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FINISHING PROCESSES
Vibrations reduce stress
levelsReprinted from European Surface Treatment Winter 1992/93 by VSR(Africa)cc August2001
Roger Claxton, The Vibratory
Stress Relieving Co.
Various vibratory methods have beenemployed, in the last 60 years, tomodify the stress patterns of
engineering components beneficially.Portable equipment developed in the
UK, has established Vibratory StressRelieving (VSR) as a viable
alternative to Thermal StressRelieving (TSR) for the reduction and
redistribution of such stress, ensuringthe stability of the component during
both machining and service. In thecase of surface coated components
where TSR is precluded, VSR isinvaluable. These stresses are
responsible for many common problems, from instability during
machining and inability to achievetight tolerances, to change in size orshape between the manufactures andcustomers inspections, either in
transit or during installation. VSRcan, in the main, eliminate such
occurrences, even in finish-machinedcomponents.
Components ranging from less than1kg to over 100t are treated, in
minutes rather than hours. Castings,welded fabrications, bars rolls and
shafts are all treated, and in a widevariety of metals. On accurate
components, the process is generallyapplied after rough machining. VSR
is even applied to some finishedmachined components. This
combined with VSRs option oftreating mixed metal fabrications,gives the potential user many
advantages
VSR versus TSRAs an alternative to TSR, VSR hasmany advantages and some
disadvantages, and both must betaken into account.
Time and cost savings with VSR areconsiderable. Floor to floor treatment
time is often as little as 15 minutes.Cost always compares favourably
with that of TSR, and often offers
substantial savings. The process doesnot significantly distort or discolour
components as the furnace would.Environmental factors are also
significant; VSR uses only a smallamount of electricity per component,
there are no flue gases, no cleaningand of course, no transport involved.
Noise was once a problem with somecomponents but as internationalresearch has refined the process,
noise has been dramatically reducedto acceptable dB levels.
Unlike TSR, the process doesnot confer any metallurgical
benefits. Depending on
requirements, this canoccasionally preclude VSR.
Conversely, the fact that VSRdoes not alter metallurgy can
allow greater freedom of design;the process is suitable for mixed
metal fabrications where there issurface hard facing, and even
rubber coating.It is often said that TSR isrequired initially to improve the
machinability of steel. This onlyapplies where thick, flame cutitems have edges to be
machined, and even then good practise dictates such items
should have been normalised prior to fabricating to ensure
good weldability. The cost of
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Shaft EG with strain gauges being VSRd by RAE
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TSR of these small items and
VSR of the finished componentsis less than the cost of thermal
treatment of the finished item.
Occasionally steel plate containsabnormal impurities, such aschrome inclusions, which make
machining difficult. As thiscondition would be unknown in
advance of machining, TSR atapproximately 625C would not
improve machinability.Obviously a high temperature
TSR is one solution. With VSR,an initial, low-cost stress relief
charge is the only expense. TSRwould involve treatment, andtransport, twice. In addition, two
thermal treatments can have a
detrimental effect on acomponent.
As well as being quicker, cheaper and
cleaner, VSR has another majoradvantage; if greater accuracy than
can be achieved with TSR is required,VSR can be reapplied near to or, with
consultation, after final machining.Effectively treating machining
stresses and improving accuracy. Aleading military instrument builder
also applies VSR to semi-finishedassemblies to give better joint
stiffness and stability, re-torquing thejoint after VSR. Finally, it should be
noted that VSR is unsuitable forstabilising severely cold-workedmaterials, or those with exceptionally
high yields, and it is only suitable forhardened components where specialmeasures are taken.
Considerations and ProcedureVSR is now widely accepted andendorsed in many company standards.
