Effect of Tempering Temperature on Microstructure and Wear Resistance of Cr-W-Mo Iron-Based Weld Overlay Layers
1. Technical Definition and Fundamental Principles
1.1 Material System Overview
The Cr-W-Mo iron-based weld overlay system is a high-performance metallic coating designed for severe abrasive and erosive-wear environments. The alloy chemistry is dominated by chromium (typically 6–12 wt%), tungsten (typically 5–15 wt%), and molybdenum (typically 1–4 wt%), with iron as the base matrix. These alloying elements promote the formation of hard carbide phases—principally M₇C₃, M₂C, and Cr₇C₃—dispersed within a tempered martensitic matrix. The resulting microstructure delivers a combination of high hardness (HRC 50–62), excellent thermal stability, and superior resistance to both sliding and impact abrasion.
1.2 Tempering Mechanism
Following weld overlay deposition, the as-welded overlay typically exhibits a fully hardened martensitic structure with retained austenite. This condition, while offering maximum hardness, is prone to microcracking, excessive residual stress, and poor toughness. Tempering is therefore an essential post-weld heat treatment (PWHT) step that:
- Decomposes retained austenite into tempered martensite and carbides, reducing the risk of delayed cracking.
- Redistributes and coarsens carbide phases, transforming fine primary carbides into more thermodynamically stable configurations.
- Relieves residual stresses accumulated during multi-pass welding, thereby improving dimensional stability and fatigue performance.
- Modulates hardness-toughness balance by controlling the degree of carbide coarsening and matrix softening.
The critical relationship between tempering temperature and the Cr-W-Mo overlay system is governed by the precipitation and coarsening kinetics of W-rich and Cr-rich carbides. At lower tempering temperatures (450–550°C), fine secondary carbides nucleate and strengthen the matrix through precipitation hardening. At higher temperatures (600–750°C), carbide coarsening accelerates, leading to progressive softening of the matrix while maintaining carbide hardness. This trade-off defines the optimal tempering window for a given service condition.
2. Category and Business Positioning
2.1 Role Within Company Capability Framework
This technical study—formalized as a structured learning reflection on tempering temperature effects—occupies a critical position within Cladding Technology Shanxi Co., Ltd's R&D and process qualification pipeline. It bridges fundamental metallurgical research with practical WPS (Welding Procedure Specification) development and product qualification. The knowledge generated directly supports:
- WPS qualification and optimization for Cr-W-Mo overlay consumables across all three manufacturing routes.
- Post-weld heat treatment procedure development compliant with ASME Section IX, AWS D10.6, and relevant GB/T standards.
- Customer-specific overlay solutions where tempering temperature must be tailored to service conditions (e.g., thermal cycling, impact loading, or elevated-temperature wear).
2.2 Strategic Value
Understanding the tempering temperature–microstructure–wear resistance relationship enables the company to deliver value-added metallurgical engineering services rather than merely executing welding operations. This positions Cladding Technology Shanxi Co., Ltd as a technical partner capable of providing optimized overlay solutions with documented performance guarantees, a significant differentiator in competitive bidding for oilfield, mining, and power generation applications.
3. Technical Purpose and Value
3.1 Core Objectives
- Determine optimal tempering temperature windows for Cr-W-Mo overlays to achieve target hardness (HRC 50–62) with acceptable impact toughness (Charpy V-notch ≥ 27 J at 25°C).
- Establish quantitative correlations between tempering temperature, carbide morphology (type, size, distribution), and dry/wet sliding wear rates.
- Define PWHT parameters (temperature, duration, cooling rate) for inclusion in qualified WPS and PQR documentation.
- Provide acceptance criteria for post-tempering hardness mapping, microstructure evaluation, and wear testing.
