Effect of Quenching and Tempering Treatment on Microstructure and Hardness of Ni60-WC Plasma Arc Weld Overlay on H13 Hot Work Tool Steel
1. Definition and Technical Principles
The Ni60-WC plasma arc weld overlay system represents a high-performance surface engineering solution combining the corrosion and wear resistance of cast Stellite 6 (Ni60) alloy with the extreme hardness contribution of tungsten carbide (WC) ceramic particles. When applied to H13 hot work tool steel substrates, this overlay addresses the critical failure modes encountered in high-temperature forming operations—namely, adhesive wear, thermal fatigue cracking, and softening of the base metal at elevated service temperatures.
Plasma arc welding (PAW) provides a highly concentrated, stable arc with precise heat input control, making it particularly suitable for overlaying hardfacing alloys onto tool steels where dilution control and thermal distortion management are paramount. The plasma jet operates at temperatures exceeding 15,000–20,000 K, enabling complete melting of both the Ni60-WC consumable and the H13 substrate surface to achieve metallurgical bonding while limiting the depth of the heat-affected zone (HAZ).
The central technical question addressed in this study is the influence of post-weld quenching and tempering (Q&T) heat treatment on the overlay's microstructure evolution and hardness profile. H13 steel is a through-hardening chromium-molybdenum hot work tool steel (ASTM A681/A681M) designed to be supplied in a quenched and tempered condition. When plasma arc weld overlay is applied, the thermal cycle of welding effectively re-heats the substrate surface, potentially altering the tempering state of the H13 base metal and the carbide morphology within the Ni60-WC overlay. Post-weld Q&T treatment serves to restore and optimize the hardness of both the overlay and the HAZ region, ensuring uniform mechanical performance across the interface.
2. Category and Business Positioning
This technology falls squarely within the company's TIG/MIG weld overlay and plasma arc hardfacing business route, specifically addressing the surface protection of hot work tooling. Within Cladding Technology Shanxi Co., Ltd's capability matrix, this entry occupies the following positions:
- Product Category: Hardfacing overlay for hot work tool steel components, including forging dies, extrusion dies, upsetting dies, and hot cracking dies.
- Substrate Material: H13 (ASTM A681), equivalent to Chinese GB/T 1299 standard 4Cr5MoSiNiRe or 4Cr5Mo1V1, a nickel-molybdenum hot work tool steel.
- Overlay System: Ni60-WC (Ni-based cast Stellite 6 with 20–30% WC by weight), providing a hardness range of HRC 50–62 as-welded, potentially exceeding HRC 62 after appropriate heat treatment.
- Process Route: Plasma arc welding (PAW) as a variant of the broader arc weld overlay family, distinguished by superior arc stability, reduced spatter, and tighter control over dilution rates.
3. Technical Purpose and Value
3.1 Engineering Objectives
The primary engineering objectives of applying Ni60-WC plasma arc overlay to H13 hot work tool steel are:
- Wear Resistance Enhancement: The WC particles embedded in the Ni-Cr matrix provide abrasive wear resistance far exceeding that of the H13 base metal (HRC 45–50 typical), extending die life by factors of 2–5 in abrasive forming applications.
- Hot Hardness Retention: The Ni-Cr matrix of Ni60 retains significant hardness at temperatures up to 600–700°C, compared to H13 which begins to soften at approximately 400°C.
- Thermal Fatigue Resistance: The Ni-based overlay provides a thermal barrier and reduces the thermal gradient at the die surface, mitigating thermal fatigue cracking.
- Rebuild Capability: The overlay system enables restoration of worn die surfaces to original or enhanced dimensions, reducing scrap rates and maintaining production continuity.
3.2 Value of Q&T Heat Treatment Study
The study of quenching and tempering effects on this overlay system is critical because the as-welded condition of Ni60-WC overlays often exhibits a heterogeneous hardness profile due to non-uniform cooling rates across the weld cross-section. The Q&T treatment achieves the following:
- Homogenization of the carbide distribution within the Ni-based matrix
- Restoration of H13 substrate hardness in the HAZ to design specifications
- Transformation of coarse primary carbides into finer, more uniformly dispersed secondary carbides
- Relief of residual welding stresses that could compromise dimensional stability during service
4. Key Process and Implementation Points
4.1 Substrate Preparation
Proper surface preparation of the H13 substrate is essential for achieving sound metallurgical bonding and minimizing dilution. The following preparation sequence is recommended:
- Grinding: Grind the overlay area to a uniform surface with 40–60 grit followed by 80–120 grit aluminum oxide or silicon carbide abrasives. The ground area should extend 5–10 mm beyond the intended overlay boundary to ensure adequate heat input distribution.
