By Steve Hornsey of VSR (Africa)
Hot dip galvanizing is a highly cost effective method of protecting structural steel fabrications 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 the (uncontrolled) release of internal stresses during the galvanizing process, can cause distortion during the hot dipping and cooling of the galvanizing operation. It is not unusual for these distortions to exceed the allowable out of straightness tolerances for structural components. 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 high success rate on components prior to them being galvanized, and thereby minimising the distortion, or as in many cases, completely eliminating the distortion. The Vibratory Stress Relieving Service has been in commercial use here in South Africa since the mid-eighties and the service is now available in all major centres of South Africa, Botswana, Zambia and Namibia.
Distortion of a component during/following hot dip galvanizing is a problem that is encountered by galvanizers worldwide. Although unacceptable distortion only occurs in a very small percentage of the tens of thousands of components that are galvanized daily, it is this very small percentage that creates problems for the galvanizer and the fabricator thus leading to designers and fabricators seeking other less efficient means of corrosion protection. Fabrications often contain a myriad of locked in stresses resulting from the original rolling process, cold working if applicable, hole punching, and through the joining (welding) processes involved. Poor conceptual design and detailing of components, coupled with a lack of knowledge as to the galvanizing process on the part of the fabricator can also contribute to the high stresses locked into the component.
Research shows that temperatures as used in the hot dip process (450-460°C) will bring the steel into the temperature range where the yield strength of most steels will be reduced by about 35%. This reduction is only temporary, as the yield strength will revert back to its normal strength upon the cooling of the material.
TASS Engineering has been actively involved in structural and architectural steel fabrication and erection for more than four decades.
Current projects:
• Standard Bank Rosebank – Standard Bank
• Telesure Head Office – Steyn City Properties
• Medupi Coal & Ash Terrace – ELB
• Medupi Tripper Car & Sishen Tripper Car – Efficient Engineering
• Heavy Duty Workshop – Efficient Engineering
• Jig Feed Bunker Retrofit – Sishen – Bateman Projects
• Workshops, stores, external yard cranes – Hotazel – United Manganese of Kalahari
• South Deep Rock Winder
• Heineken Brewery – Heineken
• Road Gantries – SANRAL
• Bus Rapid Transport System – JDA
• Klipspruit Coalmine
This reduction combined with an uncontrolled release of stresses when immersed into the galvanizing bath will often bring about the unwanted distortion. These stresses to the combined effect can result in ‘plastic’ strains. In the case of plate girders the result is web buckling distortion. The magnitude of the distortion is often a complex function of component geometry and dipping practice. Following removal from the zinc bath, the component may either be allowed to cool on the shop floor or it may be chromate dipped in a cooler temperature than the molten zinc having a quenching effect. As with heating, the changes in temperature during cooling can generate unwanted thermal stresses. Structural beams form a significant percentage of the wide product range that is suitable for hot dip galvanizing. Large fabricated plate girder beams are costly items, and owing to their size and strength, they may not be easily straightened after distortion has occurred.
A structural beam following galvanizing should always be allowed to cool whilst resting upon a flat surface as any beam at 450ºC with the corresponding reduction in yield strength while resting upon supports will experience additional forces due to the effects of gravity which will produce bending moments and associated bending stresses in the beam.
These stresses will reduce naturally over time (the ageing or weathering process). The reduction can be also accelerated by bumping during the loading operations and whilst in transit on the back of a bouncing truck/trailer which can compound the distortion, causing unexpected problems on arrival at the work 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 large quantity of fabricated beams for Forth Engineering Ltd, contractors to the Ministry of Defence. The beams ranged in length from 8m to 12m and all having additional braces welded to the webs, some of the beams would distort up to 22 mm following the galvanizing. Initially the UK Welding Institute was called upon to assist and they suggested various welding solutions, none of which solved the problem.
The Welding Institute then recommended that they try applying the VSR process to the girders at the fabricator’s workshop prior to delivery to the galvanizers.
An on site study into the galvanizing process was carried out by observing deflection for a plate girder being dipped in the vertical direction, bottom flange first into the zinc. It was established that initially the beam would bend within the elastic range, reaching its peak deflection at total submergence which would correspond to the maximum temperature differential between the upper and lower flanges. Plastic (permanent) deformation commences following this as the beam heats up and the yield point of the steel
decreases. Further temperature increases result in continuing plastic deformation with the first, the lower and the hottest flange yielding resulting in a permanent bending of the beam, with the top flange yielding to provide stress relief and a reduction of the beam distortion. This is clearly detailed in Figure 2 amazingly this distortion occurred within 31/2 minutes of total submergence in the bath of molten zinc!