3.2 Quantitative Performance Targets
| Performance Parameter | Target Range | Test Method | Standard Reference |
|---|---|---|---|
| Overlay Hardness | HRC 50–62 (surface to 2 mm) | Micro-Vickers / Rockwell C | ASTM E18 / GB/T 230.1 |
| Impact Toughness | ≥ 27 J (Charpy V-notch, 25°C) | Charpy V-Notch | ASTM E23 / GB/T 229 |
| Dry Sliding Wear Rate | ≤ 50 mg/N·m (Al₂O₃ ball-on-disc) | Ball-on-Disc Pin-on-Disk | ASTM G98 / GB/T 12444 |
| Carbide Volume Fraction | 25–45 vol% (M₇C₃ + M₂C) | SEM Image Analysis | ASTM E1245 |
| Residual Stress (post-temper) | ≤ 150 MPa (compressive preferred) | X-Ray Diffraction / Hole Drilling | ASTM E975 / ASTM E837 |
4. Key Process and Implementation Points
4.1 Tempering Temperature Regimes and Metallurgical Outcomes
| Tempering Temperature (°C) | Matrix Condition | Carbide Behavior | Hardness (HRC) | Toughness (CVN, J) | Wear Resistance Index | Recommended Application |
|---|---|---|---|---|---|---|
| As-Welded (No Temper) | Un tempered martensite + retained austenite | Fine primary + secondary carbides | 58–64 | 10–20 | High (brittle fracture risk) | Not recommended for production |
| 450–500 | Tempered martensite, minimal retained austenite | Secondary carbides nucleating, fine dispersion | 55–60 | 20–30 | Very High | Low-temperature sliding abrasion (mining buckets) |
| 550–600 | Tempered martensite, equilibrium carbides | Carbide coarsening onset, M₇C₃ dominant | 50–55 | 30–45 | High | Impact abrasion, thermal cycling (crusher hammers) |
| 650–700 | Tempered martensite → sorbite transition | Significant M₂C coarsening, Cr₇C₃ retained | 45–50 | 45–60 | Moderate-High | High-temperature wear (furnace components, 400–600°C service) |
| 750–800 | Sorbite / ferrite-pearlite | Coarse M₂C, reduced total carbide fraction | 38–45 | 60–80 | Moderate | Wear-resistance with high toughness requirement (pivots, bearings) |
4.2 PWHT Procedure Development Protocol
- Base metal compatibility assessment: Determine the maximum permissible PWHT temperature based on the base material's tempering response (e.g., 9Cr-1Mo steel base limited to 750°C to avoid over-tempering).
- Thermal gradient analysis: For thick-section overlays (>25 mm), perform finite element thermal simulation to predict peak temperature at the overlay-base interface and ensure uniform tempering without exceeding base metal limits.
- Soak time calculation: Minimum soak time shall be 1 hour per 25 mm of section thickness (per ASME Section VIII Div. 1, UG-120), with a minimum of 2 hours total.
- Heating and cooling rate control: Limit heating rate to 15°C per hour for the first 250°C above ambient, then 25°C per hour to tempering temperature. Cooling in furnace to 400°C, then air cool.
- Post-temper inspection: Perform hardness mapping at 1 mm intervals from surface to interface, microstructural examination (optical + SEM), and residual stress measurement.
4.3 Wear Testing Protocol
Wear resistance characterization shall follow a multi-modal testing approach to ensure comprehensive evaluation:
- Dry sliding wear: ASTM G98 ball-on-disc test using Al₂O₃ counterface (6 mm diameter, 10 N load, 1 m/s sliding speed, 1000 cycles). Report wear rate in mg/N·m.
- Impact abrasion: ASTM G65 sand-rubber wheel test or equivalent. Report wear rate in mg/g.
- Erosion wear: ASTM G76 air-blast erosion test with SiC particulate (63–125 μm, 30 m/s, 90° impact angle). Report mass loss in mg.