However, in order to use VSR andconform to the requirements of
BS5750, the process should bespecified by the design authority or a
written consent sought. (The authorscompany has 25 years experience of
VSR and offers a free advice servicerecognised by the The Welding
Institute in the UK.) The differences between VSR and TSR must be
considered when choosing theappropriate process: remember, VSR
does not change the metallurgicalcondition of components and this can be good or bad, depending on the
circumstances. VSR does not degradematerial properties.Where necessary, VSR can be
reapplied prior to final machining.This accounts for its success in
achieving improved accuracy and
stability compared to TSR. HoweverVSR is generally applied once, eitherbefore or after rough machining.
Components are placed on rubbersupports and the exciter is attached.
The operator then scans the frequencyrange of the equipment (0-220Hz),
noting instrumental feedback on the
components resonant responses.These are then dwelt at, on the sub-resonant base point and mid point on
the flank of each response. The peakof each response is then traversed
before progressing to the next. A testcertificate is produced for each
component either graphically or bydata logging.
Those with no experience of
VSR often ask how complexfabrications or castings can beadequately stabilised. Each
resonant response of thecomponent has an associatedloading pattern (mode).
Different modes affect
different areas of acomponent. The higher theorder of the resonant
frequency, the more complexthe loading pattern, exciting
interspersed, individualelements of the structure. Thesuperimposition of the cyclic
strains of VSR on the materialstrains, the full assessing and
maintaining between fixedstrain limits of the materials
cyclic properties, and the
consequent lowering andredistribution of stresses,render the component stable.
The potential VSR usershould remember that some
VSR systems, particularlyolder ones, have a much more
limited frequency range thanthe most up-to date models,
and would be unsuitable forsome components, not least
some of the examples givenlater in this article.
EXAMPLES
Precision pump bodiesVSR is invaluable in thetreatment of materials whoseproperties would be impaired
by a full thermal stress
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Hewson & Turrel fan on dynamic balancing machine after VSR
Repaired pump
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relieving treatment.Westinghouse have found thiswith their precision-machined
pump bodies, manufacturedfrom precipitation-hardened
aluminium alloy castings(LM25TF). Over a number of
years, the pump bodies,
measuring approximately 375x 300 x 150mm, had beensubjected to highly skilled
machining and extensiveinspection, but were
frequently rejected followingdelivery to the customer,
because of distortion up to0.15mm across the machined
surface. This wasunacceptable for the intended
use.The delicate metallurgical balance of quenching and
ageing required to achieve the
desired mechanical propertiesdoes not allow any further
TSR treatment.A finished machined and
inspected casting wassubjected to VSR, and a
distortion during treatment of between 0.10 and 0.15mm
was observed over themachined surfaces. Following
a series of trials, a satisfactorysolution was found; VSR was
applied to each casting justprior to the finish machining
operation. At this stage, therewas still 0.5mm machining
allowance on all faces. Sincethe introduction of this cycleof operations, the rejection
rate for the pump bodies has been reduced to zero withover 500 treated, and no
subsequent problems withdimensional stability have
been encountered. Nowseveral associated castings are
also treated, with the effectthat the pump life has been
considerably extended, andquieter, smoother running
conditions are achieved,
Machining tools /
supreme accuracySome eight years ago, DeanSmith & Grace became
disillusioned with thermalstress relieving when cast iron
saddles, consistently intolerance on final inspection
in the UK, were20% out onarrival in the USA,necessitating rework. VSR
solved the problem and today,rely on it solely to stabilisesaddles, beds, etc. Before that
time QA records showed that,
using thermal stress relief98% of beds were reworkedin-houseafter finalmachining
due to movement duringhandling. Subsequently, only
one of 533 beds was reworked 0.2%.
Dean Smith & Graces
subcontract machine shop alsofinishes mild steel fabricatedbeds up to 10m long and 1m x
900mm section, basically in12mm plate, but with sideway
sections up to 100 x 300mmand weight up to 12 tons. A
10m bed has a welding timeof approximately 50 hours.
