Quality Control of Weld Overlay on Wear-Resistant Components for Cement Equipment

The cement industry operates under some of the most severe abrasive and erosive conditions in all of heavy industry. Raw mill liners, kiln wear plates, ball mill liners, separator vanes, and fan blades are subjected to continuous impact and abrasion from high-hardness cement clinker, limestone, gypsum, and fly ash particles. Weld overlay—applied as a strategic surface engineering solution—provides a means to either restore worn components to serviceable dimensions or to pre-build sacrificial hardfacing layers onto new components, thereby extending service life by factors of 3 to 10 relative to the base material. This article provides a comprehensive technical analysis of quality control methodologies, process parameters, acceptance criteria, and risk management for weld overlay applied to cement equipment wear-resistant components, drawing upon industry standards, field experience, and qualification frameworks.

1. Definition and Technical Principles

1.1 Weld Overlay for Cement Equipment: Scope and Purpose

Weld overlay in the context of cement equipment refers to the deliberate deposition of one or more layers of hardfacing alloy onto a base substrate—typically low-carbon steel (Q235, Q345), medium-carbon steel (Q355, 45# steel), or cast steel (ZG200, ZG270)—to create a surface layer with significantly enhanced hardness, abrasion resistance, and sometimes impact toughness. Unlike structural welding, which prioritizes joint strength and ductility, weld overlay prioritizes surface hardness (typically 40–65 HRC for cement service), chemical stability, and the integrity of the dilution-controlled transition zone between the overlay and the base metal.

1.2 Metallurgical Principles of Hardfacing Overlay

The effectiveness of weld overlay on cement wear parts is governed by several metallurgical mechanisms:

• Carbide Formation: Alloying elements such as chromium (Cr), tungsten (W), molybdenum (Mo), vanadium (V), and cobalt (Co) form hard ceramic-like carbides (Cr₇C₃, WC, Mo₂C, VC) within the weld metal matrix. These carbides, ranging from 1–5 µm in size, provide the primary abrasion resistance mechanism against cement particles.
• Work Hardening: Certain martensitic and high-carbon austenitic overlay alloys (e.g., D2, A2, Stellite 6) exhibit strain hardening under impact loading, increasing surface hardness during service.
• Composite Structure: Multi-layer overlays create a gradient in hardness and toughness from the base metal through the transition layer to the hardfacing surface, preventing catastrophic spalling under cyclic loading.
• Dilution Control: The dilution rate—the percentage of base metal melted and incorporated into the weld metal—directly governs the hardness and microstructure of the overlay. For cement equipment, dilution must be controlled to typically 10–25% to achieve target hardness while maintaining adequate toughness.

2. Category and Business Positioning

2.1 Positioning Within the Company's Technology Portfolio

Weld overlay quality control for cement equipment falls primarily under the TIG/MIG weld overlay technology route, which is one of the three core manufacturing capabilities of Cladding Technology Shanxi Co., Ltd. This capability serves a distinct market segment compared to hydraulic explosive bonding and explosion welding:

Technology Route	Primary Application in Cement Industry	Role of Weld Overlay
TIG/MIG Weld Overlay	Hardfacing of liners, plates, vanes, and ball mill segments	Primary wear protection and component restoration
Hydraulic Explosive Bonding	Production of composite steel plates for kiln shells and heat exchanger tubes	Weld overlay may be applied as a transition or repair layer on bonded assemblies
Explosion Welding	High-performance clad pipes for pneumatic conveying systems	Overlay applied to cut edges or repair of clad surfaces

2.2 Market Value and Customer Benefit

For cement plant operators, the value proposition of professionally controlled weld overlay is quantifiable:

• Extended Service Life: Properly applied hardfacing extends liner life from 6–12 months to 24–48 months in ball mill and raw mill applications.
• Reduced Downtime: Planned overlay restoration during scheduled maintenance windows eliminates unplanned stoppages caused by liner failure.
• Energy Efficiency: Restored liners maintain optimal grinding efficiency, reducing specific energy consumption (kWh/ton) by 3–8%.
• Cost Reduction: Overlay restoration costs 30–50% less than replacement with new cast or forged liners.

