TIG Cold-Weld Remelting and Wire Overlay Process on 1Cr17 Stainless Steel: Microstructure and Performance of the Modified Layer
1. Definition and Technical Principles
The TIG (Tungsten Inert Gas) cold-weld remelting and wire overlay process on 1Cr17 stainless steel refers to a specialized surface modification technique that combines two distinct but complementary thermal-metallurgical operations: (a) remelting of a pre-existing cold-welded joint or interface to homogenize the microstructure and eliminate cold-work defects, and (b) subsequent wire-feeding weld overlay to build up a functionally graded transition layer and cladding layer with controlled alloy composition and mechanical properties.
1Cr17 stainless steel (equivalent to Chinese GB standard designation, approximately corresponding to ASTM A240 430 or AISI 430 in international nomenclature) is a ferritic stainless steel containing 17% chromium with low carbon content. Its inherent limitations include susceptibility to intergranular corrosion under certain thermal cycles, relatively lower toughness compared to austenitic grades, and limited resistance to pitting in aggressive chloride environments. The TIG remelting and wire overlay process addresses these limitations by introducing a dilution-controlled overlay layer that provides enhanced corrosion resistance, improved surface hardness, and a metallurgically sound transition zone.
The underlying metallurgical principles include:
- Controlled dilution management: By precisely controlling heat input during TIG remelting, the dilution of the base 1Cr17 ferritic substrate into the overlay layer is limited to a target range (typically 20–40%), ensuring the final microstructure of the modified layer achieves desired chromium and carbon concentrations.
- Thermal cycle optimization: The remelting step subjects the cold-welded interface to a controlled austenitizing or solution-treatment-like thermal cycle, dissolving any segregation or cold-worked grain boundaries and promoting uniform elemental distribution.
- Layered build-up strategy: The wire overlay is deposited in multiple passes with interpass temperature control, creating a gradient from the base metal through a transition zone to the final cladding surface.
- Solidification microstructure engineering: The cooling rate from the TIG process produces fine-grained columnar-to-equiaxed transition dendrites with controlled chromium carbide precipitation, balancing corrosion resistance and mechanical strength.
2. Category and Business Positioning
This technology entry falls squarely within the company's TIG/MIG Weld Overlay technology route, serving as a specialized sub-process that bridges conventional cold-weld fabrication and full cladding overlay operations. In the company's capability matrix, it occupies the following positioning:
- Process category: Surface modification and functionally graded layer fabrication via arc welding thermal processes.
- Business segment: Repair, refurbishment, and performance enhancement of existing ferritic stainless steel components, as well as pre-qualification of overlay procedures for new cladding plate/pipe production.
- Value chain position: Upstream process development and WPS qualification that feeds into downstream production of clad plates, clad pipes, and overlay-coated equipment components.
3. Technical Purpose and Value
The primary technical objectives of this process are:
- Elimination of cold-weld interface defects: Cold welding (friction stir welding, cold roll bonding, or mechanical fastening) often leaves residual stress concentrations, oxide inclusions, and incomplete metallurgical bonding at the interface. TIG remelting fully dissolves these defects, creating a sound metallurgical bond.
- Microstructural improvement of the 1Cr17 substrate: The remelting thermal cycle refines the grain structure, reduces delta ferrite connectivity in any local austenitic regions, and promotes uniform chromium distribution to mitigate sensitization risks.
- Functionally graded overlay deposition: The wire overlay builds a layer with progressively increasing corrosion resistance (by introducing higher chromium, molybdenum, or niobium content) from the substrate outward, creating a graded barrier against environmental attack.
- WPS qualification foundation: The systematic study of microstructure and properties provides the metallurgical data required for formal Welding Procedure Specification (WPS) qualification in accordance with applicable codes.
The value delivered to customers includes: extended service life of ferritic stainless steel components in corrosive environments, reduced need for full component replacement (cost savings of 40–60% compared to replacement), compliance with industry codes for pressure-containing and high-purity applications, and documented traceability of the surface modification process.
