The Effect of Different Cooling Methods on the Mechanical Properties of Wear-Resistant Weld Overlay Plates

1. Definition and Fundamental Principles

The study of cooling methods and their influence on the mechanical properties of wear-resistant weld overlay plates represents a critical metallurgical engineering discipline within the field of surfacing and cladding technology. Wear-resistant weld overlay plates are fabricated by depositing one or more layers of hard-facing or wear-resistant alloy material onto a base substrate through various welding processes. The cooling rate experienced by the deposited overlay metal during solidification and subsequent phase transformation is the single most influential parameter governing the final microstructure, hardness distribution, toughness, and overall service life of the finished product.

From a metallurgical standpoint, the cooling rate directly controls:

• Solidification microstructure: Higher cooling rates promote finer grain structures, cellular dendrite refinement, and reduced segregation in the weld metal.
• Phase transformation kinetics: The rate at which austenite transforms into martensite, bainite, or pearlite in steel-based overlay alloys is governed by the cooling rate through the critical transformation range (typically 800°C to 550°C for most hard-facing alloys).
• Hardness level: Martensitic transformation, which yields hardness values exceeding 50 HRC, requires sufficiently rapid cooling to suppress diffusional transformations.
• Residual stress magnitude: Differential thermal contraction between the overlay and base material generates residual stresses that are proportional to the cooling gradient.
• Crack susceptibility: Hydrogen-induced cracking and hot cracking risks are directly correlated with cooling rate, particularly in high-carbon and high-alloy overlay systems.

The fundamental principle underlying this technology is that the cooling method—whether air cooling, water quenching, controlled furnace cooling, or slow cooling in insulated blankets—creates distinct thermal histories that produce fundamentally different microstructural outcomes in the same overlay alloy composition.

2. Category and Business Positioning

This technical knowledge domain falls squarely within the TIG/MIG weld overlay technology route of Cladding Technology Shanxi Co., Ltd., with direct applicability to the post-weld thermal treatment (PWHT) and controlled cooling procedures that are integral to achieving specification compliance in wear-resistant cladding products.

In terms of business positioning, mastery of cooling method optimization enables the company to:

• Deliver wear-resistant overlay plates with guaranteed hardness ranges (e.g., 45–55 HRC for medium-duty, 55–65 HRC for severe-duty applications) with consistent repeatability across production batches.
• Minimize warranty claims and field failures by ensuring that the mechanical properties achieved in the laboratory are reliably reproduced in full-scale production.
• Reduce rework rates by understanding the precise thermal windows within which each cooling method operates to produce acceptable metallurgical outcomes.
• Support WPS (Welding Procedure Specification) qualification with documented cooling protocols that demonstrate conformance to applicable codes.

This knowledge area bridges the gap between metallurgical theory and production engineering, providing the technical foundation for process control decisions that directly impact product quality, customer satisfaction, and regulatory compliance.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

The systematic investigation of different cooling methods serves the following technical objectives:

1. Hardness optimization: Achieving target hardness levels in the overlay layer without inducing excessive brittleness that would compromise impact toughness or fatigue resistance.
2. Toughness preservation: Maintaining adequate fracture toughness (K_Ic values typically required above 40 MPa·m^1/2 for structural applications) while achieving high surface hardness.
3. Residual stress management: Controlling residual stress levels to below critical thresholds (typically < 200 MPa) that would cause dimensional distortion, cracking, or premature fatigue failure.
4. Microstructural uniformity: Ensuring consistent hardness and microstructure across the entire overlay surface, including at weld toes, overlaps, and edges.
5. Process repeatability: Establishing documented cooling protocols that yield consistent results regardless of ambient conditions, shift changes, or operator variations.

3.2 Business Value

The value delivered to the organization and its customers is multifaceted:

• Cost reduction: Optimizing cooling methods can eliminate the need for expensive post-weld heat treatment cycles, reducing production time and energy costs by 15–30%.
• Performance guarantee: Documented cooling protocols enable the company to make binding performance guarantees on hardness, toughness, and service life, which is a competitive differentiator in the wear-resistant materials market.
• Qualification acceleration: Understanding the relationship between cooling rate and mechanical properties allows for more efficient WPS qualification campaigns, reducing the number of trial welds required.
• Customer engineering support: Provides the technical basis for selecting appropriate cooling methods based on customer application requirements, enabling value-added engineering consultation.