3. Technical Purpose and Key Quality Control Objectives

3.1 Primary Quality Control Objectives

The quality control framework for cement equipment weld overlay encompasses the following measurable objectives:

1. Hardness Compliance: Surface hardness of the overlay must meet specified ranges (e.g., 58–65 HRC for chrome carbide hardfacing, 45–55 HRC for cobalt-based hardfacing).
2. Adhesion Integrity: Overlay must exhibit zero spalling, delamination, or cracking under specified impact and thermal cycling tests.
3. Dilution Control: Base metal dilution must remain within the specified range (typically ≤25% for single-layer, ≤15% for multi-layer applications).
4. Geometric Accuracy: Overlay thickness, coverage uniformity, and dimensional restoration must meet design specifications.
5. Microstructural Integrity: Absence of hot cracks, cold cracks, porosity, and unmelted inclusions in the weld metal and heat-affected zone (HAZ).
6. WPS/PQR Compliance: All overlay operations must be executed in accordance with qualified Welding Procedure Specifications and Performance Qualification Records.

3.2 Quality Control Hierarchy

Effective quality control operates at three levels:

• Pre-Weld Controls: Base material verification, surface preparation, consumable traceability, WPS qualification, and welder certification.
• In-Process Controls: Parameter monitoring, interpass temperature management, layer sequence verification, and real-time visual inspection.
• Post-Weld Controls: NDT, hardness testing, dilution analysis, dimensional verification, and final documentation.

4. Key Process and Implementation Points

4.1 Base Material Preparation

Proper surface preparation is the foundation of weld overlay quality. The following requirements must be met:

• Surface Cleaning: Remove all rust, scale, oil, paint, and contaminants to a minimum Sa 2.5 blast cleanliness per ISO 8501-1 or equivalent. Residual contamination leads to hydrogen-induced cracking, porosity, and reduced adhesion.
• Beveling: For overlay on thick sections or worn surfaces, a 60° V-groove or single-V preparation is typically used to ensure adequate penetration and mechanical keying.
• Preheating: Preheat base material to 150–250°C for low-carbon steel, 250–400°C for medium-carbon steel (Q345/Q355), and 300–500°C for cast steel, depending on carbon equivalent (CE) and section thickness. Preheating reduces HAZ hardness, minimizes residual stress, and prevents cold cracking.
• Moisture Control: All consumables (welding rods, electrodes) must be stored at 100–150°C in insulated ovens and transferred to the work area in heated containers to prevent moisture pickup.

4.2 Welding Process Selection and Parameters

The selection of welding process depends on component geometry, overlay alloy type, and production requirements:

Parameter	MIG (GMAW) - Wire Hardfacing	TIG (GTAW) - Stick/Powder Hardfacing	Flame Spray / Powder
Current Type	DCEN (Direct Current Electrode Negative)	DCEN or AC	N/A (fuel gas mixture)
Typical Current	150–350 A	100–250 A	N/A
Travel Speed	5–15 cm/min	3–8 cm/min	5–20 cm/min
Wire/Consumable	Cr-Cr₂C₃, Co-W-Cr, Ni-Cr-Mo	Cast irons, Stellite rods, H12/H13	HVOF/WPA powders
Shielding Gas	Ar (100%) or Ar+5% CO₂	Ar (100%)	N/A
Interpass Temperature	≤250°C	≤200°C	≤150°C
Typical Layer Thickness	3–8 mm per pass	2–5 mm per pass	1–4 mm per pass
Production Rate	High	Medium	Medium-High
Dilution	15–30%	10–25%	5–15%

4.3 Multi-Layer Overlay Strategy

For high-performance cement wear parts, a multi-layer overlay strategy is recommended to optimize the hardness-toughness balance:

1. Layer 1 (Bond/Transition Layer): A low-carbon, high-toughness alloy (e.g., 309L, 312, or nickel-based) is deposited first to act as a crack-arresting barrier between the base metal and subsequent hard layers. This layer typically achieves 30–40 HRC and provides ductility to accommodate thermal and mechanical stresses.
2. Layer 2 (Intermediate Layer): A medium-hardness alloy (e.g., D2, A2, or Cr-Mo high-speed steel) is deposited to provide a gradient transition. Target hardness: 50–58 HRC.
3. Layer 3+ (Hardfacing Surface Layer): The final layer(s) consist of high-hardness alloy (e.g., chrome carbide, cobalt-based Stellite, or tungsten carbide composite). Target hardness: 60–68 HRC. Multiple passes may be applied to achieve required thickness (typically 3–10 mm total hardfacing thickness for cement liners).

