VIBRATORY STRESS RELIEVING – It’s ADVANTAGES AS AN ALTERNATIVE TO
THERMAL TREATMENT
J.S Hornsey VSR(Africa)cc December 2004
ABSTRACT
In an introductory review, the techniques and equipment for vibratory stress relieving are described and applications
exemplified with case histories. It has been proven that the process gives stabilisation results comparable with and in many
cases exceeding those obtained with thermal treatment, whilst being quicker, cheaper, more versatile as the equipment is
completely portable and the technique offers many advantages when machined parts or heavy fabrications are involved.
INTRODUCTION
Over the last 60 years, vibratory stress relieving has evolved
from a little known art into an indispensable basic process,
which is now a well tried and established alternative to
thermal treatment for the treatment of castings, fabrications,
components requiring intricate machining operations and
non-ferrous metals. It is important to emphasize that
vibratory stress relieving is not claimed to be a substitute for
all thermal treatments although there is some common
ground just as there are areas where each process is and will
remain predominant.
Thermal and Vibratory treatment share a capability in three
areas, namely overall stress reduction, dimensional control
and dimensional stabilisation. Although total stress relief is
almost impossible to obtain by using any commercial
process, vibratory stress relieving can stabilise and stress
relieve the component at any stage of the manufacturing or
machining process without changing the materials
metallurgical condition, without scaling or discoloration and
without distortion at low cost and with minimal time
restraints to the manufacturer.
Conversely only thermal treatment will change a material’s
metallurgical properties and thermal treatment is also more
effective than vibratory stress relieving when used to
prevent incidences of brittle fracture, although more often
correct material selection is a prerequisite for the prevention
of this type of failure.
Additionally, materials that derive their mechanical
properties from transformation hardening or cold working
cannot be so thoroughly stabilised. Thus the
complementary nature of the two processes can be
appreciated. However instability in these types of materials
can be successfully treated using vibratory stress relieving.
The vibratory process involves inducing metal structures
into one or more resonant and sub resonant conditions
using portable high force exciters. Treatment periods are
short and frequencies generally in the range 10-230Hz.
With modern exciters, correctly sited, and the component
virtually undamped by means of rubber isolation mounts,
equal and often better results than are to be expected from
commercial thermal treatments are possible and are more
often obtained.
One of the main factors which has and in some cases still
hinders the acceptance of the process is the reluctance by
engineers to accept that a low cost vibratory treatment,
using only 220v and often lasting less than thirty minutes
could possibly replace an extended thermal treatment
involving high energy consumption.
A recent survey carried out by the US Department of
Energy has shown energy savings experienced by using
vibratory stress relieving in some cases exceed 500:1.
This article discusses the practical benefits of vibratory
stress relieving as compared with thermal treatment and
attempts to dispel some of the myths associated with
vibratory stress relieving it also highlights some of the
discrepancies in the various vibratory systems available. A
section dealing with vibratory stress relieving equipment
is included although it is assumed that the reader will be
broadly conversant with installations for thermal
treatment. Some of the variations in specifications for
thermal treatment are also included which will hopefully
expose the ignorance in these specifications.
PAST RESEARCH
It is possible to find technical papers, ostensibly written
about vibratory stress relieving dating as far back as 1934,
but in fact, there have been few genuine research
programs into the process.
The bulk of the papers concern either general oscillatory
testing of metals or work which the author thought to be
related to vibratory stress relieving without properly
evaluating and appreciating the process. Many tests were
limited to simple test bars, which due to restricted
budgets were treated in fatigue test machines at fixed
frequencies and amplitudes.
Recent testing (Oct 2003) in conjunction with The Anglo
American Corporation of South Africa, again due to
budget restraints used a simple welded specimen two
plates 150mm x 150mm x 25mm thick, butt welded
together. This is certainly not a good example of actual
components owing to perfect welding conditions with the
welding carried out by personnel from The South African
Institute of Welding and consequently very low
inductions of stress. The results obtained showed strain
redistribution of up to 90% and a reduction in stress of
42%
In some tests fixed foundry knock-out shakers and deburring
barrels have been used and the work audaciously
claimed to relate to vibratory stress relieving! There are
obvious reasons why most of these low budget methods
did not succeed in producing the desired result.
