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0 Preface: A heat exchanger manufactured by Zhenhai Refining & Chemical Maintenance and Installation Company has a design pressure of 1.6 MPa. During the hydrostatic test, when the pressure reached 1.0 MPa, the shell side flange near the connection fitting of the heat exchanger suddenly cracked. The crack occurred at the transition between the straight flange section and the reinforced section, with a crack length of approximately 430 mm. This paper conducts a failure analysis on the flange cracking issue on the shell side of this heat exchanger. The root cause of the flange cracking was determined by examining aspects such as the macroscopic fracture surface, scanning electron microscope fracture surface, chemical composition, mechanical properties, and microstructure ； Preventive measures and recommendations were proposed to avoid similar accidents in the future. 1 Technical data of the split flange: This heat exchanger flange is made by ring forging from 16Mn material, manufactured and inspected in accordance with JB4726-94 Class II requirements, and is supplied in a normalized state. The chemical composition and mechanical properties of the flange forgings provided by the manufacturer are shown in Table 1 and Table 2, respectively. http://www.nmtech.com.cn/jishuwang/upload/0702151543005826.jpg 2 Failure analysis of flange cracking 2.1 Macroscopic fracture analysis Figure 1 shows the macroscopic fracture morphology of the cracked flange as captured. As can be seen from Figure 1, the entire fracture surface is extremely smooth, as if cut by a knife; the fracture surface is at 90° to the inner surface of the flange, and there are no signs of plastic deformation prior to cracking. The crack occurred between the straight section of the flange and its conical neck section. It was also found at the site that no R12 transition arc was machined between the straight flange section and the conical neck section as required by the drawings, indicating significant stress concentration in that area. http://www.nmtech.com.cn/jishuwang/upload/0702151543314203.jpg Based on the macroscopic features of the fracture surface, it can be determined that the flange fracture is a brittle fracture. This suggests that the flange material may have insufficient toughness and inadequate strength reserve. For steel grades such as 16Mn, which have good comprehensive mechanical properties, the toughness of normal materials is sufficient to meet the design pressure requirements. It can be inferred that, under normal conditions during the hydrostatic test, there may be issues with the flange material. 2.2 Microfracture analysis Microfracture analysis of the cracked flange was carried out using a scanning electron microscope. Figure 2 shows the microstructural morphology of the fracture surface, obtained from a sample taken from the central part of the crack near the inner surface of the flange. As can be seen from Figure 2, the fracture exhibits the characteristic river-pattern of a cleaved fracture. This further indicates that the flange crack is a brittle crack. The flange cracked when the hydrostatic test pressure reached only half of the specified test value, indicating a significant lack of toughness in the material; there may be issues with its chemical composition or microstructure. 2.3 Chemical composition analysis: To determine whether there were any issues with the chemical composition of the flanges, samples from the cracked flanges were taken for chemical composition testing; the test results are shown in Table 3. Table 3 presents both the chemical composition of 16Mn steel as specified in JB4726-94 \"Forgings of Carbon and Low-Alloy Steels for Pressure Vessels\" and the data provided by the manufacturer, for comparison. In the chemical composition analysis, only the contents of the 5 elements that must be tested for 16Mn steel were measured. http://www.nmtech.com.cn/jishuwang/upload/0702151544284907.jpg As can be seen from Table 3, the measured chemical composition meets the requirements of the technical standard JB4726-94, and the contents of various elements are also close to the values provided by the manufacturer. This indicates that there are no issues with the chemical composition of the flange material, and the cracking of the flange was not caused by chemical factors. 2.4 Mechanical property testing To evaluate the mechanical properties of the flange material, samples from the cracked flanges were taken and subjected to tensile tests and impact tests. Among them, 3 specimens each were taken along the circumferential and axial directions for tension testing ； Due to material constraints, only 3 impact specimens were taken axially. The measurement results are shown in Tables 4 and 5. Tables 4 and 5 also provide the mechanical property values for 16Mn steel forgings as specified in JB4726-94 \"Forgings of carbon steel and low-alloy steel for pressure vessels\". As can be seen from Table 4, the mechanical property values σ0.2, σb, and δ5 of the flange material are relatively similar in the circumferential and axial directions; only the ψ value differs significantly, by 18%. The strength parameters σ0.2 and σb are both within the ranges specified in JB4726-94; however, σ0.2 is only slightly above the specified minimum value. Both σ0.2 and σb are lower than the values provided by the manufacturer, with σ0.2 differing by up to 18.6%. It is evident that the material indeed has a problem of insufficient strength reserve. As can be seen from Table 5, the measured axial impact energy AKV of the flange material is lower than the minimum value specified in JB4726-94, with a difference of 9.7% between the two ； The measured AKV value is even lower than the manufacturer’s figures; based on the minimum value provided by the manufacturer, the difference between the two is 33.2%. It can be seen that the impact toughness of the material is significantly insufficient. It can be concluded that the poor toughness of the material is the main cause of the flange cracking. 