Interpass Time Control and Its Effect on Weld Overlay Microstructure

1. Definition and Fundamental Principles

Interpass time (also referred to as dwell time or heat interval) is defined as the elapsed time between the completion of one weld pass and the initiation of the next successive pass during multi-pass weld overlay operations. In the context of bimetallic cladding and weld overlay manufacturing, interpass time is not merely a scheduling parameter—it is a critical metallurgical variable that governs the thermal cycling history experienced by both the weld metal and the adjacent base metal. The governing principle is straightforward yet profoundly impactful: the temperature of the previously deposited weld metal at the moment the next pass is initiated determines the cooling rate, grain growth behavior, phase transformations, and ultimately the microstructural integrity of the entire overlay build-up.

When interpass time is too short, the previously deposited layers retain high residual temperatures, resulting in elevated starting temperatures for subsequent passes. This leads to slower cooling rates, coarser grain structures, potential grain boundary carbide precipitation (particularly in stainless steel and nickel-based overlay systems), and reduced hardness. Conversely, when interpass time is excessively long, the base metal and prior weld layers cool to near-ambient temperatures before the next pass is applied. This creates steep thermal gradients at the interface between the new weld bead and the previously deposited material, potentially inducing high residual stresses, microcracking, and increased susceptibility to hydrogen-induced cracking in susceptible materials.

The microstructural evolution governed by interpass time can be understood through the following thermal-metallurgical relationships:

• Low interpass time (short dwell): High heat input accumulation → coarse dendritic structures, potential sensitization in austenitic stainless steels (Cr₂₃C₆ precipitation along grain boundaries), reduced hardness in martensitic overlay systems due to incomplete transformation
• Optimal interpass time: Balanced thermal cycling → refined grain structures, controlled phase distribution, appropriate hardness levels, minimal residual stress
• Excessive interpass time (long dwell): Cold start conditions → high thermal gradients, elevated residual stresses, risk of hot cracking at solidification front, potential for cold cracking in high-carbon or high-strength base metals

2. Category and Business Positioning

This technical competency falls squarely within the process engineering and metallurgical qualification domain of Cladding Technology Shanxi Co., Ltd. It represents the company's capability to deliver scientifically validated weld overlay procedures rather than relying solely on empirical field experience. In the broader industry landscape, interpass time control is a differentiating factor between commodity weld overlay services and premium, specification-driven cladding solutions.

From a business positioning perspective, mastery of interpass time effects enables the company to:

• Qualify weld overlay procedures (WPS/PQR) with documented metallurgical justification rather than trial-and-error approaches
• Provide customers with traceable, standards-compliant overlay deposits that demonstrate consistent microstructural quality
• Reduce rework rates and warranty claims by eliminating microstructure-related failures in field service
• Support certification audits by demonstrating systematic process understanding aligned with quality management requirements

3. Technical Purpose and Value

3.1 Ensuring Microstructural Integrity

The primary technical purpose of interpass time control is to achieve a target microstructure in the overlay weld deposit that satisfies the functional requirements of the service application. For example, a 309L transition layer followed by a 310 hard-facing overlay on a carbon steel pipe requires specific interpass times to ensure:

• The 309L transition layer achieves a fully austenitic microstructure without δ-ferrite excess
• The 310 hard-facing layer develops the desired eutectic microstructure with controlled carbide morphology
• The interface between layers exhibits sound metallurgical bonding without hot cracking

3.2 Residual Stress Management

Controlled interpass timing allows for thermal stress relief through the natural thermal cycling of successive passes. Properly managed heat input accumulation creates a tempering effect on the HAZ of previously deposited layers, reducing hardness peaks and residual stress concentrations. This is particularly critical in overlay applications where the final component will be subjected to cyclic thermal loading, such as in boiler tubes, heat exchanger tubesheets, and chemical reactor internals.

3.3 Productivity Optimization

While metallurgical control is paramount, interpass time also directly impacts manufacturing productivity. Excessive dwell times between passes reduce deposition rate and increase labor costs. The optimization challenge is to identify the minimum interpass time that still achieves the required microstructure and mechanical properties, thereby maximizing throughput without compromising quality.

