As the Boiler and Pressure Vessel Code continues to gain popularity worldwide, it is not uncommon that many users are mystified by the 1.1 factor used to establish the ultimate tensile strength above room temperature in developing the related design stresses of Section II Part D Stress Tables. Questions often arise about the origin and purpose of the factor, the reason for it not being applied to the yield strength, its due considerations regarding the tested tensile strength value of an alloy batch when accepting the batch for construction use, and more. Not having historic background or reliable sources for explicit explanations, some users tend to misinterpret the factor, abuse its application, incorrectly define alloy acceptance or qualification criteria, and mistakenly manipulate the safety margin in component design.
To help the Code users who struggle with these frequent and confusing issues, particularly those in the nuclear industry where rigorous criteria are required for component design and alloy acceptance or qualification, this paper is intended to demystify the 1.1 factor and facilitate knowledgeable interpretation and use of the Section II Part D Stress Tables as well as relevant Mandatory Appendices.
A brief review is first given on the background of the 1.1 factor, followed by a summary of its application in the Stress Tables. The provenance and purpose of the factor are then discussed in detail with graphic examples. Finally, current applicability and necessity of the factor are considered through a demonstration using example alloys.