ASME NTB-3-2020 pdf free download.Gap Analysis for Addressing Adequacy or Optimization of ASME Section III, Division 5 Rules for Metallic Components.
1.2.2 General Assessment
The current strain accumulation rules have geometric and service level transient design restrictions. The rules are also complicated to apply. The BPVC Section Ill, Division 5 elastic rules for strain limits evaluation are based on the decoupling of creep and plasticity. For temperatures above a certain cut off, the decoupling of creep and plasticity can no longer be justified and a unified viscoplastic model is necessary to describe the deformation behavior. The recently developed EPP methodology for strain limits evaluation of Type 304 and 316 stainless steels does not depend on the decoupling of creep and plasticity. It has been demonstrated by tests to be applicable to the full temperature range permitted code allowable stresses including very high temperatures where creep and plasticity are no longer decoupled. Cut off temperatures for the Class A materials have recently been established [12].
1.2.3 Material Specific Remarks
An Alloy 617 Code Case that includes the EPP methodologies which do not depend on the decoupling of creep and plasticity, and hence do not have the (649 C upper temperature limit for Alloy 617 as in the current strain accumulation rules, is being balloted by ASME Code committees.
1.2.4 Action Required
(1) Complete the ASME approval process for the Alloy 617 Code Case, and (2) complete the extension
the EPP methods to the remaining Class A materials.
1.3.2 General Assessment
The temperature cut off issue is the same as the simplified bounds discussed in Issue I. 2 above. The conservativism of the current BPVC Section IL Division 5 elastic rules for strain limits evaluation above the cut off temperature is currently being evaluated by ASME.