TIG Welding of Hydroelectric Turbine Runners Engineering Practice Review

Literature Overview and Background

The study by Ruan Yuezhong from Bohai Shipyard (1998) addresses a critical manufacturing challenge in large-scale hydropower equipment fabrication: the TIG welding of hydroelectric turbine runners. Turbine runners are among the most demanding welded components in the power generation industry, typically fabricated from high-strength carbon-manganese steels or low-alloy steels with plate thicknesses ranging from 30 mm to over 100 mm. The runner assembly involves complex spatial geometries with multiple weld seams intersecting at varying angles, making thermal distortion control and residual stress management paramount concerns.

This early reference, while modest in publication scope, represents a significant practical contribution from a major Chinese shipyard that possessed extensive experience in large-scale structural welding. The Bohai Shipyard, located in Tangshan, had accumulated decades of expertise in marine and power equipment fabrication, lending credibility to their process development work on turbine runner welding procedures.

Core Technical Content and Process Analysis

The fundamental challenge in TIG welding of turbine runners lies in balancing weld quality against the impracticality of using TIG as a sole filler process for thick-section components. In practice, TIG welding serves as the root pass technique, providing precise control over penetration and fusion line geometry. The typical process parameters for root pass TIG welding of runner steel plates include:

The selection of filler metal must account for the base material composition. For low-alloy steels such as 16Mn or 15MnV commonly used in runner fabrication, E71T-8 or ER70S-6 wire metals are specified for subsequent GMAW or FCAW filling passes, while the TIG root pass may use ER70S-2 or equivalent solid wire.

Welding Sequence and Distortion Control

The welding sequence strategy is perhaps the most critical engineering decision in turbine runner fabrication. The typical approach follows a symmetric, staged sequence that builds from the hub outward to the shroud, welding in a circumferential pattern to minimize angular distortion. The runner is typically assembled as a segmented shell structure with longitudinal and circumferential welds, requiring careful coordination of weld contraction forces.

Key distortion control measures include:

• Backing bars with controlled grooves to manage root geometry and prevent excessive contraction
• Interpass temperature monitoring with infrared thermocouples, maintaining temperatures between 150°C and 250°C for low-alloy steels
• Post-weld straightening using induction heating or mechanical methods within the elastic-plastic range
• Sequential welding of opposing segments to balance thermal expansion vectors

Quality Assurance and Inspection Requirements

For turbine runners operating under cyclic hydraulic loading, the inspection regime must address both volumetric and surface defects. The typical inspection hierarchy includes:

The root pass TIG weld is particularly susceptible to lack of fusion and porosity defects if the joint preparation is inadequate or if the gas shielding is compromised in difficult access positions. Overhead and vertical-up positions common in runner fabrication require meticulous technique to maintain a stable arc and adequate backside protection.

Study Insights and Engineering Reflections

Reviewing this early reference in the context of modern practices reveals several enduring principles. The emphasis on process discipline and welder skill in TIG root pass execution remains as relevant today as it was in 1998. Modern advances in hot-wire TIG and advanced pulse TIG have improved productivity, but the fundamental requirements for joint preparation quality, gas flow calibration, and interpass temperature control remain unchanged.

One area where contemporary practice has evolved significantly is the integration of residual stress measurement and mitigation. Techniques such as thermal spray peening and local hot isostatic pressing are now routinely applied to critical runner welds, whereas in 1998, stress relief relied primarily on full-scale post-weld heat treatment (PWHT) at temperatures of 580–620°C.

The study also underscores the importance of qualified welding procedure specifications (WPS) validated through NB/T 47014 or ASME IX qualification tests. For turbine runner applications governed by NB/T 47002 or relevant hydropower standards, the WPS must demonstrate adequate toughness in the weld metal and heat-affected zone, typically requiring Charpy V-notch impact energy of at least 47 J at the minimum service temperature.

This literature serves as a reminder that successful large-scale welding fabrication depends not merely on equipment capability but on systematic engineering control of every process variable from joint design through final inspection.