- Thermal cycling wear: Custom protocol: 200 cycles of 25°C → 600°C → 25°C, followed by dry sliding wear test. Evaluate thermal stability of wear performance.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure and Qualification Standards
| Standard | Scope | Relevant Requirement |
|---|---|---|
| ASME Section IX | Welding procedure and welder qualification | Essential variables for overlay welding; PWHT qualification range |
| AWS D10.6M/D10.6 | Specification for hard-faced weld overlays | Overlay thickness, hardness, impact test, bonding strength requirements |
| GB/T 13813 | Hardfacing alloy classification | Cr-W-Mo iron-based classification (Type II) |
| GB/T 12469 | Welded overlay for wear resistance | Hardness, thickness, and impact acceptance criteria |
| API RP 571 | Damage mechanisms in refineries | Wear mechanism identification for overlay selection |
| NACE SP0169 | Corrosion control in refineries | Overlay corrosion resistance requirements in wet environments |
| ISO 14224 | Classification of equipment failure | Wear failure categorization for reliability analysis |
5.2 Post-Weld Heat Treatment Standards
- ASME Section VIII, Div. 1 (UG-120): Governs PWHT requirements for pressure vessels including overlaid surfaces. Specifies temperature ranges, soak times, and rate-of-heating/cooling limits.
- ASME Section IX, QW-451: Defines PWHT as an essential variable for welding procedure qualification. Changes in PWHT temperature beyond qualified range require requalification.
- GB/T 150.4: Chinese standard for post-weld heat treatment of pressure vessels, including overlay welds. Specifies furnace temperature uniformity (±14°C across thermocouple range) and minimum holding time.
- ASTM A388: Standard specification for post-weld heat treatment of carbon and low-alloy steel weldments.
5.3 Acceptance Criteria Summary
- Hardness: Minimum HRC 50 at surface, minimum HRC 45 at 2 mm depth, with no hardness gradient exceeding 10 HRC points over 5 mm depth (per AWS D10.6).
- Impact: Charpy V-notch specimen containing overlay material shall achieve ≥ 27 J at 25°C (per AWS D10.6). For low-temperature service, minimum 20 J at -20°C.
- NDT: Magnetic particle inspection (MT) per ASTM E1444 or GB/T 15822 shall reveal no linear indications exceeding 2 mm in length or 0.5 mm in width. No cluster indications exceeding 10 mm in any dimension.
- Thickness: Overlay thickness ≥ 3 mm minimum (per customer specification, typically 3–6 mm for severe service). Thickness uniformity within ±1 mm across the overlay surface.
6. Common Risks and Controls
6.1 Over-Tempering Risk
Risk: Excessive tempering temperature or prolonged soak time leads to significant carbide coarsening, matrix softening, and reduced wear resistance. For Cr-W-Mo overlays, temperatures above 750°C can reduce hardness below HRC 40, rendering the overlay ineffective for abrasive service.
Control measures:
- Implement thermocouple monitoring at overlay surface, overlay-base interface, and furnace atmosphere (minimum 3 TCs per ASME UG-120).
- Define maximum PWHT temperature as the lower of: (a) overlay tempering limit (750°C for Cr-W-Mo), or (b) base metal tempering limit.
- Conduct hardness verification at 3 locations minimum post-temper; reject if any reading falls below HRC 45.
6.2 Under-Tempering Risk
Risk: Insufficient tempering leaves retained austenite and high residual stresses, leading to microcracking, dimensional instability, and premature failure under impact loading.
Control measures:
- Ensure minimum soak temperature and duration per WPS qualification.
- Perform residual stress measurement (XRD or hole drilling) post-temper; target ≤ 150 MPa.
- Conduct Charpy V-notch testing on weld coupons; reject if CVN < 27 J at 25°C.
6.3 Thermal Mismatch and Interface Cracking
Risk: Differential thermal expansion between Cr-W-Mo overlay (CTE ~12.5×10⁻⁶/°C) and carbon steel base (CTE ~11.5×10⁻⁶/°C) can generate interface stresses during PWHT, particularly for thick overlays on thin base plates.
Control measures:
- Limit overlay-to-base thickness ratio to ≤ 1:2 for single-side overlay without backing support.
- Apply controlled heating/cooling rates (≤ 25°C/hr) to minimize thermal gradients.
- Consider multi-stage tempering (e.g., 550°C for 2 hr, then 650°C for 2 hr) to gradually relieve interface stresses.
- Perform interface bond strength testing (shear test per ASTM F781 or equivalent) to verify overlay integrity.
