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ASME STP-NU-020-2008 — Verification of Allowable Stresses in ASME Section III Subsection NH for Alloy 800H
📐 ASME STP-NU-020-2008 is a companion report to STP-NU-019-1 prepared by the same team (R. W. Swindeman, M. J. Swindeman, B. W. Roberts, B. E. Thurgood, and D. L. Marriott), issued on November 1, 2008, with 76 pages. Its objective mirrors STP-NU-019-1, but the material is Alloy 800H (UNS N08810) — an iron-nickel-chromium alloy with good creep strength and oxidation resistance in high-temperature helium and steam environments above 750 °C, selected as a candidate structural material for the intermediate heat exchangers of high-temperature gas-cooled reactors (HTGR/VHTR) and Generation IV systems. The report is again organized in two parts: Part I verifies the base metal allowable stresses, focusing on the creep-rupture strength at 750 °C and above; Part II evaluates the strength reduction factors (SRF) of weld metal and weldments.
💡 Alloy 800H has an interesting history in the ASME system: it entered the Code through the “nuclear chemical high-temperature heat exchanger” project, and its allowables have long remained conservative. One goal of this report is to confirm whether the current values in ASME II-D and III-NH are still supported by global multi-source data, and whether supplementary allowable-stress tables (e.g., additions to III-NH Table I-14.6C) should be provided above 750 °C.
1. Material Identification and Data Sources 📚
The report first compares the chemical compositions of the Alloy 800, 800H, and 800HT variants, confirming that the analysis target is 800H (carbon controlled to 0.05–0.10% with small Al+Ti additions for high-temperature precipitation strengthening). Creep and rupture data come from more than 37 heats of global data (U.S., Japanese, and European sources) covering temperatures above 750 °C. The report’s statistical portraits are valuable: distributions of carbon content, Al+Ti content, grain size, test temperature, and rupture life give readers an intuitive view of the database’s “shape” and representativeness.
2. Part I: Base Metal Allowable Stress Verification 🔬
Part I begins with the current ASME Section II-D procedures for setting time-dependent stress allowables and the Subsection NH procedures for setting time-dependent stress intensities, then fits the Alloy 800H creep-rupture data globally with the Larson-Miller parameter and examines systematic residuals versus life, stress, and temperature. Using III-NH Table I-14.6C as the model, the report gives a complete example of how to add allowable values above 750 °C, including calculation methods, data selection, and safety factors. The key finding is that the rupture strength of Alloy 800H at 750 °C and above is significantly affected by heat-to-heat variation in carbon and Al+Ti content; the current allowables are generally reasonable, but modest adjustments based on the latest data are possible.
| Alloy Variant | Carbon Range | Al+Ti Content | Typical Temperature Range | Role in the Report |
|---|---|---|---|---|
| Alloy 800 | ≤0.10% | Low | Mid-temperature structures | Comparison baseline |
| Alloy 800H | 0.05–0.10% | Controlled range | Components above 750 °C | Analysis object (UNS N08810) |
| Alloy 800HT | 0.06–0.10% | Higher upper bound | Longer service at higher temperatures | Reference and extension |
| Filler metals Alloy 82/182, etc. | — | — | Weldments | Part II analysis objects |
3. Part II: Strength Reduction Factors for Weld Metal and Weldments 🔧
Part II identifies the typical filler metals for Alloy 800H — coated electrodes (e.g., Alloy 182), bare filler wire (e.g., Alloy 82, ERNiCr-3), and alternatives such as Alloy 117/617 and 21/33Nb — and collects tensile and rupture data on deposited weld metal and transverse weldments. Two routes quantify joint weakening: direct statistical extrapolation of deposited weld and weldment data into a “stress reduction factor” (SRF), and a parametric study of typical joint geometry (groove angle, weld thickness, specimen sampling location) to assess geometric effects. The results show that the reduction factor for Alloy 82 welds above 750 °C falls as life increases, indicating that the weak link (the heat-affected zone and the weld-metal interface) becomes more pronounced under long-term service.
⚠️ Alloy 800H weldments may control design life above 750 °C: differences in composition, microstructure, and creep ductility between weld metal and base metal make joint strength lower than base metal, and the reduction factor decreases with life. Designing weldments directly on base-metal allowables will overestimate life.
4. Key Technical Content and Data Tables 📊
- Larson-Miller fitting: a global LM fit of the 37-heat database, using residual histograms and residual-versus-life/stress/temperature plots to identify systematic bias and judge goodness of fit.
- 100,000-hour strength comparison: the report tabulates multi-source comparisons of average rupture strength for Alloy 800H at 800 °C and 100,000 h (Table 4), showing that different data selections (heat screening, lower-bound treatment) change the strength by up to about 10%.
