CLADDING TECHNOLOGY SHANXI CO., LTDInterpass Temperature Control for Austenitic Stainless Steel Overlay Welding
Overview and Technical Context
Austenitic stainless steel overlay welding is governed by a critical metallurgical constraint: the interpass temperature must remain at or below 150°C to prevent sensitization, chromium carbide precipitation, and excessive thermal distortion. This study note examines the technical rationale, practical control methods, and quality implications of interpass temperature management for 304L, 316L, and similar austenitic stainless steel overlay applications.
Metallurgical Consequences of Excessive Interpass Temperature
When the interpass temperature exceeds 150°C, the following metallurgical degradation mechanisms become active:
| Temperature Range | Degradation Mechanism | Consequence |
|---|---|---|
| 150–400°C | Accelerated chromium carbide (Cr₂₃C₆) precipitation at grain boundaries | Sensitization, reduced pitting resistance |
| 400–600°C | Maximum carbide precipitation rate; grain boundary depletion of chromium | Severe intergranular corrosion susceptibility |
| 600–800°C | Sigma phase formation in Fe-Cr-Ni systems | Embrittlement, loss of ductility |
| >800°C | Recrystallization, grain growth | Reduced mechanical properties, distortion |
For overlay applications, the consequences are particularly severe because the overlay layer is the functional surface. Intergranular corrosion in the overlay layer directly compromises the corrosion protection purpose of the cladding. In chloride-containing environments (seawater, chemical processing, sour gas service), sensitized overlay layers can fail within months rather than the expected decades of service life.
Temperature Control Methods
The following table presents the available methods for interpass temperature control, ranked by effectiveness and practicality:
| Method | Effectiveness | Practicality | Cost | Application |
|---|---|---|---|---|
| Active cooling (compressed air) | High | Moderate | Low | Large flat surfaces, thick sections |
| Active cooling (liquid nitrogen) | Very High | Low | Moderate | Thin sections, rapid cooling required |
| Thermal insulation (heat sinks) | Moderate | High | Very Low | Small sections, thin plates |
| Process interruption (waiting) | Low | High | None | Small workpieces, non-critical schedules |
| Parameter adjustment (lower heat input) | Moderate | High | None | All applications, preventive |
| Thermocouple monitoring with alarm | Monitoring only | High | Low | All applications, verification |
The most effective practical approach combines low-heat-input process parameters (lower current, higher travel speed, smaller wire diameter) with active cooling between passes. Compressed air directed at the weld zone between passes can reduce the surface temperature by 30–50°C within 1–2 minutes, depending on the section thickness and ambient conditions.
Process Parameter Optimization for Low Interpass Temperature
To minimize interpass temperature accumulation, the following parameter adjustments are recommended:
- Reduce welding current by 10–20% from maximum WPS range
- Increase travel speed by 15–25% to reduce heat input per unit length
- Use smaller wire diameter (0.8–1.0 mm instead of 1.2 mm for GMAW)
- Apply multiple thin layers (2–3 mm per layer) rather than fewer thick layers
- Use back-of-weld cooling plates or chill bars for thin sections
- Implement systematic pass sequencing to distribute heat evenly
For 304L and 316L overlay on carbon steel base plates, the interpass temperature control is especially critical because the base metal acts as a heat sink while the overlay layer accumulates heat. The temperature gradient between the overlay surface and the base metal can exceed 100°C in thick sections, requiring careful thermocouple placement to monitor the actual overlay surface temperature rather than the base metal temperature.
Verification and Quality Assurance
Interpass temperature monitoring should be performed using calibrated thermocouples (Type K or Type N) attached to the overlay surface near the weld zone. The monitoring frequency should be at least once per layer, with continuous monitoring for critical applications. The recorded temperatures must be documented in the welding log and cross-referenced with the WPS requirements.
Post-weld verification includes:
- Intergranular corrosion testing (ASTM A262 Practice E or ASTM G28 Practice A) on witness coupons
- Metallographic examination of grain boundary carbide precipitation
- Hardness profiling across the overlay/base metal interface
- Corrosion potential measurements in simulated service environments
Engineering Practice Considerations
In large-scale overlay fabrication (pressure vessel heads, heat exchanger tubesheets, reactor shells), interpass temperature control becomes a logistical challenge. The large surface area means that heat accumulates over extended welding sequences, and the cooling time between passes may be impractical for production schedules. The solution is to design the welding sequence to minimize heat accumulation—welding in a systematic pattern that moves away from previously welded areas, using multiple operators on different sections, and incorporating scheduled cooling breaks.
For in-situ repair of operating equipment, interpass temperature control is further complicated by residual heat from the base structure and limited access for cooling equipment. In these cases, the welding parameters must be further reduced, and the overlay may require more passes with thinner layers to maintain the temperature constraint.
Study Insights
The 150°C interpass temperature limit for austenitic stainless steel overlay is not an arbitrary specification but a metallurgical threshold below which chromium carbide precipitation is kinetically suppressed. The engineering challenge is not merely knowing this threshold but implementing practical control methods that maintain it throughout the entire overlay sequence—often hundreds of passes on large components. The study reinforces that interpass temperature is one of the most critical but least monitored parameters in overlay welding practice. Many shops monitor current, voltage, and travel speed meticulously while treating interpass temperature as a secondary concern. This is a fundamental quality error: for corrosion-critical overlay applications, interpass temperature control is as important as consumable selection and shielding gas composition. The integration of thermal monitoring into the quality assurance system—alongside the more traditional parameter controls—is essential for ensuring the long-term corrosion performance of austenitic stainless steel overlay layers.