Effect of Annealing Temperature on Microstructure and Hardness of Ni60/WC Plasma Arc Weld Overlay on H13 Steel
1. Definition and Fundamental Principles
1.1 Process Definition
The technical subject under analysis is the plasma arc weld overlay (PAWO) of a Ni60/WC composite hardfacing system onto H13 hot work die steel substrate, followed by post-weld annealing treatment. This process combines the metallurgical bonding capability of plasma arc deposition with the dispersion strengthening effect of tungsten carbide particles in a nickel-based matrix, and critically examines how the annealing temperature governs the resulting microstructure evolution and hardness profile of the overlay layer.
Plasma arc weld overlay operates on the principle of a high-enthalpy, ionized gas jet (typically argon with hydrogen or nitrogen additions) directed through a constricted nozzle to generate a stable plasma arc. The arc heats a consumable composite powder (Ni60 matrix with WC particles) and the substrate surface to a controlled melt pool, producing a metallurgically bonded overlay layer with controlled dilution. The plasma arc provides superior heat concentration compared to conventional arc processes, enabling narrow weld beads, low dilution ratios (typically 15–30% for single-pass), and minimal thermal distortion of the base material.
1.2 Material System
H13 Steel (Base Material): H13 is a low-alloy hot work die steel conforming to ASTM A681, characterized by approximately 5% Cr, 1.5% Ni, 1.25% Mo, and 0.4% C. It is widely used in hot forging dies, extrusion dies, and hot work tooling due to its excellent hot hardness, thermal fatigue resistance, and hot strength. However, H13 is susceptible to adhesion wear, galling, and thermal cracking under severe service conditions, necessitating surface hardening and protection through overlay technology.
Ni60 Alloy (Matrix): Ni60 (equivalent to Stellite 6 per ASTM B408) is a cast iron-nickel-chromium-cobalt alloy containing approximately 55–65% Ni, 28–34% Cr, 5–10% Co, and 0.3–1.0% C. It provides excellent corrosion resistance, thermal shock resistance, and self-lubricating properties when properly heat treated. The as-welded Ni60 contains primary chromium carbides (M7C3) that contribute to hardness but can form continuous networks detrimental to toughness.
WC Particles (Reinforcement): Tungsten carbide particles (typically 5–75 μm in size) are added to the Ni60 matrix to provide dispersion strengthening through the formation of hard WC and W2C carbide phases. The volume fraction of WC (commonly 20–40%) directly influences the hardness and wear resistance of the overlay.
1.3 Metallurgical Interaction Mechanism
During plasma arc deposition, the WC particles undergo partial dissolution at the melt pool temperatures (approximately 1500–1700°C), with some particles dissolving to form solid solution strengthening in the austenitic matrix and others remaining as undissolved or partially dissolved carbide phases. The cooling rate from the plasma arc (typically 10–50°C/s depending on heat input and bead geometry) produces a predominantly austenitic microstructure with retained carbide particles and dendritic chromium carbides at grain boundaries.
The annealing treatment serves multiple metallurgical purposes:
- Solution treatment to dissolve brittle continuous carbide networks
- Recrystallization to relieve residual stresses from welding
- Controlled precipitation to optimize the balance between hardness and toughness
- Homogenization of microsegregation in the weld metal
2. Category and Business Positioning
2.1 Technology Classification
This entry falls within the TIG/MIG Weld Overlay technology route of the company's three core manufacturing capabilities, specifically within the plasma arc weld overlay sub-category. It represents a knowledge-intensive process development activity that bridges materials science research with production qualification, establishing the process window for a specific material combination.
The Ni60/WC on H13 combination is a high-value proposition in the company's product portfolio because it addresses a critical pain point in the hot work tooling industry: the limited service life of expensive H13 die components in high-temperature, high-friction environments. By developing and qualifying this overlay system, the company positions itself as a specialist in extending the life of critical industrial tooling through advanced surface engineering.
