CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electroslag Welding Overlay of Inconel 600 on 15CrMo Forging
Literature Overview
This technical paper, published in Petrochemical Equipment Technology in 2016 by Wang Qingguo of AVIC Liming Jinxing Chemical Machinery, documents the experimental development and industrial application of strip electroslag welding (strip cladding ESW) for overlaying Inconel 600 on 15CrMo forged components. The study addresses the challenge of providing corrosion and high-temperature resistance to 15CrMo forging components used in petrochemical and hydrogenation service, where the base material lacks adequate resistance to the aggressive environment. The use of strip electroslag welding for this application represents a significant advancement in cladding technology, combining the high deposition rate of ESW with the superior corrosion and creep resistance of the Inconel 600 overlay.
Background and Material Selection
15CrMo is a chromium-molybdenum low-alloy steel widely used for high-temperature pressure vessel components, hydrogenation reactor internals, and other applications in the petrochemical and refining industries. The material has good strength and creep resistance at elevated temperatures (up to approximately 450°C) but limited resistance to corrosion, particularly in the presence of hydrogen, H2S, and other aggressive species.
Inconel 600 (UNS N06600) is a nickel-chromium-iron alloy with excellent resistance to a wide range of corrosive environments, including oxidizing and reducing acids, alkalis, and hydrogen-containing atmospheres. The alloy maintains good mechanical properties at elevated temperatures and has excellent resistance to stress corrosion cracking and intergranular corrosion. The combination of 15CrMo as the base material and Inconel 600 as the overlay material provides an optimal balance of strength, creep resistance, and corrosion resistance for hydrogenation reactor applications.
The following table summarizes the key properties of the materials involved:
| Property | 15CrMo | Inconel 600 |
|---|---|---|
| Carbon (wt%) | 0.12-0.18 | 0.10 max |
| Chromium (wt%) | 0.80-1.10 | 14.0-17.0 |
| Molybdenum (wt%) | 0.40-0.60 | 0.30-0.50 |
| Nickel (wt%) | — | 58.0-65.0 |
| Fe (wt%) | Balance | Balance |
| Density (g/cm³) | 7.85 | 8.87 |
| Thermal expansion (10⁻⁶/K) | 12.0 | 13.0 |
| Yield strength at 25°C (MPa) | 290-310 | 205 |
| Yield strength at 650°C (MPa) | 120 | 120 |
| Corrosion resistance | Poor | Excellent |
| Creep resistance | Good (up to 450°C) | Excellent (up to 650°C) |
Strip Electroslag Welding Process Description
Strip electroslag welding is a variant of the electroslag welding process in which a continuous strip of filler material, rather than a consumable electrode, is fed into the slag pool to form the weld deposit. The process is characterized by:
- High deposition rate: Typically 15-30 kg/h, significantly higher than conventional arc welding processes.
- Low dilution: The slag pool provides a protective atmosphere and minimizes the dilution of the overlay material by the base metal.
- Uniform deposit composition: The continuous strip feed ensures a consistent chemical composition throughout the overlay layer.
- Low residual stress: The slag pool acts as a thermal buffer, reducing the peak temperature and the resulting residual stress.
- Good metallurgical bond: The slow cooling rate and the absence of a solidifying shell at the interface promote a strong metallurgical bond between the overlay layer and the base material.
The following table summarizes the key process parameters for the strip ESW overlay of Inconel 600 on 15CrMo:
| Parameter | Value |
|---|---|
| Welding current | 400-600 A |
| Welding voltage | 30-40 V |
| Strip feed speed | 0.5-1.0 m/min |
| Travel speed | 50-150 mm/min |
| Strip material | Inconel 600 strip, 25 mm × 2 mm |
| Slag material | Flux-93 or equivalent |
| Preheating temperature | 250-350°C |
| Interpass temperature | 250-400°C |
| Weld layer thickness | 5-8 mm per pass |
| Number of passes | 2-4 |
Metallurgical Bond and Interface Analysis
The metallurgical bond between the Inconel 600 overlay layer and the 15CrMo base material is a critical factor in the performance and reliability of the clad component. The interface between the two materials is characterized by:
- Dilution zone: A region of mixed composition at the interface where the Inconel 600 alloy is diluted by the 15CrMo base metal. The dilution level is typically 10-20% base metal in the first pass and decreases to less than 5% in subsequent passes.
- Transition zone: A region where the microstructure transitions from the base material to the overlay material. This zone may contain a mixture of ferrite, austenite, and carbide phases.
- Overlay zone: The fully Inconel 600 alloy with a fine-grained microstructure consisting of austenite and delta ferrite.
The dilution level at the interface is a critical parameter that affects the corrosion resistance and mechanical properties of the overlay layer. A high dilution level can reduce the corrosion resistance of the overlay layer by introducing carbon and other elements that promote carbide precipitation and sensitization. The strip ESW process, with its low dilution characteristics, is particularly advantageous for this application, as it allows the overlay layer to retain its full chemical composition and corrosion resistance properties.
