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Cast in Place:


Integrating Non-Cast Components into Castings


3-D printing helped a turbine manufacturer improve the dimensional accuracy of


a part while adding fl exibility to the tooling for streamlined production. GEOFFREY D. KORFF, QUAKER CITY CASTINGS (SALEM, OHIO); AND TRAVIS B. STEWART, ELLIOTT GROUP (JEANNETTE, PENNSYLVANIA)


S


team turbines are built using a variety of cast- ings ranging from stan- dard class 25 gray iron to


400 series stainless steels. A typical single-stage turbine can contain up to 21 castings of diff erent variations for the base design, while a typi- cal multi-stage turbine can contain 30 castings or more. T is does not include any castings that would be used for add-on equipment, such as trip and throttle valves and aftermar- ket governors or for other equipment on the equipment train. T e casting material is deter- mined based on the temperature, pressure, environment, and steam conditions for the turbine applica- tion. In many cases, casting materials are interchangeable depending on these factors. For example, iron cast- ings may be used in some applica- tions for their thermal properties and ductility/dampening, while stainless steels may be used in other applica- tions where it is needed for corro- sion resistance. Material selection is important not only for the operation of the unit, but also for servicing the unit. Depending on the operating


26 | MODERN CASTING August 2017


temperatures, there may be require- ments for J-Factor calculations to avoid embrittlement in the time vs. temperature designs. Charpy impact testing may be required for low temperature service. For turbines, this is mostly environment-based and not dependent on operating temperatures. T e design of turbine castings has


changed substantially since the 1970s, when everything was typically made from iron. T ose castings were big, bulky, and over-engineered to build- in high safety factors. Contemporary designs are more precisely engineered with tighter specifi cations and closely calculated safety factors validated by FMEA models. Turbomachinery man- ufacturer Elliott Group (Jeannette, Pennsylvania) has seen an increase in requirements such as radiography, ultrasonic testing and requirements for 3.2 level certifi cations. T is caused Elliott to look at new ways to validate the process of producing its castings so they can be ordered without the requirements up front but still meet them on the back side. One example is a cast diaphragm


that is created by casting ductile iron around a series of stainess steel vanes.


T e vanes direct and redirect the


fl ow-off steam passing through a multi- stage steam turbine. T ese diaphragms can be used to generate a large amount of power or to increase effi ciency. T e current design of the part has been in existence for over 35 years and has undergone only minor changes since its conception date. T e chemistry of the iron has been the only adjustment made to the actual part. T e primary reason for changing


the chemistry was the natural trans- fers that occur between the iron and stainless steel. T ese transfers cause a lack of fusion between the dissimilar materials and create large hard spots that hamper machining. Because the pouring temperature plays a large role in the transfer, Elliott Group worked to defi ne a lower pouring temperature and used the vanes as a natural chill. T e modifi ed chemistry, along with a controlled pouring temperature, eliminated the lack of fusion and hard spots, giving the casting a uniform surface that is easily machined.


Designing a Sand Core Solution


T e process for making the basic


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