34
hardness values, was still likely to gall. In one instance, the upper and lower ring both seized
when the stationary segments came loose.
when the stationary segments came loose.
Another problem encountered with martensitic stainless steel is its susceptibility to stress
corrosion cracking when not heat treated properly. Tempering 410 stainless steel between 700
"Fahrenheit (
corrosion cracking when not heat treated properly. Tempering 410 stainless steel between 700
"Fahrenheit (
o
F) and 1,050
o
F results in decreased corrosion resistance. Rotating rings failed at
several facilities due to specifying a hardness that required tempering at 900
o
F.
To solve the galling problem, dissimilar metals should be specified. As the result of laboratory
testing, austenitic stainless steel and aluminum bronze were determined as the best combination of
materials for seal zings. In the tests, these materials were found to be not only resistant to galling
when brought into moving contact, but both materials proved to be more cavitation resistant than
the martensitic stainless steel.
testing, austenitic stainless steel and aluminum bronze were determined as the best combination of
materials for seal zings. In the tests, these materials were found to be not only resistant to galling
when brought into moving contact, but both materials proved to be more cavitation resistant than
the martensitic stainless steel.
To replace the dovetailed segments of the stationary ring, aluminum bronze segments rolled from
bar stock and held in place with radial fasteners were used. This design also proved unsuccessful.
Rings of this design failed in a relatively short time after installation. The rolled bar stock tended
to lose its rolled dimension, creating a clearance between the ring and its mating surface. The
clearance allowed the ring to move, creating a vibration that led to fatigue cracking. The rolling
process itself caused cracks to form on the outside diameter of the rings. Other factors that may
have contributed to the failure were overtorquing the fasteners, flaws in the fasteners, and stress
concentration caused by the radial boltholes.
bar stock and held in place with radial fasteners were used. This design also proved unsuccessful.
Rings of this design failed in a relatively short time after installation. The rolled bar stock tended
to lose its rolled dimension, creating a clearance between the ring and its mating surface. The
clearance allowed the ring to move, creating a vibration that led to fatigue cracking. The rolling
process itself caused cracks to form on the outside diameter of the rings. Other factors that may
have contributed to the failure were overtorquing the fasteners, flaws in the fasteners, and stress
concentration caused by the radial boltholes.
Current seal ring design calls for making the rings in one piece whenever possible. The preferred
method of manufacture is by centrifugal casting. Centrifugal casting forces the metal outward
while the lighter impurities remain on the inside diameter. The impurities are removed by
machining. When the rings are very large, centrifugal casting may not be possible, and sand
casting or ring rolled forging may be used. Ring rolled forging is a process that produces a large-
diameter seamless ling by rolling a heated, smaller-diameter ring with a thick cross section,
between two rollers. Rotating rings can also be fabricated by rolling bar stock and welding the
ends, but casting is preferable. When new stainless steel turbine runners are installed the rotating
rings are specified to be an integral part of the runner.
method of manufacture is by centrifugal casting. Centrifugal casting forces the metal outward
while the lighter impurities remain on the inside diameter. The impurities are removed by
machining. When the rings are very large, centrifugal casting may not be possible, and sand
casting or ring rolled forging may be used. Ring rolled forging is a process that produces a large-
diameter seamless ling by rolling a heated, smaller-diameter ring with a thick cross section,
between two rollers. Rotating rings can also be fabricated by rolling bar stock and welding the
ends, but casting is preferable. When new stainless steel turbine runners are installed the rotating
rings are specified to be an integral part of the runner.
The single piece rings are designed to be a shrink fit and usually don't require fasteners. If the
stationary ring has to be installed in sections, each section is cast, or the ring is cast in one piece
and cut by wire electrodischarge machining (EDM) process. EDM is specified because the width
of cut by normal sawing techniques would be greater than 0.020 inch where the width of a wire
EDM cut is less than 0.010 inch. If fasteners are required, they should be made from Nitronic 60
stainless steel. Nitronic 60, unlike most stainless steels, is resistant to galling as well as being
resistant to cavitation erosion and corrosion.
stationary ring has to be installed in sections, each section is cast, or the ring is cast in one piece
and cut by wire electrodischarge machining (EDM) process. EDM is specified because the width
of cut by normal sawing techniques would be greater than 0.020 inch where the width of a wire
EDM cut is less than 0.010 inch. If fasteners are required, they should be made from Nitronic 60
stainless steel. Nitronic 60, unlike most stainless steels, is resistant to galling as well as being
resistant to cavitation erosion and corrosion.