4.4 Critical Process Parameters for Quality Control

The following parameters must be monitored and recorded throughout the overlay process:

• Heat Input: Must be controlled within the WPS-specified range. Excessive heat input increases dilution, reduces hardness, and promotes coarse microstructure. Insufficient heat input leads to poor fusion and incomplete penetration.
• Interpass Temperature: Must not exceed the WPS-specified maximum. For cement hardfacing alloys, interpass temperatures above 250°C can cause grain coarsening and reduced impact toughness.
• Welding Sequence: Must follow a planned sequence to minimize distortion and residual stress. For large flat plates (e.g., kiln wear plates), a block welding pattern with alternating directions is recommended.
• Welding Direction: For overlay on cylindrical components (e.g., ball mill liners), welding should proceed in the circumferential direction to minimize axial distortion.
• Consumable Traceability: Each batch of welding consumables must be traceable to mill certificates, with chemical composition and hardness verified prior to use.

5. Applicable Standards and Acceptance Criteria

5.1 Welding Procedure and Performance Qualification

Weld overlay procedures for cement equipment must be qualified in accordance with recognized standards:

• GB/T 19418 (Welding procedures qualification): Covers the qualification requirements for welding procedures, including overlay welding.
• ASME Section IX (Welding, Brazing, Fusing and Bonding Qualifications): Provides qualification rules for welding procedures and welder performance qualification. Appendix X specifically addresses weld overlay.
• ISO 15614-1 (Qualification testing of welding procedures for metallic materials): International standard for welding procedure qualification.
• ISO 9606-1 (Qualification testing of welders—Welding): Requirements for welder certification in manual welding processes.
• NB/T 47014 (Qualification of welding procedures for pressure vessels and pressure parts): Applicable when cement equipment components are pressure-containing (e.g., pneumatic conveying pipes).

5.2 Acceptance Criteria for Overlay Welds

Test Method	Standard Reference	Acceptance Criteria	Frequency
Visual Inspection (VT)	GB/T 3323 / ISO 17637	No cracks, undercut > 0.5 mm, porosity > 1 mm, or incomplete fusion visible	100% of weld length
Magnetic Particle Testing (MT)	GB/T 26952 / ISO 17638	No linear indications > 2 mm; no indications in stress-critical areas	100% of overlay surface
Hardness Testing	GB/T 231.1 / ASTM E10	Within WPS-specified range (e.g., 58–65 HRC); HAZ hardness ≤ 350 HV for base steel	Every 500 mm of weld length
Dilution Analysis	ASTM E1013 (OES) / GB/T 223	Base metal dilution ≤ 25% (single layer); ≤ 15% (multi-layer surface)	Sample coupon per batch
Impact Testing (Charpy V-Notch)	GB/T 229 / ASTM E23	≥ 27 J at 0°C for transition layer; ≥ 10 J at -20°C for hardfacing layer (if required)	Per WPS qualification; periodic in production
Tensile Testing	GB/T 228.1 / ASTM E8	UTS ≥ base material UTS; elongation ≥ 15% for transition layer	Per WPS qualification
Sectioning / Macrograph	GB/T 1954 / ASTM E3	No cracks, porosity, or lack of fusion in cross-section; uniform dilution gradient	Sample coupon per batch
Adhesion Testing	ASTM G51 / NACE TM0189	No spalling or delamination under specified impact energy	Per WPS qualification

5.3 Material Standards for Overlay Alloys

• GB/T 12469 (Cast irons for welding): Covers high-chromium white cast iron hardfacing alloys (e.g., ZGCr25, ZGCr27, ZGCr30).
• ASTM A397 (Cast iron weld metal for hardfacing): Covers chrome carbide, cobalt, and nickel-based hardfacing cast irons.
• ASME SA-397 (Cast irons for welding): Similar scope to ASTM A397, applicable to ASME-coded equipment.
• GB/T 8170 (Welding consumables—Hardfacing electrodes): Covers stick electrodes for hardfacing applications.
• ISO 14270 (Welding consumables—Specification for solid filling metals): International specification for hardfacing wires and rods.

