Hardfacing Weld Overlay on WK-35 Electric Shovel Bucket Teeth: Research, Process, and Application

1. Definition and Technical Principles

Hardfacing weld overlay is a surface engineering technique in which a wear-resistant, impact-resistant, or corrosion-resistant alloy is deposited onto the surface of a base component to protect it from degradation under severe operating conditions. In the context of WK-35 electric shovel bucket teeth, this technique involves depositing specialized hardfacing alloys—typically high-carbon martensitic, austenitic, or high-chromium cast iron systems—onto the cutting edges and wear faces of the teeth using arc welding processes such as submerged arc welding (SAW), manual metal arc welding (SMAW), or gas shielded arc welding (GMAW/TIG/MIG).

The WK-35 is a heavy-duty electric rope shovel with a bucket capacity of 35 m³, widely deployed in large-scale open-pit coal and mineral mining operations in China. Its bucket teeth endure continuous impact loading, severe abrasion against rock and coal, and cyclic fatigue. Without surface protection, the base steel (typically Q345 or 16Mn structural steel) exhibits rapid wear, necessitating frequent replacement. Hardfacing overlay extends service life by a factor of 3 to 8 times, dramatically reducing downtime and maintenance costs.

The fundamental metallurgical principles governing hardfacing overlay include:

• Hardness enhancement: Hardfacing alloys achieve surface hardness of 50–70 HRC (martensitic), 25–40 HRC (austenitic), or 80–90 HRC (high-chromium cast iron), compared to the base metal's 150–250 HBW.
• Wear mechanisms addressed: Abrasive wear (two-body and three-body), adhesive wear, impact wear, and fatigue spalling are the dominant failure modes on bucket teeth.
• Dilution control: The diffusion of base metal elements into the weld overlay dilutes the alloying additions. Managing dilution (typically targeting 5–15% for martensitic hardfacing) is critical to achieving desired hardness and microstructure.
• Toughness-hardness balance: Martensitic hardfacing provides high hardness but can be brittle; austenitic hardfacing offers superior toughness and work-hardening capacity at the expense of initial hardness.

2. Category and Business Positioning

This technology entry falls under the TIG/MIG weld overlay technology route within the company's three principal technology pathways (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding). Specifically, it represents a specialized application of weld overlay in the mining equipment aftermarket service segment, targeting heavy-duty construction and mining machinery maintenance.

The business positioning of this capability is threefold:

• Aftermarket service and remanufacturing: Providing hardfacing overlay services to mining companies operating WK-35 and similar electric shovels, extending component life and reducing capital expenditure on replacement teeth.
• Process development and qualification: Developing and qualifying Welding Procedure Specifications (WPS) for hardfacing overlay that can be replicated across different shovel models and component geometries.
• Technical consultancy and knowledge transfer: Delivering research findings, process recommendations, and operator training to customer maintenance workshops, establishing the company as a technical authority in surface engineering for mining equipment.

This entry also serves as a qualification-building artifact. The "study心得" (learning心得/research心得) format indicates a structured knowledge capture exercise that documents process parameters, failure modes, and optimization strategies—critical inputs for building a corporate technical knowledge base, supporting WPS qualification dossiers, and demonstrating engineering competence to prospective customers.

3. Technical Purpose and Value

The primary technical purpose of hardfacing overlay on WK-35 bucket teeth is to extend the operational life of the teeth under the following severe conditions:

• Impact loading: Bucket teeth penetrate rock and coal at depths of 1.5–3.0 m, generating impact forces of 200–500 kN per tooth.
• Abrasive wear: Coal seams containing quartz and pyrite particles (3–8% by weight) cause severe abrasive wear on cutting edges.
• Fatigue and spalling: Cyclic loading over 5,000–15,000 operating hours induces fatigue cracking and material spalling from the tooth surface.
• Corrosive environments: In coal mines with high moisture and sulfur content, oxidative and acid corrosion accelerates surface degradation.

