Effect of Post-Weld Overlay Cooling Rate on Wear Resistance of Clad Layers
1. Technical Definition and Fundamental Principles
1.1 Core Concept
The cooling rate following weld overlay (build-up welding) is one of the most critical metallurgical variables governing the final microstructure, hardness distribution, and wear resistance of the deposited cladding layer. In the context of bimetallic cladding and weld overlay manufacturing, the post-overlay cooling rate directly determines the phase transformation kinetics within the weld metal and the heat-affected zone (HAZ), thereby controlling the formation of hard phases such as carbides, martensite, and intermetallic compounds that confer abrasion and erosion resistance.
1.2 Metallurgical Mechanisms
The weld overlay process involves the sequential melting and solidification of filler metal deposited onto a base substrate. Upon completion of the final overlay pass, the entire weld deposit assembly is subjected to a cooling cycle that proceeds through distinct temperature regimes:
- High-temperature solidification regime (above A3 or Acm temperature): Primary solidification occurs, establishing the grain structure and segregation patterns within each weld bead.
- Intermediate transformation regime (A3 to A1 for hypoeutectoid steels; Acm to A1 for hypereutectoid steels): Diffusional phase transformations proceed, producing pearlite, bainite, or ferrite-pearlite mixtures depending on cooling rate.
- Low-temperature transformation regime (below A1): Martensitic transformation may occur in high-carbon or alloyed overlay materials if the cooling rate exceeds the critical cooling rate (Vc), producing a hard, brittle microstructure.
The fundamental relationship is that faster cooling rates suppress diffusional transformations, promote finer grain structures, and can induce martensitic transformation in susceptible compositions. Conversely, slower cooling rates allow coarsening of carbides, ferrite formation, and the development of softer, more ductile microstructures.
2. Business Positioning and Technical Purpose
2.1 Strategic Value Within Cladding Technology Shanxi Co., Ltd.
This technical knowledge area represents a critical competency in the company's core capability to deliver high-performance wear-resistant clad products. The understanding of cooling rate effects enables the company to:
- Predict and control overlay hardness to meet customer specifications (typically 400–900 HV depending on application).
- Minimize cracking susceptibility in the overlay layer and at the weld interface.
- Optimize multi-pass weld overlay procedures to achieve uniform hardness distribution across the cladding layer.
- Reduce rework rates and improve first-pass quality in production weld overlay operations.
- Support WPS (Welding Procedure Specification) qualification by demonstrating technical understanding of process variables.
2.2 Technical Purpose
The primary purpose of mastering cooling rate control is to ensure that the deposited overlay layer achieves the target microstructure and wear resistance characteristics specified by the customer. This directly impacts product performance in demanding service environments such as mining, cement, power generation, and chemical processing, where component life is measured in thousands of operating hours and failure carries significant economic and safety consequences.
3. Key Process Variables and Their Influence on Cooling Rate
3.1 Primary Cooling Rate Determinants
| Parameter | Effect on Cooling Rate | Typical Range | Impact on Wear Resistance |
|---|---|---|---|
| Base material thermal mass | Higher mass = slower cooling | 10 kg to 500+ kg | Thick sections retain heat, reducing hardness |
| Preheat temperature | Higher preheat = slower cooling | 0°C to 300°C | Preheat >150°C can reduce overlay hardness by 50–150 HV |
| Interpass temperature control | Higher interpass = slower cooling of previous pass | 50°C to 250°C | Excessive interpass reduces martensite formation |
| Overlay layer thickness | Thicker deposits cool slower internally | 3 mm to 25 mm | Internal layers may be softer than surface layers |
| Welding speed and travel rate | Faster travel = less heat input = faster local cooling | 100 mm/min to 400 mm/min | Higher speed produces narrower, harder beads |
| Filler metal alloy composition | Higher C, Cr, Mo content lowers critical cooling rate | Carbon 0.5–3.5 wt% | Alloying elements promote hard phase formation |
| Post-weld cooling method | Air cooling vs. furnace cooling vs. water quench | Various | Controlled cooling enables tailored microstructure |
3.2 Microstructural Outcomes at Different Cooling Rates
| Cooling Rate (°C/s) | Dominant Microstructure | Typical Hardness (HV) | Wear Resistance Level | Cracking Susceptibility |
|---|---|---|---|---|
| <1 | Pearlite + Ferrite; coarse carbides | 200–350 | Low to Moderate | Very Low |
| 1–10 | Fine pearlite; bainite; mixed phases | 350–550 | Moderate to High | Low |
| 10–50 | Bainite; fine carbide dispersion | 550–750 | High | Moderate |
| >50 | Martensite + retained austenite; fine carbides | 750–950 | Very High | High |
4. Process Implementation and Optimization Strategies
4.1 TIG (GTAW) Weld Overlay Cooling Rate Control
In TIG weld overlay, the relatively low heat input (typically 0.5–2.0 kJ/mm) inherently produces faster cooling rates compared to MIG or submerged arc processes. Key control strategies include:
- Low heat input parameters: Using lower current (100–180 A) with higher travel speed to minimize thermal mass accumulation.
