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ASME STP-NU-019-1-2009 — Verification of Allowable Stresses in ASME Section III Subsection NH for Grade 91 Steel
📐 ASME STP-NU-019-1-2009 is a nuclear-industry technical report prepared by R. W. Swindeman (Cromtech Inc.), M. J. Swindeman (University of Dayton Research Institute), B. W. Roberts, B. E. Thurgood (Bpva Engineering), and D. L. Marriott (Stress Engineering Services), issued on March 19, 2009, with 119 pages. It is Revision 1 of STP-NU-019, and its opening pages contain a Summary of Changes that itemizes corrections made to the first revision — reference-number fixes, replaced figures, improved equation formatting, and numerical corrections (105 corrected to 100,000 and 104 corrected to 10,000). The task of the report is unambiguous: verify that the time-dependent allowable stresses (St, Smt) for Grade 91 (modified 9Cr-1Mo) steel in Subsection NH of ASME Section III are supported by the available data, in two parts — base metal (Part I) and weldments (Part II).
💡 Allowable stress is the “master switch” of high-temperature design: St sets wall thickness and Smt is the entry point to creep-fatigue evaluation, and both rest on statistical extrapolation of creep-rupture data. The value of this report is that it re-verifies these key numbers for Grade 91 against a global multi-source database and systematically assesses weldment strength.
1. Background and Material Identification 🔬
Grade 91 is a modified heat-resistant steel containing 9% Cr and 1% Mo with additions of V, Nb, and N, corresponding to specifications such as SA-213 T91 (tube), SA-335 P91 (pipe), and SA-387 Gr.91 (plate). The report first establishes the chemical composition range of the objects of analysis and compares the chemistries of welding filler metals. Data sources cover ORNL, Japan’s NIMS and its domestic data programs (JP and JCF), Germany, and other national databases, forming a creep-rupture database spanning heats, decades, and countries. The report also reproduces the NH definitions of creep stages (time to 1% total strain t1%, time to initiation of tertiary creep t3, and time to rupture tR) as the unified framework for the statistical analysis.
2. Part I: Verification of Base Metal Allowable Stresses ⚙️
Part I identifies materials, lists data sources, analyzes general trends in Grade 91 creep behavior, characterizes the creep database, specifies the criteria for setting St values and the analysis procedures, then statistically fits t1%, t3, and tR. Finally, it evaluates which criteria control St and gives conclusions and recommendations. The report compares the strength estimates for 100,000 h produced by different methods (different parametric extrapolations), noting that the degree of agreement among methods directly determines the credibility of any allowable-stress revision.
| Strength Basis | Definition | Data Needed | Role in the NH System |
|---|---|---|---|
| t1% | Time to 1% total strain in creep | Creep strain-versus-time curves | Limits deformation; one of the lower-bound criteria for St |
| t3 | Time to initiation of tertiary (accelerating) creep | Creep-rate-versus-time data | Prevents service within the instability region over design life |
| tR | Time to creep rupture | Creep-rupture tests | Rupture-controlled criterion; safety floor for St |
| St / Smt | Time-dependent allowable stress (intensity) | The three data classes above + safety factors | Wall thickness and creep-fatigue evaluation entry |
⚠️ Extrapolation of allowables is strictly bounded. Subsection NH normally permits extrapolation only to a set multiple (e.g., 3 times) of the maximum data life; anything beyond requires additional justification. The fact that 100,000 h strength estimates vary by method in this report is exactly why cross-method validation is essential in Code data revision.
3. Part II: Weldment Strength Reduction Factors 🔧
Weldments are the local weak links of high-temperature components. Part II identifies matching and non-matching filler metals for Grade 91 (typical 9Cr-1Mo-V family), collects creep-rupture test data on weld metal and weldments, and quantifies the weakness with a weldment stress rupture factor (WSRF). Several evaluation methods are used to estimate the WSRF, and the results are compared with other assessments. The effects of welding process, specimen sampling location, and joint geometry on the factor are discussed. The conclusions give recommended WSRF values for Grade 91 weldments in different temperature ranges and note that their variation is directly tied to base-metal creep strength and Type IV failure in the heat-affected zone.
4. Key Technical Details: Data Evaluation and Discussion 📊
- Multi-source data merging: combining databases (ORNL, NIMS, German, etc.) reduces single-source bias but requires handling heat-to-heat variation, test temperature differences, and data-quality differences.
- Criteria layering: St is bounded by several criteria (deformation, instability, rupture); the final value takes the lowest strength across criteria, and the report checks which criterion governs in which temperature range.