4. Key Process and Implementation Points

4.1 Comparison of Cooling Methods

Parameter	Air Cooling (Natural)	Water Quenching	Controlled Furnace Cooling	Insulated Blanket Cooling
Cooling Rate (°C/min)	5–20	100–500+	1–10	2–10
Typical Hardness (HRC)	35–45	55–65	25–40	30–42
Impact Toughness	Moderate	Low (brittle)	High	Moderate-High
Residual Stress	Medium	High	Low	Low-Medium
Crack Risk	Low	High	Very Low	Low
Cost	Lowest	Low (but risk cost)	Highest	Medium
Dimensional Stability	Good	Poor	Excellent	Good

4.2 Critical Implementation Parameters

The following parameters must be precisely controlled for each cooling method to ensure consistent and reliable results:

4.2.1 Air Cooling (Natural Cooling)

• Applicable temperature range: Initiate cooling above 500°C to avoid martensitic transformation; ambient cooling from 600–800°C.
• Environmental controls: Wind speed below 0.5 m/s; relative humidity below 70%; ambient temperature between 15–30°C.
• Stacking configuration: Plates separated by non-metallic spacers (ceramic or wooden) to ensure uniform air circulation on all surfaces.
• Monitoring: Thermocouples embedded at weld center, weld toe, and plate edge; data logged at 1-second intervals.

4.2.2 Water Quenching

• Water temperature: 20–40°C (strictly below 50°C to prevent thermal shock and quench cracking).
• Immersion timing: Quench initiated when surface temperature reaches 550–650°C (austenitization temperature).
• Immersion rate: Controlled immersion speed of 50–100 mm/min to prevent vapor barrier formation.
• Post-quench tempering: Mandatory tempering at 200–300°C for 2 hours to reduce residual stress and improve toughness without significant hardness loss.

4.2.3 Controlled Furnace Cooling (PWHT)

• Heating rate: 100–150°C per hour up to 600°C, then 50°C per hour to the target temperature.
• Soak temperature: 550–650°C for stress relief; 700–800°C for full annealing.
• Soak duration: Minimum 1 hour per 25 mm of plate thickness.
• Cooling rate: 25–50°C per hour to 300°C, then furnace cool or air cool below 300°C.
• Furnace atmosphere: Neutral or slightly oxidizing; avoid reducing atmospheres that could cause decarburization.

4.2.4 Insulated Blanket Cooling

• Blanket material: Ceramic fiber or refractory wool, minimum 50 mm thickness.
• Coverage: Complete coverage of the overlay surface; edges sealed to prevent convective heat loss.
• Removal timing: Remove blanket when surface temperature drops below 300°C to avoid thermal gradient cracking.
• Weight loading: Apply uniform pressure of 5–10 kPa to ensure intimate contact between blanket and plate surface.

4.3 Process Control Flowchart

The recommended process control sequence for wear-resistant overlay plate cooling is as follows:

1. Step 1: Complete weld overlay deposition per qualified WPS; verify overlay thickness and geometry.
2. Step 2: Measure surface temperature at multiple locations using infrared pyrometer or embedded thermocouples.
3. Step 3: Select cooling method based on target hardness/toughness requirements per customer specification.
4. Step 4: Initiate cooling at the prescribed temperature; monitor and record cooling curve continuously.
5. Step 5: Perform post-cooling inspection: visual examination, hardness testing, and dimensional verification.
6. Step 6: If hardness/toughness targets are not met, apply corrective treatment (reheat and re-cool or temper).
7. Step 7: Document all cooling parameters, curves, and test results in the batch quality record.

5. Applicable Standards and Acceptance Criteria

5.1 Welding Procedure and Qualification Standards

• GB/T 985.1–2008: Welding Procedure Qualification Test — General Requirements (determines whether cooling method changes constitute a significant variable requiring requalification).
• GB/T 19866.1–2005: Surface Engineering — Weld Cladding — Part 1: General Guidance for the Selection and Application of Cladding Systems.
• ASTM A388/A388M: Standard Specification for Carbon and Alloy Steel Plate for Wear-Resistant Service (defines hardness and impact requirements for wear plates).
• ASME Section IX: Qualification Rules for Welding, Brazing, and Fusing (QW-250 through QW-280 address PWHT requirements and cooling rate variables).
• NB/T 47014–2011: Qualification Test Procedure for Pressure Vessel Welding Procedures (Chinese national standard for pressure vessel WPS qualification).

