We were asked to help resolve a legal dispute in developing a suitable repair process on cast turbine blades exhibiting imperfections.
The turbine blades made by a particular manufacturer are a complex shape that requires them to be manufactured by casting. The ASTM A743 Grade CN6NM blades consist of a low carbon Cr-Ni-Mo stainless steel. Imperfections such as “tares and scabs” are formed during the casting process.
These imperfections are removed by grinding and repaired prior to heat treatment by quench and tempering. Weld repair is performed by using electrodes of matching composition to the parent casting. If additional imperfections are found after the Q/T, they are removed by grinding, welded, and the rough casting subjected to a stress relieving treatment (post weld heat treatment). The time and temperature of the PWHT are selected to make the casting comply with the 280 BHN.
It is sometimes necessary to make repairs of these castings to correct dimensional issues and to repair casting imperfections that go undetected until final machining. These casting imperfections are removed by grinding. Because a PWHT at this stage of manufacture many cause distortion, it is desirable to use a repair practice that does not require a stress relieving operations.
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