4. Key Process Parameters and Implementation Points

4.1 Critical Parameters Influencing Interpass Time

Parameter	Typical Range	Effect on Microstructure	Control Method
Interpass Temperature	150°C – 350°C (stainless steel overlays)	Controls cooling rate and grain growth	Infrared pyrometer or thermocouple monitoring
Interpass Temperature	50°C – 150°C (nickel-based overlays)	Prevents grain coarsening in Ni-Cr systems	Surface temperature measurement
Heat Input per Pass	0.8 – 2.5 kJ/mm (TIG overlay)	Determines thermal accumulation rate	Welding parameter control (current, voltage, speed)
Number of Successive Passes	2 – 8 (typical overlay build)	Cumulative heat input affects final microstructure	WPS specification
Base Material Thermal Conductivity	Carbon steel: 45-50 W/m·K; SS: 15-16 W/m·K	Higher conductivity materials dissipate heat faster, reducing interpass temperature	Material-specific WPS adjustment
Ambient Temperature	5°C – 40°C	Affects baseline cooling rate and required interpass time	Environmental monitoring; winter/summer WPS variants

4.2 Implementation Protocol

The following systematic approach should be adopted for interpass time management in production weld overlay operations:

1. Pre-Weld Planning: Establish target interpass temperature range based on WPS requirements, base material specification, and overlay alloy system. Reference applicable codes (ASME Section IX, AWS D10.9) for maximum allowable interpass temperatures.
2. Instrumentation: Equip each welding station with calibrated infrared pyrometers (range: 100°C–600°C) or embed thermocouples in test coupons during PQR execution. For production, IR guns with data logging capability are recommended.
3. Monitoring Frequency: Measure interpass temperature before initiating every subsequent pass after the first two (first two passes typically establish thermal equilibrium).
4. Decision Thresholds: If interpass temperature exceeds the WPS maximum, allow additional cooling time before proceeding. If temperature drops below the WPS minimum (indicating excessive dwell), apply preheat or proceed immediately.
5. Documentation: Record all interpass temperature readings in the weld log, including timestamp, measured temperature, and action taken (proceed/hold).

4.3 Microstructural Outcomes by Interpass Condition

Interpass Condition	Starting Temperature for Next Pass	Microstructural Result	Mechanical Property Impact	Risk Level
Too Short	>350°C (SS overlay)	Coarse austenite grains, sensitization (Cr carbide precipitation), possible grain boundary embrittlement	Reduced intergranular corrosion resistance, lower hardness in martensitic systems	High
Optimal	150°C – 250°C (SS overlay)	Fine-to-medium grain structure, controlled δ-ferrite content, uniform carbide distribution	Full design hardness, excellent corrosion resistance, low residual stress	Low
Moderately Long	50°C – 150°C	Fine grain structure, potentially excessive cooling rate	High hardness (may exceed specifications), elevated residual stress	Medium
Excessive	<50°C (near ambient)	Very fine grains, high thermal gradient at weld boundary, potential for microcracking	High residual stress, risk of cold cracking, possible HAZ embrittlement	High

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

• ASME Section IX: QW-301 through QW-306 govern interpass temperature requirements for weld overlay qualification. Maximum interpass temperature is typically specified in the WPS and must be maintained throughout PQR execution.
• AWS D10.9M/D10.9: Standard for Weld Overlaying of Steel—provides guidelines for interpass temperature control in overlay applications, including recommendations for specific alloy systems.
• GB/T 985.1: Chinese national standard for welder qualification—references interpass temperature as a qualifying variable.
• GB/T 3323: Radiographic testing acceptance criteria for weld overlay joints—indirectly affected by microstructure quality governed by interpass time.
• ASTM A743/A744: Cast stainless steel specifications—microstructural requirements for overlay deposits on cast components.
• API 579/ASME FFS-1: Fitness-for-Service evaluation—residual stress levels (influenced by interpass time) are a key input parameter.
• ISO 15614-1: Qualification testing of welding procedures for metallic materials—requires documentation of interpass temperature as a process variable.
• NACE MR0175/ISO 15156: For oil and gas applications—microstructural requirements for overlay deposits in sulfide stress cracking environments.

5.2 Acceptance Criteria

Interpass time control acceptance is verified through a combination of process documentation and post-weld metallurgical examination:

• Process Documentation: 100% compliance with WPS-specified interpass temperature range, verified through continuous temperature monitoring records.
• Hardness Testing: Overlay deposit hardness within specified range (e.g., 309L: 150-220 HBW; 310 hard-facing: 400-500 HV) as per AWS D10.9 or project specification.
• Macrograph Examination: Cross-sectional macrograph showing uniform weld bead geometry, no hot cracking, proper layer-to-layer fusion, and absence of lack of fusion at interpass boundaries.
• Micrograph Examination: Grain size within acceptable limits (ASTM E112 grain size determination), no sensitization (ASTM A262 Practice E intergranular corrosion test), controlled carbide distribution.
• Residual Stress Measurement: X-ray diffraction or hole-drilling method confirming residual stresses within acceptable limits (typically <100 MPa for critical applications).