- Interpass temperature monitoring: Maintaining interpass temperature between 80°C and 150°C to balance cooling rate and cracking prevention.
- Multi-pass layering: Depositing 2–4 passes of 1.5–2.5 mm each, with controlled cooling between passes to achieve uniform hardness.
- Back-gas and shield gas optimization: Using argon or helium mixtures to maintain arc stability at lower heat inputs.
- Directional welding sequence: Planning weld pass direction to control heat accumulation in the weld pool.
4.2 MIG (GMAW) Weld Overlay Cooling Rate Control
MIG weld overlay provides higher deposition rates but also higher heat input, requiring more active cooling rate management:
- Pulsed MIG welding: Using pulsed parameters to achieve high deposition rates with controlled per-pulse heat input, enabling faster cooling between pulses.
- Reduced wire feed rate: Operating at lower wire feed rates (5–8 m/min) to limit total heat input per pass.
- Shorter bead length: Limiting individual bead length to 50–100 mm to prevent excessive heat buildup.
- Active cooling aids: Using directed air or water spray on non-critical areas to accelerate cooling of completed passes.
- Filler wire selection: Choosing wires with higher carbon and alloy content (e.g., 2Cr13, 12Cr1MoV equivalent compositions) to achieve target hardness even at moderate cooling rates.
4.3 Post-Weld Thermal Treatment Options
| Method | Cooling Rate Achieved | Application Scenario | Advantages | Limitations |
|---|---|---|---|---|
| Air cooling (uncontrolled) | 5–20°C/s | Small components, thin overlays | Simplicity; no equipment required | Variable results; dependent on ambient conditions |
| Controlled air cooling | 3–15°C/s | Medium components | Reproducible; adjustable | Requires ventilation control |
| Water quench (localized) | 50–200°C/s | High hardness required; small areas | Maximum hardness achievable | Cracking risk; distortion; limited to small areas |
| Furnace cooling (controlled) | 0.5–5°C/s | Thick overlays; large components | Uniform cooling; low stress | Slower process; requires furnace capacity |
| Interpass cooling with ice/liquid nitrogen | 20–80°C/s | Critical applications requiring high hardness | Rapid, localized cooling | Specialized equipment; safety considerations |
5. Applicable Standards and Acceptance Criteria
5.1 Relevant Standards
- ASTM A240: Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip (base material specification).
- ASTM A514: Standard Specification for High-Yield-Strength Quenched and Tempered Alloy Steel Plate (substrate specification for high-strength applications).
- ASME Section IX: Welding, Brazing, and Fusing Qualifications — governs WPS qualification and PQS (Procedure Qualification Record) requirements.
- ASME Section II, Part D: Specifications for Welding Consumables — filler metal qualification requirements.
- GB/T 13912: Technical requirements for hot-dip galvanizing (relevant for post-overlay corrosion protection).
- GB/T 985.1: Hardness testing methods for welds — defines test methodology for overlay hardness verification.
- GB/T 11345: Ultrasonic testing of welds — NDT acceptance criteria for overlay integrity.
- GB/T 3323: Radiographic testing of welds — volumetric defect detection in overlay layers.
- ASTM E92: Standard Test Method for Rockwell Hardness of Metallic Materials.
- ASTM E18: Standard Test Method for Rockwell Hardness — alternative hardness verification method.
- ISO 6508: Metallic materials — Vickers hardness test.
- API 5L: Specification for Line Pipe — relevant for clad pipe applications in oil and gas.
- NACE MR0175/ISO 15156: Materials for use in H2S-containing environments — material and welding requirements for sour service.
- GB/T 12467: Welding consumables — hard-facing electrodes classification and requirements.
- NB/T 47014: Qualification test methods for welding procedures for pressure vessels.