- SRF recommendations: statistically based reduction factors for Alloy 82 welds at multiple temperatures and life points, compared with existing Code/literature assessments.
- Test matrix: Appendix 4 gives a recommended creep-rupture test program to support SRF revision above 750 °C, covering Alloy 82 weldments, Alloy 117/617 weld metal, and 21/33Nb weld metal.
5. Conclusions and Recommendations 🛠️
For the base metal, the current ASME II-D and III-NH allowables for Alloy 800H are generally supported by multi-source data, but supplemental allowable-stress tables above 750 °C are recommended based on the newly integrated data, using minimum (not average) rupture strength as the basis. For weldments, statistically based strength reduction factors must be applied, and the long-term tests listed in Appendix 4 are recommended to close the weld-data gap. The report also stresses that the high-temperature strength of Alloy 800H is sensitive to Al+Ti and carbon content, so procurement and acceptance should record and control these elements.
6. Engineering Practice Essentials 📋
- 🔍 High-temperature heat-exchanger material selection: the wall thickness of Alloy 800H IHX components in HTGR/VHTR is governed by allowables above 750 °C; design files should cite the verified allowable-stress table edition.
- 🔧 Never skip weldment reduction: Alloy 800H components with weldments must correct the allowable stress with the SRF, particularly for long-life service above 750 °C.
- 📊 Composition control: procurement specifications should bound carbon and Al+Ti ranges and record heats; composition variation is the dominant source of high-temperature strength scatter.
- 📋 Keep data traceable: LM fit constants, data selection, and safety factors should be fully recorded in the calculation files so that allowable stresses remain auditable.
- 🧭 Follow Code updates: once additions to III-NH Table I-14.6C are incorporated into the Code, the wall thickness and evaluation path of Alloy 800H high-temperature components change; track edition changes in time.
✅ Read against STP-NU-019-1, the pairing is instructive: one verifies Grade 91 steel, the other verifies Alloy 800H, using an identical framework. Reading both gives a systematic view of the “data-method-edition” revision logic behind ASME NH allowables.
7. Frequently Asked Questions ❓
❓ Q: How do you choose among Alloy 800, 800H, and 800HT?
A: The compositions are similar; 800H (N08810) and 800HT (N08811) achieve better high-temperature creep strength through controlled carbon and Al+Ti, while 800H is suited to structural components above 750 °C. Alloy 800 (N08800) serves mid-temperature and general corrosion applications. Choose by design temperature and life.
A: The compositions are similar; 800H (N08810) and 800HT (N08811) achieve better high-temperature creep strength through controlled carbon and Al+Ti, while 800H is suited to structural components above 750 °C. Alloy 800 (N08800) serves mid-temperature and general corrosion applications. Choose by design temperature and life.
❓ Q: Are SRF and WSRF the same thing?
A: Yes — both reduce the allowable strength of weldments relative to base metal. This report calls it SRF (stress reduction factor); STP-NU-019-1 calls it WSRF (weldment stress rupture factor). Both are ratio-type indicators of weldment-to-base-metal creep-rupture strength and must be statistically based.
A: Yes — both reduce the allowable strength of weldments relative to base metal. This report calls it SRF (stress reduction factor); STP-NU-019-1 calls it WSRF (weldment stress rupture factor). Both are ratio-type indicators of weldment-to-base-metal creep-rupture strength and must be statistically based.
❓ Q: Why do Alloy 800H allowables still need “verification”?
A: Because Code allowables are the product of historical data that passed through multiple extrapolations and material-specification evolutions; data sources, heat composition, and statistical methods may have become outdated. Re-fitting current, globally complete data exposes problems (e.g., over- or under-conservatism in some temperature bands) and justifies Code table revisions.
A: Because Code allowables are the product of historical data that passed through multiple extrapolations and material-specification evolutions; data sources, heat composition, and statistical methods may have become outdated. Re-fitting current, globally complete data exposes problems (e.g., over- or under-conservatism in some temperature bands) and justifies Code table revisions.
❓ Q: What is the direct use of this report for projects?
A: It provides verified allowable-stress values to cite in wall-thickness calculations; confirms that weldments must be reduced and quantifies the reduction; defines composition control and data-recording requirements for procurement; and gives background for future additions to ASME III-NH Table I-14.6C.
A: It provides verified allowable-stress values to cite in wall-thickness calculations; confirms that weldments must be reduced and quantifies the reduction; defines composition control and data-recording requirements for procurement; and gives background for future additions to ASME III-NH Table I-14.6C.