2.2 Value Chain Position
This technical entry contributes to the company's qualification building in three dimensions:
- Process Qualification: Establishing the optimal annealing temperature window (typically 900–1050°C for Ni60-based systems) enables WPS development and PQR documentation required for customer approvals
- Product Differentiation: The Ni60/WC composite overlay offers superior abrasion and adhesion resistance compared to conventional Ni60 alone, creating a premium product tier
- Engineering Support: The microstructure-hardness relationship data enables the company to provide customers with performance guarantees backed by scientific evidence
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The investigation of annealing temperature effects serves the following engineering objectives:
- Optimize hardness profile: Achieve target overlay hardness of 45–55 HRC (or 450–550 HV) while maintaining adequate toughness to prevent spalling
- Control carbide morphology: Prevent formation of continuous brittle M7C3 networks at grain boundaries that would compromise fatigue life
- Ensure metallurgical compatibility: Maintain a sound interface between the Ni60/WC overlay and H13 substrate without excessive dilution or cracking
- Establish process repeatability: Define the annealing temperature window with sufficient margin for production consistency
- Reduce residual stress: Relieve welding-induced residual stresses that could cause distortion or cracking during subsequent machining or service
3.2 Economic Value
For a typical hot forging die made from H13 steel with a service life of 20,000–50,000 strikes, the application of a properly annealed Ni60/WC overlay can extend life by 3–5 times, reducing die replacement frequency, unplanned downtime, and per-piece production costs. The annealing treatment, while adding a processing step, is essential to achieving the full performance potential of the overlay system—without proper annealing, the as-welded overlay may exhibit excessive brittleness and premature spalling.
4. Key Process and Implementation Points
4.1 Plasma Arc Weld Overlay Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Plasma Arc Current | 150–250 A | Higher current increases dilution; balance with heat input |
| Arc Voltage | 20–28 V | Determines arc power and melt pool size |
| Travel Speed | 80–150 mm/min | Lower speed increases dilution and bead width |
| Plasma Gas Flow | 3–6 L/min (Ar) | Controls arc stability and transfer mode |
| Shielding Gas | Ar + 2–5% H2 or Ar + 5% N2 | H2 increases arc enthalpy; N2 increases dilution |
| Wire Feed Rate | 0.8–1.5 m/min | Must be synchronized with travel speed |
| WC Particle Size | 5–30 μm (fine) or 30–75 μm (coarse) | Finer particles provide more uniform distribution |
| WC Volume Fraction | 20–40% | Higher fraction increases hardness but reduces toughness |
| Single Pass Dilution | 15–25% | Target value for optimal Ni60/WC properties |
| Overlay Thickness per Pass | 1.5–3.0 mm | Multiple passes for total thickness of 3–6 mm |
4.2 Annealing Temperature Effect on Microstructure
| Annealing Temperature | Microstructure Characteristics | Typical Hardness (HV) | Performance Assessment |
|---|---|---|---|
| As-welded (no annealing) | Austenite matrix + continuous M7C3 at grain boundaries + undissolved WC particles | 500–580 | High hardness but brittle; risk of spalling and intergranular cracking |
| 800–850°C | Partial dissolution of grain boundary carbides; minor recrystallization | 480–520 | Modest improvement in toughness; insufficient stress relief |
| 900–950°C (Optimal Range) | Complete dissolution of continuous M7C3 networks; fine dispersed carbides; recrystallized austenite | 440–490 | Optimal balance of hardness and toughness; good fatigue resistance |
| 1000–1050°C | Full solution treatment; grain growth begins; possible chromium depletion at grain boundaries | 400–450 | Reduced hardness; potential for sensitization if cooled slowly |
| 1100°C and above | Excessive grain growth; significant WC dissolution; possible substrate softening | 350–400 | Unacceptable loss of hardness; substrate properties compromised |
4.3 Annealing Process Implementation
The annealing treatment must be carefully controlled to achieve the target microstructure:
- Heating Rate: 5–10°C/min to prevent thermal shock cracking in the overlay layer, especially for thick overlays (>4 mm) or complex geometries
- Soak Temperature: 900–950°C held for 1–2 hours (depending on component thickness and overlay depth)
- Soak Duration: Minimum 30 minutes per 25 mm of component thickness, with a minimum of 1 hour for complete carbide dissolution
- Cooling Method: Furnace cooling to below 600°C, followed by air cooling; rapid quenching (water/oil) is generally avoided as it may cause cracking in the overlay