6. Common Risks and Controls

6.1 Technical Risks

Risk	Cause	Impact	Control Measure
Cold Cracking in HAZ	High carbon equivalent base metal; insufficient preheat; rapid cooling	Overlay spalling; component failure	Preheat to specified temperature; use low-hydrogen consumables; control cooling rate with insulation blankets
Hot Cracking in Weld Metal	Excessive heat input; improper alloy composition; high sulfur/phosphorus	Reduced load-bearing capacity; premature failure	Control heat input per WPS; verify consumable chemistry; use multi-layer strategy with ductile transition layer
Excessive Dilution	High current; slow travel speed; deep weld penetration	Reduced surface hardness; loss of abrasion resistance	Optimize welding parameters; use multi-layer approach; verify dilution via OES on sample coupons
Porosity	Moisture in consumables; contaminated base surface; improper gas shielding	Reduced adhesion; stress concentration points	Store consumables in heated ovens; ensure Sa 2.5 surface cleanliness; maintain gas flow rate and nozzle condition
Overlay Spalling	Thermal mismatch; brittle single-phase microstructure; lack of transition layer	Catastrophic loss of wear protection	Use multi-layer strategy with ductile transition; control interpass temperature; avoid single-phase hardfacing alloys
Distortion	Excessive residual stress; improper welding sequence; high heat input	Dimensional inaccuracy; assembly interference	Follow planned welding sequence; use back-step welding; apply backing plates; post-weld stress relief if required
Hydrogen-Induced Delayed Cracking	Moisture in electrode coating; hydrogen diffusion into high-strength HAZ	Cracks appearing hours to days after welding	Use low-hydrogen electrodes (≤ 5 mL/100g); apply post-weld bake-out at 250–300°C for 2–4 hours

6.2 Process and Management Risks

• Welder Skill Variability: Manual welding processes (TIG, stick electrode) are highly operator-dependent. Control: Implement welder certification per ISO 9606-1 or GB/T 15169, with periodic requalification and skill assessment.
• Consumable Quality Variation: Different manufacturers or batches of hardfacing consumables may have varying chemical compositions and hardness. Control: Maintain approved supplier lists (ASL); verify each batch via mill certificates and incoming inspection (hardness spot-check, chemical analysis).
• Inadequate Documentation: Missing or incomplete welding records compromise traceability and qualification. Control: Implement a Welding Log system recording all parameters, operator ID, consumable batch, and inspection results for each component.
• Environmental Conditions: Wind, humidity, and temperature affect welding quality. Control: Set up welding bays with wind protection; monitor ambient temperature and humidity; restrict outdoor welding when wind speed exceeds 8 m/s or humidity exceeds 80%.

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Application)

The TIG/MIG weld overlay route is the primary technology for cement equipment wear parts. Key application scenarios include:

• Ball Mill Liners: Multi-layer overlay (transition + hardfacing) applied to 16Mn or Q345 steel liners. Chrome carbide hardfacing (ZGCr27) achieves 60–65 HRC, extending liner life from 6 months to 24+ months.
• Raw Mill / Kiln Wear Plates: MIG overlay with Co-W-Cr alloy (Stellite 6 equivalent) on 45# steel plates. Applied in circumferential weld beads with 2–3 mm overlap for uniform coverage.
• Separator Vanes and Classifier Components: TIG overlay with high-speed steel (H12/H13) on worn vane edges. Restores original profile while adding 2–5 mm of wear-resistant material.
• Fan Blades and Duct Liners: MIG overlay with nickel-based alloy (Ni-Cr-Mo) for combined corrosion and abrasion resistance in flue gas ducts.
• Cement Mill Gears and Shafts: TIG overlay with low-alloy steel (e.g., 5CrMnMo) for surface hardening and wear restoration of critical rotating components.

7.2 Hydraulic Explosive Bonding Route (Supporting Application)

While hydraulic explosive bonding is primarily used for producing clad plates and tubes, weld overlay quality control principles apply in supporting roles:

• Repair of Bonded Assemblies: Local damage or delamination in explosively bonded clad plates can be repaired using TIG weld overlay with a compatible filler alloy, followed by hardness verification to ensure bond integrity is maintained.
• Edge Treatment: After cutting or machining of clad plates, the exposed base metal at cut edges may require weld overlay to restore the composite structure or provide a transition layer for subsequent welding operations.
• Welding Procedure Qualification Support: Overlay welding procedures developed for cement equipment can be adapted for welding on clad materials, with dilution control being the critical common factor.