Recent tests carried out amongst others by the University
of Strathclyde and the Department of Trade and Industry
has redressed some of the inadequacies of the older test
methods, a list of papers and tests will be included in the
bibliography at the end of this paper.
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EQUIPMENT
Although any equipment can satisfy the “easy to treat /
little need” category, only the best equipment with
optimum force / frequency characteristics and maximum
“g” tolerance successfully treats the most challenging end
of the range. As examples and research show, it is a range
that spans the entire materials and engineering spectra.
Gone are the days when heat treatment contractors took
an adversial attitude to vibratory stress relieving. Some
have purchased their own VSR equipment; and many
others use an on-site service. As well as enabling them to
treat parts hitherto too large for their furnace, vibratory
stress relieving opens up completely new areas of
business. However, for coded components such as
pipework and pressure vessels etc. thermal stress relief
must be used as only this gives the required metallurgical
benefits. There have been many requests to include
vibratory stress relieving into the various codes but the
reluctance to do so is still dominant in the industry
Stability is the main requirement for which vibratory
stress relieving is applied. When VSR is used stability
more than matches that of thermal stress relieving.
Stability can be improved by re-applying VSR to
components in near finished condition thus saving
components that might otherwise have been scrapped.
Vibratory stress relieving does not reduce rigidity or affect
material properties or fatigue life.
There are various VSR systems, some effective some less
so. The only common denominator being that the
component to be treated is placed upon rubber isolators
and subjected to a cyclic force. Recent research has
identified the successful and not so successful processes.
The three main VSR approaches are resonant (R-VSR),
modal sub-resonant (SB-VSR) and sub harmonic (SHVSR).
The British “VCM series” is the only equipment range
that is specifically designed for R-VSR. It has superior
frequency/force ranges and a remarkable tolerance to
high “g” forces. The formula 62 and Fouriermatic
systems claim to be successful for resonant VSR but
research mentioned below casts doubts on their
effectiveness – possibly because of poor frequency range,
“g” tolerance etc. Practice seems to support this.
RESONANT VSR
This has evolved over a 40-year period. For the VCM
series mid 1997 saw major research-led changes in both
approach and equipment specification. In well defined
areas of application. R-VSR is now 100% successful in its
main objective stress relief-component stabilisation. The
treatment of components from less than 1kg to in excess
of 100 ton is commonplace.
Procedures stipulate a progression up the peaks to
resonance, consisting of a pause at the foot to allow any
critically high stresses to diminish, prior to treating at the
mid height region and then a short defined number of
cycles at the actual peak. As long as the mean stress is
allowed to float the resulting cyclic imposed, strains
progressively add to the residual strains in the material to
cause stress reduction and distribution as with TSR. For
the most uniform stress relief and stability, as many as the
natural frequencies as possible are reached. The greater
the equipment’s range and the more complex the loading
pattern then the better the treatment.
Research and over 40 years of application have shown
that there is no damage due to high resonance. This is
because critically high-imposed stresses are impossible to
achieve as damping increases dramatically with high cyclic
strain.
R-VSR is normally applied before machining, ideally
though it should be applied after rough machining as it
then also reduces machining stresses. Application before
final grinding achieves even closer tolerances. Treatment
at this or the finished stage eliminates micro movement
occurring between leaving the customer or in service. The
most accurate and stable components are R-VSR treated.
In general, even using old style R-VSR, where suitable
components have been excited at one or more resonant
frequencies, the results have been stress reductions of
30% or more depending mainly on the equipment used.
Meanwhile an AC vibrator system with a ‘g’ tolerance of
over 80g can obviously be expected to be the most
efficient means of stress relief Strachen showed an 80%
reduction with mild steel welded specimens and a 60%
reduction in stainless steel welded pieces. Zveginceva
found over a 40% decrease and Zubchenko showed a
73% reduction with large mild steel welded bedplates.