The fabrication is solely VSRtreated. Operation procedure
is to fabricate, apply VSR,inspect, rough machineremoving up to 35mm to
produce slideway profile, ship
to Dean Smith & Grace, applyVSR and finish machine to
five microns in 6m bygrinding. No machinability
problems are encountered atany stage, despite extensive
machining of flame cut edgesup to 100mm thick. For QA
purposes, a graphic record isproduced. This is also used by
the end customer as valuabledesign feedback data with
regard to the vibrationcharacteristics of the beds.
Following comparison trialsin 1990 between thermal
stress relieving and VSR,Bradford Universityrecommended VSR for
stabilising the famous ButlerElgamill machine beds. Theseare of similar size and require
similar accuracy, but are castiron and have slideways on
two adjacent slides.
Repaired pumps and fansHewson & Turrel have found
VSR indispensable for twovastly different product
groups: fans/impellers and pump housing repairs.
Aluminium, stainless steel,cast steel and chrome
molybdenum steels, steel pump housings passing
corrosive or abrasive mediawear badly in service, and are
built up by MMA and MIGwelding then machined backto correct dimensions. Prior to
the introduction of VSR,service leaks occurred.Housings treated after rough
machining, at frequenciesbetween
80Hz and 170Hz, were foundto change shape up to 0.8mm
TIR. On Finish machining andin service, components have
been stable with no leaks.Similar advantages are
reported from Turkeysleading metallurgy laboratory,
Zumre Metallurgi, but heremarine valve stems and bodies
350mm diameter with 4140steel base metal are first
preheated to 250-350C, thendeposited with Stellite gr6while the part is vibrating at
an undisclosed highfrequency. This technique hasresulted in an approximate
20% reduction in pinholes andflaws generally, improved
dilution and given an easiersurface to machine as the bead
profile is reduced byapproximately 50%. (If the
damage is more than 6mmdeep, the crater is first built up
with SS GR316L by TIG andthen Stellite deposit overlaid.)
New and reconditioned,Stellite deposited plastic
moulding machine screws andbarrels are similarly treated at
many companies worldwide,both to stabilise and to makestraightening easier.
The other main use of VSR atHewson & Turrel is tostabilise fans and impellors
ranging in size from 800mm
diameter x 100mm to 2mdiameter x 900mm infabricated mild steel and
stainless steel. Sometimesthese are repaired
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Another type of repaired pump HSG
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components, and sometimesreplacements. After fabricating, but prior to
dynamic balancing, thecomponents are subjected to
VSR. Since introducing thistreatment, no fans or
impellors have gone out of
balance in service, even underhot conditions hitherto a
troublesome area. Installationsare now much quieter and lastlonger between overhauls.
Novenco Aerex, the UKslargest fan and impellormanufacturer, have had their
own VSR unit for many yearsand endorse the benefits stated
above. Their Canadian plantalso uses VSR. In both cases
the system paid for itself in 4 5 months. Rubber coated,
steel fan blades have beentreated to overcome
instability.
Moulds and diesThe periodic application of
VSR to moulds and dies toreduce the build up of in
service surface stresses hasreceived much publicity
recently especially in theUSA. This application isneither new nor demanding,
and most VSR can be used.For over 20 yearscomponents, typically 1m x
200-300mm have been roughmachined and VSRd by
many leading companies. Sub
resonant vibrations aretraditionally used for solidcomponents. More flexible
components are treated by thenormal resonant method.
Coopers Payen Ltd found thatcomplex surface stress
patterns resulting from
induced stress relievingduring grinding, made itimpossible to obtain the
required P20 die and mouldcomponents (typically 300 x
100 x 250 x 8mm). Treatingthe components on a vibrating
table has proved veryeffective. Components are
required flat and parallel towithin a few microns, even at
high temperatures
Minesweeping multiplanes
flatness of 5mm to 500mmMultiplanes for minesweeping weresubject to severe vibration and stressduring operation, and suffered from
cracking which leads to prematurefailure. The cracking was of acorrosion fatigue type. The
components were fabricated from13mm thick 316 and 347 stainless
steel, and varied from approximately1.15m to 1.65m square by about
250mm deep. The root runs were TIGcapped with five runs of manual
metal arc, using 19/12 Oerlikon fillermaterial.
