TIG Welding of Hydro Turbine Runners

Literature Overview

This 1998 technical paper by Ruan Yuezhong from Bohai Shipyard addresses the TIG welding fabrication and repair of hydro turbine runners—a component that combines demanding structural requirements with unique metallurgical challenges. Hydro turbine runners are large, complex-shaped components that convert hydraulic energy into mechanical rotation, typically fabricated from cast steel, stainless steel, or bronze. The paper discusses both new fabrication welding and repair welding scenarios, providing practical guidance for one of the most challenging applications of TIG welding in heavy engineering.

Technical Background

Hydro turbine runners present several unique welding challenges:

Complex geometry: Curved blade profiles, varying thickness (5–30 mm), and difficult access positions
High cyclic loading: Millions of pressure cycles from water hammer effects
Corrosive environment: Continuous exposure to water with dissolved oxygen and minerals
Large dimensions: Runner diameters ranging from 0.5 m (micro-hydro) to 12+ m (large hydro)
Fatigue-critical application: Weld defects can initiate crack propagation under cyclic loading

Materials Used in Turbine Runners

Material	Application	Welding Challenges
Cast carbon steel (ZG200-400)	Small to medium runners	High carbon equivalent; cracking susceptibility
304/316 stainless steel	Corrosive water environments	Hot cracking; sensitization
Bronze (Cu-10Sn)	High-corrosion applications	High thermal conductivity; porosity
Duplex stainless steel	High-pressure applications	Phase balance control
16Mn low-alloy steel	Large capacity runners	Hydrogen cracking; PWHT required

Welding Process Parameters

Parameter	Cast Steel Runner	Stainless Steel Runner	Bronze Runner
Polarity	DCEP	AC (60% positive)	AC (65% positive)
Shielding gas	Ar + 1% O₂	Pure Ar	Pure Ar
Current density	8–12 A/mm²	10–15 A/mm²	12–18 A/mm²
Travel speed	60–120 mm/min	100–200 mm/min	150–300 mm/min
Preheating	250–400 °C	Not required	200–300 °C
Filler metal	ER70S-6 / E7018 equivalent	ER308L / ER316L	Cu-10Sn equivalent
Interpass temp	≤250 °C	≤150 °C	≤200 °C
PWHT	600–650 °C / 2h	Not required	Not required

Welding Sequence Strategy

The welding sequence for turbine runner blades is critical to managing distortion and residual stress:

Symmetric welding: Weld opposite blades simultaneously or in alternating sequence
Start from thick to thin: Begin at the blade root (thick section) and progress to the tip
Tack weld placement: Use closely spaced tack welds (every 30–50 mm) to minimize gap distortion
Back-welding technique: For blade-to-shroud joints, back-welding reduces toe cracking

Defect Analysis and Quality Control

Defect Type	Criticality	Detection Method	Acceptance Criteria
Cracks (cold/hot)	Critical	MT + UT + PT	Zero acceptance
Porosity	Major	RT + UT	≤2 mm per 100 mm length
Lack of fusion	Critical	UT + RT	Zero acceptance
Inclusion	Major	MT + UT	≤1 mm for structural welds
Undercut	Minor	Visual + MT	≤0.5 mm depth
Excess reinforcement	Minor	Visual + dimensional	≤3 mm max

Fatigue Life Considerations

The fatigue life of TIG-welded turbine runner joints is significantly influenced by:

Weld toe geometry: Smooth transition from weld to base metal reduces stress concentration (Kt < 1.5 preferred)
Surface finish: Post-weld grinding or TIG dressing of weld toes can improve fatigue life by 30–50%
Residual stress: Compressive residual stress at the weld toe (achieved through shot peening or low-heat-input welding) extends fatigue life
Weld quality: Porosity and lack of fusion act as crack initiation sites under cyclic loading

Engineering Practice Insights

From extensive experience with turbine runner fabrication, I emphasize several practical considerations that this paper addresses:

Positional welding challenges: Many blade welds must be completed in overhead or vertical positions, requiring exceptional torch control and filler wire manipulation skills.
Heat input management: Excessive heat input causes grain coarsening in the HAZ, reducing fatigue resistance. For cast steel runners, heat input should be limited to 1.0–1.5 kJ/mm.
Repair welding considerations: Repair welds on existing runners must account for pre-existing residual stresses, which can be relieved by preheating to 300–400 °C before welding.
Dimensional accuracy: TIG welding allows precise control of weld reinforcement, which is critical for maintaining blade profile accuracy that affects hydraulic efficiency.

Repair Welding Case Example

A typical repair scenario involves crack repair on a stainless steel runner blade. The recommended procedure is:

Remove all cracked material using mechanical grinding (not thermal cutting)
Prepare a U-groove with 60° included angle and 2 mm root face
Preheat to 150 °C for austenitic stainless steel
Weld with ER308L or ER316L in multiple passes with interpass temperature ≤150 °C
Post-weld TIG dressing of all weld toes
Shot peen the weld toe region
Perform full UT and PT inspection before returning to service

Study Insights and Implications

This paper captures practical wisdom from a shipyard environment where turbine runner fabrication demands both structural integrity and hydraulic precision. The TIG process is uniquely suited for turbine runner welding because it provides the precision, cleanliness, and heat input control necessary for fatigue-critical applications. However, the paper implicitly acknowledges the productivity limitation of TIG for large-scale fabrication—this is why modern turbine runner manufacturing increasingly combines TIG for critical joints (blade-to-shroud, blade root) with automated processes for less critical areas.

The enduring value of this study lies in its process parameter recommendations, which remain applicable to modern turbine runner fabrication. The fundamental metallurgical challenges—cracking prevention, fatigue optimization, and dimensional control—have not been eliminated by newer technologies; they have been refined through better understanding of weld metal behavior under cyclic loading.