Treatment at a succession of modes, each having a
different strain pattern was shown by Polnov to cause
substantial reduction and redistribution of stresses. At the
limit 1% stress relief makes the difference between
instability and stability.
With the advent of the 5-220Hz range of VSR machines,
Jesensky Bonthuys Ohol and Sagalevich have shown
reductions of 40-80% using resonant frequencies. The
higher percentage figure will not be achieved if the
researchers did not invoke the cyclic properties of the
material. Much is to be learned from the excellent
research by Walker, Waddell & Johnstone .
Manufactures of vibrating plant use R-VSR for stress
relief and fitness for purpose testing and thereby extend
warranties on screen, deck support frames, moulds etc.
MODAL SUB-RESONANT VSR
If when attempting R-VSR, only the base of the peak is
achievable (due to the peak being just out of range).
Treatment would be classed as modal sub-resonant VSR
i.e. the mode shape would be evident, but the peak not
achievable. Optimum results are obtained if up to 10
times the number of cycles required for R-VSR are
applied in inverse proportion to the magnitude of the
cyclic response.
Where only modal sub-resonant treatment is possible
Waddell has proved that, given sufficient cycles,
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considerable stress relief occurs with no reduction in
fatigue life.
Practice supports this. The time for treatment varies from
equipment to equipment.
Strain measurements have indicated that modal SR-VSR
is most effective against high tensile stresses, whereas RVSR
works well either on both high tensile and high
compressive stresses. For stability after machining and in
service, both tensile and compressive stress peaks must
be lowered if they are approaching yield value. After all,
stability is the main requirement for which R-VSR or
modal SR-VSR is applied. When resonance is used,
stability more than matches that of thermal stress relief,
as it can be re-applied near finished machine size.
It is best carried out with the equipment used for R-VSR
processing because of its superior frequency range.
VCM 90/905 machines which have twice the range of
any other equipment.
SUB-HARMONIC VSR (MetaLax)
If neither of the above conditions are met (due to
resonant responses being way beyond the range of the
equipment), conventional wisdom indicates that no stress
relief is possible. This seems to be the domain of subharmonic
VSR. Treatment is said to take place at the foot
of a minute sub-harmonic of a true resonant peak. Sales
literature states that the process depends on energy
absorption being at a maximum near the foot of a subharmonic
peak. Because exciter force increases with the
square of the speed one might logically expect the highest
sub-harmonic peak to be the most effective for treatment,
however the manufacturers, actually advocate treatment
at a low one. This possibly indicates that their equipment
has poor ‘g’ tolerance. The mechanism by which SH-VSR
is said to work has no connection with either R-VSR or
modal SR-VSR.
The diagram used to promote the process and its
mechanism appears unconvincing if drawn to scale. SHVSR
claims to vibrate the atoms and move them relative
to one another in the strained crystal lattice of the
material. This seems farcical, as the energy used is so low
that the vibration usually cannot be either felt or heard.
RESEARCH
Researchers have investigated aspects of VSR for over 40
years. Some were legitimately exploring its boundaries but
others have toyed with test-pieces and procedures not
remotely connected with VSR resulting in some
misconceptions. All the research reported below was
conducted with actual VSR equipment, assisted by the
equipment manufactures or their direct agents. Whereas
historically research projects in the mid/late nineties have
consistently disproved the effectiveness of American dc
resonant, non-resonant and sub harmonic equipment. A
2-year Dutch/German EU project tested two dc types of
equipment – SRE Co, Formula 62 and VSR Eng Martin
LT120/MX800 re: stress reduction and fatigue of
components. Little or no benefit was found. British EU
and later DTI projects tested two other dc types; both
automatic Bonal Meta-lax sub harmonic system and
VSR Eng. KD16 Fourier scan re stress reduction and
stability. The projects lasted nearly six years and little or
no benefit was found. Particularly difficult components
were tested as an adjunct to the DTI project.