- Cleaning: Remove all grinding debris, oil, and contaminants using solvent cleaning (acetone or naphtha) followed by dry compressed air blow-off.
- Preheating: Preheat H13 substrate to 300–400°C using induction heating or torch heating. This temperature range reduces the thermal gradient, minimizes HAZ cracking susceptibility, and prevents hydrogen-induced cracking in the martensitic H13 microstructure.
4.2 Plasma Arc Weld Overlay Process Parameters
The following table summarizes typical plasma arc welding parameters for Ni60-WC overlay application on H13 steel:
| Parameter | Range / Value | Notes |
|---|---|---|
| Plasma Gas | Argon (Ar) | Purity ≥ 99.99%; flow rate 20–40 L/min |
| Shielding Gas | Argon (Ar) | Purity ≥ 99.99%; flow rate 15–25 L/min |
| Plasma Current | 80–160 A | Depends on transfer mode and consumable diameter |
| Transfer Mode | Free transfer or semi-transferred | Free transfer preferred for minimal dilution |
| Consumable Wire | Ni60-WC, φ1.6–φ2.4 mm | Cast Stellite 6 with 20–30% WC |
| Travel Speed | 200–500 mm/min | Higher speed reduces dilution and HAZ width |
| Stick-out Length | 15–25 mm | Consistent stick-out ensures uniform bead profile |
| Interpass Temperature | ≤ 200°C | Maintain to prevent excessive grain growth |
| Preheat Temperature | 300–400°C | Induction or torch preheating |
| Post-Weld Cooling | Controlled air or furnace cooling | Avoid water quenching to prevent HAZ cracking |
4.3 Quenching and Tempering Treatment Parameters
The post-weld Q&T treatment is the defining variable studied in this entry. The following parameters govern the heat treatment cycle:
| Treatment Stage | Temperature | Medium | Soak Time | Purpose |
|---|---|---|---|---|
| Quenching | 1020–1050°C | Salt bath or vacuum furnace | 15–30 min per 25 mm thickness | Austenitize H13 and overlay; dissolve carbides |
| Quench Medium | — | Air quench (vacuum) or oil quench | — | Transform austenite to martensite |
| First Temper | 540–580°C | Furnace | 2× thickness (hours) | Temper martensite; relieve residual stress |
| Second Temper | 540–580°C | Furnace | 2× thickness (hours) | Stabilize microstructure; secondary hardening |
Important Note: The quenching temperature must be carefully controlled. Temperatures above 1080°C risk excessive grain growth in the H13 substrate and excessive dissolution of WC particles in the overlay, leading to reduced hardness and potential cracking. Temperatures below 1000°C result in incomplete austenitization, leaving retained carbides that compromise the uniformity of the martensitic transformation.
4.4 Multi-Pass Overlay Considerations
For overlay thicknesses exceeding 3 mm, multi-pass welding is required. The following guidelines apply:
- Pass 1 (Bonding Pass): Minimum thickness of 1.0–1.5 mm; this pass establishes metallurgical bond with the H13 substrate and may exhibit 30–50% dilution.
- Pass 2 (Build-up Pass): Intermediate thickness; dilution decreases to 10–20% as the previous pass provides a Ni-based substrate.
- Pass 3+ (Surface Passes): Final surface passes with dilution typically below 10%, ensuring the overlay composition closely matches the nominal Ni60-WC specification.
- Interpass Grinding: Light grinding between passes (if required) to remove undercut and ensure proper bead overlap of 50–60%.
5. Microstructure Analysis
5.1 As-Welded Condition
In the as-welded condition, the Ni60-WC overlay microstructure typically consists of:
- γ-Ni Matrix: Face-centered cubic austenitic nickel matrix containing dissolved Cr, Mo, and Co.
- Primary WC Particles: Tungsten carbide particles ranging from 5–50 μm, distributed within the Ni matrix. These may partially dissolve at the weld fusion boundary depending on local temperatures.
- Secondary Carbides: Cr7C3, Mo2C, and Ni3Mo carbides precipitate during solidification and cooling, particularly in regions of higher cooling rate.
