ASME STP-PT-025-2009 — Extended Fatigue Exemption Rules for Low Cr Alloys into the Time-Dependent Range for Section VIII Div 2 Construction

🔄 ASME STP-PT-025-2009 is a Pressure Technology research report prepared by Charles Becht IV and Charles Becht V (Becht Engineering Company) under project 07-03 (BPVC#1). The new ASME Section VIII, Division 2 permits high-strength low-chromium alloys (1¼Cr-½Mo, 2¼Cr-1Mo, 2¼Cr-1Mo-V, 9Cr-1Mo-V, etc.) to be used above 700°F and into their respective creep ranges, which makes “creep-fatigue” interaction the life-limiting failure mode for fatigue analysis. This report extends the existing fatigue exemption rules of Div 2 into the time-dependent (creep) range, proposing exemption criteria based on the “shakedown plus stress relaxation reset” concept, and provides relaxation/damage accumulation curves and cyclic fatigue data for each low-Cr alloy.

1. Background and Problem Statement 📌

Div 2 allows high-strength materials (e.g., 2¼Cr-1Mo-V, 9Cr-1Mo-V) above 700°F into their creep range, where the life-limiting failure mode is “creep-fatigue” — life shorter than that expected from either creep or fatigue acting independently. The report observes that:

• Current fatigue exemption rules cover only the time-independent (elastic/plastic) range and do not address damage accumulation during high-temperature hold times in the creep range.
• Low-Cr alloys exhibit cyclic softening or hardening at high temperature, so the analysis must be corrected with actual material data.
• Industry — especially refining — runs much equipment near or in the creep range and needs simple, reliable exemption criteria to avoid over-conservative analysis or expensive detailed evaluation.

2. Shakedown and Stress Reset Concepts 🔧

The report establishes a core criterion flow:

• Shakedown: if the cyclic stress-strain hysteresis closes after a few cycles (elastic shakedown), the stress amplitude stops growing and fatigue damage accumulates at a bounded rate.
• Initial stress and strain range: for a shaken-down cycle, the strain range is determined from elastic analysis combined with the shaken-down stress state.
• Reset to allowable stress: creep relaxation during the high-temperature hold lowers peak stress to the allowable stress level — effectively “resetting” the stress before the next condition — which substantially reduces damage in subsequent cycles.
• Creep-fatigue interaction diagram: the internationally used bilinear damage-interaction diagram evaluates the creep damage from hold periods.

3. Intermediate Cycles and Low-Cycle Exemption 🧪

The report further analyzes “intermediate cycles”: when switching between two operating conditions, the high-temperature hold at the second condition relaxes part of the stress established by the first, producing a stress reset (Figures 10/11) that affects total damage. For the high-peak-stress locations common in refining equipment (e.g., the acute corner of a nozzle, Figure 12), an example illustrates the strain-limit ratio and damage after 10 cycles with a 2-year hold time (Figure 13).

• Low-cycle exemption: proposes criteria to exempt low-cycle conditions that shake down to elastic action with sufficient hold-time relaxation from detailed creep-fatigue evaluation.
• Intermediate-cycle treatment: proposes a cycle-counting and damage-summation method that accounts for stress reset during condition transitions.

4. Engineering Practice Essentials 🛠️

• 🔍 Criteria first: first check whether the structure shakes down after the initial cycles and whether the hold time produces sufficient stress relaxation; if both hold, apply the exemption instead of a full creep-fatigue analysis.
• 🔧 Match material data: the exemption criteria rely on alloy-specific data in Appendices A–D; fatigue behavior of 2¼Cr-1Mo differs markedly with heat treatment (normalized-and-tempered, quenched-and-tempered, PWHT), so data must correspond to the product condition.
• 📊 Hold-time sensitivity: the longer the hold time and the higher the temperature, the larger the creep damage share; the Figure 9 interaction diagram quantifies whether “fatigue exemption plus creep damage” is acceptable.
• 📋 Conservative principle: data scatter against literature values (Appendices B/D) shows some alloys are discrete; exemption decisions should take the conservative side.

5. Frequently Asked Questions ❓