- Atmosphere: Protective atmosphere (nitrogen or vacuum) to prevent oxidation of the overlay surface; if air annealing is used, a post-annealing cleaning step is required
4.4 Interface Considerations
The metallurgical interface between the Ni60/WC overlay and H13 substrate is critical for joint integrity:
- Dilution Control: The first pass should achieve dilution below 25% to maintain the Ni60/WC properties; subsequent passes should achieve dilution below 15%
- Transition Zone: A gradient zone exists at the interface where H13 alloying elements (Cr, Mo, Ni) diffuse into the overlay and Ni60 elements diffuse into the substrate; this zone should be analyzed for hardness and microstructure
- Cracking Risk: The coefficient of thermal expansion mismatch between austenitic Ni60 (approximately 13×10⁻⁶/°C) and ferritic/martensitic H13 (approximately 12×10⁻⁶/°C) creates thermal stresses; annealing relieves these stresses
- Hardness Gradient: The transition zone typically shows hardness of 40–48 HRC, providing a gradual transition from the overlay (45–52 HRC) to the substrate (38–42 HRC)
4.5 Pre-Weld Substrate Preparation
- Substrate Preheat: H13 steel should be preheated to 200–350°C to reduce thermal gradient and minimize cracking risk
- Machining: The overlay area should be machined to a smooth surface (Ra ≤ 3.2 μm) with proper undercut preparation (typically 60° groove, depth 2–3 mm) to ensure adequate penetration and bonding
- Cleaning: Remove all oils, coolants, and contaminants from the substrate surface using solvent cleaning or mechanical brushing
- Interpass Temperature: Maintain interpass temperature between 150–300°C for multi-pass overlays
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
| Material/Component | Applicable Standard | Key Requirements |
|---|---|---|
| H13 Substrate | ASTM A681 / GB/T 1299 | Composition, hardness (38–44 HRC after tempering), impact toughness |
| Ni60 Overlay Wire | ASTM B408 / AWS A5.15 (ENi-CrAI-6) | Chemical composition, carbon content, Cr/C ratio |
| WC Powder | ASTM B869 / GB/T 3745 | Purity (>99%), particle size distribution, specific surface area |
| Composite Powder | ASTM B408 (composite) / ISO 14273 | WC content, powder morphology, flowability, moisture content |
5.2 Process and Quality Standards
- WPS/PQR Development: AWS D10.9 (Welding Procedure Specification and Welding Performance Qualification for Hardfacing), AWS D10.6 (Specification for Welding Procedures for Hardfacing)
- Weld Overlay Qualification: AWS D10.9 covers qualification requirements including hardness testing, macrograph examination, dilution measurement, and impact testing for hardfacing applications
- Non-Destructive Testing: ASTM E165 (Magnetic Particle Testing), ASTM E1417 (Magnetic Particle Test Method), ASTM E2312 (Acoustic Emission Testing), ISO 17637 (Ultrasonic Testing)
- Hardness Testing: ASTM E92 (Rockwell Hardness), ASTM E182 (Vickers Hardness), ISO 6508 (Vickers Hardness of Metals)
- Microstructural Examination: ASTM E3 (Metallographic Preparation), ASTM E1245 (Image Analysis), GB/T 13298 (Metallographic Examination of Steel)
- Post-Weld Heat Treatment: AWS D10.9 Annex (post-weld heat treatment guidelines), manufacturer's recommended annealing schedule
- Acceptance Criteria: NACE MR0175 (if sour service), API 571 (damage mechanisms), customer-specific specifications
5.3 Acceptance Criteria for Ni60/WC Overlay on H13
| Test Parameter | Acceptance Criteria | Test Method |
|---|---|---|
| Overlay Hardness | 45–55 HRC (450–580 HV) | ASTM E92 / ASTM E182 |
| Interface Hardness | 38–48 HRC (gradual transition, no sharp drop) | ASTM E182 (Vickers, 500g) |
| Dilution (first pass) | ≤25% by optical emission spectroscopy or metallographic width measurement | AWS D10.9 / OES |
| Overlay Thickness | As specified (typically 3–6 mm total) | Ultrasonic thickness gauge |
| Surface Quality | No cracks, porosity, undercut, or excessive spatter | Visual + MPI (ASTM E165) |
| Internal Defects | No cracks or porosity exceeding acceptance criteria | MT (ASTM E165) + UT (ISO 17637) |
| Impact Toughness | Charpy V-notch ≥10 J at 20°C (for thick sections) | ASTM E23 / GB/T 229 |
| Corrosion Resistance | No intergranular corrosion after annealing | ASTM A262 Practice E (if applicable) |
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measures |
|---|---|---|
| Overlay Cracking | High carbon content, excessive dilution, rapid cooling, high residual stress | Control dilution <25%, maintain interpass temperature, anneal after welding, use low-carbon filler for first pass |
| Spalling/Delamination | Poor metallurgical bond, excessive hardness mismatch, thermal fatigue | Ensure proper surface preparation, control dilution, avoid excessive hardness (>58 HRC), anneal to relieve stress |
| Substrate Softening | Excessive heat input, overheating during annealing, long soak time at high temperature | Control plasma arc parameters, limit annealing temperature to ≤950°C, use protective backing |
