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

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°(degree)C) will bring the steel into the temperature range where it will lose about a reduction of anything of about 35% in the yield strength of most steel grades. 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 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.

The principle source of the distortion in large beams is a variation in the longitudinal stresses over the cross sections of the beams. Longitudinal stresses run parallel to the length of the beam and there are three common types of distortion that can result from the variation in these stresses.  These are detailed in Fig.1

The more complex distortion such as twisting is a result of a combination of longitudinal, transverse and sheer stresses. Other minor influences upon the degree of distortion are sometimes caused by the liquid drag forces incurred as the beam is withdrawn from the zinc bath. This will also depend upon the position of the support points as the effect will be maximised when the beam is being withdrawn as the yield strength of the steel will have been reduced and the beam will be lacking the buoyancy effect from the molten zinc.

 

Fig. 1 Typical distortion in beams

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 the 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 many 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 Fig. 2 amazingly this distortion occurred within 3½ minutes of total submergence in the bath of molten zinc!

 

Fig. 2 Time estimates of distortion during dipping and removal

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

A photograph of one of the beams undergoing a VSR treatment is detailed in fig. 3.

Fig. 3 Beam VSR treated prior to galvanizing.

Beam dimensions – Height 1050mm, Width 390mm, Length 6-12m, Web thickness 16mm, flange thickness 30mm

Steel properties at 30ºC – Modulus of elasticity 200GNmˉ2, Coefficient of expansion 1.2×10ˉ5Kˉ1, Thermal Conductivity 45Wmˉˡ Kˉˡ, Yield Point 277Nm m²

Galvanising conditions – Bath temperature ±455 ºC, Dipping angle 30º to horizontal with webs vertical, Dipping velocity average 500mm / min, Removal velocity average 1m / min

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 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 was 1m in height by 10m in length with a flange width of around 450mm. They were fabricated using structural grade steel. 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. (The distortion simulated that as can be seen detailed at item 3 in fig.1).

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 at 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 had proven 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.

 Examples of fabricated structural beams undergoing VSR treatment to effect a reduction in the distortion.  (Courtesy Tass Eng)

The fabricated beams as shown above are being treated with the intent of removing the buckling distortion that had been caused by the varying temperature gradients that had been induced during the hot dip galvanising process and the results obtained were as required.

However, technically 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 expensive galvanised 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 galvanising process being carried out. It is after all these high and often uneven stresses that are locked into the component that when rapidly released by the galvanising temperatures then bring about the unwanted distortion. A beam treated in this manner if the galvanising procedure and subsequent storage of the beam whilst cooling is correct would exhibit little or no distortion along its axis.

Of the hundreds of components that are treated daily in South Africa using the on-site VSR service it is unknown what percentage requires the services of the 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.

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

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 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 of the on-site services of The VSR Co. (UK).

Vibratory Stress Relieving can now be found in all major centers of South Africa. The process is quick, and it is clean with no scaling or discoloration to the component, and more importantly, there is no unwanted change in 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 1 kg to in excess of 150,000 kg, the process can be carried out either at the fabricators own premises or at the galvanizing plant.

Further information, if required, is available from the VSR website http://www.vsr-africa.com