6.4 Carbide Network Formation
Risk: Inadequate consumable mixing or improper welding parameters can produce continuous carbide networks along grain boundaries, severely reducing toughness despite adequate hardness.
Control measures:
- Use consumables with controlled carbon content (1.5–3.5 wt%) and balanced Cr:W:Mo ratio.
- Maintain interpass temperature ≤ 250°C to prevent excessive grain growth and carbide network formation.
- Perform metallographic examination of transverse and longitudinal sections; reject if continuous carbide networks span > 50% of grain boundaries (ASTM E126 grain boundary carbide rating).
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
The TIG (GTAW) and MIG (GMAW) weld overlay routes are the primary methods for depositing Cr-W-Mo iron-based overlays in Cladding Technology Shanxi Co., Ltd's fabrication operations. The tempering temperature study directly informs WPS development for these processes:
- Multi-pass overlay strategy: A typical Cr-W-Mo overlay requires 2–4 passes to achieve 3–6 mm thickness. The tempering temperature selection must account for the total overlay thickness and the thermal history of each pass. Thicker overlays (≥ 5 mm) generally require higher tempering temperatures (600–650°C) to ensure uniform stress relief throughout the overlay depth.
- WPS essential variables: Per ASME Section IX, QW-250 (for GMAW) and QW-118 (for GTAW), the PWHT temperature is an essential variable. The tempering study defines the qualified PWHT temperature range for each consumable type, enabling efficient WPS coverage.
- Process integration: For TIG overlay on thin-section components (e.g., valve trim, pump sleeves), lower tempering temperatures (450–550°C) are preferred to minimize distortion while maintaining wear performance. For MIG overlay on heavy-duty components (e.g., mining bucket teeth, crusher hammers), higher temperatures (550–650°C) provide better toughness for impact abrasion.
7.2 Hydraulic Explosive Bonding (HEB) Route
While hydraulic explosive bonding is primarily used for dissimilar metal cladding (e.g., carbon steel/stainless steel, carbon steel/nickel alloy), the tempering temperature knowledge from Cr-W-Mo overlay studies contributes to:
- Post-bond heat treatment design: HEB-clad components often require PWHT to relieve bonding-induced residual stresses. The tempering temperature–microstructure relationship established for Cr-W-Mo overlays informs the selection of PWHT parameters for clad plates and pipes where Cr-W-Mo weld overlay is subsequently applied to the clad surface.
- Composite component qualification: For components combining HEB cladding (corrosion resistance) with Cr-W-Mo weld overlay (wear resistance), the PWHT procedure must simultaneously satisfy the bonding interface integrity requirements and the overlay hardness requirements. The tempering study provides the metallurgical basis for defining compatible PWHT windows.
- Thermal simulation calibration: Finite element thermal models used to predict HEB bonding quality and subsequent PWHT effects are calibrated using data from Cr-W-Mo overlay tempering studies, improving prediction accuracy for multi-layer composite components.
7.3 Explosion Welding (EW) Route
Explosion welding produces metallurgical bonds between dissimilar metals through high-velocity impact. The tempering temperature study supports EW applications in the following ways:
- Post-explosion heat treatment: Explosion-welded clad plates (e.g., 16Mn/Cr-Mo steel) require PWHT to relieve the extreme residual stresses generated during the explosion process. Tempering temperature data from Cr-W-Mo overlay studies provides reference for selecting PWHT parameters that maintain clad interface integrity while achieving target hardness in any subsequent overlay layer.
- Wear-resistant clad plate production: Explosion welding can produce Cr-Mo steel/carbon steel clad plates where the Cr-Mo layer provides wear resistance. The tempering study directly informs the PWHT procedure for these clad plates, ensuring the Cr-Mo layer achieves target hardness (HRC 50–55) without compromising the explosion weld interface.