5.2 Acceptance Criteria for Wear-Resistant Overlay Layers
| Parameter | Typical Acceptance Criteria | Test Method | Standard Reference |
|---|---|---|---|
| Overlay hardness | ≥ specified minimum (typically 450–850 HV per customer spec) | Vickers or Rockwell C | ASTM E18 / ASTM E92 / GB/T 985.1 |
| Hardness uniformity | Maximum variation ≤ ±10% across overlay surface | Grid hardness mapping (5×5 minimum) | Customer specification |
| Overlay thickness | Within ±0.5 mm of nominal (or per drawing tolerance) | Ultrasonic thickness measurement | GB/T 11345 |
| Weld defects | No cracks, porosity >0.5 mm, or incomplete fusion | RT / UT / PT / MT | GB/T 3323 / GB/T 11345 |
| Interface bonding | Full fusion; no delamination at substrate-overlay interface | UT or destructive sectioning | ASME Section IX |
| Tensile strength (if applicable) | ≥ 90% of filler metal minimum specified tensile strength | Tensile coupon test | ASME Section IX |
| Wear resistance (if specified) | ≥ specified minimum per ASTM G99 or equivalent | Abrasive wear test | ASTM G99 / ASTM G65 |
6. Common Risks and Control Measures
6.1 Risk Identification
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Cracking in overlay layer | Excessive cooling rate in high-carbon/high-alloy fillers | Component rejection; safety hazard | Control interpass temperature; use appropriate preheat; select lower carbon filler | Insufficient hardness | Too slow cooling rate; excessive preheat; thick deposits | Product failure in service; customer complaint | Implement controlled cooling; reduce interpass temperature; use multi-pass strategy |
| Hardness non-uniformity | Inconsistent cooling across deposit; thermal mass variation | Localized wear failure; uneven service life | Standardize welding sequence; monitor and control interpass temperature uniformly |
| Interface delamination | Thermal stress from differential cooling between substrate and overlay | Catastrophic overlay failure in service | Use transition layers; control preheat; apply gradual cooling |
| Excessive dilution | High heat input; poor technique; large bead size | Reduced overlay hardness; substrate properties dominate | Use low heat input parameters; optimize travel speed; consider backing plate or transition layer |
6.2 Process Control Protocol
- Pre-weld assessment: Evaluate base material thermal properties, component geometry, and mass to predict cooling behavior.
- WPS development: Establish documented welding parameters including heat input limits, interpass temperature ranges, and cooling method specifications.
- Welder qualification: Ensure welders are qualified per ASME Section IX or NB/T 47014 for the specific overlay process and materials.
- In-process monitoring: Use infrared thermography or contact thermocouples to monitor interpass temperatures in real-time.
- Post-weld cooling control: Implement the specified cooling method (controlled air, furnace, or active cooling) as documented in the WPS.
- Non-destructive examination: Perform UT, PT, and/or MT inspection per the applicable standard before hardness testing.
- Hardness verification: Conduct grid hardness testing on the completed overlay surface and document results against acceptance criteria.
- Corrective action: If hardness is below specification, implement rework procedures (additional overlay passes with controlled cooling) rather than post-weld heat treatment which may compromise hardness.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Applications
The cooling rate knowledge is most directly applicable to the TIG/MIG weld overlay route, which constitutes the primary production method for wear-resistant cladding at Cladding Technology Shanxi Co., Ltd. Specific applications include:
- Miner and crusher components: Hardfacing overlay of 12–20 mm thickness on jaw crusher plates, cone liners, and impact crusher hammers. Cooling rate control ensures hardness of 600–800 HV with minimal cracking in the thick overlay deposits.
- Cement industry components: Overlay of grinding mill liners, selection plates, and bucket elevator components. Moderate cooling rates (10–30°C/s) achieve the target 500–650 HV hardness while maintaining acceptable toughness.
- Power generation components: Wear-resistant overlay on coal handling equipment, fan blades, and boiler components. Cooling rate management ensures consistent hardness across large, thin-walled components where thermal mass is low.
- Chemical processing components: Corrosion-wear resistant overlay on pumps, valves, and heat exchanger tubes. Cooling rate control balances hardness requirements with the need for adequate corrosion resistance (avoiding excessive retained austenite that may be susceptible to pitting).
7.2 Hydraulic Explosive Bonding Applications
In hydraulic explosive bonding, the cooling rate concept applies differently but remains relevant:
- Post-bonding thermal effects: The high-pressure impact event in hydraulic explosive bonding generates localized heating at the bonding interface. The subsequent cooling rate of the bonded interface affects the microstructure of the bond line, including grain refinement and phase stability.
- Subsequent overlay on bonded substrates: When a hydraulically bonded clad plate undergoes additional weld overlay for increased thickness or specialized surface properties, the cooling rate of the overlay deposit is influenced by the thermal properties of the bonded assembly (dual-material thermal conductivity).
- Post-bonding stress relief: Controlled cooling after the bonding event helps manage residual stresses in the bonded assembly, preventing delayed cracking or distortion.
7.3 Explosion Welding Applications
In conventional explosion welding, cooling rate considerations are integral to the process:
- Interface microstructure: The explosive bonding event creates a high-temperature, high-strain-rate environment at the interface. The cooling rate immediately following detonation determines the microstructural evolution of the bond line, including the formation of intermetallic compounds in dissimilar metal bonds.
- Multi-layer explosion welding: When producing multi-layer clad plates (e.g., stainless steel/carbon steel/stainless steel), the cooling rate between explosion events affects the thermal state of previously bonded layers, influencing bonding quality in subsequent layers.
