Treatment process for a central bore through a centrifugal compressor wheel to create a deep cylindrical zone of compressive residual hoop stress on a fractional portion of the bore length, and compressor wheel resulting therefrom
A process for cold working of the inner surface of a bore in a centrifugal compressor wheel along only a fractional portion of the bore length (i.e., along less than a full axial length of the bore), thereby creating a zone of compressive residual hoop stress in the metal surrounding the bore where the wheel needs the beneficial residual stress. The process purposefully avoids cold working of the bore at locations adjacent to high-stress areas and features of the wheel, where cold working in such locations could negatively impact the wheel's overall life.
1. A cold-working process comprising the steps of:
providing a centrifugal compressor wheel fabricated of a ductile metal having a tensile yield strength, the compressor wheel comprising a hub and a plurality of blades joined to the hub and extending radially outwardly from the hub, each blade having a blade root fillet where the blade joins with the hub, said blade root fillets each having a leading edge, the compressor wheel having a first face and an opposite second face, the hub of the compressor wheel defining a bore that extends centrally through the hub along a downstream axial direction, the bore having an axial length;
selecting a fractional portion of the axial length of the bore defined between a starting point and an ending point of said fractional portion, wherein the starting point is spaced axially downstream from the leading edges of the blade root fillets and the ending point is spaced axially downstream from the starting point, and providing the bore to have a counterbore located upstream of the starting point of the fractional portion of the bore, the counterbore having a diameter exceeding an initial diameter of the fractional portion of the bore;
disposing a mechanical tool in the counterbore, the tool having a diameter that exceeds the initial diameter of the fractional portion of the bore; and
cold working the metal at an inner surface of the bore beyond the tensile yield strength in a hoop-wise direction about the bore, along only said fractional portion of the bore, by drawing the tool along the downstream axial direction through the fractional portion of the bore, the bore not being cold worked outside said fractional portion, so as to induce compressive residual hoop stresses in the metal adjacent the inner surface along said fractional portion.
2. The cold working process of claim 1 , wherein the mechanical tool comprises a split sleeve that surrounds a mandrel, the split sleeve having an axial length corresponding to the axial length of the fractional portion of the bore, wherein the cold working step comprises disposing the split sleeve within the fractional portion of the bore and drawing the mandrel axially through the split sleeve to expand the split sleeve radially outwardly.
3. The cold working process of claim 1 , wherein the providing step further comprises providing the compressor wheel as a twin-impeller having a first impeller and a second impeller arranged back-to-back, the first impeller defining the first face and the second impeller defining the second face of the compressor wheel, wherein the counterbore is located adjacent the first face of the compressor wheel and extends to the starting point of the fractional portion of the bore, and further providing the bore to have a second counterbore extending from the ending point of the fractional portion of the bore toward the second face of the compressor wheel, the second counterbore having a diameter exceeding an initial diameter of the fractional portion of the bore before the cold working step.
4. An article produced by a process comprising the steps of:
providing a centrifugal compressor wheel fabricated of a ductile metal having a tensile yield strength, the compressor wheel comprising a hub and a plurality of blades joined to the hub and extending radially outwardly from the hub, each blade having a blade root fillet where the blade joins with the hub, said blade root fillets each having a leading edge, the compressor wheel having a first face and an opposite second face, the hub of the compressor wheel defining a bore that extends centrally through the hub along a downstream axial direction, the bore having an axial length;
selecting a fractional portion of the axial length of the bore defined between a starting point and an ending point of said fractional portion, wherein the starting point is spaced axially downstream from the leading edges of the blade root fillets and the ending point is spaced axially downstream from the starting point, and providing the bore to have a counterbore located upstream of the starting point of the fractional portion of the bore, the counterbore having a diameter exceeding an initial diameter of the fractional portion of the bore;
disposing a mechanical tool in the counterbore, the tool having a diameter that exceeds the initial diameter of the fractional portion of the bore; and
cold working the metal at an inner surface of the bore beyond the tensile yield strength in a hoop-wise direction about the bore, along only said fractional portion of the bore, by drawing the tool along the downstream axial direction through the fractional portion of the bore, the bore not being cold worked outside said fractional portion, so as to induce compressive residual hoop stresses in the metal adjacent the inner surface along said fractional portion.