They were treated using the Meta-lax and VCM 90
equipment. The results showed that Meta-lax
brought about little change whereas the VCM 90 was
on par with thermal stress relief (see bar diagram).
Sumarising recent research, it clearly shows that:
VSR can be as effective as TSR given the
best R-VSR equipment.
A cyclic version of a simple stress overload is
one mechanism that is at work given sufficient
amplitude.
Given sufficient energy, a beneficial effect on
the distorted crystal lattice of the material
occurs.
No reduction in fatigue life occurs using any
form of modern VSR equipment.
INDUSTRIAL EXAMPLES
The widespread use of, and the general satisfaction with
vibratory stress relieving has been shown by the extent to
which it has been accepted by virtually all sectors of
industry; so extensive in fact that it is impossible to truly
represent the entire spectrum here.
No specific example of mild steel fabrications or cast iron
/ cast steel castings is given here as it is well accepted that
where no metallurgical changes are required, vibratory
stress relieving is as good as thermal stress relieving for
stabilising and stress relieving beams, bases, columns,
gearboxes, bedplates etc.
But it is quicker, cleaner and cheaper as witnessed by
thousands of regular users over the last 45 years in
virtually every applicable engineering field. The
acceptance and usage of VSR in South Africa alone has
increased by an average of 69% annually since 1992.
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Fan Impellors and Rotating Equipment:
Vibratory Stress Relieving is used to stabilise fans and
impellors ranging in size from 800mm diameter x
100mm to 2m diameter x 900mm in fabricated mild steel
and stainless steel. Sometimes these are repaired
components, and sometimes replacements. After
fabricating, but prior to dynamic balancing, the
components are subjected to VSR. Since introducing this
treatment, no fans or impellors have gone out of balance
in service, even under hot conditions – hitherto a
troublesome area. Installations are now much quieter and
last longer between overhauls. Novenco Aerex, the UK’s
largest fan and impellor manufacturer, have had their own
VSR unit for many years and endorse the benefits stated
above. Their Canadian plant also uses VSR. In both cases
the system paid for itself in 4 – 5 months. Rubber coated,
steel fan blades have been treated to overcome instability.
Picture Courtesy Rotary Machine Equipment South Africa
Rolls, bars and shafts:
Bowing of shafts whether during machining, weld
depositing of worn items or in service had proved to be a
virtually insurmountable problem.
Particularly difficult materials such as duplex stainless
steel, nitronic50, E4340PQ etc. are stabilised using this
method, saving companies a fortune in material, time and
labour costs.
The following photograph shows one of twenty-four
unstable EN19 steel, forged drive shafts, being VSR
treated and monitored using surface strain gauges. The
results showed that VSR reduced surface stress to safe
limits, stabilising the component while not reducing
fatigue life or altering material properties. Also, VSR and
strain measurements clearly identified the shafts which
had been correctly TSR’d and those which had not.
20 off EN19 Shafts treated prior to final machining
Machine Tools and Baseplates
Vibratory stress relieving being carried out on a cast iron precision
machine bed.
Some twenty three years ago, Dean Smith & Grace
became disillusioned with thermal stress relieving when
cast iron saddles, consistently in tolerance on final
inspection in the UK, were 20% out on arrival in the
USA, necessitating rework. VSR solved the problem and
today, DSG rely on it solely to stabilise saddles, beds, etc.
Before that time QA records showed that, using thermal
stress relief 98% of beds were reworked in-house after
final machining due to movement during handling.
Subsequently, only one of 533 beds was reworked – 0.2%.
Dean Smith & Grace’s subcontract machine shop also
finishes mild steel fabricated beds up to 10m long and 1m
x 900mm section, basically in 12mm plate, but with
sideway sections up to 100 x 300mm and weight up to 12
tons.
A 10m bed has a welding time of approximately 50 hours.