- HAZ Microstructure: The H13 substrate HAZ exhibits tempered martensite with varying tempering temperatures from the fusion line outward, creating a gradient of hardness.
5.2 Post-Q&T Condition
After quenching and tempering treatment, significant microstructural changes occur:
- Overlay Matrix: The γ-Ni matrix undergoes no phase transformation (remains austenitic), but the tempering cycle promotes precipitation of fine M6C-type carbides (Cr, Mo, W)6C, which contribute to secondary hardening.
- WC Particles: Partial dissolution of WC during austenitization followed by reprecipitation during tempering produces a more uniform distribution of fine WC and M6C carbides, enhancing hardness uniformity.
- HAZ: The H13 HAZ undergoes full martensitic transformation during quenching, followed by tempering to the target temper state. This eliminates the tempered martensite gradient present in the as-welded condition and restores substrate hardness to HRC 48–52.
- Interface: The metallurgical bond at the overlay/substrate interface becomes more homogeneous, with reduced risk of interfacial cracking during thermal cycling.
5.3 Hardness Profile Comparison
| Zone | As-Welded Hardness (HRC) | Post-Q&T Hardness (HRC) | Change |
|---|---|---|---|
| Ni60-WC Overlay Surface | 55–60 | 60–65 | +5–8 HRC |
| Overlay/HAZ Interface | 48–55 | 52–58 | +4–7 HRC |
| H13 HAZ (near fusion line) | 40–48 (over-tempered) | 48–52 (restored) | +5–10 HRC |
| H13 Base Metal (remote) | 45–50 | 48–52 | +3–5 HRC |
The data above illustrates the dual benefit of Q&T treatment: it increases overlay hardness through carbide redistribution and precipitation hardening, while simultaneously restoring the H13 substrate hardness in the HAZ that was reduced by the welding thermal cycle.
6. Applicable Standards and Acceptance Criteria
6.1 Material Standards
- H13 Substrate: ASTM A681/A681M (Hot Work Tool Steels); equivalent to GB/T 1299 (Tool Steel); JIS G4401; EN ISO 4892 (X39CrMo17-1 or similar).
- Ni60 Overlay: ASTM A213 Type C (Cast Stellite 6); equivalent to GB/T 16864 (Ni-based cast alloys); ISO 3513 (Ni-based hardfacing alloys).
- WC Content: Typically 20–30% by weight; verified by optical emission spectrometry (OES) or inductively coupled plasma (ICP) analysis per ASTM E1019 or ASTM E1251.
6.2 Welding and Heat Treatment Standards
- Welding Procedure Specification: ASME Section IX (Welding, Brazing, and Fusing Qualifications); AWS D10.9 (Specification for Welding of Castings); GB/T 19866 (Welding Procedure Specification qualification).
- Heat Treatment: ASTM A681/A681M (quench and temper requirements for H13); ISO 9517 (Heat treatment of steel — General technical delivery conditions).
- Hardness Testing: ASTM E18 (Rockwell Hardness); ASTM E92 (Vickers Hardness); ISO 6508 (Vickers Hardness Testing).
- NDT: ASTM E23 (Drop Weight Test); ASTM E165 (Magnetic Particle Inspection); ASTM E305 (Radiographic Testing); ASTM E109 (Visual Examination).
6.3 Acceptance Criteria
| Inspection Item | Acceptance Criterion | Method |
|---|---|---|
| Overlay Hardness | ≥ HRC 55 (as-welded); ≥ HRC 60 (post-Q&T) | ASTM E18 / ISO 6508 |
| Substrate HAZ Hardness | ≥ HRC 45 (restored to within 5 HRC of base metal) | ASTM E18 |
| Dilution Rate | ≤ 30% for single pass; ≤ 15% for multi-pass | OES / ICP chemical analysis |
| Crack-Free Interface | No cracks at overlay/substrate interface | MPI (ASTM E165) or Microsectioning |
| Overlay Thickness | Within ±0.5 mm of specified thickness | Ultrasonic thickness or micrometer |
| Surface Quality | No undercut, porosity, or spatter; smooth finish | Visual (ASTM E109) |
7. Common Risks and Controls
7.1 Thermal Cracking in HAZ
Risk: H13 steel is susceptible to quench cracking and temper embrittlement. The welding thermal cycle, if not properly controlled, can produce a HAZ with reduced toughness and potential cracking during subsequent Q&T treatment.
