Casting at Three Gorges15 January 2001
Electroslag casting has been used to manufacture runner blades for hydro turbines for over 15 years in China. The Shenyang Research Institute of Foundry is now using the process to construct wicket gates for the Three Gorges project
ELECTROSLAG casting (ESC) is based on electroslag resmelting (ESR) which is used in the metallurgical field as a means of refining and improving the crystal structure of metals to increase their mechanical properties.
Metals formed by the ESR process exhibit several advantages:
• Impurities are removed, the molten steel is refined and the material’s properties are improved.
• A strong internal structure is obtained through self-shrinkage.
• The cast crystal structure of the small secondary dendrite structure is improved.
ESC allows refining and casting to be carried out at the same time. ESC materials have all the advantages and properties of ESR, but also have the following additional advantages:
• The use of a metal mould gives a precise casting profile. The mould can also be reused. This makes ESC especially suitable for manufacturing large batches of high quality parts.
• As some pieces of the consumable electrodes can be welded together, the electrode can be fabricated using basic melting equipment and then cast together.
The main differences between ESC and ESR are:
• ESC equipment requires much more control and operational accuracy. The conventional electroslag furnace for ESR can not satisfy ESC requirements.
• As the shape of the castings vary, the design of the consumable electrode, mould and ESC process are much more complex than in ESR. All of these are adjusted according to the variations in the casting process.
Although it does not have a complex shape, casting a wicket gate is difficult. The gate needs to have a high quality internal structure which is difficult to achieve through the sand casting process. The main problems are shrinkage and porosity which can affect the safe operation of the turbines. In order to resolve these problems, the Shenyang Research Institute of Foundry (SRIF) has successfully developed a new technique for the ESC wicket gate. The ESC wicket gate has a precise profile with a grinding margin of only 3mm for each passing flow. SRIF has manufactured more than 630t of ESC wicket gates for many projects with large and medium hydrogenerating units (see table below).
SRIF, in co-operation with Harbin Electrical Machinery Company, has also developed a new process of ESC section-varied plate and die pressing, and successfully manufactured the blades for the 225MW Tiansheng Bridge hydro turbine. Firstly, a section-varied blade, with a 10-15mm machining excess on the flat surface and a 3mm grinding excess on the curved surface, was cast using the ESC process. The blade is then heat treated, excess material is removed and the die pressing process is applied to shape the blade.
Wicket gates for Three Gorges
SRIF is currently preparing to manufacture wicket gates for the Three Gorges project in China. The size of the gates are shown in the table below. The following preparation work has been carried out for this project:
• Developing a computer simulation technique for ESC section-varied blades to improve the accuracy and reliability of the ESC process.
• Designing a consumable electrode, mould, ESC centre with a capacity of 2000t/year and a maximum workpiece up to 20t with a total investment of about US$4.2M.
• Constructing a piece of a full size wicket gate for the Three Gorges project and carrying out tests on its quality.
TablesESC wicket gates