ACONA’s commitment to finding innovative solutions is also reflected in its close collaboration with students and universities in the surrounding region.
In addition to employing numerous working students, we actively support and cooperate with students on their bachelor's and master's theses.
A notable example is the master’s thesis of Urs Henke, conducted in cooperation with ACONA. His research systematically investigates whether mechanical vibratory stress relief can serve as a suitable alternative to conventional stress relief annealing for reducing residual stresses in metallic components.
Residual Stresses in Metallic Components
During the welding of cylinder tubes, significant residual stresses are generated due to local temperature gradients, phase transformations, and uneven cooling rates.
On the one hand, the release of these residual stresses during subsequent machining processes can lead to undesirable distortions, thereby compromising dimensional accuracy. On the other hand, residual stresses are superimposed on the stresses acting on the component during operation, which can reduce the service life of welded structures.
Stress Relief Annealing – The Proven Method in the Metalworking Industry
Stress relief annealing is a heat treatment process that gently reduces residual stresses generated during welding. Through controlled heating, holding, and cooling, it improves dimensional stability and extends the service life of components.
The Process:
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1.Heating:
Slow heating to a temperature of 450–650 °C.
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2.Hold::
Hold at this temperature for a defined period of time.
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3.Cooldown:
Slow, controlled cooling, ideally within the furnace.
What advantages does vibration stress relief offer compared to stress relief annealing?
As a thermal process, stress relief annealing consumes large amounts of energy for heating, soaking, and controlled cooling of large cylinder tubes. This results in high costs and significant CO₂ emissions. The time required is also considerable: hours of heating to 550–650 °C, followed by several hours of holding time and subsequent controlled cooling in the furnace.
The advantages of vibration stress relief? Exactly these factors are intended to be avoided!
Vibration stress relief – a more economical alternative for ACONA?
A metallurgical process without heat
Vibration stress relieving uses mechanical vibrations in a targeted way to reduce residual stresses, which may, for example, arise as a result of the welding process. The component to be treated and the vibration motor are rigidly connected, and subsequently brought into oscillation near the natural resonance frequency of the component.
Process of vibration stress relief
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1.Modal analysis:
Resonance frequencies are determined using a modal hammer and an accelerometer. This is achieved by applying controlled impact impulses.
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2.Automatic treatment
Approximately one hour of controlled vibration in multiple frequency steps.
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3.Documentation:
Digital documentation of the frequencies
Key advantages of this method compared to conventional stress relief annealing include lower energy consumption, shorter treatment times, and the elimination of logistical efforts, since the process can be carried out in-house.
What is the potential of vibration stress relief?
The Experiment
To experimentally verify the reduction of residual stresses compared to the thermal stress relief annealing process in welded cylinder tubes, two identical test specimens were manufactured under the same welding conditions and measured before and after the stress relief methods using the X-ray diffraction method.
Using the non-destructive X-ray method, residual stresses could be compared at exactly the same positions before and after treatment, in contrast to destructive methods such as the hole-drilling method, thereby minimizing measurement errors.
Stress relief annealing reduced residual stresses at the measurement points by at least 80%, whereas vibration stress relief showed a significantly lower reduction in residual stresses. This indicates a considerably higher effectiveness of the thermal process. Vibration stress relief produces reproducible, but limited, relaxation effects.
In another chapter of the study, an increase in service life was determined for welded cylinder tubes subjected to pulsating loads. According to the FKM guideline, the theoretical service life increases by 23% through stress relief annealing and comparable processes.
Result: Vibration stress relief can represent an alternative.
In general, it can be stated that vibration stress relief may represent an alternative to stress relief annealing depending on the specific application. This is particularly due to significant savings in emissions, space requirements, and costs, as well as the easier integration into existing production processes.
A precise and reliable verification of the increase in service life of welded components must be carried out through experimental dynamic testing. Depending on the specific conditions and application, it must be assessed to what extent vibration stress relief can be considered a possible alternative to stress relief annealing.
The process may be suitable for applications in which a lower level of residual stress reduction is already sufficient.
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