🎓 Lesson 15
D5
Post-Weld Heat Treatment Alternatives for Super Alloys
Post-weld heat treatment alternatives are special heating-and-cooling steps used after welding super alloys to fix internal stresses and improve strength—without using the traditional, time-consuming oven-based method.
🎯 Learning Objectives
- ✓ Explain the metallurgical rationale for avoiding conventional PWHT in gamma-prime strengthened super alloys
- ✓ Analyze residual stress profiles before and after applying induction-based local PWHT using thermal simulation outputs
- ✓ Design a laser shock peening (LSP) parameter set (pressure, spot overlap, pulse count) to achieve ≥200 µm compressive layer depth in IN718 weldments
- ✓ Apply ASTM E2899–23 guidelines to select and qualify an alternative PWHT method for a field-repaired gas turbine vane
📖 Why This Matters
In mining and energy infrastructure—especially in underground hard-rock ventilation systems, high-pressure slurry piping, and SAG mill liners—super alloy components (e.g., IN625, IN718, Haynes 282) are increasingly welded onsite. Conventional furnace PWHT is often impossible due to size, geometry, or operational downtime constraints. Choosing the wrong alternative can cause catastrophic intergranular cracking or sigma-phase embrittlement. Knowing *which* alternative works—and *why*—is not just convenient—it’s a safety-critical design decision.
📘 Core Principles
Super alloys rely on coherent gamma-prime (γ′) precipitates for strength; conventional PWHT (>1000°C for hours) overages these precipitates, reducing creep resistance. Alternatives instead target *localized* thermal management: induction heating provides rapid, controlled surface heating (5–20 mm depth) with minimal heat-affected zone (HAZ) growth; laser shock peening introduces deep compressive residual stresses without thermal input; and rapid thermal cycling (RTC) exploits kinetic suppression of deleterious phases (e.g., δ-Ni3Nb in IN718) via sub-second heating/cooling rates. All alternatives must satisfy two metallurgical constraints: (1) avoid crossing the solvus temperature of strengthening phases, and (2) reduce peak tensile residual stress to <25% of yield strength at service temperature.
📐 Induction Heating Penetration Depth
The standard depth of penetration (δ) determines effective heating thickness and guides coil design. It quantifies where eddy current density drops to 1/e (~37%) of surface value—critical for ensuring uniform stress relief without overheating the HAZ.
Standard Penetration Depth
δ = √(ρ / (π × f × μ))Calculates electromagnetic energy penetration depth in conductive materials during induction heating.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| δ | Penetration depth | m | Depth at which current density falls to 1/e of surface value |
| ρ | Electrical resistivity | Ω·m | Material-specific resistance to eddy current flow |
| f | Frequency | Hz | Alternating current frequency of induction power supply |
| μ | Absolute magnetic permeability | H/m | μ = μ₀ × μᵣ; μ₀ = 4π × 10⁻⁷ H/m, μᵣ ≈ 1.0–1.2 for aged IN718 |
Typical Ranges:
IN718 thin-section weld repair: 8–15 mm
IN625 thick-wall liner: 20–40 mm
💡 Worked Example
Problem: Calculate penetration depth for induction heating of IN718 (ρ = 1.26 × 10⁻⁶ Ω·m, μᵣ = 1.02) at 30 kHz frequency.
1.
Step 1: Convert resistivity ρ = 1.26 × 10⁻⁶ Ω·m and relative permeability μᵣ = 1.02 → absolute permeability μ = μ₀ × μᵣ = (4π × 10⁻⁷) × 1.02 ≈ 1.282 × 10⁻⁶ H/m
2.
Step 2: Apply formula δ = √(ρ / (π × f × μ)) = √(1.26e-6 / (π × 30,000 × 1.282e-6))
3.
Step 3: Compute numerator: 1.26e-6; denominator: π × 3e4 × 1.282e-6 ≈ 0.1208 → δ = √(0.01043) ≈ 0.102 m = 102 mm
Answer:
The calculated penetration depth is 102 mm—exceeding typical weld HAZ depth (5–15 mm). Therefore, 30 kHz is *too low*; switching to 200 kHz yields δ ≈ 39 mm, and 1 MHz yields δ ≈ 12 mm—optimal for thin-section IN718 welds.
🏗️ Real-World Application
At Vale’s Sossego copper mine (Brazil), a cracked IN718 slurry pump casing was repaired onsite using GTAW + laser shock peening (LSP). Conventional PWHT would have required dismantling the 4.2-ton assembly and 48-h furnace cycle—causing 14-day production loss. Instead, LSP (6 GW/cm², 8 ns pulse, 70% spot overlap, 3 passes) applied over the weld toe reduced peak tensile residual stress from +410 MPa to −220 MPa and extended fatigue life by 3.2× per ASTM E468 strain-gauge validation. The procedure was qualified per AWS G2.4-2022 and approved by DNV GL for Class I pressure boundary service.