Soft Iron Shot Strip TIG Weld Overlay Process Comparative Study
1. Definition and Technical Principles
Soft iron shot strips (also referred to as low-carbon mild steel strips or low-hardness iron shot strips) are elongated, thin cross-sectional workpieces typically composed of low-carbon steel (C ≤ 0.25%, with controlled Mn and Si content) that serve as critical consumable components in pneumatic and centrifugal shot blasting equipment. The TIG (Tungsten Inert Gas) weld overlay process applied to these strips involves depositing a specialized erosion-resistant or hard-facing alloy onto the surface of the base strip using a non-consumable tungsten electrode, with the weld pool protected by a high-purity shielding gas (typically argon or argon-helium mixtures).
The fundamental principle underlying TIG weld overlay on soft iron shot strips is the creation of a functionally graded interface between the ductile base material and the wear-resistant overlay layer. The process relies on precise thermal input control to achieve adequate metallurgical bonding while minimizing dilution of the overlay alloy into the base metal. For shot blasting applications, the overlay must withstand repeated high-velocity impacts from abrasive media (typically 0.8–1.2 mm iron shot traveling at velocities of 70–110 m/s), necessitating overlay materials with hardness values exceeding HV 400–600 while maintaining sufficient toughness to resist spalling and cracking.
The comparative study referenced in this entry systematically evaluates multiple TIG welding process configurations—including variations in current type (AC/DC), electrode geometry, wire feed parameters, travel speed, shielding gas composition, and interpass temperature—to determine optimal process windows for consistent, defect-free overlay production on soft iron shot strips.
2. Category and Business Positioning
Within the company's technology portfolio, soft iron shot strip TIG weld overlay falls under the TIG/MIG Weld Overlay technology route, representing a high-precision, lower-production-volume segment of the cladding business. This positioning distinguishes it from:
- Hydraulic Explosive Bonding (HEB): Used for large-area plate cladding where bulk volume and uniform thickness are paramount.
- Explosion Welding (EW): Employed for high-integrity clad plate/pipe fabrication requiring metallurgical bonding across large surfaces.
The soft iron shot strip overlay segment addresses a specialized niche market: the shot blasting equipment industry, including blast machine manufacturers, foundry operators, and heavy industrial maintenance contractors. The business value proposition centers on extending component service life by 3–8 times compared to unclad soft iron strips, reducing total cost of ownership through decreased replacement frequency and improved blasting efficiency.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Erosion Resistance Enhancement: Achieve overlay hardness ≥ HV 450 on the wear surface while maintaining base material ductility for strip forming and installation.
- Process Consistency: Establish repeatable welding parameters that produce uniform overlay thickness (typically 0.5–2.0 mm) across strip lengths of 300–1200 mm.
- Defect Minimization: Reduce porosity, lack of fusion, cracking, and undercut to below industry acceptance thresholds.
- Cost Optimization: Balance overlay performance against material and processing costs to deliver competitive pricing.
3.2 Comparative Study Value
The process comparative study serves as a foundational knowledge asset for the company's technical development team. By systematically documenting performance differences between welding configurations, the study enables:
- Standardized WPS (Welding Procedure Specification) development
- Training materials for welding operators
- Quality baseline establishment for incoming inspection
- Technical justification for customer qualification submissions
4. Key Process and Implementation Points
4.1 Process Parameter Comparison Matrix
| Parameter | Configuration A (DCEN) | Configuration B (AC) | Configuration C (DCEN + Pulsed) | Recommended Range |
|---|---|---|---|---|
| Welding Current | 120–180 A | 100–160 A (RMS) | 80–140 A (peak) / 40–70 A (trough) | 100–170 A |
| Voltage | 18–22 V | 16–20 V | 17–21 V | 17–22 V |
| Travel Speed | 300–500 mm/min | 250–450 mm/min | 350–550 mm/min | 300–500 mm/min |
| Wire Diameter | 1.6 mm | 1.6 mm | 1.6 mm | 1.2–2.0 mm |
| Wire Feed Speed | 1.5–2.5 m/min | 1.2–2.2 m/min | 1.8–3.0 m/min | 1.5–2.8 m/min |
| Shielding Gas | 100% Ar | 100% Ar | Ar + 5% O₂ | 100% Ar or Ar + 2–5% O₂ |
| Gas Flow Rate | 12–15 L/min | 12–15 L/min | 12–15 L/min | 12–18 L/min |
| Interpass Temperature | ≤ 150°C | ≤ 120°C | ≤ 180°C | ≤ 150°C |
| Electrode Type | W-L15 (1.5% La) | W-L15 (1.5% La) | W-L15 (1.5% La) | W-L15 or W-20 |
| Overlay Hardness (HV) | 420–480 | 400–460 | 450–530 | ≥ 450 |
| Overlay Thickness (mm) | 0.8–1.5 | 0.6–1.2 | 1.0–2.0 | 1.0–1.5 |
4.2 Critical Implementation Steps
- Base Material Preparation: Shot strips must be cleaned of surface contaminants (oil, rust, scale) using grinding or shot blasting. Surface roughness should be controlled to Ra ≤ 6.3 μm to ensure consistent arc stability.