Controls:
- Maintain preheat temperature of 300–400°C to reduce cooling rate in the HAZ.
- Control interpass temperature below 200°C to prevent excessive grain growth.
- Use low hydrogen consumables and ensure proper gas shielding to minimize hydrogen pickup.
- Avoid water quenching immediately after welding; use controlled air cooling or furnace cooling.
- Perform stress relief annealing (600–650°C for 2 hours) between welding and Q&T if required by the WPS.
7.2 WC Particle Dissolution and Coarsening
Risk: Excessive austenitization temperature or prolonged soak time during Q&T can dissolve WC particles, leading to reduced hardness and loss of the primary wear resistance mechanism.
Controls:
- Limit austenitization temperature to 1020–1050°C (not exceeding 1080°C).
- Control soak time to 15–30 minutes per 25 mm of effective thickness.
- Use vacuum furnace or salt bath for precise temperature control.
- Verify WC retention through microstructural examination of cross-sections.
7.3 Dilution and Overlay Composition Deviation
Risk: High dilution rates result in overlay composition that deviates significantly from nominal Ni60-WC, reducing hardness, corrosion resistance, and hot hardness.
Controls:
- Use free transfer plasma mode to minimize arc force and substrate melting.
- Employ multi-pass welding to progressively reduce dilution.
- Verify dilution through chemical analysis of the overlay cross-section at multiple depths.
- Adjust plasma current and travel speed to optimize dilution rate.
7.4 Interfacial Cracking and Delamination
Risk: Mismatch in thermal expansion coefficients between Ni-based overlay (CTE ≈ 13×10⁻⁶/K) and H13 steel (CTE ≈ 12×10⁻⁶/K) can cause interfacial cracking during Q&T cooling or thermal cycling in service.
Controls:
- Ensure sound metallurgical bond through proper surface preparation and adequate heat input.
- Use intermediate transition layers (e.g., NiCrBSi) if dilution or interfacial issues persist.
- Apply controlled cooling rates during Q&T to minimize thermal stress at the interface.
- Perform bend testing or macrosectioning to verify interface integrity.
8. Application Scenarios Across Company Technology Routes
8.1 TIG/MIG Weld Overlay Route
Plasma arc welding is a specialized variant within the arc weld overlay family. The Q&T treatment study directly informs TIG and MIG overlay procedures for H13 components where post-weld heat treatment is required. Key applications include:
- Forging Die Rebuild: Rebuilding worn cavity surfaces on H13 forging dies with Ni60-WC overlay followed by Q&T to restore die hardness and enhance wear resistance.
- Extrusion Die Protection: Applying Ni60-WC overlay to the bearing surfaces of H13 extrusion dies for aluminum extrusion, where abrasive wear from oxide particles is the primary failure mode.
- Hot Cracking Die Surface Treatment: Overlaying the contact surfaces of H13 hot cracking dies to resist hot metal adhesion and reduce die life loss from thermal fatigue.
- WPS Qualification: The Q&T parameter study provides the technical basis for qualifying welding procedure specifications (WPS) per ASME Section IX or GB/T 19866, ensuring that post-weld heat treatment is an integral part of the qualified procedure.
8.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding is primarily used for clad plate and pipe production with Ni-based cladding layers, the microstructural and hardness data from this Q&T study provides valuable reference for:
- Post-Bonding Heat Treatment: Understanding how Q&T affects Ni-based alloy layers informs the design of post-bonding heat treatment cycles for hydraulic explosive bonded Ni60/steel clad plates.
- Interface Integrity: Knowledge of carbide redistribution and matrix evolution during Q&T helps predict potential interface degradation in bonded clad products subjected to subsequent heat treatment.
- Material Selection: The Q&T response of Ni60-WC provides comparative data for selecting between WC-containing and WC-free Ni alloys for hydraulic explosive bonding applications where post-bonding heat treatment is required.
8.3 Explosion Welding Route
For explosion-welded clad products incorporating Ni-based alloys, the Q&T study contributes to:
- Post-Weld Heat Treatment Design: Explosion welding produces severe plastic deformation and strain hardening in the cladding layer. Q&T treatment is often required to relieve residual stresses and restore ductility. The Ni60-WC Q&T data provides guidance on optimal temperature and time parameters.
- Hardness Mapping: The hardness profile data from the Q&T study enables comparison with explosion-welded Ni-based clad hardness profiles, facilitating quality assurance and performance prediction.
