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ASME STP-PT-058-2013 — Temper Bead Qualification Hardness Acceptance Criteria
🔧 ASME STP-PT-058-2013 is a technical report sponsored by ASME ST-LLC and produced jointly by TWI Ltd (David Abson, Adrienne Barnes, Sayee Raghunathan) and Rolls-Royce plc (Steve Jones). When in-service defects appear in pressure-retaining equipment — especially in hardenable steels such as Cr-Mo grades — the temper bead technique allows repair welding to be completed without a full postweld heat treatment (PWHT). The objective of this report is to provide a scientific, workable hardness acceptance criterion for qualifying temper bead repair procedures, answering the long-debated engineering question: “what HAZ hardness level is acceptable, and how should it be verified?”
💡 The core principle of the temper bead: subsequent weld beads “self-temper” the previous bead and the heat-affected zone beneath it, re-refining the hardened coarse-grained region so that the toughness of the parent material is restored without a full PWHT — but only if the process parameters are controlled and strict acceptance criteria are applied.
1. Overview and Controlled Deposition Repair Strategies 📌
Part I of the report systematically reviews the engineering strategies for controlled deposition repair:
- Repair using near-matching weld metal: consumables with composition and strength close to the parent metal, relying on a controlled bead sequence for self-tempering.
- Repair using Ni-base weld metal: nickel-based consumables for higher-alloy parent metals, reducing HAZ hardenability and improving high-temperature performance.
- HAZ hardness: the key indicator of hardening and cracking susceptibility; the report compiles reported HAZ hardness values for C-Mn, C-Mn-Ni-Cr-Mo, 0.5Cr-0.5Mo-0.25V, 1.25Cr-0.5Mo, 2.25Cr-1Mo, and 9Cr-1Mo steels under controlled deposition repairs (Tables 1–6).
- Residual stresses: the residual stress state of repair welds directly affects crack-driving force; the report discusses how controlled deposition affects the residual stress distribution.
- Repair of service-aged material: when repairing long-service, aged material, parent-metal degradation must be considered in the qualification.
| Steel Family | Typical Grades | Reported HAZ Hardness Range | Repair Concern |
|---|---|---|---|
| C-Mn steel | SA-105, SA-516, etc. | HV 250–380 (depending on cooling) | Hardenability in thick sections |
| C-Mn-Ni-Cr-Mo steel | SA-508, etc. | HV 300–420 | Hydrogen cracking susceptibility |
| 0.5Cr-0.5Mo-0.25V steel | SA-182 F22V class | HV 320–450 | Reheat cracking, hardness control |
| 1.25Cr-0.5Mo steel | SA-387 Gr.11 | HV 300–400 | Temper embrittlement, interlayer tempering |
| 2.25Cr-1Mo steel | SA-387 Gr.22 | HV 320–430 | Temper embrittlement, creep properties |
| 9Cr-1Mo steel | SA-387 Gr.91 | HV 350–470 | Martensitic structure, hardness-toughness link |
⚠️ Hardness is not a simple “lower is better” or “higher is worse” issue: excessively low HAZ hardness may mean insufficient strength or overtempering, while excessively high hardness means residual hardened structure and reduced toughness. Acceptance criteria must balance strength, toughness, and cracking resistance.
2. Part II: Experimental Investigation 🔬
Part II of the report carries out a systematic welding program to provide measured evidence for the hardness acceptance criteria:
- Parent steels and consumables: P4-class (e.g., 1.25Cr-0.5Mo) and P5A-class (e.g., 2.25Cr-1Mo, 9Cr-1Mo) parents, with several consumable combinations including near-matching and nickel-based wires.
- Welding details: one-layer and four-layer controlled deposition procedures, including deliberately “optimum” and “poor” procedures for comparison (Appendix A).
- Testing: hardness testing of weld metal and HAZ (Tables 11, 12) and Charpy impact testing (Tables 13–16), plus investigation of the relationship between HAZ hardness and HAZ Charpy toughness.
