Partial Bore Cold Working in Centrifugal Compressor Wheels
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Solution Overview
Problem
Centrifugal compressor wheels suffer from low-cycle fatigue (LCF) failure due to cracks starting at the surface of the through bore, which is exacerbated by high stresses and variable wall thickness, making it challenging to apply cold working effectively without reducing the wheel's overall life.
Innovation Solution
A partial-bore cold working treatment is applied to create a zone of compressive residual hoop stress along a fractional portion of the bore, avoiding high-stress areas to prevent deleterious overlapping stresses, using tools like radially expandable mandrels or split sleeves to selectively cold work the bore, ensuring the treatment covers areas susceptible to LCF failure while minimizing impact on blade root fillets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If cold working is applied to the entire bore length, then LCF life is improved through compressive residual stresses, but deleterious overlapping stresses occur at high-stress areas like blade root fillets that reduce wheel life
Solution Approach 1:
The patent applies cold working only to a fractional portion of the bore length (typically 50-75% of total bore length) rather than the entire bore. This localized treatment creates beneficial compressive residual stresses in the critical mid-section of the bore while deliberately excluding high-stress areas such as blade root fillets and counterbore regions, thereby avoiding deleterious stress overlap and achieving optimal LCF life extension.
2Strength
If cold working is applied to high-stress areas, then compressive residual stresses are created, but the overall wheel life is reduced due to stress overlap
Solution Approach 1:
The treatment selectively applies cold working to specific axial portions of the bore where compressive residual stresses are most beneficial for LCF resistance, while intentionally avoiding regions with pre-existing high stresses such as blade root fillets. This spatial differentiation of treatment intensity optimizes the strength-life tradeoff.
Solution Approach 2:
Instead of applying cold working to the full bore length, the patent applies it to only a fractional portion (typically 50-75% of the bore length), focusing the treatment on the most critical stress regions while leaving non-critical areas untreated to avoid harmful effects.
3Adaptability or versatility
If the bore shape has variable wall thickness, then manufacturing flexibility is improved, but applying cold working becomes challenging due to high stress features
Solution Approach 1:
The cold working process parameters (such as mandrel size, expansion force, and treatment length) are customized for each specific bore geometry and wall thickness distribution. This localized adaptation of treatment parameters enables effective cold working application to complex bore shapes with variable wall thickness while avoiding high-stress features.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process significantly extends the LCF life of centrifugal compressor wheels by inducing beneficial residual compressive hoop stresses in high-stress areas, reducing the risk of crack initiation and improving the wheel's durability without negatively affecting adjacent high-stress features.
Implementation Method 1
cold working of the inner surface of the bore along only a fractional portion of the bore length
Implementation Method 2
creating a zone of compressive residual hoop stress in the metal surrounding the bore
Data Source
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AI summary
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.