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Vibratory Stress Relieving Equipment

Residual Stresses are stresses developed in the work piece because of welding, casting, forging
or any other processing. The presence of residual stress in component can cause distortion.
This distortion is the cause of dimensional instability. Sometimes this distortion is so severe that it may lead to total destruction of the component.

There are various methods of relieving residual stresses. The earlier techniques include hammering the work piece with a wooden mallet and dropping a casting or weldment from a height to pile of sand. This probably set up vibrations which eliminated some of the residual stresses caused during casting or fabrication process. The more recent methods are ageing and annealing. These earlier methods of relieving stresses formed the first step towards what is known as vibratory stress relief

The vibratory stress relief process can be described as a ‘method of applying Controlled low amplitude, low frequency vibrations in metal components to Obtain dimensional stability and control for machining tolerances’. It is an Economical, fast and convenient method of reducing stresses and is used during process to achieve the dimensional stability

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Theory

Stress relieving by any method is associated with the micro plastic deformation of atoms. This
is achieved by injecting additional energy in the form of thermal or mechanical.

In case of thermal stress relieving, the residual stress value becomes higher than the yield stress value at relieving temperature. Hence the material will immediately undergo plastic deformation effecting reduction of residual stress value. Mechanically, the vibration energy is introduced into the work piece so that the applied stress is superimposed on the existing residual stress. The resultant stress value crosses the yield point of the material and will undergo plastic deformation effecting reduction of residual stress value. The energy injected should have sufficient magnitude to enable the stress atom to cross the yield point. This is achieved by injecting the vibration energy at natural frequency of the work piece.

VSR Applications
  • Dimensional stabilization after machining, welding, forming, casting, forging or any other operations.
  • Avoidance of stress corrosion cracking through reduction of surface tensile stress.
  • Overall Stress reduction.
Advantages of VSR Process
  • VSR causes no metallurgical changes and does not reduce yield strength, tensile or fatigue life as thermal Relive (TSR) Process.
  • The equipment is simple, inexpensive and portable. Handling cost are minimized drastically by bringing the process to work piece.
  • The operating cost are dramatically lower as very little power and time are consumed for the entire process as compared to TSR
  • The VSR process can be used on any size, shape and weight of work piece. No change in equipment is needed.
  • No undesirable side effects, such as oxidation or scaling. The process is clean and generates no smoke, fumes or gases as in the case of thermal Process
COMPARISION OF STRESS RELIVING PROCESSES
Ageing Thermal Vibratory
Principle Due to thermal expansion and contraction Plastic deformation by injecting thermal energy. Plastic deformation by injecting Vibration energy.
Equipment None Expensive, Large furnace Considerably less expensive, VSR equipment.
Area Of application All types of cast and fabricated components. All types of cast and fabricated components. Same as thermal but nor suitable for cold worked material
Investment Large,Storage facility required High, Furnace and material handling equipment required. Low, Job Handling saved.Equipment goes to work piece.
Operating Time 1 to 2 years to complete the job 2 or more days to complete the job 30 to 45 minutes to complete the job
Reliability Good Very Good As reliable as thermal
Versatility Good Limited by size of Furnace Versatile, Can Handle any Shape,size and weight of jobs.
Post Treatment Mechanical cleaning required Mechanical cleaning required No cleaning required.
Effect on Mechanical Properties Slight due to corrosion Tensile strength and yeild strength changes. No effect on mechanical properties.