The fabrication is solely VSR treated. Operation
procedure is to fabricate, apply VSR, inspect, rough
machine removing up to 35mm to produce sideway
profile, ship to Dean Smith & Grace, apply VSR and
finish machine to five microns in 6m by grinding. No
machinability problems are encountered at any stage,
despite extensive machining of flame cut edges up to
100mm thick
Treatment of fabricated pump and gearbox bases.
5
Gearbox Casings
In this example the manufacturer had a reclamation
problem involving the rewelding and finish machining of
lightweight gearboxes already in a part machined
condition. The components had already undergone one
heat treatment prior to the reclamation operation, during
which distortion had taken place sufficient to indicate that
a further thermal treatment would render them suitable
only for scrap. After consultation VSR was attempted on
a reclaimed sample of the weakest component, the
gearbox hood, following a rigorous dimensional check.
The hood was satisfactorily crack detected and finish
machined and all other items successfully treated, thereby
avoiding complete remanufacture. VSR is used
extensively for this type of fabrication which requires
close machining tolerances.
Picture courtesy Sasol Synthetic Fuels South Africa
Treating the parts that thermal stress relieving
cannot treat
There are thousands of components in need of stabilising
that cannot be thermally stress relieved but can be treated
using a VSRS.
Here follow some typical examples:
(a) Precision conveyor rolls for nuclear waste
disposal, having an outer 304L stainless-steel
shell, of 819mm diameter x 884mm face, welded
to 789mm-diameter mild-steel end plates and
bosses with integral En8 120mm-diameter shaft.
AC-VSRS was specified by Sandvik, approved
by British Nuclear Fuels and NIS based on
Sandvik’s twelve years of complete satisfaction
with the AV-VSRS.
(b) Mild-steel rolled hollow-section (RHS)
fabricated ‘A’ Frames with reinforcements for a
vehicle front chassis, with integrally-welded cast
steel “Rose’ universal joints, are manufactured in
a jig to tight tolerance. Prior to VSRP being
applied, 106 sets were produced and all distorted
in service, causing wear. For over 1000 sets,
VSRS treating at three resonances between 35
and 180Hz has rendered all completely stable.
(c) Three designs of steel armour-grade investment
casting, one with a welded-on tie-bar in the fully
heat-treated and final metallurgical condition,
were found to be grossly unstable during
machining. The largest had a 300 x 400mm
picture-frame face, associated bore and pad
faces 400mm apart, in the first batch of four,
movement continued for two months after
machining. TIR allowable in all planes is better
than 3 microns. A special VSRP was applied,
prior to machining, by mounting the component
at its center of gravity on a small pad on a 400 x
400mm jig table with a vibrator mounted on the
underside. Treatment: 11 modes of vibration
between 5 and 220Hz, which has rendered all
subsequent batches completely stable.
(d) A VCM80 AC-VSRS was specified by Short
Bros. For this work at their subcontractors and
they have used their own VCM80 for stabilising
mild-steel and aluminum composite fabrications
for many years. A VCM90 system was ordered
in January 1991.
(e) A failure rate of approximately 40% has been
reduced to zero on parts of mining and
quarrying equipment since Trellex (Trelleborg
Group) and Skega introduced the VSRP to
complex mild-steel fabricated components,
often only 14mm x 2mm in section x 4m long.
(f) In the same industry, some vibrating screens
now carry a three-year guarantee, thanks to the
AC-VSRP.
Vibrating Screen VSR after assembly
(g) Typically, screens are mild-steel fabrications
from 1m x 3m to 3m x 10m and 100 to 200mm
deep – usually a complex lattice of RHS, angle
and tubular members. If thermally stress
relieved, they usually distort badly and need
mechanical or thermal straightening, often
defeating the object of the original thermal
treatment. When no stress relief or thermal
treatment was used, butt welds that lacked
preparation and had their bead ground off,
leaving a weak joint, failed in service. Now, with
6
the introduction of VSRP, welders know that a
poor joint will break in the weld shop so they
ensure good joints. This ‘fitness for purpose’
testing is regarded by Goodwin Barsby, Parker,
Kue Ken, Pegsons, Babcock Power, etc. as a
good reason to use the VSRP, as in-service life
has, on average, tripled.