- Joint Preparation: For multi-pass overlay, bevel grooves of 30°–45° included angle are machined or ground to create a mechanical key for overlay retention. Single-pass overlay on flat surfaces requires no groove preparation.
- Welding Sequence: Multi-pass overlay should follow a zigzag or weave pattern with overlapping passes of 50–70% to ensure uniform coverage. Each pass should be oriented to minimize residual stress accumulation.
- Post-Weld Treatment: Controlled cooling (air cooling or furnace cooling at ≤ 200°C/h) prevents microcracking in the overlay layer. For high-hardness overlays, tempering at 200–300°C for 1 hour may be applied to reduce residual stress.
- Dimensional Verification: Post-overlay strip dimensions must be verified for width tolerance (±0.5 mm), straightness (≤ 1 mm/m), and overlay thickness uniformity (±0.2 mm).
4.3 Overlay Material Selection
| Overlay Alloy Type | Composition (wt%) | Hardness (HV) | Application |
|---|---|---|---|
| High-Cr Iron-based | Cr 20–28, C 1.5–3.0, Mo 2–4 | 500–650 | Severe shot erosion |
| Medium-Cr Iron-based | Cr 10–18, C 1.0–2.0, Mo 1–3 | 420–520 | Moderate shot erosion |
| Cr-Mo Hardfacing | Cr 8–12, Mo 4–6, C 0.8–1.5 | 450–550 | Combined erosion/impact |
| Ni-Cr Hardfacing | Ni 60–70, Cr 15–20, B 2–4 | 400–500 | High-temperature erosion |
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 985.1 — Welding — Symbols for fusion welding and thermal cutting — Part 1: Symbols on engineering drawings
- GB/T 3375 — Welding, brazing and cutting — Terms and definitions
- GB/T 19804.1 — Welding procedure qualification — Part 1: General rules for ferrous metals
- GB/T 19804.2 — Welding procedure qualification — Part 2: Qualification of TIG and plasma arc welding
- GB/T 19866 — Welding procedure specification — General rules
- GB/T 26512 — Non-destructive testing of welds — Magnetic particle testing
- GB/T 11345 — Non-destructive testing of welds — Ultrasonic testing
- GB/T 13912 — Metallic coatings — Zinc hot-dip coatings on wrought iron and steel (for post-overlay corrosion protection)
- ASTM A395 — Standard specification for carbon steel wire for welding (weld wire qualification)
- ASTM A516/A516M — Specification for pressure vessel plates, carbon steel, suitable for welding (base material reference)
- ASME Section IX — Qualification rules for welding, brazing, and bonding procedures and personnel
- ISO 9606-1 — Qualification testing of welders — Arc welding — Part 1: Steel
- NACE MR0175/ISO 15156 — Materials for use in H₂S-containing environments (where applicable)
5.2 Acceptance Criteria
| Inspection Method | Acceptance Criteria | Reference Standard |
|---|---|---|
| Visual Inspection (VT) | No undercut > 0.5 mm; no surface porosity > 1 mm; overlay uniformity ±0.3 mm; no surface cracks | GB/T 3375, ISO 17637 |
| Magnetic Particle Testing (MT) | No linear indications > 2 mm length; no cluster of indications > 5 mm | GB/T 26512, ASTM E709 |
| Hardness Testing | Overlay hardness ≥ HV 450 (average); base metal hardness ≤ HV 200; hardness transition zone gradient ≤ 50 HV/mm | GB/T 231.1, ASTM E92 |
| Microstructure Examination | No centerline cracking; no unmelted particles; dilution ≤ 30% for single pass; ≤ 40% for multi-pass | ASTM E3 |
| Tensile/Bend Test (coupon) | Overlay layer tensile strength ≥ 550 MPa; no cracking in 180° bend test | GB/T 228, GB/T 232 |
| Impact Test | Charpy V-notch impact energy ≥ 27 J at -20°C (where specified) | GB/T 229, ASTM E23 |
6. Common Risks and Controls
6.1 Process Risks
| Risk | Cause | Detection Method | Mitigation/Control |
|---|---|---|---|
| Overlay Cracking | Excessive thermal input; high carbon dilution; rapid cooling | MT, visual, macro examination | Limit interpass temperature ≤ 150°C; control dilution; apply controlled cooling |
| Porosity | Inadequate shielding; contaminated base/wire; excessive travel speed | VT, ultrasonic testing (UT) | Maintain gas flow ≥ 12 L/min; pre-clean surfaces; stabilize wire feed |
| Lack of Fusion | Insufficient current; excessive travel speed; improper joint geometry | UT, sectioning | Increase current 10–15%; reduce travel speed; verify groove preparation |
| Undercut | Excessive current; improper electrode angle; too fast travel speed | VT | Reduce current 5–10%; maintain electrode angle 75–85°; slow travel speed |
| Spalling/Delamination | Poor metallurgical bond; high residual stress; thermal mismatch | Impact testing; service failure analysis | Ensure adequate base material preheat; multi-pass with interpass grinding; post-weld stress relief |
| Dimensional Distortion | Asymmetric heat input; clamping constraints | Dimensional measurement | Alternate welding sides; use fixture clamps; control welding sequence |
6.2 Quality Control Measures
- Incoming Inspection: Verify base strip chemistry (C, Mn, Si, S, P content per mill certificate), dimensions, and surface condition before overlay.
