ASME Section VIII, Division 2 Part 5—Updated Creep Guidelines for 2025: What Engineers Need to Know

The 2025 Edition of ASME Section VIII, Division 2 brings a number of updates to high-temperature design practices. The changes to Part 5 expand the treatment of creep, clarify the role of primary and secondary stresses in ratcheting, and modernize the use of isochronous stress–strain curves for creep-buckling. For EPCs, owner-operators, and analysts working in high-temperature service, these updates represent a welcome alignment between legacy Code rules and the methods engineers actually use today. 

A Broader Framework for Creep

Earlier editions of Div. 2 provided limited creep guidance and lacked the breadth needed for thorough and safe analysis. The 2025 updates expand that framework significantly:

• Clearer treatment of elastic stress methods in the creep regime
• Adjusted allowable stresses for time-dependent behavior for ratcheting evaluation
• Use of the isochronous stress–strain curve methodology described in ASME FFS-1/API 579 for buckling in the creep regime

The result is a more holistic alignment between how real materials behave at temperature and how the Code requires engineers to check them.

Isochronous Stress–Strain Curves For Creep-Buckling

One of the most practical improvements is the renewed emphasis on isochronous stress–strain curves. Although familiar to engineers working under Section III (e.g., legacy N-47 / NB-3200 approaches), and ASME FFS-1/API 579 Chapter 10, they have not been widely used in many Division 2 workflows.

Isochronous curves define:

• The strain corresponding to
• A given stress, at
• A specific temperature, for
• A specific time duration.

For example, 304 stainless steel at 950 °F exhibits noticeably different curves depending on whether the load duration is 1 hour, 1,000 hours, or 100,000 hours. At 1,000 °F, the entire family of curves shifts downward. The updated Div. 2 guidance ensures these relationships are properly incorporated when evaluating time-dependent deformation, rather than relying solely on instantaneous elastic properties. The use of the isochronous stress-strain curves is used solely in a nonlinear type analysis, where the material deformation exceeds the material yield at elevated temperature, such a buckling analysis.

This directly benefits designs where even modest creep strain accumulation must be controlled—such as heater coils, hot piping manifolds, high-temperature nozzles, or tube connections.

Clarity in Ratcheting and the Role of Bree Type Diagrams

Perhaps the most engineer-friendly enhancement is the formal recognition of Bree-style primary + secondary load interaction diagrams. These diagrams have long been used informally by analysts to understand when incremental plastic strain (ratcheting) will occur, but the Code treatment has been scattered.

The 2025 Edition clarifies:

• How primary membrane loads interact with secondary bending
• Where the boundary lies between reverse plasticity and incremental strain
• How differing yield strengths during loading/unloading affect behavior
• How phase shifts in complex load cycles should be treated

This clarity is especially helpful for pressure-bearing components that sit at the intersection of sustained pressure loading and cyclic thermal bending—where ratcheting, not fatigue, may govern.

Consistency Between Piping and Vessel Codes

B31.3 §304.7.2 increasingly directs analysts to Section VIII, Div. 2 non-linear methods for strain-limited evaluations. The 2025 improvements to Div. 2 therefore benefit piping assessments as well—particularly for:

• Reinforced branch connections at high temperature
• Local strain checks where biaxial stress reduces ductility
• Scenarios involving large pressure + seismic load combinations
• Unlisted components such as laterals
• D/T > 100 components where diameter to thickness ratio is outside the scope of ASME B31J.

The updated framework supports more consistent results across piping and vessel analyses and reduces the ambiguity that previously surrounded high-temperature screening.

What This Means for Engineering Workflows in 2025

More Accurate Time-Dependent Predictions

Designers now have clearer guidance on when creep screening is sufficient and when full isochronous curve–based evaluation is required.

Better Insight Into Ratcheting Behavior

The updated Bree extension diagrams help engineers quickly determine whether a load combination will lead to incremental strain or simple elastic shakedown.

Clearer Allowables and Load Case Structures

Adjustments to Part 5 allowables and load cases remove historical inconsistencies and streamline high-temperature assessments.

Alignment With FEA-Based Design

Division 2’s evolution continues to support workflows where elastic-plastic FEA, time-dependent material curves, and load-cycling simulations form the basis of design decisions.