- Interface metallurgy optimization: The knowledge of carbide precipitation kinetics at various tempering temperatures helps predict and control the microstructure evolution at the explosion weld interface during PWHT, preventing intermetallic compound formation or excessive softening of the Cr-Mo layer.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This tempering temperature study serves as a foundational document for building a comprehensive WPS/PQR qualification matrix for Cr-W-Mo overlay welding. Key contributions include:
- Qualified PWHT range definition: Establishes the temperature range (450–700°C) and soak time requirements for Cr-W-Mo overlays, enabling WPS qualification coverage per ASME Section IX.
- Essential variable documentation: Provides the metallurgical justification for PWHT temperature as an essential variable, supporting efficient WPS qualification and minimizing unnecessary requalification.
- Third-party certification readiness: Documentation aligned with AWS D10.6, ASME Section IX, and GB/T 12469 requirements facilitates third-party certification audits (e.g., AWS Certified Welding Inspector, ASME N-stamp).
8.2 Product Delivery Enhancement
- Reduced rework rates: By defining optimal tempering parameters, the study reduces the incidence of post-temper hardness failures and impact test rejections, improving first-pass yield rates.
- Accelerated delivery schedules: Pre-qualified PWHT parameters eliminate the need for trial-and-error tempering cycles, reducing production cycle time by an estimated 15–25% for overlay components.
- Consistent quality across production batches: Standardized tempering procedures based on this study ensure batch-to-batch consistency in overlay hardness, microstructure, and wear performance.
8.3 Customer Value Creation
- Extended component service life: Optimized tempering parameters deliver overlays with the best hardness-toughness balance for the specific service condition, extending component life by 2–5× compared to unoptimized tempering.
- Reduced unplanned downtime: Improved impact toughness and residual stress control minimize the risk of overlay spalling and cracking, reducing unplanned shutdowns in continuous-process industries.
- Customized overlay solutions: The ability to select tempering temperature based on service conditions (temperature, impact severity, wear mode) enables truly customized overlay solutions, adding significant value beyond standard catalog products.
- Documented performance guarantees: Quantitative wear test data and hardness/toughness specifications derived from this study support performance-based warranties and service life guarantees, building customer confidence and competitive advantage.
9. Implementation Recommendations
9.1 Immediate Actions
- Integrate tempering temperature selection criteria into all active Cr-W-Mo overlay WPS documents, ensuring PWHT parameters are explicitly defined and qualified.
- Establish a tempering temperature decision matrix (based on service temperature, impact severity, and required hardness) for use by field application engineers.
- Conduct hardness and microstructure verification on the last 3 production batches of Cr-W-Mo overlays to validate current PWHT procedures against the study's recommendations.
9.2 Medium-Term Development
- Develop and qualify multi-stage tempering procedures for thick-section overlays (>6 mm) to optimize the hardness profile from surface to interface.
- Extend the tempering temperature study to include elevated-temperature wear testing (400°C, 500°C, 600°C) to define service temperature limits for each tempering regime.
- Establish a database correlating tempering parameters, consumable chemistry, welding process variables, and wear performance for predictive overlay design.
9.3 Long-Term Strategic Direction
- Pursue patent protection for optimized tempering procedures that deliver proprietary hardness-toughness-wear performance combinations.
- Develop automated PWHT monitoring systems with real-time thermocouple data logging and alarm functions to ensure consistent tempering quality at scale.
- Collaborate with research institutions to extend Cr-W-Mo overlay system understanding to advanced characterization techniques (TEM, EBSD, atom probe tomography) for next-generation overlay development.
10. Conclusion
The effect of tempering temperature on the microstructure and wear resistance of Cr-W-Mo iron-based weld overlay layers represents a fundamental metallurgical knowledge area that directly underpins the technical competitiveness of Cladding Technology Shanxi Co., Ltd. By systematically understanding and controlling the tempering temperature variable, the company can deliver overlay products with optimized hardness-toughness-wear performance, reduced production variability, and documented qualification compliance. This knowledge asset supports all three manufacturing technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—and creates measurable value for customers through extended component life, reduced downtime, and customized engineering solutions. The systematic implementation of findings from this study into WPS qualification, production procedures, and customer-facing technical documentation will strengthen the company's position as a premier provider of wear-resistant overlay solutions in the industrial markets it serves.