Trater, since the end of the Nineties, has had the equipment to perform this type of treatment and has performed hundreds of interventions on many types of items for various uses and has carried out a very in-depth study (Protocol 3019 financed by the MIUR(Ministero dell’Istruzione, dell’Università e della Ricerca) Ministry of Education, University and Research, with ministerial decree 19982) on the use of this technology to obtain the relieving of the items.
The results of the research conducted in the years from 2006 to 2009 and the feedbacks obtained on concrete cases of application of the process are, however, conflicting, having in some cases had an effective relieving of the residual stresses of the treated structures, with stability of the items during their mechanical processing; in other cases complete failures, with the same treatments’ performance. Surely there are no metallurgical effects. The hardness of the welds is not reduced, which will therefore continue to function as stress concentration corridors; the high hardness is also sign of their fragility. Today Trater does not offer this method as an alternative to the classic heat treatment and, if it is requested, we clearly indicate to the customer the limits and its potential ineffectiveness. We do not recommend applying it, for example:
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VSR PROCESS THEORY
With this process, mechanical vibration energy is supplied to the item through the rotation of eccentric masses. The effect of the vibrations adds to the stress state present in the item. In the points where the total stresses are higher, dislocations’ sliding phenomena occur; the result is a more stable structural condition with final residual stresses, at the microscopic level, low or redistributed in the material.
The abundance of defects in the atomic structure facilitates the sliding of dislocations with the relieving of residual stresses. It is known, however, that own characteristics of materials can counteract their movement by modifying the mobility of a dislocation. In pure metals and alloys, grains’ boundaries and the presence of other dislocations disadvantage the relieving; in the alloys the solute atoms and the particles of another phase present act as "nails" to the slidings. Material with a strong work hardening state, fine grain (with many grain boundaries), or which has undergone heat treatments with precipitations will be difficult to stress-relieve with this method.
TREATMENT METHOD
The geometry, mass and material of each item to be subjected to VSR treatment is studied by our technicians for the determination:
- dof the position(s) in which the vibrator, which will transmit the vibrations to the item, will be installed
- of its support on the appropriate shock absorbers
- of the position of the accelerometer capable of detecting the way the item vibrates.
- The operational method of treatment involves:
- the application of the vibrator to the item through an effective mechanical connection, in the position(s) determined in the study;
- the support of the product on the shock absorbers;
- in the positioning of the accelerometer;
- in a first phase of scanning of all the vibration frequencies (from 25Hz to 83 Hz) with recording of the absorption values of the vibrator’s power current and in which, automatically or through operator analysis, the specific vibration frequencies are selected;
- in making the item vibrate at the frequencies identified in the previous phase with the recording of the initial and final amperometric absorption of the motor current;
- in repeating the scanning of the entire operating frequencies range with the data recording.
TECHNICAL STAFF
The treatment has a duration that can vary from one hour to several operating days in relation to the dimensions of the item, its geometry, complexity and weight.
When it is possible to carry out the treatment in the oven, Trater recommends, however, heat treatment. The relieving results are certain and repeatable.
