Process Test Research on 90mm Wide Band Double-Layer Electroslag Welding Overlay of Stainless Steel

Literature Overview

This study presents a comprehensive process development investigation for producing 90 mm wide stainless steel weld overlay layers using double-layer electroslag welding (ESW) technology. The research addresses the practical challenge of achieving wide, uniform overlay deposits on large-diameter pressure vessel shells and thick-walled components where conventional strip cladding or multi-pass SAW overlay would be economically impractical. The work encompasses systematic parametric studies on welding current, travel speed, slag composition, and electrode configuration, with extensive metallographic, mechanical, and corrosion testing of the resulting overlay layers.

Core Technical Content

Process Configuration and Parameters

The 90 mm wide double-layer ESW overlay process employs a specially designed consumable electrode assembly consisting of two parallel stainless steel electrode strips, each approximately 45 mm wide, separated by a refractory spacer to maintain a defined gap. The process produces two adjacent overlay tracks in a single pass, effectively doubling the deposition width compared to single-strip ESW. The following table presents the optimised process parameters:

The slag composition was carefully formulated to provide adequate fluidity at the welding temperature while maintaining sufficient viscosity to contain the molten pool and prevent excessive metal loss through the slag. The basicity of 2.5-3.0 was selected to promote stable slag-metal reaction, effective deoxidation, and adequate inclusion flotation.

Microstructural Characteristics

Metallographic examination of the optimised overlay layers revealed a columnar grain structure growing from the fusion boundary into the weld metal, with grain width increasing from approximately 80-120 micrometres near the base metal interface to 200-350 micrometres at the surface. The double-layer configuration produced a characteristic microstructural profile where the first layer developed slightly coarser grains due to the higher cooling rate associated with the initial base metal thermal mass, while the second layer exhibited somewhat finer grains owing to the preheating effect of the still-warm first layer.

The fusion boundary between the carbon steel base plate and the stainless steel overlay layer showed a narrow dilution zone of approximately 0.5-1.2 mm into the base metal. Chemical analysis of this dilution zone indicated chromium content transitioning from the base plate level (approximately 0.2%) to the overlay specification (18-20%) over a distance of 0.8 mm, with the carbon content dropping from 0.17% to below 0.08%. This dilution zone is critical for corrosion resistance assessment, as the chromium content must exceed approximately 12% to provide adequate passivation.

Mechanical and Corrosion Performance

The mechanical properties of the double-layer overlay deposits met or exceeded the requirements specified in NB/T 47002 and ASTM A263:

The bond strength testing, conducted per ASTM A263 using a specially designed tensile specimen with the load applied parallel to the fusion line, demonstrated robust metallurgical bonding between the overlay layer and the base plate. The intergranular corrosion testing confirmed that the overlay material retained full austenitic stability without sensitisation-induced chromium depletion at grain boundaries, validating the PWHT procedure.

Process Development Challenges and Solutions

Slag Stability and Metal Transfer

One of the principal challenges in wide-band ESW overlay is maintaining stable slag-metal interaction across the full width of the electrode assembly. The study identified that the central region between the two electrode strips experienced reduced shielding effectiveness, leading to localised oxidation and increased inclusion content. This was addressed by incorporating a third, narrower filler strip (15 mm wide) positioned in the gap between the two main electrodes, which simultaneously improved slag coverage and provided additional metal deposition in the centre region.

The slag viscosity was found to be highly sensitive to the CaF2 content, with values below 8% leading to excessive metal loss through the slag, while values above 15% caused poor slag fluidity and incomplete fusion at the fusion boundary. The optimised slag composition contained 52-58% CaO, 18-22% SiO2, 8-12% CaF2, and 12-16% Al2O3, providing an optimal balance of fluidity, basicity, and thermal stability.

Thermal Management and Residual Stress Control

The double-layer configuration introduced complex thermal cycling effects that required careful management. The first layer, deposited on the cold base plate, experienced a peak cooling rate of approximately 8-12 degC/s at the fusion boundary, while the second layer, deposited on the still-warm first layer, experienced a reduced cooling rate of 4-6 degC/s. This differential cooling resulted in asymmetric residual stress distributions, with the first layer exhibiting higher tensile residual stresses (approximately 180-220 MPa) compared to the second layer (approximately 120-160 MPa).

To mitigate residual stress effects, the study evaluated several post-weld treatment options. Full PWHT at 1050 degrees Celsius for 2 hours per 25 mm of thickness, followed by slow cooling in the furnace to below 300 degrees Celsius, effectively reduced residual stresses to below 50 MPa. However, this treatment caused significant grain coarsening in the overlay layer. An alternative approach using low-temperature stress relief at 450-500 degrees Celsius for 4 hours achieved approximately 60% stress reduction with minimal microstructural change, and was recommended as the preferred post-weld treatment for production applications.

Engineering Practice Integration

Application to Large-Diameter Pressure Vessel Shells

The 90 mm wide double-layer ESW overlay process is particularly advantageous for fabricating large-diameter pressure vessel shells where the overlay width requirement exceeds the practical limits of single-strip ESW (typically 45-60 mm). For a hydrogenation reactor shell with an internal diameter of 3.5 meters and a required overlay coverage of the full internal surface, the 90 mm wide process reduces the number of overlay passes by approximately 40% compared to single-strip ESW, translating into significant cost savings in electrode consumption, welding hours, and production cycle time.

The process is also well-suited for overlaying thick-walled components (wall thickness exceeding 80 mm) where the high deposition rate of ESW (approximately 15-25 kg/h for the double-layer configuration) provides a substantial productivity advantage over GTAW or GMAW overlay methods.

Quality Assurance Considerations

The following quality assurance measures are recommended for production implementation:

1. Pre-weld slag qualification testing to verify fluidity, basicity, and deoxidation capacity
2. In-process monitoring of arc voltage and current stability with automated alarm for parameter deviation exceeding ±10%
3. Visual inspection of the slag surface for indications of metal loss or excessive oxidation
4. Ultrasonic testing (UT) of the fusion boundary using dual-probe technique at 2.5 MHz to detect lack of fusion and delamination
5. Hardness mapping across the full overlay width to verify uniformity and absence of localised hardening
6. Spot corrosion testing on production coupons taken from each heat of overlay material

Study Insights and Conclusions

The 90 mm wide double-layer ESW overlay process represents a significant advancement in overlay welding technology for large-scale pressure vessel fabrication. The systematic parametric optimisation presented in this study provides a reliable process window that can be directly transferred to production environments. The key engineering insight is that the double-layer configuration not only doubles the effective overlay width but also creates a beneficial thermal interaction between the two layers that moderates cooling rates and improves microstructural quality. However, the process requires careful attention to slag stability, thermal management, and post-weld treatment to achieve consistent quality across the full overlay width. Engineers implementing this process should invest in thorough procedure qualification and operator training, as the process sensitivity to parameter variation is greater than single-strip ESW due to the complex interaction between the two electrode assemblies.