Principles for substituting materials in pressure vessels

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When material substitution is required for pressure vessels, the basic principles are as follows: 01 The original design must not be altered; the structural dimensions and specifications specified in the original design must not be changed, nor must the drawings be modified. The substitute steel must meet the conditions and requirements of the original design. 02 When the mechanical properties of the substitute steel are superior to those of the originally specified steel, the thickness of the substitute steel shall not be reduced. When its mechanical properties are slightly inferior, the thickness of the substitute steel shall not either be increased, and a recheck of strength or stiffness must be conducted. 03 When using alternative steel materials, compatibility with adjacent steel materials must be considered, as well as any changes in manufacturing, inspection, etc. that may result from this. 04 In domestic and foreign steel standards, the relationships between substitute steel grades and the originally designed steel grades—such as similarities and differences in type—should be treated differently. ①The correspondence relationship refers to the following relationship between the corresponding grades of different **: (a) they are either both grades of pressure-processed materials or grades of cast material ； (b) having a standard chemical composition range that is very close, or the same, standard chemical composition range ； (c) Have very similar performance, or the same performance. Using steel grades that are equivalent to those specified in the original design allows the issue of material substitution to be simplified, and this approach is often employed in such substitutions. When using steel grades that are substitutes for those specified in the original design, attention must also be paid to any differences in specific sensitive properties resulting from variations in certain key chemical components. ②A close relationship refers to the following relationship between the corresponding grades of different **: (a) they are either both grades of pressure-treated materials or grades of casting materials ； (b) Having a standard chemical composition range that is close ； (c) It has performance that is close to that of. Using steel grades similar to those of the original design materials as substitutes is feasible; although their chemical compositions and properties are similar, it is necessary to be aware of differences in performance under certain conditions. For example, there are differences in the intergranular corrosion resistance between stabilized austenitic stainless steels and unstabilized austenitic stainless steels ； Differences in corrosion resistance, pitting resistance, and crevice corrosion resistance between molybdenum-containing austenitic stainless steels and molybdenum-free austenitic stainless steels in reducing media ； In addition to differences in corrosion resistance, austenitic stainless steels with different carbon contents also require attention to variations in their mechanical properties under high- and low-temperature conditions. Tube 20 has a higher allowable stress and lower plasticity than tube 10, but it remains within the acceptable range; therefore, tube 10 and its foreign equivalent grades can be used interchangeably with tube 20 and its foreign equivalent grades. Tube 10 and tube 20 are not equivalent grades but rather similar grades. ③Different types of steel can also be used as substitutes; for example, austenitic stainless steel can be used in place of carbon steel or low-alloy steel. In general, it is better to use a superior material instead of a inferior one, rather than using an inferior material in place of a superior one. When using different types of steel as substitutes, in addition to checking strength and stability, it is also necessary to consider the manufacturing process properties such as welding of dissimilar materials. Additionally, the compatibility between the substitute steel and the adjacent steels should be considered, such as the thermal stresses caused by differences in thermal expansion coefficients, the galvanic corrosion that may occur when there are significant differences in electronegativity, and the fact that carbon steels and low-alloy steels have better resistance to chloride-induced stress corrosion, whereas austenitic stainless steels are more sensitive to such stress corrosion. Economic feasibility is also an issue that needs to be considered when substituting different types of steel. There are many factors to consider when using different types of steel substitutes, and they are rarely used in engineering projects.