Why is it rare to specify an upgraded base material during design for products manufactured through prolonged normalizing and tempering?
Thread Content
In the manufacturing of pressure vessel heads, it is common that after hot pressing, the heads need to undergo normalizing or normalizing plus tempering treatment to return to their serviceable state. If stress relief treatment is also required for the entire equipment, then an additional stress relief process SR for the whole equipment must be carried out; if the equipment is treated in segments or if there are revisions, the time required for the SR process will increase further. For the end caps, this is equivalent to a heat treatment process that involves normalizing followed by overall SR treatment for the equipment, or normalizing followed by tempering followed by overall SR treatment. For example, when an end cap undergoes 1 hour of normalizing plus 8 hours of SR treatment, it faces significant challenges in terms of both strength and toughness reduction. To ensure the strength of the weld joints, higher-grade welding materials are used. However, for equipment that requires such prolonged combined heat treatments during manufacturing, it is rare to see designs where the base material is chosen with a higher strength level to compensate for the decrease in strength of the raw material after such treatment. Thus, a question arises: why isn’t the design taking into account the need to select a material with a higher strength level to compensate for the decline in mechanical properties caused by prolonged heat treatment? For example, if the design requirements specify the use of a material with an Rm value of 490 Mpa for calculations, then a material with Rm=490 Mpa should be chosen. After normalizing the equipment and subjecting it to prolonged heat treatment, the actual tensile strength drops to 450 Mpa; thus, heat treatment results in a reduction of 40 Mpa in tensile strength. Then why not consider using a material with Rm=550 Mpa during the design phase? In that case, even after prolonged heat treatment, the strength would still remain between 510 and 500 Mpa, which would still meet the initial design requirement of 490 Mpa
When designing pressure vessels, it is generally believed that the performance of the product can be ensured by selecting appropriate materials and implementing strict welding quality control. As for heat treatments such as normalizing and tempering, although they do result in a decrease in the strength of the material, this reduction in strength is usually predictable and calculable. Furthermore, the main purposes of normalizing and tempering are to improve the toughness of the material and eliminate internal stresses, thereby enhancing the overall performance and safety of the material. An important reason why the use of upgraded base materials was not generally considered during design was cost and material availability. Using materials of higher strength grades is usually more costly, not only due to the cost of the materials themselves but also possibly due to increased processing costs. At the same time, high-strength materials may not be as easily available as conventional materials. Furthermore, pressure vessel design typically includes a certain safety factor, which means that even if the strength of the material decreases after prolonged heat treatment, its actual performance still meets the design standards and safety requirements. Therefore, when making design choices, it is not always necessary to increase the strength of the base material in order to compensate for the effects of heat treatment. In summary, the lack of widespread consideration for raising the strength grade of the base material during design is mainly due to factors such as cost, material availability, and safety factors. Under certain circumstances, if the effect of heat treatment on the material’s properties exceeds acceptable limits, designers may consider using materials with higher strength or adopting other compensatory measures. .