5.2 Mechanical Testing Standards

• GB/T 231.1–2018: Metallic Materials — Brinell Hardness Test (for hardness verification on overlay surfaces).
• GB/T 230.1–2018: Metallic Materials — Rockwell Hardness Test (primary method for HRC hardness measurement on overlay plates).
• GB/T 229–2020: Metallic Materials — Charpy Pendulum Impact Test (toughness verification).
• ASTM A370: Standard Practices for Mechanical Testing of Steel Products (comprehensive mechanical property verification).
• ISO 6508-1: Metallic Materials — Vickers Hardness Test — Part 1: Test Method (microhardness mapping across the overlay cross-section).

5.3 Acceptance Criteria

Property	Light Duty (Air Cool)	Medium Duty (Controlled Cool)	Severe Duty (Quench + Temper)
Surface Hardness (HRC)	35–45	45–55	55–65
Impact Energy at 25°C (J)	≥ 47	≥ 27	≥ 15 (with temper)
Impact Energy at -40°C (J)	≥ 27	≥ 15	≥ 10 (with temper)
Residual Stress (MPa)	≤ 150	≤ 200	≤ 250 (post-temper)
Overlay Thickness (mm)	3–5	5–10	8–15
Adhesion Strength (MPa)	≥ 300	≥ 300	≥ 300

5.4 NDT Standards

• GB/T 11345–2013: Non-destructive Testing of Welds — Ultrasonic Testing (for subsurface defect detection).
• GB/T 11346–2017: Non-destructive Testing of Welds — Radiographic Testing (for volumetric defect detection).
• GB/T 18851–2015: Non-destructive Testing of Welds — Magnetic Particle Testing (for surface crack detection post-cooling).
• API 570: Piping Inspection Code (for in-service wear plate assessment criteria).

6. Common Risks and Controls

6.1 Quench Cracking

Risk Description: Rapid water quenching of high-carbon or high-alloy overlay deposits can induce severe thermal gradients that exceed the material's tensile strength, resulting in quench cracks. This is particularly prevalent in martensitic hard-facing alloys with carbon content above 0.5%.

Controls:

• Limit water temperature to below 40°C and maintain controlled immersion speed.
• Apply preheating to 200–300°C before quenching to reduce thermal shock.
• Implement mandatory post-quench tempering within 2 hours of quench completion.
• Perform magnetic particle inspection (MPI) on all quenched surfaces before tempering.
• Limit plate thickness to below 25 mm for water quench applications without preheating.

6.2 Hydrogen-Induced Delayed Cracking

Risk Description: Slow cooling methods (insulated blanket, furnace cooling) can allow dissolved hydrogen in the weld metal to diffuse to grain boundaries and defects over extended time periods, causing delayed cracking hours or days after welding.

Controls:

• Use low-hydrogen filler metals (hydrogen content < 5 mL/100g) for all overlay welding.
• Implement bake-out treatment at 100–150°C for 1–2 hours immediately after welding to promote hydrogen diffusion out of the metal.
• Maintain welding wire and flux in properly conditioned storage (dried at 150–300°C for 2–4 hours before use).
• Limit ambient humidity to below 80% during welding operations.
• Apply interpass temperature controls (150–250°C) to limit hydrogen absorption during multi-pass builds.

6.3 Hardness Non-Uniformity

Risk Description: Inconsistent cooling rates across the plate surface—particularly at edges, corners, and near the backing plate—can result in hardness variation exceeding acceptable limits (typically > 10 HRC difference between center and edge).

Controls:

• Use multi-point thermocouple monitoring (minimum 5 points per plate) during cooling.
• Implement symmetric cooling configurations for edge regions (double-sided blankets or symmetric quench fixtures).
• Map hardness at a minimum of 9 points per plate (3×3 grid) for acceptance testing.
• Apply edge bead geometry optimization to ensure uniform overlay thickness at plate edges.
• Establish cooling rate tolerance bands (±20% of target rate) and reject plates outside these bands.