A photograph of one of the beams undergoing a VSR treatment is detailed in Figure 3. Figure 4 shows beam and galvanizing details.
Initially the first beam received a frequency scan which was recorded upon a graphic print-out for further reference purposes. The beams were basically identical and as their natural frequencies are in part determined by size, shape and mass, it was assumed that the other beams would be very similar in their modal response.
The trial beams were then VSR treated at their first 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 tolerance of 10 mm rendering them all fit for service with no further rework after galvanizing. The procedure was then adopted to include VSR on all beams prior to galvanizing.
Tass Engineering (Pty) Ltd, a well-known and respected Johannesburg based company specialising in structural steel had been contracted to fabricate a large quantity of beams for Eskom’s Medupi Ash and Coal Project. The average size of these beams were 1m in height by 10m in length with a flange width of around 450mm. The welding process was completed using submerged arc welding, (a process which can minimise welding stresses owing to the lower cooling rate of the welding).
Following the hot dip galvanizing process areas of buckling distortion were measured along the webs of the beams, the specified tolerance of which was 7mm. Following galvanizing the worst amount of the distortion measured was 11mm and as such the entire batch of beams was rejected by the on-site inspector. (See item 3 of Figure 1 for a drawing of the distortion).
After trying, without too much success, various methods of mechanical straightening of these beams, the VSR Witbank office was approached with a view to vibratory stress relieving the remaining batch of beams with the required end result being that of limiting web distortion to within tolerance if not completely eliminating the distortion.
Treatment of these beams averaged around 25 minutes each with the first resonant frequency being around 34Hz. Following the VSR treatment the worst of the distortion had been greatly reduced, in some instances by as much as 8mm with the remainder of
the buckling all being brought back to within the required tolerance.
Prior to VSR treatment the expert staff at Tass Engineering had attempted to press out some of the distortion but owing to the high residual stresses within the structure this proved to be impossible. Although VSR had brought the beams back into tolerance, a mechanical press was used to remove some of the remaining high spots and owing to the material relaxation brought about by the successful stress relief process the beams were now easy to process.
The fabricated beams were treated with the intent of removing the buckling distortion that was caused by the varying temperature gradients induced during the Hot Dip Galvanizing Process and the results obtained were as required. Technically though this is not the ideal scenario for the treatment of beams of this type and size. Through the additional handling of the beams and combined with the attachment of the VSR equipment (using heavy duty clamps) to the component there is always the risk of damaging small areas of the xpensive galvanized coating which of course is undesirable to the end user. This can lead to improper repairs to the coating which then shortens the lifespan of the corrosion protection.
A far more satisfactory result would have been achieved by the inclusion of the VSR process at the final stage of the beams production or just prior to the galvanizing process being carried out. When the high (and often uneven) stresses in the components are released by the raised temperatures during the galvanizing process, unwanted distortion usually occurs. A beam treated in this manner if the galvanizing procedure and subsequent storage of the beam whilst cooling is correct would exhibit little or no distortion along its axis. It is unknown what percentage of the hundreds of components that are treated daily in South Africa using on-site VSR services, requires the services of hot dip galvanizers as no survey has ever been carried out.
What is now known is that where stress relief or component stability is required VSR can match that of thermal stress relief. A fact which is proven by the thousands of different users of the service on the hundreds of different components ranging from fan impellors, machine and pump base plates, through to heavy fabrications. A detailed listing of users is available if required.
Components which are currently being treated in the UK prior to hot dip galvanizing include the long complex fabrications which are used as jigs in the manufacture of complex aircraft wings by Airbus Industries of Broughton North Wales.
Vibratory Stress Relieving can now be found in all major centres of South Africa. The process is quick, and it is clean with no scaling or discolouration to the component, and more importantly there is no unwanted change to the materials properties or loss of material yield strength. The system is also fully portable, running off a 220v single phase supply, with no atmospheric pollution as is in comparison to a thermal stress relieving oven.
One must always remember that VSR is not a replacement for thermal stress relieving, it is merely an acceptable alternative, and there still remain some applications that will require the use of a furnace. Where a metallurgical change is not required VSR is fast becoming the preferred method of stress relieving owing to time and cost savings.
Treatment capacity ranges from less than 1kg to in excess of 150 000kg, the process can be carried out either at the fabricator’s own premises or at the galvanizing plant.