4. Key Process Implementation Points
4.1 TIG Remelting Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Welding current | 80–150 A | Controlled penetration to remelt interface without excessive base metal dilution |
| Travel speed | 3–6 mm/s | Ensures adequate heat input for complete interface remelting while limiting HAZ width |
| Shielding gas | Pure Ar (99.99%) or Ar/He 75/25 mix | Argon provides adequate shielding; helium addition increases heat input for deeper remelting |
| Gas flow rate | 12–18 L/min | Maintains laminar flow and prevents backflow contamination |
| Electrode diameter | 2.0–3.2 mm | 2.0 mm for precise remelting; 3.2 mm for thicker cold-weld interfaces |
| Interpass temperature | ≤150°C | Prevents excessive grain growth and minimizes sensitization in 1Cr17 substrate |
| Preheat temperature | 100–200°C | Reduces thermal gradient and residual stress; eliminates surface moisture/contamination |
| Heat input | 0.5–1.2 kJ/mm | Optimized for complete remelting with minimal dilution |
4.2 Wire Overlay Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Wire material (typical) | ER309, ER310, ER316L, or ER410 (per ASTM A5.9) | Selected based on target overlay composition and dilution requirements |
| Wire diameter | 1.0–1.6 mm | Finer wire allows better dilution control and layer uniformity |
| Welding current | 60–120 A (DCEN) | DCEN provides deep, narrow penetration for layer-by-layer build-up |
| Travel speed | 4–8 mm/s | Controls bead width and overlap for uniform coverage |
| Number of overlay passes | 2–6 (depending on target thickness) | Multiple passes reduce dilution and improve compositional uniformity |
| Target overlay thickness | 0.5–3.0 mm | Depends on service requirement for corrosion/erosion resistance |
| Interpass temperature | ≤100°C (between overlay passes) | Minimizes sensitization and controls grain coarsening in overlay |
| Bead overlap | 50–70% of bead width | Ensures full coverage without excessive local heat concentration |
4.3 Process Sequence
- Surface preparation: Grind cold-weld interface to bare metal (Sa 2.5 per ISO 8501-1), remove all oxide, oil, and contamination. Verify surface cleanliness by visual and solvent wipe testing.
- Preheat: Apply preheat to 100–200°C using induction heater or torch preheat. Verify with infrared pyrometer at multiple points.
- TIG remelting: Execute remelting weld along the cold-weld interface using parameters from Table 4.1. Use weave pattern if interface width exceeds single-bead capacity. Maintain constant travel speed.
- Post-remelting inspection: Visual inspection for porosity, undercut, or incomplete fusion. Perform dye penetrant testing (PT) per ASTM E709 on remelted zone.
- Wire overlay deposition: Deposit overlay layer using parameters from Table 4.2. Begin at remelted interface and extend 5–10 mm beyond on both sides. Maintain strict interpass temperature control.
- Post-overlay treatment: Depending on application, apply solution heat treatment (1050–1100°C for austenitic overlay) or stress relief (600–650°C for ferritic overlay). Alternatively, allow controlled air cooling for martensitic-free microstructure in certain grades.
- Final inspection and documentation: Complete all NDT requirements and compile WPS/PQR documentation.
4.4 Microstructure Characterization Targets
| Zone | Expected Microstructure | Target Properties |
|---|---|---|
| Base metal (1Cr17) | Fine ferritic grains, no sensitization | Hardness ≤250 HB; no intergranular corrosion per ASTM A262 Practice E |
| Remelted HAZ | Refined ferritic/duplex grains, uniform Cr distribution | Hardness 200–280 HB; adequate toughness (≥20 J Charpy if applicable) |
| Transition layer (first overlay pass) | Mixed ferrite/austenite or martensite-free microstructure | Dilution 25–40%; Cr content 18–22 wt% |
| Final overlay surface | Columnar-to-equiaxed dendritic structure | Hardness 180–260 HB; pitting resistance ≥ base metal |
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- GB/T 985.1–2008 (Welding procedure specification – General principles) – Basis for WPS documentation in Chinese market applications.
- GB/T 19418–2004 (Welding procedure qualification test – General requirements) – Governs PQR testing for the overlay process.
- ASME Section IX, QW-440 through QW-460 – Weld overlay qualification requirements if the overlay is part of a pressure vessel or piping system.
- ASME B31.3 (Process Piping) – Applicable when overlay is used on process piping containing corrosive fluids.
- ASTM A5.9/A5.9M (Standard Specification for Welding Rods, Electrodes, and Bare Filler Metals for Shielded Metal Arc and Gas Metal Arc Welding) – Wire filler metal qualification.
- ISO 13919 (Welding – Welding procedure specification) – International WPS documentation format.
5.2 Material and Performance Standards
- GB/T 4237–2015 (Cold-rolled stainless steel plates and sheets) – Base material specification for 1Cr17.
- GB/T 1221–2009 (Stainless steels – Heat-resisting, acid-resistant, and cryogenic steels) – Chemical composition and mechanical properties of 1Cr17.
- ASTM A240/A240M – Graded stainless steel plate (Grade 430 equivalent to 1Cr17).
- NACE MR0175/ISO 15156 – If overlay is used in sour service environments containing H₂S.
- ASTM A262 (Standard Practices for Detecting Susceptibility to Intergranular Corrosion in Stainless Steels) – Practice E (critical pitting temperature) for overlay surface evaluation.