The following table summarizes the dilution levels and resulting properties at the overlay-base material interface:
| Pass Number | Dilution Level (%) | Overlay Hardness (HV) | Corrosion Rate (mm/year) |
|---|---|---|---|
| 1 | 15-20 | 180-200 | 0.05-0.10 |
| 2 | 8-12 | 160-180 | 0.02-0.05 |
| 3 | 3-5 | 150-170 | 0.01-0.02 |
| 4 | <3 | 150-160 | <0.01 |
Mechanical Property and Corrosion Resistance Testing
The overlay layer was subjected to a comprehensive testing program to evaluate its mechanical properties and corrosion resistance:
| Test Method | Test Condition | Result | Acceptance Criteria |
|---|---|---|---|
| Tensile test | 25°C | 580 MPa | ≥ 520 MPa |
| Tensile test | 650°C | 180 MPa | ≥ 150 MPa |
| Hardness test | 25°C | 155-170 HV | 140-200 HV |
| Impact test | 25°C | 45 J | ≥ 27 J |
| Impact test | -20°C | 35 J | ≥ 20 J |
| Intergranular corrosion | ASTM A263 Practice E | No intergranular attack | No intergranular attack |
| Creep test | 650°C, 100 MPa, 10,000 h | < 1% elongation | < 2% elongation |
| Hydrogen permeation | 25°C, 1 MPa H2 | < 0.5 μL/cm²·h | < 1.0 μL/cm²·h |
The results demonstrate that the Inconel 600 overlay layer provides excellent mechanical properties and corrosion resistance, meeting or exceeding the acceptance criteria for all tested parameters. The low dilution level achieved by the strip ESW process is a key factor in achieving the required corrosion resistance, as it minimizes the introduction of carbon and other elements that can promote carbide precipitation and sensitization.
Application and Field Experience
The strip ESW overlay of Inconel 600 on 15CrMo forgings has been successfully applied to hydrogenation reactor internals, high-temperature heat exchanger tubesheets, and other critical components in the petrochemical industry. The following case study illustrates the practical application of this technology:
Application: Overlay of Inconel 600 on a 15CrMo forged reactor tube sheet for a hydrogenation reactor operating at 380°C and 15 MPa with a hydrogen partial pressure of 12 MPa.
Overlay specification: 3-pass strip ESW overlay, total thickness 18 mm, Inconel 600 strip.
Quality assurance: UT inspection of all overlay passes, MT inspection of the final surface, hardness profiling across the overlay thickness, and intergranular corrosion testing of the overlay layer.
Service performance: The overlay layer has been in service for over 5 years with no evidence of corrosion, cracking, or delamination. The UT inspection performed during a scheduled shutdown showed no indications of defects at the overlay-base material interface.
Defect Analysis and Countermeasures
The following defects were encountered during the development and application of the strip ESW overlay process:
| Defect | Cause | Countermeasure |
|---|---|---|
| Incomplete fusion at interface | Insufficient preheating, too high travel speed | Increase preheat to 300°C, reduce travel speed to 80 mm/min |
| Cracking in overlay layer | High residual stress, hydrogen embrittlement | Apply post-weld baking at 400°C for 2 h, use low-hydrogen flux |
| Excessive dilution | Too deep penetration, too low current | Reduce current by 10%, increase travel speed |
| Slag inclusion | Inadequate slag removal between passes | Ensure complete slag removal, use a slag-cleaning brush |
| Porosity | Moist flux, inadequate gas shielding | Bake flux at 300°C for 4 h, ensure proper gas flow |
The most critical defect was incomplete fusion at the overlay-base material interface, which was traced to insufficient preheating of the base material. The implementation of a rigorous preheating protocol, with the preheat temperature verified by infrared thermography, eliminated this defect in subsequent applications.
Key Questions and Reflections
The application of strip ESW for the overlay of Inconel 600 on 15CrMo forgings raises several important questions for engineers involved in cladding and bimetal product manufacturing. First, the long-term stability of the overlay-base material interface under cyclic thermal loading is a critical consideration for components that experience repeated heating and cooling cycles. The thermal expansion mismatch between the two materials (12.0 × 10⁻⁶/K for 15CrMo and 13.0 × 10⁻⁶/K for Inconel 600) can generate significant residual stresses at the interface, which may lead to cracking under cyclic loading. Second, the effect of the overlay layer on the fatigue performance of the component is not well understood, and further research is needed to establish the fatigue life of clad components under cyclic loading.
The use of strip ESW for this application represents a significant advancement in cladding technology, as it combines the high deposition rate and low dilution of ESW with the superior corrosion and creep resistance of Inconel 600. The technology is particularly well-suited for large-scale industrial applications where the cost and time of overlay are critical factors. However, the technology also has limitations, including the requirement for specialized equipment, the need for precise process control, and the limited flexibility in adapting to complex geometries.
Study Insights and Implications
The strip ESW overlay of Inconel 600 on 15CrMo forgings demonstrates that advanced cladding technologies can provide significant improvements in the corrosion resistance and service life of high-temperature pressure vessel components. The key to success lies in the careful selection of the overlay material, the optimization of the welding process parameters, and the implementation of a rigorous quality assurance program. For engineers involved in the design and fabrication of hydrogenation reactors and other high-temperature pressure vessels, this technology offers a practical and cost-effective solution to the challenge of providing corrosion and creep resistance to low-alloy steel components.
Reference Value and Outlook
This study provides a valuable reference for engineers involved in the development and application of strip ESW overlay technology for high-temperature pressure vessel applications. The documented process parameters, testing results, and field experience can be adapted for similar applications involving low-alloy steel base materials and nickel-based alloy overlays. Future developments in strip ESW technology, including the use of advanced filler materials, improved flux compositions, and automated process control, will further enhance the reliability and performance of strip ESW overlay applications. The continued development of non-destructive testing techniques, particularly phased array ultrasonic testing and terahertz imaging, will improve the ability to assess the quality of overlay layers and ensure long-term service reliability.