2.5 Microstructural analysis Microstructural analysis was conducted on samples taken radially from the flange along the crack surface of the cracked flange; Figures 3(a) and (b) show the microstructures near the inner and outer surfaces of the flange, respectively. http://www.nmtech.com.cn/jishuwang/upload/0702151545443767.jpg As can be seen from Figure 3, the microstructure on the crack surface near the inner surface of the flange appears in a band-like pattern; ferrite and pearlite are arranged in bands that run parallel to the fracture surface ； The microstructure near the outer surface of the flange shows no distinct banding. In accordance with CB/T13299-91 \"Method for evaluating the microstructure of steel\", the banding grade of the inner surface is rated as grade 4, while that of the outer surface is grade 1. The microstructure of 16Mn forgings after normalizing should consist of fine and uniform ferrite along with lamellar pearlite distributed in clusters; ferrite and pearlite must be intermixed with each other for the material to possess good overall mechanical properties. Banded structure appeared on the inner surface of the cracked flange material, while the banded structure on the outer surface was not apparent. The material specifications indicate that banding occurred during the forging process, and this banding was not properly eliminated during the normalizing heat treatment intended to remove it; as a result, this banding pattern appeared within the flange material. 3 Results and Discussion Based on the knowledge of heat treatment of metal materials, the causes of banding structure are rolling on the outside, as well as the segregation of sulfur and phosphorus and inclusions within the steel ingot. Therefore, banded structure is likely to occur during the forging process. Banded structure induces anisotropy in the mechanical properties of steel, meaning that the strength and toughness are high along the direction of the bands, while they are low and poor in the transverse direction. Therefore, once band-like tissue appears, measures must be taken to reduce or eliminate it ； At this point, annealing cannot be used to eliminate it; instead, normalizing must be employed, with the normalizing temperature being strictly controlled. Studies have shown that alloy steels with manganese as the main element tend to develop a banded structure due to the dendritic segregation of manganese, and this phenomenon is quite severe. Steel grades in the 16Mn series belong to one of these categories. According to the experimental results in the literature, the banded structure reduces the impact energy of the steel in a direction perpendicular to the bands to only one-third of that in a direction parallel to the bands. Therefore, for cracked flanges with strip-like structures, whose longitudinal impact toughness is already low, their transverse impact toughness will be even worse. As a result, at lower test pressures, brittle cracking of the flange occurred at the stress concentration areas. 4 Conclusions Through the failure analysis of the flange cracking on the shell side of the heat exchanger, the following conclusions are drawn: (1) The flange cracking is a type of brittle fracture. The poor toughness of the flange material and insufficient strength reserve are the main reasons for flange cracking. (2) Chemical composition analysis of the flange material showed that it is essentially consistent with the components specified for the design, meeting the requirements of technical standards. The flange cracking is not caused by the chemical composition. (3) As determined through the mechanical property testing of the flange material, its strength parameter σ0.2 is slightly higher than the minimum value specified in the technical standards; thus, there is insufficient strength reserve ； The impact toughness value does not meet the requirements; the impact energy is below both the minimum value specified in the technical standards and the values provided by the manufacturer. (4) Analysis of the metallographic structure of the flange material revealed uneven distribution of the structure, with a band-like pattern present near the inner surface; the direction of these bands was parallel to the fracture surface. This led to an even greater reduction in the radial toughness of the flange material, ultimately causing it to crack during the hydrostatic test. 5 Preventive Measures In light of the causes of cracking in heat exchanger flanges, and to prevent similar accidents from occurring again, the following preventive measures are proposed: (1) Strictly control every stage of the flange manufacturing process, including smelting, forging, and heat treatment. (2) Special attention must be paid to the machining of areas on the flange where stress concentration may occur, and the requirements specified in the design must be strictly followed. (3) Strengthen quality inspection.