7.3 Explosion Welding Route (Supporting Application)

• Clad Pipe Repair: Explosion-welded clad pipes used in cement pneumatic conveying systems may require overlay repair at damaged sections. The overlay procedure must be qualified to ensure no damage to the existing clad layer.
• End Preparation: Weld overlay can be used to build up worn or damaged pipe ends prior to butt welding, ensuring adequate wall thickness and composite layer continuity.
• Post-Weld Treatment: After welding of clad components, overlay may be applied to the HAZ to restore hardness and microstructure, particularly where the base metal has been sensitized or embrittled.

8. Qualification Building and Certification Framework

8.1 WPS/PQR Development for Cement Equipment Overlay

Each unique combination of base material, overlay alloy, welding process, and application requires a qualified WPS and corresponding PQR. The qualification process includes:

1. WPS Development: Define essential variables including welding process, consumable type and size, current range, voltage range, travel speed, gas flow rate, preheat temperature, interpass temperature, and post-weld treatment.
2. Coupon Preparation: Fabricate test coupons from production-representative base material, with dimensions per ASME Section IX or GB/T 19418.
3. Welding Execution: Perform overlay welding on coupons by certified welders using the proposed parameters.
4. Testing and Evaluation: Conduct all required tests (hardness, tensile, impact, sectioning, dilution analysis) and evaluate against acceptance criteria.
5. PQR Documentation: Record all parameters, test results, and conclusions in a formal PQR.
6. WPS Approval: Upon successful PQR, issue the WPS for production use, with all essential variables locked within qualified ranges.

8.2 Welder Certification

Welders performing overlay operations on cement equipment must be certified per ISO 9606-1 or GB/T 15169, with qualification covering:

• Welding process (GTAW, GMAW, SMAW)
• Consumable type and diameter
• Base material group
• Position (flat, vertical, overhead)
• Section thickness range
• WPS reference

Certification validity is typically 6 months for overlay welding (due to the specialized nature of the process) and must be renewed through periodic performance testing.

8.3 Quality Management System Integration

Weld overlay quality control must be integrated into the company's overall Quality Management System (QMS) per ISO 9001 and, where applicable, ISO 3834-2 (Requirements for quality assurance systems for welding of metallic materials). Key integration points include:

• Document control for WPS, PQR, and welding logs
• Supplier evaluation and approval for consumables
• Calibration management for testing equipment (hardness testers, OES analyzers, MT equipment)
• Internal audit procedures for welding operations
• Corrective and preventive action (CAPA) processes for non-conforming welds
• Traceability system linking each delivered component to its welding records

9. Practical Implementation Checklist

The following checklist provides a practical framework for ensuring quality control of weld overlay on cement equipment:

1. Before Production:
   • Verify WPS is current and covers the specific application
   • Confirm welder certification is valid
   • Verify consumable certificates and perform incoming hardness check
   • Prepare welding bay (ventilation, wind protection, lighting)
   • Calibrate all monitoring and testing equipment
2. During Production:
   • Verify base material identification and heat number
   • Confirm surface preparation to Sa 2.5
   • Apply preheat and verify temperature with calibrated thermocouple
   • Record all welding parameters (current, voltage, speed, gas flow) per weld
   • Monitor interpass temperature and stop welding if limits are exceeded
   • Perform 100% visual inspection of each weld pass
3. After Production:
   • Perform MT or PT inspection on 100% of overlay surface
   • Conduct hardness testing at specified intervals
   • Prepare dilution test coupon and submit for OES analysis
   • Verify dimensional accuracy against drawing
   • Complete welding log and inspection report
   • Package and ship with quality documentation

10. Conclusion

Quality control of weld overlay on cement equipment wear-resistant components is a multidisciplinary challenge that integrates metallurgy, welding engineering, process control, and quality management. The systematic application of qualified WPS/PQR, certified welders, controlled consumables, rigorous NDT, and comprehensive documentation ensures that overlay welds deliver the specified hardness, adhesion, and durability required in the demanding environment of cement production.

For Cladding Technology Shanxi Co., Ltd., mastery of weld overlay quality control is not merely a technical capability—it is a qualification asset that enables market entry into the cement industry, a delivery guarantee that differentiates the company from competitors, and a customer value driver that translates into measurable operational savings for cement plant operators. The integration of this capability with the company's hydraulic explosive bonding and explosion welding routes creates a comprehensive surface engineering portfolio that addresses the full spectrum of wear, corrosion, and composite material requirements in heavy industry.