Interpass Temperature Control for Duplex Stainless Steel Cladding to Prevent Brittle Phase Precipitation

Overview of the Technical Challenge

Duplex stainless steels such as UNS S31803 (2205) and UNS S32750 (2507) are widely specified in aggressive chloride environments, hydrogenation reactors, and offshore platforms due to their balanced ferrite-austenite microstructure, which provides both high yield strength and superior pitting resistance. However, the very microstructural features that make these alloys valuable also make them extremely sensitive to thermal exposure during cladding or weld overlay operations. When the interpass temperature exceeds approximately 150 °C, the prolonged residence in the critical temperature range of 600–900 °C during subsequent welding passes accelerates the precipitation of sigma phase (σ, FeCr intermetallic) and chromium nitrides (Cr2N, CrN). These brittle phases deplete the surrounding matrix of chromium and nitrogen, causing a simultaneous and severe degradation of both corrosion resistance and mechanical toughness. In engineering practice, this manifests as intergranular pitting, reduced impact energy, and premature cracking in service — failures that can compromise the integrity of pressure vessels and heat exchangers operating under harsh chemical conditions.

Mechanism of Brittle Phase Formation and the Critical Temperature Window

The precipitation kinetics of sigma phase in duplex stainless steels follow the C-curve model, where the nose of the curve typically lies between 700 and 850 °C. Although the weld pool itself reaches far higher temperatures, the heat-affected zone (HAZ) and previously deposited overlay layers spend significant time in the precipitation-sensitive range during the cooling cycle between passes. When the interpass temperature is allowed to rise above 150 °C, the cumulative time at elevated temperature increases dramatically, shifting the microstructure toward the sigma-phase-forming region. For 2205, the recommended ferrite content is 40–60% (FE%); thermal excursions can reduce the effective ferrite stability and promote sigma formation preferentially at ferrite-austenite boundaries. For 2507, which contains higher nitrogen (3.0–3.5%), chromium nitrides form more readily, further destabilizing the phase balance.

The following table summarizes the critical parameters governing sigma phase formation in duplex stainless steel cladding:

Process Strategy: Low Heat Input Combined with Rapid Cooling

The engineering solution to prevent brittle phase precipitation is a dual-pronged approach: maintaining low interpass temperatures and minimizing the heat input per pass to limit the thermal cycle severity. Low heat input reduces the peak temperature and narrows the HAZ, thereby reducing the volume of material exposed to the precipitation-sensitive range. Rapid cooling, achieved through air cooling, water-spray quenching (where permitted), or the use of chill plates, further compresses the time spent in the critical temperature window.

In practice, the following process parameters are typically employed for duplex stainless steel weld overlay:

A critical observation from field experience is that the interpass temperature limit of 150 °C is not merely a guideline but a mandatory requirement for both 2205 and 2507 cladding. Exceeding this limit, even briefly, can initiate irreversible sigma-phase precipitation that cannot be fully reversed by post-weld solution heat treatment, particularly in thick-section clad plates or multi-pass overlay builds. The welder must be trained to use infrared pyrometers or contact thermocouples to monitor the interpass temperature continuously, and the welding procedure specification (WPS) must explicitly state the 150 °C ceiling as a stop-weld criterion.

Defect Analysis and Quality Assurance

When interpass temperature control is inadequate, several defects become prevalent. Sigma phase precipitation can be detected through metallographic examination using selective etchants such as glycerol-based reagents that reveal sigma phase as dark particles at grain boundaries. Intergranular corrosion testing per ASTM G48 Method B (65 °C, 10% HClO4) will reveal preferential grain boundary attack if sigma phase is present. The impact test results will show a significant drop in absorbed energy, often below the 47 J threshold specified in NB/T 47002 and ASME VIII Div.1.

From a quality control perspective, the following checks are essential:

• Visual inspection for weld bead profile and interpass cleaning.
• Metallographic examination for ferrite content (ASTM E1245) and sigma phase presence.
• Intergranular corrosion testing per ASTM G48.
• Charpy V-notch impact testing at -40 °C (or the service temperature).
• Hardness mapping across the overlay thickness to detect localized embrittlement.

Integration with Engineering Practice

In the fabrication of clad-plate pressure vessels per GB/T 150 or NB/T 47002, the overlay welding of duplex stainless steel cladding on carbon steel or low-alloy steel backing plates is a common configuration. The thermal mismatch between the base material and the overlay material creates additional challenges: the carbon steel base plate absorbs heat rapidly, but the low thermal conductivity of the duplex overlay layer causes heat to accumulate at the interface. This makes interpass temperature monitoring even more critical at the first overlay pass, where the base metal can act as a heat sink initially but then as a heat reservoir once the overlay layer thickens.

A practical approach adopted in several fabrication shops involves the use of sacrificial copper chill plates attached to the back of the base plate to promote rapid heat extraction. Additionally, the sequence of welding passes should be planned to minimize the time between passes — welding in short, continuous sequences with minimal interruption reduces the risk of the interpass temperature drifting upward due to ambient heat or residual heat from adjacent operations.

Key Reflections and Study Insights

The most important lesson from studying duplex stainless steel interpass temperature control is the recognition that the 150 °C limit is not arbitrary but is rooted in the fundamental precipitation kinetics of the sigma phase and chromium nitrides. Engineers must understand that this limit applies regardless of the welding process — whether SAW, GMAW, or FCAW — and regardless of the overlay thickness. The temptation to increase interpass temperature to improve deposition efficiency or reduce distortion must be firmly resisted, as the consequences of brittle phase formation are catastrophic and irreversible in service.

Furthermore, the training of welders on this topic requires more than simply stating a temperature limit. Welders must understand the metallurgical consequences of exceeding the limit, so that they appreciate the importance of the stop-weld-and-cool protocol. This understanding can be reinforced through micrograph comparisons showing the difference between a properly controlled weld and one with sigma phase precipitation. The cost of rework — including grinding out a thick overlay build and restarting — far exceeds the marginal time saved by allowing a higher interpass temperature.

In conclusion, interpass temperature control at or below 150 °C is the single most critical process parameter for preserving the corrosion resistance and toughness of duplex stainless steel weld overlay. It is a non-negotiable requirement for 2205 and 2507 cladding, and its enforcement requires a combination of rigorous WPS specification, real-time temperature monitoring, welder training, and post-weld metallurgical verification. Engineers who internalize this principle will produce reliable, long-life clad components that perform as intended in their intended service environments.