The quantifiable value delivered includes:

Value Metric	Without Hardfacing	With Hardfacing Overlay	Improvement
Service life (operating hours)	80–150 h	400–1,200 h	3–8× extension
Replacement frequency	Every 2–4 weeks	Every 3–6 months	75–90% reduction
Annual maintenance cost per shovel	¥300,000–500,000	¥80,000–150,000	60–75% savings
Downtime per year	120–180 h	30–50 h	65–75% reduction

4. Key Process and Implementation Points

4.1 Base Metal Preparation

Proper surface preparation is the foundation of a successful hardfacing overlay. The following steps are mandatory:

1. Removal of existing wear surface: Grind or mill the worn cutting edge to a uniform profile. Remove all oxide scale, rust, oil, and prior weld deposits to a minimum depth of 2 mm below the original surface.
2. Preheating: Preheat the base metal to 250–400°C (depending on base steel carbon equivalent and thickness) using induction heating or gas torches. This reduces the cooling rate, minimizes residual stresses, and prevents cold cracking.
3. Geometry assessment: Verify the tooth profile against the original manufacturing drawings. Correct any geometric deviations (e.g., excessive wear on the cutting edge, deformation of the tooth body) before overlay application.
4. Surface roughness: Achieve a surface roughness of Ra 12.5–25 μm on the prepared surface to ensure adequate mechanical bonding of the first overlay pass.

4.2 Hardfacing Alloy Selection

The selection of hardfacing alloy is governed by the dominant wear mechanism and the mechanical properties of the base metal:

Alloy Type	Typical Composition	Hardness (HRC)	Primary Wear Resistance	Application Suitability
High-Carbon Martensitic (e.g., D2, H13)	2.0–2.5% C, 11–13% Cr	55–62	Abrasive (coal/rock)	Cutting edges, high-abrasion zones
Austenitic (e.g., Stellite 6, H21)	0.15% C, 25% Cr, 5% Mo, 7% Co	25–35	Impact + abrasive	Impact-prone zones, high-temperature service
High-Chromium Cast Iron (e.g., N100, Ni-Hard)	1.5% C, 15–20% Cr, 4–6% Ni	60–70	Severe abrasive	Chipping edges, high-abrasion coal seams
Composite (Hard + Soft alternating)	Martensitic + Austenitic layers	30–60	Combined impact + abrasion	General-purpose overlay on full tooth surface

4.3 Welding Process Parameters

The following table summarizes typical welding parameters for hardfacing overlay on WK-35 bucket teeth using different processes:

Parameter	SMAW (Manual)	SAW (Submerged Arc)	GMAW (MIG)
Welding Current	180–280 A	400–600 A	200–350 A
Welding Voltage	25–32 V	22–28 V	22–30 V
Travel Speed	20–40 mm/min	100–200 mm/min	30–60 mm/min
Wire/Flux Diameter	φ3.2–φ4.0 mm	φ3.0–φ4.0 mm	φ1.2–φ1.6 mm
Interpass Temperature	250–350°C	300–400°C	250–350°C
Number of Passes	2–4	1–3	2–5
Overlay Build-up Height	5–12 mm	6–15 mm	4–10 mm

4.4 Multi-Pass Overlay Strategy

For WK-35 bucket teeth, a multi-pass overlay strategy is recommended to manage dilution, residual stress, and microstructural uniformity:

1. Pass 1 (Transition/Build-up): Use a transition alloy (e.g., 309L or H12) to bridge the metallurgical gap between the base steel and the final hardfacing alloy. This pass controls dilution and prevents cracking at the base metal/weld interface.
2. Pass 2 (Hardfacing): Apply the primary hardfacing alloy (e.g., H13 or Stellite 6) to achieve the target hardness and wear resistance.
3. Pass 3 (Final Hardfacing, if required): Apply a second layer of hardfacing alloy to further reduce dilution and ensure consistent hardness across the full overlay thickness.