(h) Beams, 5m long x 140 x 300mm section,
fabricated from RTQ60 material, bowed 2mm
during rough machining. Thermal stress
relieving was not permissible on metallurgical
grounds. The VSRP has completely solved the
problem for British Steel.
Treatment of crusher support beams
(i) Deloro Stellite use an AC-VSRS at both their
UK and Canadian plants to stabilise carpet knife
blades. These are typically mild-steel bar, 5m x
150 x 10mm, grooved out and deposited with
stellite along one long edge. This edge is ground
to expose the stellite and form a cutting edge.
The mild-steel section behind is slotted to give
adjustments for the holding screws. Up to ten
year ago, it was common for 50% of the wear
tolerance to be lost due to movement taking
place during transport (typically UK to Italy).
Since introducing the VSRP, no movement has
occurred and tighter tolerances are maintained.
Hundreds of examples are on file: large copper plates
fully machined, screwed and dowelled; flow-brazed
aluminum instrument frames; powder-coated enameled
25mm x 25mm mild-steel angle instrument frames for
Marconi; mixed-metal fabrications for Helio tank turrets
and a wide variety of materials such as Inconel, Zeron,
duplex stainless steel, Ferraliu, titanium, P20 (1.7% Cr
steel), aluminum in TF condition, composite metal /
plastic, metal / rubber fabrications, etc. for these and
many other applications, the VSRP is invaluable.
However, it must be remembered that the VSRP cannot
be used on pressure vessels, pipework or any parts where
metallurgical change is necessary. To its benefit VSRP can
be used on all non-ferrous materials and on the hardened
materials that are commonplace on most mining and
quarrying components. No reduction or softening of
material properties occur with the correct treatment
Treatment of Large Fabrications
119,000kgs Tippler cage VSR after welding prior to machining
Treatment of the above tippler cage and its associated
components was carried out on behalf of Saldanha Steel
at DCD Dorbyl. The tippler cage weighed 119,000kgs
and treatment was in the range of 2 hours at resonant
frequencies, the complete assembly including the end
rings was in excess of 400,000kgs all of which was VSR
treated. A modern VSR system has the capacity to treat a
singular component of up to 200ton.
Opencast dragline buckets with weights of up to
70,000kgs are treated on a regular basis. Many are treated
after a major repair some are treated after fabrication at
the OEM suppliers. VSR has been proven to reduce
cracking and some manufactures in the USA are claiming
an increase in service life of 400% a figure suspected to
be grossly exaggerated although published in a leading
welding publication.
Feedback received from mines in South Africa would
suggest a figure of around 45% to be more realistic as
VSR could in no way influence the (wear and tear)
characteristics of a bucket with the exception of no
softening of the materials occurring during the stress
relief process
VSR treatment of a dragline bucket after repairs
7
CONCLUSIONS
Based on hitherto attained research results and experience
in practical use of VSR weldments the introduction of
VSR into practice can be recommended. The Vibratory
stress relieving can be employed for stabilisation of the
size of suitable weldments prior to their machining and
servicing as a replacement of stress relief annealing. The
VSR process is used for lowering of residual stresses and
stabilisation of the size of different weldments such as
frames of forming machines, machine frames, grey cast
iron castings, etc. which were up to now subjected to
stress relief annealing.
VSR does not negatively affect the static dynamic
strength of welded joints and weldments, fracture and
notch toughness and homogeneity of welded joints.
Based on the attained data the implementation of
VSR procedures as a replacement of stress relief
annealing for the stabilisation of weldments, castings
and forging leads to high savings of production costs
to our national economy. The saving of thermal
energy has to be emphasized first of all because in
the VSR procedure it does not exceed 1% of the
energy required for the annealing of weldments and
in average only 0.4% of production costs.
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