Zhuojin Pipe Fitting Co., LtdStrip Submerged Arc Overlay Welding of Inconel 625 on 12Cr2Mo1 Forging
Literature Overview
This 2024 study by Zhan Lining et al., published in Chemical Equipment and Piping (Vol. 61, Issue 1), addresses a critical engineering challenge in the chemical and petrochemical industry: the overlay welding of Inconel 625 onto 12Cr2Mo1 forged components for heat exchanger tube sheets. The research was conducted by the Hebei Provincial Special Equipment Supervision and Inspection Research Institute and the Hebei Provincial Innovation Center for Pressure Vessel Inspection and Evaluation, reflecting the practical engineering needs of equipment integrity management.
Technical Background and Requirements
Service Environment
The target application is a high-temperature hydrogenation/circulating hydrogen heat exchanger tube sheet, which operates under:
- High temperature: 350–450°C
- High pressure: 15–30 MPa
- Corrosive environment: Hydrogen-containing, potentially with H2S
- Thermal cycling: Startup and shutdown cycles
Material Selection Rationale
| Component | Material | Properties |
|---|---|---|
| Base forging | 12Cr2Mo1 (ASTM A387 Gr. 22 equivalent) | High-temperature strength, hydrogen resistance |
| Overlay | Inconel 625 (UNS N06625) | Excellent corrosion resistance, high-temperature strength |
| Weld strip | EQ62-50 | Inconel 625 composition strip |
| Flux | ES200 | Low-hydrogen, low-alloy flux |
The selection of Inconel 625 for overlay is driven by its exceptional resistance to:
- High-temperature oxidation
- Hydrogen attack (reduction cracking)
- Chloride stress corrosion cracking
- Sulfidation and high-temperature corrosion
Process Development and Procedure Qualification
Welding Method
The study employs strip submerged arc welding (SAW), which offers several advantages for overlay applications:
- High deposition rate: 5–10 kg/h compared to 1–2 kg/h for electrode or wire SAW
- Consistent bead quality: Continuous strip provides uniform composition
- Low dilution: Proper technique achieves 10–20% base metal dilution
- Good penetration control: Adjustable through current, voltage, and travel speed
- Low hydrogen content: Submerged flux provides excellent protection
Test Piece Configuration
The procedure qualification test pieces were fabricated using a combination of:
- Circumferential overlay: Simulating the actual tube sheet geometry
- Straight-run overlay: For mechanical property testing
This approach ensures that the qualified procedure reflects actual production conditions.
Welding Parameters
| Parameter | Value |
|---|---|
| Weld strip | EQ62-50 (Inconel 625 equivalent) |
| Flux | ES200 |
| Current | 500–700 A |
| Voltage | 28–35 V |
| Travel speed | 150–250 mm/min |
| Strip width | 20–25 mm |
| Preheat | 150–200°C |
| Interpass temperature | <300°C |
| Post-weld heat treatment | Solution treatment at 1050–1100°C for 1–2 h |
Performance Evaluation Results
Chemical Composition
The overlay deposit composition meets Inconel 625 specifications with minor variations due to dilution:
- Ni balance
- Cr: 20–23%
- Mo: 8–10%
- Nb: 3–4%
- Fe: <5% (controlled by dilution)
Mechanical Properties
| Property | Requirement | Test Result | Status |
|---|---|---|---|
| Transverse bend | 180° bend, no cracking | Pass | Acceptable |
| Hardness (as-welded) | ≤ 300 HV | 250–280 HV | Acceptable |
| Hardness (solution treated) | ≤ 250 HV | 200–230 HV | Acceptable |
Microstructure
The overlay microstructure consists of:
- Primary phase: Austenite (γ) — FCC structure
- Secondary phase: δ-ferrite (b) — BCC structure
- Precipitates: NbC and Nb2C carbides (stabilized by Nb addition)
The δ-ferrite content is typically 5–15%, which is within acceptable limits for Inconel 625 weldments. Excessive δ-ferrite (>25%) could reduce ductility and increase susceptibility to intergranular corrosion.
Corrosion Performance
The corrosion testing results are critical for this application:
| Condition | Corrosion Rate (mm/a) |
|---|---|
| As-welded | 1.50 mm/a |
| Solution treated | 1.17 mm/a |
| Ratio (as-welded / solution treated) | 1.28 |
Standard requirement: JB/T 4756—2006 requires the ratio to be ≤ 1.5.
The result of 1.28 satisfies this requirement, confirming that the welding process does not introduce excessive sensitization or microstructural degradation that would impair corrosion resistance.
Engineering Application and Field Performance
Production Application
Based on the qualified procedure, the overlay welding was applied to an actual high-temperature hydrogenation/circulating hydrogen heat exchanger tube sheet. The production overlay included:
- Multiple passes to achieve required thickness (typically 2–3 mm)
- Careful control of dilution to maintain Inconel 625 properties
- Post-weld solution treatment to optimize microstructure and corrosion resistance
- Comprehensive inspection including visual, dye penetrant, and hardness testing
Long-Term Performance
The overlay-welded tube sheet has been in stable service for three years without:
- Welding defects (cracks, porosity, incomplete fusion)
- Corrosion defects (pitting, intergranular attack, crevice corrosion)
- Hydrogen damage (reduction cracking, blistering)
- Mechanical failure (fatigue, creep)
This field performance validates the procedure qualification results and demonstrates the reliability of the overlay welding approach for this application.
Quality Control and Defect Prevention
Critical Quality Factors
| Factor | Control Method | Acceptance Criteria |
|---|---|---|
| Dilution control | Parameter optimization, first-pass inspection | Fe < 5% in overlay |
| Cracking prevention | Preheat, interpass temperature control |