Strip Electrode Wide-Band Submerged Arc Cladding of Stainless Steel on Hydropower Generator Top Cover

Literature Overview

This study, published in Welding Technology (2009, Vol. 38, Issue 10), authored by Li Hua from Toshiba Hydroelectric Equipment (Hangzhou) Co., Ltd., addresses the practical engineering challenge of applying a wear- and corrosion-resistant stainless steel cladding layer onto the top cover of a hydropower generator unit. The base material is ZG20SiMn cast steel, a medium-carbon low-alloy cast steel commonly used in large hydroelectric power equipment housings and covers. The cladding is achieved using a strip electrode wide-band submerged arc welding (SAW) process with H134 stainless steel strip electrode and SJ315 sintered flux. The resulting cladding layer achieves a hardness of HRC 45 with a lath martensite microstructure, satisfying both abrasion resistance and corrosion resistance requirements.

Core Technical Points and Process Parameters

The selection of H134 strip electrode and SJ315 sintered flux is technically significant. H134 is a martensitic stainless steel strip electrode designed for submerged arc welding applications where high hardness and good corrosion resistance are required. The SJ315 sintered flux provides adequate deoxidation, slag protection, and alloying contribution to the weld metal. The critical process parameters identified in this study are as follows:

The use of a strip electrode rather than a wire electrode enables significantly higher deposition rates, which is critical for large-scale cladding operations on bulky components such as generator top covers. The wide-band SAW process allows for broad coverage with fewer passes, reducing production time and cost while maintaining consistent quality across the cladding surface.

Microstructure Analysis and Engineering Significance

The lath martensite microstructure observed in the cladding layer is a direct result of the rapid solidification cooling rate characteristic of submerged arc welding combined with the alloy composition of the H134 electrode. Lath martensite, as opposed to plate martensite, generally offers superior toughness at comparable hardness levels because the lath structure provides more effective crack deflection paths. This is particularly important for the generator top cover application, where the cladding must withstand cyclic mechanical loading from turbine operation, hydraulic pressure fluctuations, and thermal cycling during startup and shutdown sequences.

The interpass temperature control at 100 °C is a carefully chosen compromise. Too low an interpass temperature would result in excessive cooling rates that could promote brittle martensite transformation and increase the risk of cracking in the HAZ. Too high an interpass temperature would reduce the cooling rate, potentially leading to grain coarsening and a decrease in hardness. The 100 °C limit ensures that the weld metal solidifies with a fine lath martensite structure while avoiding the formation of brittle phases in the heat-affected zone of the ZG20SiMn base metal.

Engineering Practice Integration and Reflections

In practice, the wide-band strip electrode SAW process has several advantages over conventional wire electrode SAW for large-area cladding applications. The deposition rate can be 3 to 5 times higher, making it economically viable for large components where multiple layers of cladding are required. However, the process requires precise control of the electrode feed speed, travel speed, and flux coverage to maintain consistent weld geometry and avoid defects such as lack of fusion, porosity, and undercut.

From an FMEA (Failure Mode and Effects Analysis) perspective, the primary failure modes for this cladding process include: cold cracking due to insufficient preheat, hot cracking in the weld metal due to improper flux coverage, and insufficient dilution control leading to hardness variations. The countermeasures identified in this study — preheating to 100 °C, strict interpass temperature monitoring, and proper flux application — effectively address these risks.

A noteworthy reflection from this literature is the practical orientation of the work. The authors did not merely optimize parameters in a laboratory setting but validated the process on an actual hydropower generator top cover component. This bridge between laboratory metallurgy and field application is essential for ensuring that the cladding process delivers reliable performance under real operating conditions. The HRC 45 hardness level represents a well-balanced combination of wear resistance and ductility, suitable for the operating environment of a hydroelectric power station where the top cover is exposed to water erosion, cavitation, and mechanical abrasion from internal components.

This study provides a valuable reference for engineers working on cladding applications for large hydroelectric power equipment, particularly when the objective is to extend component life through surface hardening while maintaining structural integrity.