5.3 NDT and Acceptance Standards
- GB/T 3323.1–2019 (Non-destructive testing of welds – Radiographic testing) – RT acceptance for overlay welds.
- GB/T 11345–2013 (Non-destructive testing of welds – Ultrasonic testing) – UT acceptance criteria for overlay thickness and internal defects.
- ASTM E709 (Standard Practice for Visual Examination of Welds) – Visual acceptance criteria.
- GB/T 1954–2004 (Non-destructive testing – Magnetic particle testing) – MT for surface defect detection on ferritic overlay.
- ASME Section V, Article 2 and Article 4 – RT and MT acceptance criteria (Level 1 or 2 depending on service severity).
- ISO 17635 (Non-destructive testing of welds – General recommendations) – NDT selection and acceptance framework.
5.4 Acceptance Criteria Summary
| Inspection Method | Acceptance Criteria | Reference Standard |
|---|---|---|
| Visual (VT) | No cracks, undercut ≤0.5 mm, ripple uniform, no porosity visible | ASTM E709 / GB/T 3375 |
| Dye Penetrant (PT) | No linear indications; round indications ≤1 mm | ASTM E709 / GB/T 18851 |
| Magnetic Particle (MT) | No linear indications; round indications ≤1.5 mm | GB/T 1954 / ASME V Art. 7 |
| Ultrasonic (UT) | No indications exceeding Q1 level; overlay thickness uniformity ±10% | GB/T 11345 / ASME V Art. 4 |
| Hardness | Overlay: 180–260 HB; HAZ: within 20% of base metal | GB/T 231.1 / ASTM E18 |
| Dilution (metallographic) | 25–40% base metal dilution in first overlay pass | ASTM E3 / Company WPS |
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Mechanism | Control Measures |
|---|---|---|
| Cracking in overlay or HAZ | Excessive cooling rate in 1Cr17 ferritic substrate; hydrogen embrittlement from moisture contamination | Maintain preheat ≥100°C; use dry shielding gas; control interpass temperature; consider post-weld heat treatment at 600–650°C |
| Excessive dilution | High heat input or single-pass deposition with large wire diameter | Use multiple thin passes; reduce current; increase travel speed; use finer wire (1.0–1.2 mm); verify dilution by optical emission spectroscopy (OES) on each pass |
| Porosity in overlay | Inadequate shielding gas coverage; surface contamination (oil, oxide, moisture) | Verify gas flow rate and nozzles; use trailing gas shield; strict surface preparation per ISO 8501-1 Sa 2.5; solvent wipe test before welding |
| Sensitization of 1Cr17 base metal | Prolonged exposure to 450–850°C range during welding | Limit total heat input; minimize welding time; use pulsed TIG to reduce peak temperature; post-weld solution treatment if required |
| Delamination at overlay interface | Incomplete fusion due to insufficient remelting depth; oxide inclusion at interface | Verify remelt depth by macrograph examination; use AC TIG for oxide cleaning; ensure adequate penetration (minimum 0.3 mm into base metal) |
| Residual stress and distortion | Thermal gradient between overlay and substrate; constrained geometry | Apply backing plate; use tack welds for fit-up; implement stress-relief PWHT; use balanced weld sequence (center-out or symmetric) |
6.2 Quality Control Measures
- Pre-weld verification: Confirm base material heat number, chemical composition (via PMI/OES), and mechanical properties. Verify WPS/PQR currency and welder qualification (per GB/T 15169 or ASME Section IX).
- In-process monitoring: Record welding parameters (current, voltage, travel speed) continuously. Monitor interpass temperature with calibrated thermocouples. Inspect each pass visually before proceeding.
- Post-weld verification: Complete all specified NDT. Perform metallographic cross-section examination on coupon samples to verify dilution, microstructure, and absence of defects. Conduct hardness traverse across overlay/HAZ/base metal.
- Documentation: Compile complete weld records including parameter logs, NDT reports, heat treatment records, and final inspection certificates for customer traceability.
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This process is the core competency within the TIG/MIG weld overlay technology route. Key applications include:
- Clad plate fabrication: Production of 1Cr17 base plate with corrosion-resistant overlay for chemical processing equipment, heat exchanger tubesheets, and reactor internals. The remelting step ensures sound bonding between the cold-welded or mechanically bonded substrate and the overlay.
- Clad pipe manufacturing: Overlay of pipe interiors with higher-grade austenitic or duplex stainless steel for oil and gas, pulp and paper, and pharmaceutical applications. The graded transition layer prevents cracking at the dissimilar metal interface.
- Equipment repair and refurbishment: Restoration of worn or corroded 1Cr17 components (valve bodies, pump casings, tank internals) by remelting damaged areas and rebuilding with overlay material. This extends service life by 3–5 years in typical applications.
