A Study Into the Effects of Residual Stresses and Hot Dip Galvanizing

8/14/2019 A Study Into the Effects of Residual Stresses and Hot Dip Galvanizing

1/4

Hot dip galvanizing is an effective method of protecting steel fabrications and other structural components

against corrosion, its usage can be traced back almost one hundred years. A disadvantage of the galvanizing

process however, is that the large temperature gradients that are generated in fabrications, coupled with therelease of internal stresses during the galvanizing process, can often cause severe distortion during the

galvanizing operation. This paper looks at the causes of the distortion and the usage of a system known as

Vibratory Stress Relieving (VSR) as a means of either preventing or greatly minimising this distortion. The VSR

system is already being used in many parts of the world with a very high success rate on components prior to

them being galvanized, and thereby minimising the distortion, or as in many cases, completely eliminating the

distortion.

Distortion of a component following hot dip galvanizing is a problem that is encountered by Galvanizersworldwide. Although unacceptable distortion only occurs in a very small percentage of the tens ofthousands of components that are galvanized daily, it is this very small percentage that often casts doubt

and mistrust upon the galvanizing process and can thus further lead to fabricators often seeking other lessefficient means of protective coatings.Fabrications often contain a myriad of locked in stresses, these stresses are often caused through the coldworking of the metal, hole punching, and through the joining processes involved. Poor design, and in manycases poor engineering practices coupled with a lack of education as to the galvanizing process on the partof the fabricator can account for another source of unacceptably high stresses in the component.The temperatures as used in the HDG process will bring about a reduction of anything up to 50% in theyield strength of most materials. Although this reduction is only temporary, as the yield strength will revert

back to its normal strength upon the cooling of the material; this reduction combined with an uncontrolledrelease of stresses when immersed into the galvanizing bath will often bring about the unwanted and oftendamaging distortion.

Zinc temperatures of 450-460C will induce additional thermal gradients and stresses into the productbeing galvanized. These stresses may often be relieved at the galvanizing temperatures by conversion to plastic strains, possibly accompanied by various forms of buckling distortion. The magnitude of thedistortion is often a complex function of component geometry and dipping practice. Following removalfrom the zinc bath, the item may either be allowed to cool on the shop floor or it may be quenched bydipping into a bath of water. As with heating, the changes in temperature during cooling can generatethermal stresses.Structural beams form a significant percentage of the wide product range that is suitable for hot dipgalvanizing. Large fabricated beams are costly items and, owing to their size and strength, they may not beeasily straightened once having distorted.

Fig 1 Typical distortion in beams

A Study into the effects of residual stresses and the hot dip galvanizing process.

A means of preventing unwanted distortion. By JS Hornsey B.Sc of VSR (Africa )cc.November 2008

The principle source of the distortion in large beams is a

variation in the longitudinal stresses over the cross sectionsof the beams. Longitudinal stresses run parallel to thelength of the beam and there are three common types ofdistortion that can result from the variation in thesestresses. These are detailed in Fig.1The more complex distortion such as twisting is a result ofa combination of longitudinal, transverse and sheerstresses. Other minor influences upon the degree ofdistortion are sometimes caused by the liquid drag forcesincurred as the beam is withdrawn from the zinc bath. Thiswill also depend upon the position of the support points as

the effect will be maximised when the beam is beingwithdrawn as the yield strength of the steel will have beenreduced and the beam will be lacking the buoyancy effectfrom the molten zinc.


2/4

A structural beam following galvanizing should always be allowed to cool while resting upon a flat surface as anybeam at 450C with the corresponding reduction in yield strength while resting upon supports will experienceadditional forces due to the effects of gravity which will produce bending moments and further bending stresses inthe beam. These stresses will reduce naturally over time, the reduction can be also accelerated by the bumpingduring loading and whilst in transit and this then compounds the distortion, causing further problems on arrival at thework site.A particularly severe problem of beam distortion following galvanizing was noted by our associate company VSR(UK). The galvanizers were Hereford Galvanizing who at the time was contracted for the galvanizing of a largequantity of fabricated beams for Forth Engineering Ltd, contractors to the Ministry of Defence. The beams ranged inlength from 8m to 12m and all having additional braces welded to the webs, some of the beams would distort up to22 mm following the galvanizing. Initially the UK Welding Institute was called upon to assist and they suggestedvarious welding solutions, none of which worked. The Welding Institute then recommended that they try adding theVSR process whilst at the fabricators prior to the galvanizing. An on site study into the galvanizing process wascarried out and it was established that initially the beam was bending in its elastic state, reaching its peak deflectionat total submergence which would correspond to the maximum temperature differential between the upper and lowerflanges. Plastic (permanent) deformation commences following this as the beam heats up and the yield point of thesteel decreases. Further temperature increases result in continuing plastic deformation with the first, the lower andhottest flange yielding resulting in a permanent bending of the beam, with the top flange yielding to provide stressrelief and a reduction of the beam distortion. This is clearly detailed in Fig 2, amazingly this distortion occurred

within 3, 5 mins of total submergence!

