English -- Standard Toughness Requirements for Piping

Abstract

Scope: INTRODUCTION This Code provides requirements for evaluating the suitability of materials used in piping systems for piping that may be subject to brittle failure due to low-temperature service conditions. While low-temperature service is usually considered to be below ambient temperature, brittle failure can occur at temperatures above ambient temperature for certain combinations of materials, thicknesses, and stress levels. The definition of “low-temperature service” as used in this Code, therefore, varies widely across the many applications for which piping systems are used. For a building service air line, low temperature may be 0°C (32°F), whereas for a cryogenic piping system, it could easily be −185°C (−300°F).1 However, the principles used to evaluate the suitability of a piping system as related to service temperature by evaluating the toughness of the material can be applied across a wide temperature range, and this Code has been established to provide uniform guidance in this area. This Code may be invoked in whole or in part by various piping codes and/or specifications and is only mandatory when so invoked. Suitability of piping systems for low-temperature service is a function of several variables, including material properties, design loadings, and fabrication procedures. The three primary factors that generally control the susceptibility for brittle fracture are material toughness, crack size, and tensile stress level. There are a wide variety of services in which low-temperature suitability need not even be considered; however, a screening criterion is necessary to determine this. One objective of this Code is to provide a simple approach to evaluate whether additional consideration is necessary to evaluate suitability for low-temperature service. This is done by establishing a low-temperature service limit for various materials. Services at or warmer than this limit are not considered low temperature, and additional considerations relative to suitability are not required. For services colder than this limit, various requirements are provided that, when met, qualify the material for lowtemperature services. These requirements include impact testing, qualification of welding and other fabrication procedures, and limiting the design loadings. The low-temperature service limit established herein is based on a reasonable degree of assurance that at this temperature the material will have a ductile failure mode. The actual ductile-to-brittle transition temperature for a given material specification will vary based on actual heat chemistry of the material and subsequent processing. For critical applications, the design engineer can select materials with a lower low-temperature service limit or require impact testing. On less-critical applications, material with a higher low-temperature service limit may be acceptable. The final selection is left to the referring code and the design engineer (when permitted by the referring code). To keep the number of sets of requirements to a minimum, material groupings have been established, and a unique set of requirements has been provided for each group. These groups are assigned “Tnumbers” for easy reference. Although most materials used in piping systems are listed, some are not, and these unlisted materials are not addressed in this Code. Where permitted by the code or specification invoking this Code, these requirements may be used for unlisted materials. The invoking code or specification may establish the correct T-number group for the material or may invoke the testing and other requirements of this Code using the worst-case assumption that the design minimum temperature is colder than the temperatures that would allow exemption from any of the requirements of this Code. The guidelines for establishing the correct Tnumber group are provided in Nonmandatory Appendix B. 1 For guidance on cryogenic valves, refer to MSS SP-134, Valves for Cryogenic Service Including Requirements for Body/Bonnet Extensions.