For equal strength matches, why does ASME use tensile strength as the criterion, while EN uses yield strength?
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Dear teachers, I recently went to study * and have a question: Since both national standard welding materials and ASME standards use tensile strength as the criterion for equivalent strength matching, what is the purpose of this? Would it be more reasonable to use yield strength as the benchmark for equivalent strength matching, similar to European standards? Since container design is based on keeping deformation within the elastic range as a limit, plastic deformation marks the onset of failure, and plasticity is bounded by the yield strength; therefore, in structural calculations, the yield strength serves as the key criterion. It would be more logical for welded joints to meet design requirements based on the yield strength as well. Then why do national standards and ASME use the tensile strength as the benchmark? From a design or other perspective, why does ASME use tensile strength as the basis for equivalent strength matching?
ASME standards achieve equivalent strength matching based on tensile strength, primarily for historical and safety reasons. Historically, ASME standards have focused on the ultimate state of materials when being established, that is, the maximum force that materials can withstand under the most extreme conditions. Tensile strength reflects a material’s ability to withstand tensile stress until it breaks, which is crucial for ensuring the overall safety of structures. On the other hand, the EN standard focuses on the yield point of materials, which is related to European design concepts. Europe places emphasis on the service condition of materials and their ability to withstand operational loads under normal conditions. Yield strength is the maximum stress that a material can endure before deforming, and this is very important for determining the material’s performance and safety margin. There is a rationale behind the establishment of each standard, and it is necessary to choose which one to use based on actual conditions and design requirements. The adoption of tensile strength as the equal-strength matching criterion for national standards and ASME standards is likely based on the requirements in engineering practice regarding the ultimate state of materials, as well as considerations of consistency with previous standards. .
Under normal circumstances, the selection of welding materials for carbon steel is based on matching the tensile strength; this is because the allowable stress for carbon steel containers is calculated based on their tensile strength； The selection of welding materials for stainless steel is based on a matching process determined by their chemical composition. The wall thickness calculation for stainless steel containers is based on the allowable stress determined from the yield strength.
This post was last edited by Claymore2000 on 2024-3-7 13:49. Thank you for your answer. Regarding this issue, there is a small anecdote: for the welding of the composite pipe 16MnII+S30408, the base layer was filled entirely with 309 material up to the top – how was this approved in terms of design? For composite pipes, due to diameter constraints that limit internal construction, the welding sequence of coating – transition layer – base layer is often used (while the conventional sequence is base layer – transition layer – coating). Since the coating is welded first, in order to prevent subsequent base layer welds from diluting or fusing with the coating, more base layer material is welded during the actual process, occupying a large portion of the base layer’s thickness. Additionally, the standard allows for a process in which a 309 transition layer is used until the base layer reaches the top (as illustrated in the special case on the right side of Figure A). However, under the same pressure, the allowable stresses of the low-alloy steel 16MnII and the weld metal of 309 differ significantly, resulting in large differences in the calculated wall thicknesses. Therefore, when using 309 welding material to fill the base layer all the way to the top, how is it possible to address the difference in allowable stresses between 16MnII and 309 in the design? Given the standard that supports the requirement of “filling the base layer with 309” (as specified in Figure B), would the design team approve such an approach on site?