Centrifugal Compressor Wheel Bore Treatment for Low-Cycle Fatigue
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Solution Overview
Problem
Centrifugal compressor wheels suffer from low-cycle fatigue (LCF) failure modes, particularly cracks initiating at the bore surface or in the back disk, leading to premature failure.
Innovation Solution
A treatment process involving cold working of the bore to induce compressive residual hoop stresses and surface peening to create compressive residual stresses on the compressor wheel surfaces, with cold working achieving at least 1.2% retained expansion and peening performed at an intensity of 0.15 mm to 0.6 mm on the Almen N scale.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the compressor wheel operates under cyclic loading, then it performs its compression function, but low-cycle fatigue cracks initiate at the bore surface or back disk leading to premature failure
Solution Approach 1:
The patent applies preliminary anti-action by inducing compressive residual stresses in the bore surface and back disk regions before the compressor wheel is subjected to cyclic loading. This is achieved through two operations: (1) cold working the bore to create compressive residual hoop stresses in the metal surrounding the bore, and (2) surface peening to induce compressive residual stresses in the wheel surfaces. These pre-induced compressive stresses counteract the harmful tensile stresses that would otherwise initiate fatigue cracks, thereby extending LCF life.
Solution Approach 2:
The patent changes the stress state parameter in the critical regions of the compressor wheel. By cold working the bore, the material undergoes plastic deformation that permanently alters the residual stress distribution, creating a zone of compressive residual hoop stresses. Similarly, surface peening modifies the surface stress state by inducing compressive residual stresses. These parameter changes in the stress field transform the harmful tensile stress environment into a beneficial compressive stress environment, preventing crack initiation.
2Reliability
If cold working is applied to the bore to induce compressive residual stresses, then LCF life is extended, but the manufacturing process complexity increases
Solution Approach 1:
The cold working operation is performed as a preliminary action during the manufacturing process, before the compressor wheel is put into service. By inducing the compressive residual stresses during manufacturing rather than requiring a separate post-processing step, the patent integrates the reliability enhancement into the existing manufacturing workflow. The bore is cold worked using conventional machining equipment, and surface peening is applied to complete the treatment, both before the wheel undergoes its first operational cycle.
3Reliability
If surface peening is performed to induce compressive residual stresses, then crack initiation is prevented, but the manufacturing process time increases
Solution Approach 1:
The surface peening operation is applied locally to the critical regions of the compressor wheel, specifically targeting the back disk and other surfaces prone to fatigue crack initiation. Rather than treating the entire wheel uniformly or requiring extensive post-processing, the peening is focused on the specific areas where compressive residual stresses are most needed to prevent crack initiation. This localized approach minimizes the additional manufacturing time while maximizing the reliability benefit.
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
Significantly extends the LCF life of the centrifugal compressor wheel by mitigating cyclic tensile stresses and inducing compressive residual stresses, thereby enhancing the wheel's durability.
Implementation Method 1
cold working the metal at an inner surface of the bore beyond the tensile yield strength in a hoop-wise direction about the bore, so as to induce compressive residual hoop stresses in the metal adjacent the inner surface of the bore
Implementation Method 2
surface peening is performed on at least a portion of the compressor wheel to induce compressive residual stresses in surfaces of the wheel
Data Source
AI summary
A process for treating a centrifugal compressor wheel includes a combination of cold expansion of at least part of the bore of the wheel to induce residual compressive stresses in a region around the bore, and surface peening at least parts of the compressor wheel such as the back disk and portions of the blades.