6. Common Risks and Controls

6.1 Risk Matrix

Risk	Cause	Consequence	Control Measure
Sensitization of austenitic overlay	Interpass temperature exceeding 350°C; excessive heat accumulation	Intergranular corrosion failure in service; rejection per ASTM A262	IR temperature monitoring with alarm at WPS limit; WPS revision for high-deposition-rate sequences
Hot cracking at interpass boundaries	Excessive interpass time creating cold-start conditions with high thermal gradient	Surface cracks, lack of fusion, overlay spallation	Minimum interpass temperature enforcement; preheat application; controlled ramp-up of subsequent passes
Excessive hardness in martensitic overlay	Interpass time too long, resulting in high cooling rate	Brittleness, susceptibility to cracking under thermal cycling	Maximum interpass time specification; post-weld heat treatment (PWHT) per AWS D10.9
Welding operator non-compliance	Lack of real-time temperature feedback; reliance on subjective estimation	Inconsistent microstructure across production batch	Automated temperature monitoring systems; mandatory weld log completion; periodic audit of temperature records
Environmental variability	Seasonal ambient temperature changes affecting cooling rates	Summer: shorter effective interpass time; Winter: longer effective interpass time	Seasonal WPS supplements; wind sheltering for outdoor operations; heated enclosures for cold-weather welding

6.2 Corrective Actions

When interpass temperature deviations are detected during production:

1. Immediate: Halt welding if temperature exceeds maximum allowable limit. Allow cooling to specified range before resuming. Document deviation in weld log.
2. Post-Weld: If deviation occurred during PQR execution, conduct additional metallurgical examination (hardness survey, macrograph, intergranular corrosion test) to verify deposit quality before approving the procedure.
3. Systemic: If repeated deviations occur, conduct root cause analysis (5-Why or fishbone diagram), revise WPS interpass temperature parameters, retrain operators, and implement automated monitoring upgrades.

7. Application Across Company Technology Routes

7.1 TIG Weld Overlay (GTAW Overlay)

In TIG weld overlay—the company's primary technology route for precision overlay on piping, valves, and small-diameter components—interpass time control is particularly critical due to the relatively high heat input concentration and the common use of austenitic and nickel-based overlay alloys. TIG overlay typically involves 2-4 passes per build, and each subsequent pass directly affects the microstructure of the previous layer.

Key Implementation Considerations for TIG:

• Due to the focused arc and relatively slow travel speeds (50-150 mm/min), interpass temperatures rise rapidly. For a 309L transition layer on carbon steel, the interpass temperature can reach 300-400°C within 1-2 minutes of completing the first pass. Real-time IR monitoring is essential.
• For multi-layer hard-facing overlays (e.g., CoCr-based on 309L transition), the interpass temperature for the hard-facing passes should be controlled to 100-200°C to maintain carbide integrity and avoid excessive grain growth in the cobalt matrix.
• WPS qualification per ASME Section IX requires that the interpass temperature range used during PQR be maintained during production. The company's technical learning on interpass time effects directly supports WPS qualification and production consistency.

7.2 MIG Weld Overlay (GMAW Overlay)

MIG weld overlay is employed for higher-deposition-rate applications, including large-diameter pipe cladding and thick-section equipment. The higher wire feed rates and gas shielding characteristics of MIG create different thermal accumulation patterns compared to TIG, requiring distinct interpass time management strategies.

Key Implementation Considerations for MIG:

• MIG overlay typically deposits 3-6 kg/h of weld metal, creating significant thermal mass. Interpass temperatures may remain elevated for longer periods, requiring extended cooling times before subsequent passes.
• For stainless steel overlay on carbon steel using 309L/316L wire, the interpass temperature should be maintained below 250°C to prevent sensitization. The high deposition rate means that 3-5 minutes of natural cooling may be insufficient; forced air cooling or extended dwell may be necessary.
• Interpass time effects on MIG overlay are particularly relevant for build-up welding of heavily worn components (e.g., pump impellers, valve seats) where multiple passes are required to achieve the specified overlay thickness.

