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ASME STP-PT-080-2016 — Average Isochronous Stress-Strain Curves and External Pressure Charts for 9Cr-1Mo-V Steel
📐 ASME STP-PT-080-2016 is a technical report sponsored by the ASME Pressure Technology Codes & Standards committee and ASME ST-LLC, jointly prepared by Global Engineering & Technology, LLC and Cromtech, Inc., and issued in June 2016. It develops two categories of essential design data for 9Cr-1Mo-V steel (Grade 91): first, average isochronous stress-strain curves and equations across a range of temperatures and hold times; and second, external pressure charts and equations for external-pressure design. The report documents the complete derivation path — from creep models to engineering curves — providing the data foundation for updates of ASME high-temperature analysis and external-pressure design rules.
💡 Isochronous stress-strain curves are the core input for high-temperature inelastic analysis (creep-fatigue assessment, design by analysis): each curve tells you the total strain (elastic + plastic + creep) a material permits at a given stress, temperature, and hold duration. External-pressure charts are the direct basis for buckling design of thin-walled components under external pressure.
1. Background and Material 📌
Grade 91 (9Cr-1Mo-V; T91/P91) is the workhorse material of supercritical boiler and high-temperature piping, offering an excellent combination of high-temperature strength, creep performance, and steam-oxidation resistance. Codes, however, need more complete high-temperature constitutive data: traditional code data often provide only short-time and creep-rupture properties, lacking isochronous curves usable for inelastic analysis and creep-deformation calculations.
The report is organized in three parts: Part 1 establishes the Grade 91 creep model and determines its parameters; Part 2 generates the average isochronous stress-strain curves and equations; Part 3 develops the external pressure charts and equations on the basis of the minimum isochronous curves.
2. Creep Models and Parameter Determination 🔧
The report compares and adopts several creep-rate formulations:
| Model / Approach | Basic Idea | Role in this Report |
|---|---|---|
| Logarithmic creep rate formulation | Creep rate decays with log time; suited to primary-creep-dominated long-duration behavior | Base creep law for generating isochronous curves |
| Ellis creep form | Explicit rate-time expression for creep | Cross-validation with the logarithmic formulation |
| Model parameter determination | Regress parameters such as α1, α3, ε̇0, ε̇01 from test data | Tailor the creep model to Grade 91 |
| Plasticity | Short-time plastic deformation and strain hardening | Provides the plastic strain component in isochronous curves |
| Nonlinear combined model (NLCM) | Nonlinear combination/superposition of creep and plasticity | Identified for its limitations in the creep regime, used as a cross-check |
⚠️ The report specifically discusses the limitations of the nonlinear combined model (NLCM) for creep: in long-duration regimes where creep dominates, NLCM can produce non-physical results. The report therefore builds the isochronous curves with explicit logarithmic-creep-rate / Ellis formulations; engineers should verify the applicable temperature and time range of any constitutive model they use.
3. Generating the Isochronous Curves and External Pressure Charts 📊
The isochronous curves are generated on the decomposition “total strain = elastic strain + plastic strain + creep strain”:
- Elastic strain: from modulus and temperature, εe = σ/E(T).
- Plastic strain: from short-time stress-strain relations (including strain hardening).
- Creep strain: integrated from the creep model at time t and temperature T.
- Summing the three and plotting over a series of hold times (e.g., 100, 1000, 10000, 100000 hours) yields the isochronous curve family; averaging over the data at each temperature gives the average isochronous curves.
- External pressure charts: buckling design needs minimum (conservative) curves; the report generates external-pressure charts and equations from the minimum isochronous curves, linking to the Section VIII Division 1 UG-28 external-pressure design procedure.
🚨 Distinguish “average” from “minimum” curves: use average curves for deformation and fatigue analysis, but minimum (conservative) curves for external-pressure charts and allowable values. Mixing the two results in assessment errors.
4. Engineering Practice Essentials 🛠️
- 🔍 High-temperature inelastic analysis: input the average isochronous curves into FE or simplified methods to compute accumulated strain under a load-time history for creep-fatigue interaction assessments (e.g., NH Subsection / Section VIII-2 high-temperature rules).
- 📊 External-pressure design: use the report’s charts/equations with the UG-28 flow (or the applicable code) to determine allowable external pressure; thin large-diameter components (shell-side external pressure, vacuum vessels) should be checked for buckling first.
- 🔧 Data consistency: verify the Grade 91 data edition and code references actually in use (whether post-2016 code updates have adopted these curves), and keep base-metal and weld assessment data consistent.
- 📋 Parameter conversion: the appendix gives unit-conversion procedures for α1, α3, ε̇0, ε̇01 — avoid unit errors that change results by orders of magnitude.
5. Frequently Asked Questions ❓
❓ Q: What is an isochronous stress-strain curve?
A: It is a family of stress-versus-total-strain curves, each for one fixed temperature and one fixed hold time (e.g., 1,000, 10,000, or 100,000 h). Reading the curve gives the creep-deformation capability of the material at given temperature and time — the basic data for high-temperature design and life assessment.
A: It is a family of stress-versus-total-strain curves, each for one fixed temperature and one fixed hold time (e.g., 1,000, 10,000, or 100,000 h). Reading the curve gives the creep-deformation capability of the material at given temperature and time — the basic data for high-temperature design and life assessment.
❓ Q: What is the difference between average and minimum curves, and where is each used?
A: Average curves represent typical (50%) material behavior and are used for deformation prediction and inelastic analysis; minimum (conservative) curves represent low-end properties and are used for external-pressure charts and allowable-stress safety assessments. They must not be interchanged.
A: Average curves represent typical (50%) material behavior and are used for deformation prediction and inelastic analysis; minimum (conservative) curves represent low-end properties and are used for external-pressure charts and allowable-stress safety assessments. They must not be interchanged.
❓ Q: Why are the external-pressure charts related to isochronous curves?
A: Buckling depends on the material’s compressive stress-strain behavior; at high temperature, creep-deformation capability directly affects the critical buckling pressure, so the external-pressure charts must be generated from the minimum isochronous curves (reflecting long-time high-temperature behavior), not from ambient curves alone.
A: Buckling depends on the material’s compressive stress-strain behavior; at high temperature, creep-deformation capability directly affects the critical buckling pressure, so the external-pressure charts must be generated from the minimum isochronous curves (reflecting long-time high-temperature behavior), not from ambient curves alone.
🔍 Q: Why is Grade 91 of particular interest?
A: Grade 91 has become the standard material for high-temperature components in supercritical and ultra-supercritical units; its creep properties and long-term microstructural stability are critical to plant safety and economics, and the codes need complete high-temperature constitutive and external-pressure data for design and life assessment.
A: Grade 91 has become the standard material for high-temperature components in supercritical and ultra-supercritical units; its creep properties and long-term microstructural stability are critical to plant safety and economics, and the codes need complete high-temperature constitutive and external-pressure data for design and life assessment.