4.5 Post-Weld Heat Treatment

Post-weld heat treatment (PWHT) is essential for martensitic hardfacing alloys to relieve residual stresses and achieve the desired microstructure:

• Tempering: For martensitic hardfacing (e.g., H13), temper at 540–580°C for 2–4 hours in a controlled atmosphere furnace. This transforms the brittle as-quenched martensite into tempered martensite, improving toughness while maintaining hardness of 50–55 HRC.
• Stress relief: For austenitic hardfacing (e.g., Stellite 6), stress relief at 800–900°C for 1–2 hours is sufficient. Austenitic alloys do not require tempering.
• Cooling rate: Cool at a controlled rate of ≤50°C/h to prevent thermal cracking and minimize residual stress.

5. Applicable Standards and Acceptance Criteria

5.1 Standards Governing Hardfacing Weld Overlay

Standard	Title / Scope	Relevance to WK-35 Bucket Teeth
GB/T 11351	Welding consumables for hardfacing	Classification and specification of hardfacing electrode/wire types
GB/T 13917	Welding consumables—Welding hardfacing electrodes	Electrode qualification and performance testing
GB/T 1985	Welding consumables—Submerged arc hardfacing wire and flux	SAW hardfacing consumable specification
ASTM A402	Standard Specification for Hardfacing Welding Electrodes and Rods	International reference for hardfacing consumable classification
ASME Section IX	Qualification Rules for Welding, Brazing, and Fusing	WPS/PQR qualification for hardfacing overlay procedures
NB/T 47014	Qualification tests for welding procedure specification for pressure vessels	Domestic Chinese qualification standard (applicable by analogy)
ISO 14271	Welding consumables—Welding hardfacing electrodes and rods	International hardfacing consumable classification
NACE MR0175 / ISO 15156	Mandatory Requirements for Materials for H₂S-Containing Environments	Applicable if bucket teeth are used in H₂S-containing coal seams

5.2 Acceptance Criteria for Hardfacing Overlay

1. Visual inspection (VT): The overlay surface shall be free of cracks, pores, undercut exceeding 0.5 mm, and incomplete fusion. Surface roughness shall not exceed Ra 6.3 μm after grinding.
2. Hardness testing: Overlay hardness shall be verified at three points across the width and at two depths (1 mm and 3 mm from surface). Martensitic hardfacing shall achieve ≥50 HRC; austenitic hardfacing shall achieve ≥25 HRC. Hardness gradient at the interface shall not exceed 30 HV/mm.
3. Dilution testing: Dilution shall be measured by spark emission spectrometry or optical emission spectrometry (OES). Maximum dilution shall not exceed 15% for the first hardfacing pass and 10% for the final pass.
4. Macrographic examination: Cross-section examination shall reveal uniform microstructure, no unmelted inclusions, and no cracks at the base metal/weld interface. Overlay thickness shall be within ±1 mm of the specified dimension.
5. Mechanical testing: Transverse tensile specimens shall achieve ultimate tensile strength ≥400 MPa (martensitic) or ≥500 MPa (austenitic). Impact energy (Charpy V-notch at 20°C) shall be ≥27 J for martensitic hardfacing after tempering.
6. Wear testing: Taber abrasion or dry sand-rubber wear tests shall demonstrate wear resistance ≥3× that of the base metal.

6. Common Risks and Controls

6.1 Cracking

• Risk: Hot cracking in the hardfacing overlay due to high sulfur and phosphorus content in the base metal or excessive cooling rate. Cold cracking at the base metal/weld interface due to hydrogen-induced cracking in high-carbon-equivalent base steels.
• Controls:
  • Preheat to ≥250°C to reduce cooling rate below 10°C/s.
  • Use low-hydrogen welding consumables (diffusible hydrogen ≤6 mL/100g for SMAW).
  • Apply a transition layer (e.g., 309L) to absorb sulfur and phosphorus from the base metal.
  • Maintain interpass temperature ≥200°C to prevent hydrogen entrapment.