| WC Particle Agglomeration | Non-uniform powder mixing, excessive melting, poor powder flow | Use well-mixed composite powder, control arc parameters to avoid excessive melting, use smaller particle sizes |
| Sensitization/Intergranular Corrosion | Slow cooling through 450–850°C range, chromium carbide precipitation at grain boundaries | Avoid slow cooling through sensitization range, use protective atmosphere during annealing |
6.2 Process Risks
- Porosity: Caused by contaminated powder, inadequate shielding gas, or wet flux. Control: use dry powder (<0.1% moisture), ensure gas flow, clean substrate
- Undercut: Caused by excessive travel speed or insufficient current. Control: optimize parameter combinations during WPS development
- Excessive Dilution: Caused by high current, low travel speed, or inadequate groove preparation. Control: maintain first pass dilution <25%, use proper groove geometry
- Thermal Distortion: Caused by high heat input on thin sections. Control: use backing plates, fixturing, and low heat input parameters
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route
The Ni60/WC plasma arc weld overlay on H13 is a flagship application within the company's TIG/MIG weld overlay technology route. Key application scenarios include:
- Hot Forging Dies: Application of 3–5 mm Ni60/WC overlay on punch faces and die cavities of H13 hot forging dies to extend life from 20,000 to 80,000+ strikes in automotive stamping and forging operations
- Extrusion Dies: Overlay of Ni60/WC on H13 extrusion die land surfaces to resist adhesion wear and galling during aluminum or steel extrusion
- Hot Work Tooling: Protection of H13 guide plates, sliders, and wear plates in hot rolling mills and hot working operations
- Repair Applications: Restoration of worn H13 components by building up material with Ni60/WC overlay followed by machining to final dimensions
The plasma arc process is preferred for this application due to its ability to produce narrow, deep penetration beads with controlled dilution, making it suitable for both new overlay application and repair of existing worn surfaces. The annealing step is critical for production parts and should be included in the WPS as a mandatory post-weld treatment.
7.2 Hydraulic Explosive Bonding Route
While the Ni60/WC plasma arc overlay technology is primarily associated with the weld overlay route, the knowledge gained from annealing temperature studies contributes to the hydraulic explosive bonding (HEB) route in the following ways:
- Surface Preparation for HEB: Understanding the microstructure of Ni60/WC overlays enables the company to provide pre-clad H13 components where the Ni60/WC layer serves as a transition or functional layer before hydraulic bonding of additional cladding materials
- Hybrid Cladding Systems: The annealing temperature data supports the development of multi-layer cladding systems where a Ni60/WC weld overlay layer is hydraulically bonded to a wear-resistant outer layer (e.g., high-chromium cast iron or ceramic)
- Post-Bonding Heat Treatment: The annealing temperature windows established for Ni60/WC overlays inform the post-bonding heat treatment schedule for hybrid cladding assemblies, ensuring metallurgical compatibility across all layers
7.3 Explosion Welding Route
The connection to the explosion welding route is established through the following scenarios:
- Explosion-Welded Billet Preparation: H13 base material with Ni60/WC explosion-welded cladding can be subsequently machined and annealed; the annealing temperature data ensures the explosion-welded interface is not adversely affected by the heat treatment
- Explosion Cladding of Ni-Based Alloys: The metallurgical knowledge from Ni60/WC overlay studies (carbide dissolution behavior, grain growth kinetics, residual stress relaxation) is directly applicable to explosion-welded Ni-based cladding systems
- Post-Explosion Annealing: Explosion-welded Ni60/WC cladding on H13 requires post-weld annealing to relieve the high residual stresses generated during the explosion process; the temperature windows established in this study (900–950°C) are directly applicable
- Multi-Process Cladding Solutions: For complex geometries requiring both explosion-welded and weld-overlay cladding, the annealing study provides the thermal budget data needed to ensure all layers can withstand a common post-weld heat treatment without degradation
8. Qualification Building and Customer Value
8.1 Qualification Development
This technical entry represents a critical step in the company's qualification building process:
- WPS Development: The annealing temperature data directly feeds into the Welding Procedure Specification, establishing the mandatory post-weld heat treatment parameters. Without this data, the WPS would be incomplete and non-compliant with AWS D10.9 requirements.