- Parametric extrapolation: time-temperature parametric methods (such as Larson-Miller) are compared for their ability to predict long-term strength in the mid- and high-temperature regions of Grade 91.
- Scarcity of weld data: long-term creep-rupture data for weldments are far fewer than for base metal, so the report stresses the need for supplementary testing and gives priorities.
5. Conclusions and Recommendations 🛠️
For base metal, the available data generally support the NH allowable-stress framework for Grade 91, but fine-tuning of St in selected temperature ranges is recommended based on cross-method validation, with careful separation of average versus minimum strength usage. For weldments, creep-rupture strength is lower than base metal and must be reduced by the WSRF; recommended factors are statistically based and vary with temperature and life region. The report also recommends additional long-term tests for secondary and tertiary filler metals and special joint geometries to close data gaps.
✅ Direct value for designers: this report provides the complete evidence chain for Grade 91 base metal and weldment allowable stresses — usable for auditing wall-thickness calculations, selecting weldment reduction factors in design packages, and defining long-term creep data requirements in procurement specifications.
6. Engineering Practice Essentials 📋
- 🔍 Never mix St and Smt in wall-thickness work: St is used in primary stress limits; Smt is the stress basis for creep-fatigue evaluation. They rest on different criteria and data — do not interchange them.
- 🔧 Evaluate weldments separately: Grade 91 weld seams in high-temperature piping and vessels must use the reduced joint strength in St calculations, or the failure risk will be systematically understated.
- 📊 Watch for Type IV cracking: creep failure in Grade 91 welds occurs preferentially in the fine-grained heat-affected zone (Type IV), which is also why the WSRF falls at lower temperatures; design selection and welding-procedure review should pay particular attention.
- 📋 Data traceability: design files should record the Code edition, data sources, and safety factors behind every allowable stress used, so that future audits and re-verification remain complete.
- 🧭 Version management: this report is Revision 1 and contains an errata list; always check the revision number and corrections when citing, and avoid referencing superseded content.
7. Frequently Asked Questions ❓
❓ Q: What is the difference between St and Smt?
A: St is the time-dependent allowable stress used for primary-stress limits under major loads; Smt is the time-dependent allowable stress intensity used as the stress basis in creep-fatigue damage accumulation. Both derive from creep-rupture data, but with different criterion combinations and safety factors — use each for its intended purpose.
A: St is the time-dependent allowable stress used for primary-stress limits under major loads; Smt is the time-dependent allowable stress intensity used as the stress basis in creep-fatigue damage accumulation. Both derive from creep-rupture data, but with different criterion combinations and safety factors — use each for its intended purpose.
❓ Q: Why do weldments need a separate strength reduction factor?
A: The microstructure, precipitates, and residual-stress state of weld metal and the heat-affected zone differ from the base metal; long-term creep-rupture strength is usually lower, and failure tends to initiate in the fine-grained HAZ (Type IV). Without reduction, joint strength is overestimated, so the Code system applies a WSRF.
A: The microstructure, precipitates, and residual-stress state of weld metal and the heat-affected zone differ from the base metal; long-term creep-rupture strength is usually lower, and failure tends to initiate in the fine-grained HAZ (Type IV). Without reduction, joint strength is overestimated, so the Code system applies a WSRF.
❓ Q: What is the “Summary of Changes” in Revision 1?
A: It itemizes the corrections from the initial revision — reference-number fixes, replaced figures, improved equation formatting, and numerical corrections such as 105 to 100,000 and 104 to 10,000. Always read the revised edition.
A: It itemizes the corrections from the initial revision — reference-number fixes, replaced figures, improved equation formatting, and numerical corrections such as 105 to 100,000 and 104 to 10,000. Always read the revised edition.
❓ Q: How does this report relate to STP-NU-013 and STP-NU-020?
A: STP-NU-013 covers negligible-creep criteria and creep-fatigue methods for Grade 91; STP-NU-020 performs the same kind of allowable-stress verification for Alloy 800H; this report is the primary allowable-stress verification for Grade 91. Together they form the “material-data-method” evidence chain for the ASME-NH revision.
A: STP-NU-013 covers negligible-creep criteria and creep-fatigue methods for Grade 91; STP-NU-020 performs the same kind of allowable-stress verification for Alloy 800H; this report is the primary allowable-stress verification for Grade 91. Together they form the “material-data-method” evidence chain for the ASME-NH revision.