- Transition layer deposition: When overlaying austenitic materials (304, 316, 310) onto 1Cr17 ferritic substrate, this process provides the critical transition layer that prevents cracking due to coefficient of thermal expansion mismatch and solidification cracking susceptibility.
7.2 Hydraulic Explosive Bonding Route (Supporting Application)
In hydraulic explosive bonding (water-jet assisted explosive welding), the TIG remelting and overlay process serves as a post-bonding finishing and reinforcement step:
- Interface refinement: After hydraulic explosive bonding of 1Cr17 to a corrosion-resistant cladding layer, the TIG remelting process can be applied to selected areas where the explosive bond quality is marginal or where a metallurgical bond is required instead of a mechanical interlock.
- Edge sealing: The edges of explosively bonded clad plates often have incomplete bonding or oxide contamination. TIG overlay along the edges provides a continuous protective barrier and prevents ingress of corrosive media between layers.
- Thick cladding build-up: When the required cladding thickness exceeds what can be achieved by single explosive bonding (typically limited to 3–6 mm), the TIG overlay process builds up additional material on top of the explosively bonded layer to achieve total thicknesses of 8–15 mm.
7.3 Explosion Welding Route (Complementary Application)
In conventional explosion welding, this process provides:
- Pre-weld preparation: Remelting of 1Cr17 surface prior to explosion welding to remove surface contamination, oxide layers, and residual stresses from prior forming operations, improving explosion bond quality and reducing defect density at the interface.
- Post-explosion repair: Localized repair of explosion weld defects (separation zones, oxide inclusions) using TIG remelting and overlay to restore structural and corrosion integrity without reworking the entire component.
- Functionally graded interface creation: For applications requiring a gradual transition in properties (e.g., thermal expansion matching between 1Cr17 and a nickel-based cladding), the TIG overlay process creates intermediate alloy compositions between the explosion-welded layers.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This process development directly supports the company's qualification portfolio by:
- Generating validated WPS/PQR packages for TIG overlay on 1Cr17 substrates, enabling bidding on projects requiring code-compliant weld overlay (ASME, NB/T standards).
- Establishing welder qualification procedures and skill certification programs for specialized overlay welding on ferritic stainless steels.
- Building metallurgical databases (microstructure, hardness, corrosion resistance, dilution curves) that support rapid WPS development for future projects with similar material combinations.
- Meeting certification body requirements (TÜV, DNV, Lloyd's Register) for welding procedure approval in pressure equipment and offshore applications.
8.2 Product Delivery
In terms of product delivery capability:
- Enables the company to offer "repair and re-overlay" services that reduce customer downtime by 60–80% compared to full component replacement.
- Supports the production of custom clad plate and pipe configurations where 1Cr17 base material meets structural requirements but a corrosion-resistant surface is needed for process fluid contact.
- Allows the company to accept orders with tight delivery schedules by providing a faster alternative to explosion welding for thinner cladding requirements (≤3 mm overlay thickness).
- Provides a quality assurance pathway through documented process parameters, enabling repeatable production and reduced scrap rates.
8.3 Customer Value
The technical value proposition to customers includes:
- Cost savings: Overlay repair of existing 1Cr17 equipment costs 40–60% less than replacement with new clad components, while restoring performance to like-new condition.
- Extended service life: Properly executed overlay extends component life by 3–8 years depending on service severity, providing significant ROI on capital expenditure.
- Code compliance: Fully documented WPS/PQR packages with NDT verification enable customer compliance with regulatory and insurance requirements.
- Performance verification: Microstructural and mechanical property data provides customers with confidence in the long-term reliability of the overlay in their specific service environment.
- Reduced downtime: On-site or shop-floor overlay repair can be completed in days rather than the weeks required for component replacement, minimizing production losses.
9. Conclusion
The TIG cold-weld remelting and wire overlay process on 1Cr17 stainless steel represents a sophisticated surface engineering capability that integrates metallurgical science, welding process expertise, and quality management into a unified technology package. By systematically controlling heat input, dilution, interpass temperature, and filler metal selection, this process creates functionally graded overlay layers with optimized microstructure and performance characteristics. The resulting modified layer provides enhanced corrosion resistance, adequate mechanical strength, and sound metallurgical bonding to the ferritic substrate.
For Cladding Technology Shanxi Co., Ltd., this capability strengthens the TIG/MIG weld overlay technology route as the primary production method while supporting the hydraulic explosive bonding and explosion welding routes through interface preparation, edge sealing, and repair applications. The associated WPS qualification, NDT verification, and metallurgical documentation provide the quality assurance framework required for code-compliant product delivery to demanding industrial customers across chemical processing, oil and gas, power generation, and marine sectors.