- PQR Documentation: Performance Qualification Records must demonstrate that the qualified procedure produces overlay properties meeting acceptance criteria. The microstructure and hardness data from annealing temperature studies provide the evidence base for PQR acceptance.
- Customer Approval: Major customers (automotive OEMs, aerospace companies, oil and gas operators) require documented process development data before approving new overlay systems. This technical entry provides the scientific foundation for customer technical reviews.
- ISO 9001 Compliance: The systematic approach to process development, including parameter optimization, documentation, and verification, demonstrates the company's commitment to quality management system requirements.
8.2 Product Delivery Enhancement
The knowledge gained from this study directly enhances product delivery capabilities:
- Faster Turnaround: With established annealing parameters, production can proceed without trial-and-error heat treatment, reducing delivery time by 20–30%
- Higher Yield: Knowledge of the optimal temperature window reduces the risk of annealing defects (overheating, sensitization), improving first-pass yield
- Consistent Quality: Standardized annealing procedures ensure consistent overlay properties across production batches, supporting customer confidence and repeat orders
- Customization Capability: Understanding the temperature-hardness relationship enables the company to offer customers hardness-optimized overlays tailored to specific service conditions
8.3 Customer Value Proposition
The technical depth demonstrated by this entry creates significant customer value:
"By providing customers with scientifically validated overlay solutions—where every parameter, from plasma arc settings to annealing temperature, is backed by microstructural evidence—the company differentiates itself from competitors who rely on rule-of-thumb approaches. This translates to longer overlay life, fewer field failures, and ultimately lower total cost of ownership for the customer."
- Performance Guarantee: The company can offer guaranteed overlay hardness ranges (45–55 HRC) backed by annealing process control
- Life Extension Data: Customers receive documented evidence that the Ni60/WC overlay extends H13 die life by 3–5×, enabling ROI calculations
- Technical Support: Engineering staff can provide customers with detailed microstructure reports, hardness profiles, and dilution measurements as part of the delivery package
- Failure Analysis Capability: Understanding of the annealing temperature-microstructure relationship enables rapid diagnosis of overlay failures in service
9. Implementation Recommendations
9.1 Production Implementation
- Standardize Annealing Procedure: Develop a standard operating procedure (SOP) specifying heating rate (5–10°C/min), soak temperature (925±15°C), soak time (1.5 hours minimum), and cooling method (furnace cool to 600°C, then air cool)
- Implement In-Process Monitoring: Use thermocouples embedded in the workpiece or attached to the surface to verify actual temperatures during annealing; record temperature-time curves for each batch
- Establish Hardness Verification: Perform hardness testing on every production batch at defined locations (overlay surface, interface, and substrate) to verify compliance with acceptance criteria
- Document All Parameters: Maintain detailed records of plasma arc parameters, powder lot numbers, annealing temperatures, and hardness results for traceability
9.2 Continuous Improvement
- Parameter Optimization: Conduct periodic studies to refine the annealing temperature window based on production experience and customer feedback
- Microstructure Mapping: Use advanced characterization (SEM/EDS, XRD) to correlate microstructure with field performance data
- Process Automation: Develop automated plasma arc welding and furnace control systems to minimize operator variability
- Knowledge Transfer: Train production operators on the metallurgical significance of annealing parameters to promote process awareness and quality culture
10. Conclusion
The investigation of annealing temperature effects on Ni60/WC plasma arc weld overlay microstructure and hardness on H13 steel represents a foundational technical capability that underpins the company's weld overlay product line. By establishing the optimal annealing window of 900–950°C, the company achieves a critical balance between hardness (450–490 HV) and toughness that meets the demanding requirements of hot work tooling applications.
This technical knowledge directly contributes to qualification building through WPS/PQR development compliant with AWS D10.9, enhances product delivery through standardized and documented processes, and creates customer value through performance guarantees backed by scientific evidence. The insights gained are transferable across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—providing a unified metallurgical foundation for the company's multi-process cladding solutions.
The continued investment in such process development studies positions Cladding Technology Shanxi Co., Ltd. as a technically competent partner capable of delivering high-performance surface engineering solutions that extend the life and reliability of critical industrial components.