• Version
• 126 Downloads
• 794.56 KB File Size
• 1 File Count
• December 15, 2016 Create Date
• December 15, 2016 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - Effects of Some Solution Treatments Followed by an Aging Treatment on the Life of Small Cast Gas Turbine Blades of a Cobalt-Chromium-Base Alloy - II - Effect of Selected Combinations of Soaking Time, Temperature, and Cooling Rate

In order to expedite the heat—treating of the turbine blades, two
electric furnaces were used, one having an atmosphere produced by the
combustion of natural gas and the other an atmosphere of inert gas.
Blades of groups 5 and 6 were treated in the combusted natural-gas-
atmosphere furnace. The remaining groups were heat-treated in the inert
atmosphere furnace and all blades were then aged in the furnace having
the inert-gas atmosphere.

A gas turbine supplied with hot gases from a turbojet combustion
chamber was used in evaluating the performance of the turbine blades.

The turbine operating temperature was indicated by‘a thermocouple that
measured gas temperature in the inlet duct 12 inches upstream of the
turbine inlet. This apparatus is described in references 1 and 4.

The turbine wheel had a total diameter of 12.5 inches and contained
142 blades. All the blades used in this program were installed in the
wheel at the start of the investigation. The wheel was dynamically
balanced prior to initial operation, and thereafter, as necessary.

The blade evaluation was conducted in the following manner:
Combustion air was supplied to the turbine and the wheel was motored at
approximately 6000 rpm for 5 minutes. Combustion was initiated and the
wheel brought to operating conditions, a shaft speed of 22,50Qt200 rpm and
an indicated inlet gas temperature of 1650i15° F. At these conditions,
achieved in approximately 3 minutes, the blade temperature was estimated
from previous operating experience to be 15000 F. Operating time was
measured from the beginning to the termination of combustion. Upon failure
of a blade, indicated by a change in the pitch of sound emitted from the
unit, combustion was immediately terminated and the air flow reduced to a
value such that the wheel motored at 6000 rpm. This air flow was main-
tained for 10 minutes in order to cool the assembly. Shutdowns were
effected as quickly as possible to minimize the effects of vibration
caused by wheel unbalance. The turbine wheel was removed and the failed
blade, or blades, replaced by as—cast blades. Severexy cracked blades
were considered failures in order to minimize shutdowns and to facilitate
operation. Blade fragments, when thrown from the wheel, became imbedded in
asbestos packing and were thus preserved and did not ricochet to damage
remaining blades.