Fig 2 Time estimates of distortion during dipping and removal

A photograph of one of the beams undergoing a VSR treatment is detailed in fig. 3. Fig 4 shows beam andgalvanizing details

Fig 3 Beam VSR treated prior to galvanizing. Fig 4 Beam and galvanizing details.

Initially the first beam received a frequency scan which was recorded upon a graphic print out for further referencepurposes. The beams were basically identical and as their natural frequencies are in part determined by size, shapeand mass, it was assumed that the other beams would be very similar in their modal response. The trial beams werethen treated at their 1st bending mode in each plane for 8 minutes, a total of just 24 minutes treatment per beam.Following the galvanizing process the beams maintained a tolerance of within 7mm, well within the specified

Beam dimensions Steel properties @ 30C

Height 1050mm Modulus of elasticity 200GNm2Width 390mm Coefficient of expansion 1.2x105K1Length 6-12m Thermal Conductivity 45Wm K Web Thickness 16mm Yield Point 277Nm mFlange Thickness 30mm

Galvanizing conditions

Bath temperature 455 CDipping angle 30 to horizontal with webs verticalDipping velocity average 500mm / minRemoval velocity average 1m / min

During the removal process, the top flange cools slightlymore rapidly, thereby increasing the stress over the rest ofthe beam. With the lower portion of the web and the bottomflange being hotter, it yields even more resulting in a smallincrease in the plastic (permanent) distortion.


3/4

tolerance of 10 mm rendering them all fit for service with no further rework after galvanizing. The procedure wasthen adopted to include VSR on all beams prior to galvanizing.In South Africa, Rosati Industries a well known company based in Isithebe Natal has been contracted to manufacturea large quantity of mobile pull out seating stands, the individual components of which are to be galvanized, refer Fig5 which shows one of the sections undergoing VSR treatment.The first batch of the seating decks that was sent for galvanizing distorted so badly it rendered them fit for scrap

purposes only refer Fig 6.

Fig 5. Seating platform prior to galvanizing VSR treated Fig 6. Items scrapped following galvanizing.

As can be seen in the photographs the components do not present an ideal shape for galvanizing owing to the thinmaterial, combined with numerous right angle bends, stiffeners and excessive stitch welding.VSR (KZN) was subsequently approached with a view to trialing the VSR process on the thin fabricated sections

prior to the galvanizing operation. The first item was treated at very low frequencies in both the bending andtorsional modes for a total of nine minutes. This item was then sent for hot dip galvanizing, and on completion ofgalvanizing it remained perfectly flat and required no rework, saving an immense amount of time and money in

wasted material and galvanizing costs. Tests are now underway with the manufacturing of a jig thereby allowing theVSR operator to treat up to 21 components in one batch.

Fig 7. VSR treated plate following galvanizing Fig 8. Test jig for treating batches of 21 plates

There is still considerable testing to be carried out regarding batch treatment on these plates in order for the VSRoperator to establish the correct frequencies and forces that are required to treat multiple plates, as each plate must beallowed to resonate freely in order to effect stress reduction.

Further components that were treated individually exhibited either minimal or no distortion. Owing to the largevolume of items that would require galvanizing the components can also be treated in pairs using a splitter box andtwo exciters which would then enable two plates to be treated in less than ten minutes. The success rate to date has

been such that Rosati Industries are now enquiring about purchasing their own VSR system for their in house work.


4/4

Components which are currently being treated in the UK prior to hot dip galvanizing include the long complexfabrications shown in fig 9 which are used as jigs in the manufacture of aircraft wings by Airbus Industries ofBroughton North Wales.

Of the hundreds of components that are treated daily in South Africa by using the on-site VSR service it is unknownwhat percentage requires the services of the hot dip galvanizers as no survey has ever been carried out.What is known is that where stress relief or component stability is required VSR can match that of thermal stressrelief a fact which is proven by the thousands of different users of the service on hundreds of different componentsranging from fan impellors, machine and pump base plates, through to heavy fabrications. A detailed listing of usersis available if required.

Vibratory Stress Relieving can be found in all major centers of South Africa. The process is quick, and it is cleanwith no scaling or discoloration to the component, most importantly it is fully portable, running off a 220v singlephase supply. Treatment capacity ranges from less than 1 kg to in excess of 150,000 kg, the process can be carriedout either at the fabricator or at the galvanizing plant.

Further information if required is available upon our website http://www.vsr-africa.com or the author can becontacted at

VSR(Africa)cc

PO Box 12272

Leraatsfontein

1038

Tel 013 6500702 / 013 6500287

[email protected]

Fig 9.Various bases being VSR treated at Airbus Industries North

Wales UK before the Hot Dip Galvanizing process. The

galvanized sections are visible in this photograph.

Owing to the extremely tight tolerances that are specified

during the manufacture and the post machining of these jigs

the only way that stability could be assured was to remove all

of the locked in stresses prior to galvanizing. The nearest

furnace for thermal stress relief would have involved a return

trip of over 250kms and with the complex geometry of these

components it was likely that further thermal distortion would

have occurred.

The easiest and the most cost effective solution was the use ofthe on-site services of VSR (UK).

Documents

A Study Into the Effects of Residual Stresses and Hot Dip Galvanizing