- Post-explosion stress relief: Controlled cooling of the explosion-welded assembly prevents the development of excessive residual stresses that could compromise long-term structural integrity.
- Integration with weld overlay: Explosion-welded clad plates frequently undergo additional weld overlay (e.g., hardfacing on the exposed cladding surface). The cooling rate of this secondary overlay must account for the composite thermal properties of the explosion-welded base.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Demonstrated mastery of cooling rate control is essential for:
- WPS/PQR qualification: ASME Section IX and NB/T 47014 require that welding procedure qualifications demonstrate adequate understanding of essential variables. Cooling rate, controlled through interpass temperature and heat input parameters, is a qualifying variable that must be documented and controlled.
- Customer-specific qualification: Major customers (particularly in mining, oil and gas, and power generation) require demonstrated process knowledge during supplier qualification audits. Technical documentation of cooling rate control procedures provides evidence of process competence.
- ISO 9001 / ISO 3834 compliance: The quality management system requires documented control of process parameters. Cooling rate control protocols fulfill requirements for process monitoring and measurement.
- API Q1 / API Q2 qualification: For oil and gas industry customers, API quality management system requirements mandate documented control of all process parameters affecting product performance, including welding thermal cycles.
8.2 Product Delivery Enhancement
- Reduced rework: Systematic cooling rate control reduces the incidence of hardness non-conformance, directly decreasing rework rates and improving on-time delivery performance.
- Expanded capability range: Understanding of cooling rate effects enables the company to accept orders for a wider range of hardness specifications (from 400 HV to 900 HV) by selecting appropriate cooling strategies.
- Consistent quality: Standardized cooling rate protocols ensure batch-to-batch consistency in overlay hardness, meeting customer requirements for uniform performance across production runs.
- Thick overlay capability: Knowledge of cooling rate effects in multi-pass deposits enables reliable production of thick overlay layers (up to 25 mm) with uniform hardness, a capability that commands premium pricing.
8.3 Customer Value Delivery
- Extended component life: Optimized cooling rate produces the maximum achievable hardness and wear resistance, directly extending service life of customer components by 2–5× compared to baseline.
- Reduced total cost of ownership: Longer component life translates to fewer shutdowns, reduced replacement frequency, and lower operational costs for the customer.
- Technical consulting value: The company can provide customers with cooling rate optimization recommendations for their specific application, adding engineering value beyond simple fabrication.
- Risk mitigation: Controlled cooling rate reduces cracking risk, providing customers with confidence in the structural integrity and safety of clad components.
9. Practical Implementation Guidelines
9.1 Recommended Cooling Rate Targets by Application
| Application | Target Hardness (HV) | Recommended Cooling Rate (°C/s) | Interpass Temperature (°C) | Filler Metal Example |
|---|---|---|---|---|
| Crusher jaw plates | 600–750 | 15–40 | 80–120 | 12Cr1MoV equivalent |
| Ball mill liners | 500–650 | 10–25 | 100–150 | Cr-Mo-B alloy |
| Coal handling components | 450–600 | 8–20 | 100–180 | Cr-Mo medium carbon |
| High-wear mining parts | 700–900 | 30–80 | 50–100 | High-Cr (26Cr) or high-carbon |
| Corrosion-wear components | 400–550 | 5–15 | 150–250 | Stainless steel (309L/310L base) |
9.2 Cooling Rate Monitoring Methodology
- Thermocouple placement: Install K-type or N-type thermocouples at representative locations on the overlay surface (minimum 3 points per component).
- Temperature recording: Record temperature vs. time data at 1-second intervals during the cooling phase following the final overlay pass.
- Cooling rate calculation: Determine cooling rate as the negative slope of the temperature-time curve in the transformation range (800°C to 500°C for most overlay materials).
- Hardness correlation: Establish empirical correlation between measured cooling rate and achieved hardness for the specific filler material and base combination.
- Process adjustment: Use cooling rate-hardness correlation to adjust interpass temperature, welding parameters, or cooling method to achieve target hardness.
10. Conclusion
The influence of post-weld overlay cooling rate on wear resistance represents a fundamental metallurgical principle with direct, quantifiable impact on product performance. For Cladding Technology Shanxi Co., Ltd., systematic mastery of cooling rate control enables the production of high-quality, consistent, and specification-compliant wear-resistant clad products across all technology routes. This technical competency directly supports WPS qualification, reduces manufacturing defects, expands the range of achievable hardness specifications, and delivers measurable value to customers through extended component service life and reduced total cost of ownership.
Implementation of structured cooling rate control protocols — including documented WPS parameters, in-process temperature monitoring, post-weld cooling method standardization, and hardness verification — transforms this metallurgical knowledge into a repeatable manufacturing capability that provides competitive differentiation in the wear-resistant cladding market.