Initially, thermal treatment was usedto increased service life. This
consisted of heating to a temperaturebetween 1,000 and 1,050C, followed
by rapid air cooling. In addition, theweld toes were ground to a radius of
about 12mm. Replacement of TSR byVSR resulted in a doubling of servicelife. Shot peening the weld toes to
induce compressive stresses into thesurface was used after VSR, in anattempt to obtain further increases.
VSR became standard procedure onthese components after sea trials.
Rolls and ShaftsFigure 1 shows one of eight unstable
alloy steel, forged drive shafts, 24mmx 2m in A1, T condition being VSR
treated and monitored using surfacestrain gauges. The results showed that
VSR reduced surface stress to safelimits, stabilising the component
while not reducing fatigue life oraltering material properties. Also,
VSR and strain measurements clearlyidentified the shafts which had been
correctly TRSd and those which hadnot.
Hard power enamelling to soft
rubber liningKnowing about VSR has saved many
an engineers skin. Problems
frequently occur either because stressrelieving is thought unnecessary, orbecause it is omitted by mistake.
During the machining of pads on 10fabricated RHS beams, 150mm x
150mm x 6mm x 7m for a mechanicalanalysis machine, unacceptable
bowing occurred. As the components
had a powder enamel finish, TSR,was out of the question. All thebeams were subjected to VSR, there
was no cracking of the coating, andthe components were subsequently
completely stable.Similar problems were overcome
with a 1m x 5m x 9m enlargingcamera slideway, this time with a
spray hammer finish, which wasunstable during final tests.
Cast steel pipes not called upon tobe stress relieved, were lined with anabrasive and corrosive resistant,
rubber surface coating. In service the
flanges distorted; after uncoupling forroutine maintenance work, they
would not reseal. They weredismantled, VSRd re-skimmed and
gave no further trouble. Allsubsequent parts have been VSRd
prior to matching and
there have been no furtherproblems.
Extremely precise instrumentsneed to be surface finishedusing the chemical blacking
process. In case where the
components are required to bestable and accurate to within afew microns, problems occur.
This is due to prolonged soak,in a caustic medium at 290C,
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Cooper-Payen dies/moulds
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which is necessary to producethe FE304 surface coat. VSR,applied just before final
grinding and blacking, hascompletely overcome
instability.In the plastics, paper, chemical
and powder industries, there
have been many cases wheremirror finishes have beenrequired, often introducing
high surface stress. Wheneverinstability occurs on a
component, VSR comes to therescue and is subsequently
written into the manufacturingspecification for future
production.
Surface hardened
componentsToday there is a greater variety
of surface hardeningtechniques than ever. VSR cansolve many of the associated problems. With weld
depositing or surfacetransformation using lasers,induction, carbonisation, Arc
lamps, etc, surface stressingdue to volumetric changes can
cause problems with long termstability. As milling introduces
approximately 10 times morestress than planning, VSR is
also commonly applied toovercome machining
instability. VSR is appliedeither before or after rough
machining as appropriate, andgives stability and flatness
commensurate
With the machine producing
the surface.
Galvanised beamsFollowing an enquiry from a
hot dip galvanising company
some years ago, VSR is nowregularly used by structuralsteel joist manufacturers when
the end product is to begalvanised. Stresses and stress
relieving induced duringdipping influence the welding
and straightening stresses,often causing columns and roof
beams to drift out of acceptablelimits. The process is most
effective in beams and mesh baskets, but appears to have
little effect on sheet metalcomponents. The procedurehas largely eradicated the
problem of components,particularly asymmetric ones,bowing. All for as little as 20
per beam!
Paper Reprinted by
VSR(Africa)cc
August 2001
For more information
Tel 27 135 6500702
27 135 6500287
Fax27 135 6501308
Website
http://www.vsr-
africa.com
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