7.3 Hydraulic Explosive Bonding and Explosion Welding

While hydraulic explosive bonding and explosion welding do not involve traditional multi-pass welding sequences, the principles of thermal cycling and microstructural evolution remain relevant in the following contexts:

• Post-Weld Heat Treatment: Clad plates and pipes produced by explosion welding often require post-weld heat treatment (PWHT) to relieve residual stresses and temper the cold-worked interface. The interpass time analogy applies here: the thermal profile during PWHT (ramp rate, soak temperature, cooling rate) must be controlled to achieve the desired microstructure at the bond interface, similar to how interpass time controls microstructure in weld overlay.
• Weld Overlay Repair of Explosion-Welded Components: When explosion-welded clad pipes require field repair or end preparation, TIG/MIG weld overlay is used to repair the cladding. The interpass time during this repair welding directly affects the microstructure at the repair zone, which must be compatible with the explosion-welded interface microstructure.
• Hybrid Processes: In some applications, explosion welding is followed by weld overlay of a third layer (e.g., explosion-welded 316L on carbon steel, then TIG overlay of hard-facing alloy). The interpass time during the weld overlay step must account for the thermal mass and thermal conductivity of the underlying explosion-welded layers.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

The systematic understanding of interpass time effects on microstructure directly supports the company's qualification infrastructure:

• WPS/PQR Development: Each qualified welding procedure specification includes documented interpass temperature limits validated through PQR execution with metallurgical examination. This technical knowledge ensures that WPS parameters are scientifically justified rather than arbitrarily set.
• Material Qualification: When qualifying new overlay alloy combinations (e.g., a novel CoCr hard-facing on a specific base material), interpass time studies are conducted as part of the qualification matrix to establish the optimal processing window.
• Audit Readiness: Documentation of interpass time monitoring during PQR and production welds demonstrates compliance with ASME Section IX, AWS D10.9, and ISO 15614-1 requirements during certification audits by TUV, Lloyd's Register, or API.

8.2 Product Delivery

• Consistency: Standardized interpass time protocols ensure that every weld overlay component—regardless of shift, operator, or production batch—exhibits consistent microstructural quality, reducing variability and improving first-pass yield.
• Traceability: Weld logs documenting interpass temperatures provide full traceability from process parameters to final product quality, supporting customer quality audits and failure investigation.
• Productivity: Optimized interpass times (minimum time for required quality) maximize deposition rates and reduce manufacturing cycle time without compromising product integrity.

8.3 Customer Value

• Service Life Extension: Properly controlled microstructures in overlay deposits translate to longer service life in corrosive, erosive, and wear environments. Customers in power generation, petrochemical, and mining sectors benefit from reduced unplanned shutdowns and extended equipment life.
• Specification Compliance: Many end-user specifications (particularly in oil and gas, nuclear, and pharmaceutical industries) require documented interpass temperature control as part of the quality assurance package. The company's technical capability in this area enables compliance with demanding customer requirements.
• Engineering Support: The company can provide customers with technical data packages demonstrating microstructural quality (macrographs, micrographs, hardness surveys, corrosion test results) that validate the overlay deposit's suitability for the intended service conditions.

9. Summary and Recommendations

The effect of interpass time on weld overlay microstructure represents a fundamental process variable that bridges the gap between welding execution and metallurgical outcome. For Cladding Technology Shanxi Co., Ltd., mastery of this parameter is not merely an academic exercise—it is a production-critical competency that directly impacts product quality, qualification integrity, and customer satisfaction.

Recommendations for Continued Development:

1. Implement automated interpass temperature monitoring systems with data logging and alarm functions across all TIG and MIG welding stations.
2. Develop material-specific interpass time databases for all overlay alloy systems in current production, incorporating base material, ambient temperature, and heat input variables.
3. Conduct periodic interpass time sensitivity studies (DOE format) for new alloy combinations to establish optimal processing windows before PQR qualification.
4. Integrate interpass temperature data into the company's quality management system for trend analysis and continuous improvement.
5. Extend interpass time knowledge to the repair and maintenance division, ensuring that field repair welding of cladded components maintains the same metallurgical discipline as shop fabrication.

"The microstructure of a weld overlay deposit is not determined by the welding parameters of the final pass alone—it is the cumulative result of every thermal cycle experienced from the first bead to the last. Interpass time is the variable that connects these cycles, and its control is the difference between a functional overlay and a metallurgically sound one." — Principle of Weld Overlay Metallurgy