- Metallography: reveals the refinement and tempering effect of the temper bead on the coarse-grained HAZ.
| Study Element | Method / Scope | Key Result Direction |
|---|---|---|
| Number of layers | 1-layer vs. 4-layer controlled deposition | Multi-layer self-tempering markedly reduces peak HAZ hardness |
| Parent class | P4 (Cr-Mo), P5A (Cr-Mo-V / 9Cr) | Higher-alloy parents are more sensitive to the process window |
| Hardness testing | HAZ microhardness, full weld hardness | Establishes a “process-parameters-to-hardness-to-toughness” map |
| Charpy impact | HAZ vs. parent toughness comparison | Hardness and impact toughness show a clear inverse trend |
3. HAZ Hardness-Toughness Relationship and Acceptance Recommendations 📏
The conclusions and recommendations chapter states explicitly:
- There is a clear inverse correlation between HAZ hardness and HAZ Charpy impact toughness; hardness can serve as an indirect acceptance indicator for toughness, but should be validated with Charpy tests.
- Differentiated maximum HAZ hardness limits are recommended for different parent-metal classes, together with minimum tempering-sufficiency process controls (interpass temperature, bead thickness, and sequence).
- For nickel-based consumable repairs, attention should be paid to the hardness gradient at the weld/parent interface rather than a single peak value.
🚨 Temper bead procedures must be qualified on representative parent material and thickness: HAZ hardenability varies significantly with thickness, preheat, and interpass temperature. A “generic” qualification divorced from the actual service conditions is not representative and may mask a real hydrogen-cracking risk.
4. Engineering Practice Essentials 🛠️
- 🔍 Process window control: preheat, interpass temperature, bead thickness, and sequence are the four key variables of self-tempering; they must be defined and controlled in the welding procedure specification (WPS/PQR).
- 🔧 Hardness measurement locations: HAZ hardness should be measured at the weakest coarse-grained region and around the entire fusion line to avoid missing peak zones.
- 📊 Toughness verification: combine hardness acceptance with Charpy impact tests, especially for long-service or high-restraint repair locations.
- 📋 Code compliance: temper bead procedures must be qualified per the applicable code clauses (e.g., ASME IX and the governing fabrication code); the hardness criteria in this report can serve as acceptance reference values.
5. Frequently Asked Questions ❓
❓ Q: What is a “temper bead”?
A: A temper bead is a controlled deposition welding technique in which subsequent weld beads use their thermal cycles to self-temper the coarse-grained heat-affected zone of earlier beads, refining it and lowering hardness and cracking susceptibility — allowing repair welding of pressure equipment without a full postweld heat treatment.
A: A temper bead is a controlled deposition welding technique in which subsequent weld beads use their thermal cycles to self-temper the coarse-grained heat-affected zone of earlier beads, refining it and lowering hardness and cracking susceptibility — allowing repair welding of pressure equipment without a full postweld heat treatment.
❓ Q: Why use HAZ hardness as the acceptance indicator?
A: HAZ hardness directly reflects the degree of hardened structure (martensite/bainite) and correlates with hydrogen cracking and brittle fracture risk, and it is fast and inexpensive to measure. This report further demonstrates a good correlation with Charpy impact toughness, making hardness suitable as a qualification acceptance parameter.
A: HAZ hardness directly reflects the degree of hardened structure (martensite/bainite) and correlates with hydrogen cracking and brittle fracture risk, and it is fast and inexpensive to measure. This report further demonstrates a good correlation with Charpy impact toughness, making hardness suitable as a qualification acceptance parameter.
❓ Q: Are the hardness acceptance criteria the same for all materials?
A: No. Carbon steel, Cr-Mo steel, and 9Cr steel differ in hardenability and acceptable hardness levels. The report recommends setting maximum HAZ hardness limits per parent-metal class, judged together with strength and toughness requirements.
A: No. Carbon steel, Cr-Mo steel, and 9Cr steel differ in hardenability and acceptable hardness levels. The report recommends setting maximum HAZ hardness limits per parent-metal class, judged together with strength and toughness requirements.
🔍 Q: Does HAZ hardness still matter with nickel-based consumables?
A: Yes, but with a different focus: the nickel weld metal itself is soft, so attention shifts to the hardness gradient near the fusion line and the interface microstructure, to avoid local embrittlement from dilution.
A: Yes, but with a different focus: the nickel weld metal itself is soft, so attention shifts to the hardness gradient near the fusion line and the interface microstructure, to avoid local embrittlement from dilution.