- WPS Development: For explosion welding procedures requiring post-weld heat treatment, the Q&T parameter ranges established in this study serve as a technical foundation for procedure qualification per ASTM A266 or ISO 14732.
9. Contribution to Qualification Building, Product Delivery, and Customer Value
9.1 Qualification Building
This technical entry represents a critical knowledge asset for the company's welding procedure qualification program. Specifically:
- WPS Development: The Q&T parameter ranges (austenitization temperature, tempering temperature, soak times) can be directly incorporated into welding procedure specifications for Ni60-WC overlay on H13 substrates, ensuring that heat treatment is a qualified, documented part of the process.
- WPQ Support: Welder performance qualifications (WPQ) for plasma arc overlay can reference the Q&T study to demonstrate that the welder's deposited weld metal achieves the required hardness after post-weld heat treatment.
- Customer Audit Readiness: Having documented Q&T parameter studies demonstrates technical competence to customers and third-party auditors, supporting compliance with quality management standards such as ISO 9001 and ISO 3834.
- Standard Compliance: The study supports compliance with ASTM A681 (H13 heat treatment requirements) and ASME Section IX (WPS qualification), which require documented heat treatment procedures for overlay welds on tool steels.
9.2 Product Delivery
For product delivery, the Q&T study enables:
- Process Optimization: Precise Q&T parameters reduce the risk of hardness non-conformance, minimizing rework and improving first-pass yield rates.
- Consistency: Standardized Q&T cycles ensure consistent hardness and microstructure across all production batches, supporting reliable product performance.
- Efficiency: Optimized soak times and temperature ranges reduce furnace cycle times, improving throughput and reducing energy consumption.
- Traceability: Documented Q&T parameters enable full traceability from raw material through welding to final heat treatment, supporting quality records and customer documentation requirements.
9.3 Customer Value
The technical knowledge from this study directly translates to customer value:
- Extended Service Life: Customers receive H13 tool components with Ni60-WC overlays that achieve HRC 60–65 after Q&T, representing a 30–50% hardness improvement over the as-welded condition and significantly extended die life in abrasive forming operations.
- Reduced Downtime: Consistent overlay performance reduces unplanned die failures and production stoppages, directly impacting customer OEE (Overall Equipment Effectiveness).
- Cost Savings: Extended die life and reliable rebuild capability reduce customers' cost per part by minimizing die replacement frequency and production interruptions.
- Technical Partnership: The depth of technical understanding demonstrated through this study positions the company as a technical partner rather than a simple service provider, enabling collaborative problem-solving for complex surface engineering challenges.
10. Conclusion and Recommendations
The study of quenching and tempering effects on Ni60-WC plasma arc weld overlay on H13 hot work tool steel is a foundational technical contribution to the company's hardfacing overlay capability. The key findings—that Q&T treatment increases overlay hardness by 5–8 HRC, restores H13 HAZ hardness to design specifications, and promotes more uniform carbide distribution—provide actionable process parameters for production implementation.
The following recommendations are proposed for further development:
- Parameter Optimization: Conduct a systematic DOE (Design of Experiments) study to optimize the austenitization temperature and tempering temperature for maximum hardness uniformity across the overlay cross-section.
- Wear Testing: Complement hardness data with dry sliding wear tests (ASTM G99) and abrasive wear tests (ASTM G65) to quantify the improvement in wear resistance after Q&T treatment.
- Thermal Fatigue Testing: Evaluate the thermal fatigue resistance of Q&T-treated overlays through thermal cycling tests to validate performance in hot work die applications.
- WPS Documentation: Formalize the Q&T parameters into qualified welding procedure specifications per ASME Section IX and GB/T 19866, incorporating the heat treatment as a required post-weld operation.
- Training Material: Develop training materials based on this study for welders, inspectors, and quality engineers to ensure consistent implementation of the optimized Q&T process.
- Multi-Route Integration: Extend the Q&T knowledge base to TIG and MIG overlay processes, as well as to post-bonding heat treatment for hydraulic explosive bonded and explosion-welded clad products, creating a unified technical framework across all three technology routes.
By systematically applying the findings from this Q&T study, Cladding Technology Shanxi Co., Ltd. can deliver higher-performance Ni60-WC overlay solutions for H13 hot work tooling, strengthen its qualification portfolio, and deliver measurable value to customers through extended component life, reduced downtime, and improved cost efficiency.