop cold cracks. Trace amounts of S and P in steel increase the likelihood of crack formation. Based on the above analysis, it can be seen that welding 20 MnMo thick forgings is quite challenging; therefore, a suitable welding process along with appropriate preheating and post-heating treatments are necessary. 4 Treatment method: The welding connection of the cylinder end flange to the inner sleeve is the method adopted in the original design. Based on the above analysis, it can be seen that welding this material is quite difficult; most manufacturers do not possess the welding techniques or heat treatment facilities necessary for working with 20 MnMo. Based on the technical capabilities of each factory and the principles of simplicity and effectiveness, depending on the connection structure between the end flange of the high-pressure cylinder and the lining sleeve as well as the degree of damage in the heat-affected zone of the welds, the following treatments can be applied accordingly. 4.1 When the sleeve connection flange does not fall off: Some manufacturers, when removing the heat exchange internals, cause damage to the inner sleeve due to improper lifting methods, leaving behind the sleeve connection flange. In this case, an angle grinder can be used to cut away the remaining inner sleeve, ensuring that the inner diameter of the connection flange is smooth. The inner sleeve should be rolled out according to the dimensions specified in the original drawings; a slight flare should be created on it. After it is inserted into the outer cylinder, the inner sleeve and the connection flange should be spot-welded together, taking care to ensure that the arc welding does not damage the outer cylinder. The node diagram at the connection point is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0703141526415408.jpg Figure 2: Schematic diagram of the connection between the rolled inner sleeve and the connecting flange. 1 – Flange on the end face of the cylinder ； 2—Heat exchange internals ； 3—Inner sleeve ； 4—Connection flange 4.2 Connection flange structure without an inner sleeve; another connection structure is shown in Figure 3. http://www.nmtech.com.cn/jishuwang/upload/0703141527158449.jpg Figure 3: Schematic diagram of the connection between the inner sleeve and the outer cylinder without a connecting flange. 1 – Flange on the end face of the outer cylinder ； 2—Heat exchange internals ； 3—Inner sleeve: When removing the inner sleeve in this type of structure, special care must be taken to avoid using an oxygen-acetylene flame for cutting; instead, an angle grinder should be used to remove the upward flange on the inner sleeve body, thereby separating it from the outer sleeve. As shown in Figure 4(a). During restoration installation, after the cylinder is in place, only manual arc spot welding is used, and the arc must not damage the outer cylinder. The structure after welding is shown in Figure 4. http://www.nmtech.com.cn/jishuwang/upload/0703141528057532.jpg Figure 4: Schematic diagram of the outer cylinder body and the connections after the inner sleeve has been removed. 1 – Flange on the end face of the outer cylinder body ； 2—Remnant flanging of old inner sleeve ； 3—Inner sleeve 4.3 Floating seal ring compensation method: In cases where the flange connecting the inner sleeve has come loose or cracks have formed, and a depression has been created as a result of grinding, as shown in Figure 5. http://www.nmtech.com.cn/jishuwang/upload/0703141528436801.jpg Figure 5: Schematic diagram showing the indentation formed after grinding the end flange of the outer cylinder. In this case, due to the change in structure, it is not possible to fix the inner sleeve using the original method. Three tension rods can be considered to secure the inner sleeve, with a floating seal ring used to prevent gas short circuits. The scheme is shown in Figure 6. http://www.nmtech.com.cn/jishuwang/upload/0703141529162042.jpg Figure 6: Schematic diagram of the connection between the inner sleeve and the cylinder using a floating seal ring as a compensation mechanism. 1 – Flange on the end face of the cylinder ； 2—Heat exchange internals ； 3—Inner sleeve ； 4—Fixing ring ； 5—Floating seal ring ； 6—Stretching: When carrying it out, pay attention to the following points. (1) It is necessary to take into account that the airflow channels of the original structure cannot be changed. The position of the floating seal ring must isolate the gas outlet from the vapor line of the level gauge; the floating separator ring should be placed between the gas outlet pipe and the vapor line of the level gauge. (2) The distance between the inner sleeve and the upper tube sheet of the heat exchange internals must ensure the original gas window area. (3) The inner diameter of the floating ring is 6–10 mm larger than the outer diameter of the inner sleeve, facilitating center adjustment. (4) The outer diameter of the floating ring is 2–4 mm smaller than the inner diameter of the outer cylinder. This ensures sealing while also meeting the ellipticity requirements of the inner diameter of the outer cylinder. (5) The inner diameter of the fixing ring is 1 mm larger than the outer diameter of the inner sleeve as specified in the drawings, thereby preventing the inner sleeve from becoming out of round. (6) The outer diameter of the fixed ring is 10–15 mm smaller than that of the floating ring. (7) The axial clearance between the fixed ring and the floating seal ring is 1–1.5 mm when used for cold exchange, and 2–3 mm when used for hot exchange. This connection method has been used by two manufacturers with good results. It is also worth mentioning that for a plant mentioned in Part 2 of this article, after its high-pressure heat exchanger had to be taken out of service on a 32 MPa pressure production line due to welding defects and grinding issues, the user entity, following the author’s advice, conducted strength checks and used the equipment at a lower pressure by modifying its internal structure; care was taken to avoid welding on the flanges made of 20 MnMo material. This approach enabled the replacement of a container rated for 13 MPa, which needed to be purchased externally, with success. No crack defects were found at that location during the subsequent comprehensive inspections of pressure vessels in 2003 and 2006.