- In-Process Monitoring: Record welding parameters (current, voltage, travel speed, gas flow) for each production batch. Implement parameter deviation alerts for values exceeding ±10% of WPS specifications.
- Post-Weld Inspection: 100% visual inspection; MT on ≥ 20% of production (or 100% for critical applications); hardness verification on every batch; dimensional check per sample plan.
- Traceability: Maintain welder qualification records, material heat numbers, and WPS/PQR references for each production lot.
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route)
Soft iron shot strip overlay is the core application domain for this technology entry. Key scenarios include:
- Shot Blasting Machine Liners: Overlay of mild steel or iron shot strips used as blast wheel components, blast chamber linings, and conveyor wear strips in foundry and surface treatment facilities.
- Custom Strip Fabrication: Production of pre-overlay shot strips to customer specifications (length, width, thickness, overlay hardness) for OEM blast equipment manufacturers.
- Repair Overlay: On-site or in-plant TIG repair of worn shot strips, extending component life without full replacement.
- Multi-Layer Overlay: For severe service conditions, two-layer or three-layer overlay schemes combining a transition layer (e.g., 309L or E7018) with a hardfacing top layer.
7.2 Hydraulic Explosive Bonding (HEB) — Indirect Application
While soft iron shot strips are primarily a TIG overlay application, HEB technology contributes to the supply chain through:
- Base Material Preparation: Production of clad steel plates (carbon steel base + stainless steel or alloy steel cladding) used in blast machine housing fabrication, where the soft iron strips are subsequently installed.
- Composite Component Manufacturing: HEB-produced clad plates serve as structural components in blast equipment that house the overlay-treated shot strips.
7.3 Explosion Welding (EW) — Complementary Application
Explosion welding technology supports the shot strip overlay business through:
- High-Performance Clad Substrates: EW-produced clad plates (e.g., 16Mn base + 304 stainless cladding) provide corrosion-resistant blast chamber walls that complement the erosion-resistant overlay strips.
- Process Knowledge Transfer: Fundamental understanding of high-velocity impact bonding mechanics from EW research informs overlay dilution control and interface metallurgy optimization for TIG overlay processes.
- Customer Package Solutions: Integrated delivery of EW clad blast chamber housings combined with TIG-overlay shot strips provides a complete blast equipment package.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR Development: The comparative study data directly supports the development of qualified Welding Procedure Specifications (WPS) compliant with GB/T 19804.2 and ASME Section IX requirements. Multiple process configurations enable flexibility in qualification coverage.
- Welder Qualification: Process knowledge enables standardized welder qualification testing per ISO 9606-1, ensuring consistent operator capability across production shifts.
- Customer Qualification Submissions: Documented process comparisons with performance data serve as technical evidence in customer qualification packages, accelerating approval timelines.
8.2 Product Delivery Enhancement
- Process Optimization: Identified optimal parameter windows reduce production cycle time by 15–25% compared to unoptimized processes, enabling faster order fulfillment.
- Defect Reduction: Systematic process understanding reduces first-pass defect rates from typical 5–8% to below 2%, minimizing rework and improving on-time delivery.
- Scalability: Process knowledge enables reliable scaling from prototype/batch production to volume manufacturing without quality degradation.
8.3 Customer Value Creation
- Service Life Extension: Properly executed overlay extends shot strip service life from 200–400 hours (unclad) to 1,500–3,200 hours, reducing customer downtime and replacement costs by 60–80%.
- Technical Advisory: Process expertise enables value-added consulting services, including application-specific overlay material selection and maintenance schedule recommendations.
- Customization Capability: Multiple process configurations allow tailoring of overlay properties (hardness, toughness, thickness) to specific customer operating conditions, creating competitive differentiation.
- Quality Assurance: Documented process control and NDT verification provide customers with traceable quality documentation meeting international standards (ISO 9001, API Q1).
9. Conclusion and Forward Development
The soft iron shot strip TIG weld overlay process comparative study represents a critical knowledge asset for the company's weld overlay technology capabilities. By systematically documenting process-performance relationships across multiple welding configurations, the study establishes a technical foundation for standardized production, qualification compliance, and customer-specific optimization.
Future development directions include:
- Integration of automated TIG welding systems with real-time parameter monitoring and adaptive control
- Extension of overlay material library to include advanced ceramic-reinforced composite overlays for extreme erosion environments
- Cross-application of process knowledge to MIG overlay for higher-production-volume shot strip manufacturing
- Development of digital twin models for overlay process simulation and virtual qualification
This technical capability positions the company as a specialized provider of erosion-resistant cladding solutions, bridging the gap between standard welding services and high-performance surface engineering, with direct applicability across all three technology routes in the company's cladding portfolio.