6.4 Dimensional Distortion

Risk Description: Differential thermal contraction between the overlay and base material during cooling can cause plate warping, bowing, or cross-sectional distortion, particularly in thin plates or asymmetric overlay configurations.

Controls:

• Apply rigid backing fixtures during welding and cooling to constrain deformation.
• Use symmetric multi-pass overlay sequences that distribute heat input evenly.
• Limit maximum single-pass heat input to prevent localized thermal distortion.
• Implement post-weld straightening procedures (mechanical or thermal) for plates exceeding flatness tolerance of 1 mm/m.
• Design overlay plate geometries with uniform thickness to minimize differential contraction.

6.5 Adhesion Failure

Risk Description: Excessive cooling rates can generate interface stresses at the overlay/base material boundary that exceed the bond strength, resulting in delamination or spalling of the overlay during service.

Controls:

• Verify base material cleanliness (SA 2.5 minimum per SSPC-SP 10) prior to overlay welding.
• Apply a transition layer (e.g., 309L or 316L) when joining dissimilar materials to reduce interface stress.
• Perform shear bond testing (minimum 3 specimens per batch) to verify adhesion strength ≥ 300 MPa.
• Control interpass temperature between 100–200°C to maintain proper metallurgical bonding.
• Implement post-weld stress relief at 550–650°C for critical applications.

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay Route

The TIG/MIG weld overlay route is the primary application domain for cooling method optimization. In this route, the cooling method is directly integrated into the welding procedure specification and represents a critical process variable.

Specific Applications:

• Hard-facing overlay on carbon steel: For Cr-C-Mo hard facing alloys (e.g., D2, H13 composition), controlled air cooling from 600°C produces optimal martensite-bainite microstructure with 50–58 HRC hardness and acceptable toughness.
• Multi-layer overlay builds: For thick overlay deposits (8–15 mm), progressive cooling between passes (maintaining interpass temperature at 150–250°C) followed by final air cooling provides the best balance of hardness and toughness.
• Stainless steel overlay on carbon steel: Slow cooling (insulated blanket) is preferred to minimize carbon pickup at the interface and prevent intergranular corrosion susceptibility in the heat-affected zone.
• Nickel-based overlay alloys: These alloys (e.g., Stellite, Inconel) are inherently crack-resistant and can tolerate water quenching for maximum hardness without cracking risk, making aggressive cooling viable.

Process Integration: The cooling method selection is documented in the WPS and verified during WPS qualification testing. Changes in cooling method constitute a significant variable requiring requalification per GB/T 985.1 and ASME Section IX.

7.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding (water pressure bonding) does not involve welding heat input, the cooling method principles apply to the post-bonding thermal conditioning of the bonded interface and any subsequent weld overlay applied on top of the bonded clad plate.

Specific Applications:

• Post-bonding stress relief: Hydraulic explosive bonding generates significant residual stresses at the bonded interface. Controlled furnace cooling (stress relief at 550–620°C) is applied to the bonded clad plate before any subsequent machining or welding operations.
• Overlay welding on bonded clad plates: When a wear-resistant overlay is subsequently applied via MIG welding on a hydraulically bonded clad plate, the cooling method must account for the existing residual stress state from the bonding process. Insulated blanket cooling is typically specified to minimize additional stress buildup.
• Interface integrity preservation: Aggressive cooling methods (water quench) applied after overlay welding on bonded clad plates can compromise the bonded interface integrity by generating differential thermal contraction across the dissimilar material interface. Slow cooling is mandatory.

7.3 Explosion Welding Route

In explosion welding, the cooling rate during the initial bonding event is inherently extremely high (on the order of 10^4–10^5 °C/s at the collision interface), producing a unique metastable microstructure. However, the subsequent thermal processing of the explosion-welded clad plate is governed by the same cooling method principles.

