Zhuojin Pipe Fitting Co., LtdTIG Welding of Hydro Turbine Runner with Dissimilar Steel Materials
Literature Overview
This paper by Ruan Yuezhong, published in Welding journal (1998, Vol. 6, pp. 24-25) from Bohai Shipyard, documents the application of TIG welding for joining dissimilar steel components in a hydro turbine runner manufactured for the Wuqiangxi Power Station in Hunan Province. The runner comprises an upper crown and lower ring made of A216 carbon steel, and 13 blades fabricated from X5CrNi13-4 martensitic stainless steel. The welding challenge arises from the significant disparity in strength grades between the two materials, making the selection of an appropriate welding process particularly demanding.
Core Technical Challenge
The fundamental difficulty in this application lies in the heterogeneous material interface. A216 is a low-carbon structural steel with relatively low hardness and moderate strength, while X5CrNi13-4 is a 13% chromium martensitic stainless steel with substantially higher hardness and strength. This mismatch creates several metallurgical concerns:
| Parameter | A216 Carbon Steel | X5CrNi13-4 Martensitic SS |
|---|---|---|
| Carbon content | ~0.25% | ~0.12% |
| Chromium | Trace | ~13% |
| Nickel | Trace | ~4% |
| Hardness (as-received) | ~120 HB | ~280 HB |
| Thermal conductivity | ~45 W/m·K | ~15 W/m·K |
| Coefficient of thermal expansion | ~12 ×10⁻⁶/K | ~10 ×10⁻⁶/K |
The large difference in thermal conductivity means that heat input from the welding arc will preferentially flow into the carbon steel side, creating asymmetric cooling rates at the joint. This can lead to localized hardening in the heat-affected zone (HAZ) of the stainless steel side due to rapid cooling, while the carbon steel side may experience excessive grain growth. Additionally, chromium diffusion from the stainless steel into the carbon steel can create a chromium-depleted zone susceptible to intergranular corrosion, while carbon migration toward the stainless steel side can form brittle carbides at the interface.
Welding Process Selection and Parameters
The author selected TIG welding (GTAW) as the primary joining method, citing two key advantages: the ability to precisely control heat input and the capacity to achieve high-quality welds without excessive spatter or distortion. The rationale behind this choice is well-founded for dissimilar steel applications where thermal management is critical.
Key process considerations for this application would include:
- Heat input control: Maintaining low to moderate heat input (typically 0.8-1.5 kJ/mm for this type of joint) to minimize the width of the HAZ and reduce the extent of chromium depletion and carbon migration zones.
- Welding direction strategy: Welding from the stainless steel side toward the carbon steel side, or using a weave pattern that spends more time on the carbon steel side to balance heat distribution.
- Filler metal selection: Using a high-chromium, nickel-containing filler (such as ER309L or ER410) to bridge the composition gap between the two base metals and prevent cracking at the fusion line.
- Preheating: Moderate preheating (100-150°C) to reduce the cooling rate on the carbon steel side and minimize the risk of hydrogen-induced cracking.
- Interpass temperature control: Maintaining interpass temperatures below 150°C to avoid excessive grain growth and tempering of the martensitic microstructure.
Metallurgical Considerations at the Dissimilar Interface
The fusion zone microstructure in dissimilar steel TIG welds is governed by the dilution ratio between the two base metals. In the case of A216 and X5CrNi13-4, the Schaeffler diagram can be used to predict the weld metal microstructure. The A216 side contributes ferrite-forming elements, while the X5CrNi13-4 contributes austenite-forming and martensite-forming elements. Without careful control, the weld metal may solidify as a brittle martensitic structure, particularly if the dilution from the stainless steel is high.
The HAZ on the stainless steel side is particularly vulnerable to hardening. X5CrNi13-4 in the as-welded condition contains a high proportion of martensite, and further heating during welding can cause grain coarsening and increased hardness in the coarse grain HAZ. This can lead to reduced ductility and increased susceptibility to cracking under residual stress. Post-weld heat treatment (PWHT) at 620-680°C for stress relief and tempering of the martensitic regions is often necessary for this type of joint.
Engineering Practice and Quality Control
The paper notes that the TIG welding process achieved the product technical requirements and reached the level of similar products manufactured abroad. This suggests that the process was validated through comprehensive quality control measures, likely including:
- Visual inspection (VT) for surface quality and weld geometry
- Penetrant testing (PT) for surface-breaking defects
- Radiographic testing (RT) for volumetric defects such as porosity, lack of fusion, and incomplete penetration
- Mechanical testing including tensile tests, bend tests, and hardness surveys across the joint
- Metallographic examination of the fusion zone and HAZ microstructures
For a critical component such as a hydro turbine runner, which operates under cyclic loading and hydrodynamic pressure, the integrity of the weld joints is paramount. Any defects at the dissimilar material interface could propagate under fatigue loading, leading to catastrophic failure. The use of TIG welding, with its inherent precision and cleanliness, provides the best chance of achieving defect-free welds in this challenging application.
Study Insights and Implications
This case study illustrates the practical application of TIG welding to solve a real-world engineering problem involving dissimilar steel joining. The key takeaway is that process selection is not merely a matter of convenience but must be driven by the metallurgical requirements of the specific material combination. In modern practice, similar challenges arise in the welding of power plant components, nuclear reactor internals, and other high-integrity applications where dissimilar steel joints are unavoidable. The principles established in this 1998 paper—precise heat input control, careful filler selection, and comprehensive quality verification—remain directly applicable to contemporary welding engineering.