6.2 Excessive Dilution

• Risk: High dilution (>20%) reduces overlay hardness below the required threshold, rendering the hardfacing ineffective.
• Controls:
  • Use multi-pass overlay with a transition layer to progressively reduce dilution.
  • Employ narrow weld beads with low heat input (≤1.5 kJ/mm for SAW).
  • Verify dilution after each pass using OES and adjust parameters accordingly.
  • Use overlay alloys with higher alloy content to compensate for dilution (e.g., H13 with 13% Cr instead of H12 with 11% Cr).

6.3 Spalling and Delamination

• Risk: The hardfacing overlay may spall off the base metal under impact loading if the interface bond strength is insufficient or if residual stresses are excessive.
• Controls:
  • Ensure thorough surface preparation (grinding to bare metal, removal of all oxide and contamination).
  • Apply adequate preheat and PWHT to relieve residual stresses.
  • Use a composite overlay strategy (hard + soft alternating layers) to improve interface toughness.
  • Verify interface bond strength through macrographic examination and microhardness gradient testing.

6.4 Distortion and Dimensional Deviation

• Risk: Thermal distortion of the bucket tooth geometry during multi-pass overlay can alter the cutting profile, reducing excavation efficiency.
• Controls:
  • Use symmetric welding sequence (start from the center and weld outward in both directions).
  • Apply backing bars or rigid fixtures to constrain movement during welding.
  • Monitor dimensional accuracy after each pass using CMM or template gauges.
  • Grind the final overlay surface to the original tooth profile after welding and PWHT.

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay (Primary Route)

The WK-35 bucket teeth hardfacing application is a direct deployment of the company's TIG/MIG weld overlay capability. Key aspects include:

• Process adaptability: MIG welding (GMAW) is preferred for high-productivity multi-pass overlay on flat or gently curved tooth surfaces. TIG welding (GTAW) is used for narrow, high-precision overlay on the cutting edge where geometric control is critical.
• Equipment requirements: MIG welding station with pulsed current capability, gas shielding (Ar + 2–5% CO₂ or pure Ar), and wire feed system rated for φ1.2–φ1.6 mm hardfacing wire.
• WPS development: The research findings from this entry directly inform the development of qualified WPS for hardfacing overlay, which can be extended to other mining equipment components (e.g., conveyor rollers, dragline buckets, crusher hammers).

7.2 Hydraulic Explosive Bonding (Complementary Route)

While hydraulic explosive bonding (HEB) is primarily used for producing clad plate and pipe with uniform bimetallic layers, it can serve as a complementary technology for WK-35 bucket teeth in the following scenarios:

• Pre-clad bucket teeth: HEB can be used to produce clad steel plate from which bucket teeth are subsequently fabricated. The clad layer (e.g., 3 mm Stellite 6 on 20 mm Q345 base) provides a wear-resistant surface without the need for post-fabrication weld overlay.
• Advantages: HEB produces metallurgical bonds without dilution, achieving 100% alloy integrity in the clad layer. The bonding strength exceeds the base metal strength (typically >300 MPa shear strength).
• Limitations: HEB is limited to flat plate geometries. Complex bucket tooth shapes require post-fabrication machining, which may be less cost-effective than direct weld overlay for high-volume production.

7.3 Explosion Welding (Complementary Route)

Explosion welding (EW) is another method for producing clad plate that can be applied to bucket tooth fabrication:

• Clad plate production: EW can produce large-format clad plate (e.g., 2000 mm × 6000 mm) with a wear-resistant overlay layer (e.g., Ni-Hard or Stellite 6) on structural steel. These plates can be cut and formed into bucket teeth.
• Advantages over weld overlay: EW produces a fully metallurgical bond with no heat-affected zone (HAZ) in the clad layer, preserving the full hardness and wear resistance of the overlay alloy.
• Applicability: EW is most suitable for high-volume production of standardized bucket teeth where the cost of explosion welding setup is amortized over large quantities.