Specific Applications:

• Post-explosion-weld stress relief: The extreme thermal gradients from explosion welding generate very high residual stresses. Controlled furnace cooling at 550–650°C for stress relief is mandatory before any further processing. The cooling rate from the stress relief temperature must be controlled at 25–50°C/hour to prevent re-introduction of significant residual stresses.
• Thermal diffusion bonding optimization: When explosion-welded clad plates undergo subsequent thermal treatment (e.g., solution treatment for stainless steel cladding), the cooling rate from the solution temperature determines the final precipitate distribution and mechanical properties of the cladding layer.
• Weld overlay on explosion-welded clad: When additional weld overlay is applied to an explosion-welded clad plate (e.g., adding a hard-facing layer on top of an explosion-welded stainless steel cladding), the cooling method must be carefully selected to avoid cracking at the explosion-welded interface. Insulated blanket cooling or furnace cooling is recommended.

8. Qualification Building and Certification Value

8.1 WPS Qualification Support

Systematic understanding of cooling methods directly supports the development and qualification of Welding Procedure Specifications. During WPS qualification testing, the cooling method is a fundamental process parameter that must be:

• Defined in the WPS with specific temperature limits, cooling rates, and monitoring requirements.
• Verified during qualification welding by measuring and recording actual cooling curves.
• Tested for mechanical property compliance (hardness, impact, tensile) at the specified cooling conditions.
• Documented in the WPQR (Welding Procedure Qualification Record) with all test results.

8.2 Production Authorization

For production authorization under standards such as GB/T 19418 (Welding Procedure Specification), the cooling method must be demonstrated as a controlled and repeatable process. This requires:

• Equipment capability verification (furnace temperature uniformity, thermocouple calibration, cooling rate control accuracy).
• Operator qualification demonstrating understanding of cooling method selection and monitoring procedures.
• Statistical process control data from a minimum of 3 consecutive production batches demonstrating consistent results.
• Documented corrective action procedures for cooling deviations.

8.3 Customer Value and Market Positioning

The technical expertise in cooling method optimization provides significant customer value:

• Custom performance specification: Ability to tailor mechanical properties to specific customer application requirements (e.g., high hardness for abrasion resistance vs. high toughness for impact loading).
• Reduced customer risk: Documented and qualified cooling procedures reduce the probability of field failures, providing customers with confidence in product performance.
• Technical differentiation: Demonstrates engineering sophistication and commitment to quality that differentiates the company from competitors who may use generic or undocumented cooling practices.
• Accelerated project timelines: Pre-qualified cooling procedures eliminate the need for customer-specific trial production, reducing time-to-delivery.

9. Quality Management Integration

9.1 Documentation Requirements

All cooling method implementations must be documented in accordance with the company's Quality Management System (QMS) aligned with ISO 9001 and relevant industry standards. Required documentation includes:

• Cooling procedure work instructions (CWI) for each cooling method.
• Equipment calibration records for thermocouples, pyrometers, and data loggers.
• Cooling curve records for each production batch (minimum 1 record per plate).
• Hardness test records with location mapping.
• Impact test records (for critical applications).
• Non-conformance reports and corrective actions for any cooling deviations.

9.2 Process Monitoring and Control

Real-time monitoring of cooling parameters is essential for maintaining process control:

• Temperature monitoring: Continuous recording at minimum 5 thermocouple locations per plate.
• Cooling rate calculation: Automated calculation of cooling rate through the critical range (800–500°C).
• Alarm thresholds: Automatic alerts when cooling rate deviates from specification by more than ±20%.
• Data retention: All cooling data retained for minimum 10 years per API 570 and customer requirements.

10. Conclusion

The systematic understanding and application of different cooling methods in wear-resistant weld overlay plate manufacturing represents a fundamental technical competency that directly impacts product quality, performance reliability, and customer satisfaction. By mastering the metallurgical principles governing cooling rate effects on microstructure and mechanical properties, Cladding Technology Shanxi Co., Ltd. can deliver precisely engineered wear-resistant products that meet or exceed customer specifications across all three technology routes.

This technical knowledge base supports the company's commitment to quality excellence, regulatory compliance, and continuous improvement, while providing a solid foundation for WPS qualification, production authorization, and customer engineering support. The integration of cooling method optimization into the overall quality management system ensures that every production batch benefits from the cumulative learning and technical expertise accumulated through systematic study and documented practice.