7.4 Comparative Analysis of Technology Routes

Criterion	TIG/MIG Weld Overlay	Hydraulic Explosive Bonding	Explosion Welding
Geometry flexibility	High (complex 3D shapes)	Low (flat plate only)	Low (flat plate only)
Dilution	5–15% (manageable)	0% (no dilution)	0% (no dilution)
Production volume	Low to medium	Medium to high	High
Capital investment	Low (welding equipment)	Medium (HEB press)	High (explosion facility)
Overlay thickness	4–15 mm	1–10 mm	1–10 mm
Repairability	Excellent (field repair)	Poor (requires re-cladding)	Poor (requires re-cladding)
WK-35 bucket tooth suitability	Primary method	Complementary (pre-clad)	Complementary (pre-clad)

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

• WPS/PQR development: The research findings documented in this entry provide the technical basis for developing and qualifying Welding Procedure Specifications (WPS) and Performance Qualification Records (PQR) for hardfacing overlay. These qualifications are essential for demonstrating compliance with ASME Section IX, NB/T 47014, and customer-specific requirements.
• Operator certification: The process parameters and quality criteria established in this research serve as training material for welding operator certification programs. Certified operators are a prerequisite for delivering qualified hardfacing overlay services to mining customers.
• NDT capability: The acceptance criteria defined in this entry (visual inspection, hardness testing, macrographic examination, mechanical testing) inform the development of the company's NDT capability, including ultrasonic testing (UT), magnetic particle testing (MT), and radiographic testing (RT) for hardfacing overlay quality assurance.

8.2 Product Delivery

• Standardized product offering: The research enables the company to offer standardized hardfacing overlay packages for WK-35 bucket teeth, with defined alloy selections, process parameters, and acceptance criteria. This standardization reduces delivery lead time and ensures consistent quality across multiple customer sites.
• On-site and off-site delivery: The research findings support both off-site (workshop) hardfacing overlay and on-site (mine workshop) hardfacing overlay. On-site delivery requires portable welding equipment, field NDT capabilities, and mobile hardness testing equipment—all of which are specified in the research.
• Scalability: The process parameters and alloy selections developed for WK-35 bucket teeth can be extended to other mining equipment (WK-4, WK-8, WK-10 electric shovels; dragline buckets; conveyor rollers), enabling the company to scale its hardfacing overlay business across the mining equipment aftermarket.

8.3 Customer Value

• Cost reduction: By extending bucket tooth service life by 3–8×, the hardfacing overlay service reduces annual maintenance costs by ¥200,000–400,000 per WK-35 shovel, delivering immediate and quantifiable ROI to mining customers.
• Uptime improvement: Reduced replacement frequency translates to 75–90% fewer maintenance shutdowns, directly increasing mining production output and revenue.
• Technical partnership: The depth of research and process knowledge demonstrated in this entry positions the company as a technical partner rather than a commodity service provider, enabling long-term customer relationships and recurring revenue streams.
• Knowledge transfer: The "study心得" format ensures that process knowledge is documented, standardized, and transferable, reducing dependence on individual operators and enabling consistent quality delivery across multiple sites and shifts.

9. Conclusion

The research and application of hardfacing weld overlay on WK-35 electric shovel bucket teeth represents a high-value, technically demanding application of the company's TIG/MIG weld overlay capability. The systematic approach to alloy selection, process parameter optimization, dilution control, post-weld heat treatment, and quality assurance—documented in this entry—provides a replicable framework for extending hardfacing overlay services to other mining equipment components and customer segments.

This entry serves as a cornerstone for building the company's qualification portfolio (WPS/PQR), standardizing product offerings, and delivering measurable cost and uptime benefits to mining customers. The integration of hardfacing weld overlay with the company's hydraulic explosive bonding and explosion welding capabilities further broadens the technical scope, enabling tailored solutions for different production volumes, geometries, and performance requirements. As the company continues to expand its surface engineering capabilities, this research entry exemplifies the depth of technical expertise and customer-focused value delivery that differentiate the company in the competitive mining equipment aftermarket.