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Institute of Materials Finishing
The Southern Branch of the IMF held a mini symposium entitled “Sparks will Fly” on Wednesday 4th October at Lloyds Register Global Technology Centre based in Southampton.
Meeting 4th October 2017
The meeting was in conjunction with The Welding Institute and was a first in terms of a joint venture.
The subjects chosen for the evening were:
- A History of Diving Helmets.
- The chemical cleaning of weld scale prior to Surface Finishing
- Determining the Thickness of TSA Coatings on Stainless Steel
- The importance of Surface Inspection for crack detection in welded materials
The first speaker was John Bevan, chairman of the Historical Diving Society.
The museum based at Gosport in Hampshire is claimed to be best diving museum in the world together with the best collection of diving equipment ranging from recreational to commercial/professional to military.
The talk was aimed at the development of the diving helmet which began in 1823 and was originally targeted at being a smoke helmet. Charles Dean from Deptford was a caulker in the shipbuilding yard and he was concerned about entering rooms and ships which were full of smoke to risk rescuing people and property and he invented a smoke helmet. The patent was for a copper helmet attached to a leather dress fed with air from a wooden bellows with the exhaust being fed from a pipe which ran down his leg in the hope that it would keep the smoke out of the dress.
The process did not really work and the fire brigades were not interested and they did not think it would carry on.
His brother John Dean thought that it might be more beneficial being used under water as the only method originally was the use of a diving bell.
One of the original experimental helmets was on show and it was likened to wearing a bucket upside down on the head. Air was pumped in keeping the head free of water but a problem was that if the diver leaned over, water would rush in from the back of the helmet and risk possible drowning.
The dress material was substituted by a waterproof material invented by a Mr Macintosh and this gave a degree of dryness to the diver but did not stop the water from still rushing in if the diver bent over.
They experimented with many different alternatives allowing the air to leak out through special rivets but these also led to problems with the diver not being able to see due to the stream of bubbles.
John went on to talk about how the glass was sealed against the helmet to prevent water ingress and also how the glass was protected from damage with copper bars being placed in front.
The supply of air was always constant with pipes supplying the input and exhaust from the helmet, the exhaust being led down through the leg.
There were further refinements and variations on the helmet and by 1839 the two brothers were successfully using a working helmet and other people were expressing an interest and the helmet was now being sold.
There was still the issue of air leaking if the diver bent over and this was still a big drawback with the “open diving helmet”.
Further refinement by a man called George Edwards led to a sealing of the “dress” to the helmet. This was known as the “closed dress” and it left the diver completely dry and sealed off from the water allowing him to bend over and move more freely.
From 1840 this helmet became the principle system for diving helmets and remained the same until 1985 in this country.
In 1985 the Health & Safety executive outlawed the use of this type of diving equipment as unsafe even though it had worked well for the last 150 years with a high safety record; the reason being was that it did not have a secondary emergency air supply. This was corrected by the use of the diver carrying an emergency small tank of air in case the main supply failed.
Modern day helmets (Kirby Morgan) are now made using fibre glass materials as compared to copper and are capable of being used in “deep diving” (>50m in depth) compared with “shallow diving (<50m in depth).
Deep Diving requires a gas supply of oxygen/helium compared to air for shallow diving, the reason being that air is narcotic at depths greater than 50m.
The one disadvantage of the modern helmet is that because of the design, the helmet sits on the head with a neck seal to prevent water ingress. The older style had a collar allowing the helmet to sit on the shoulders and therefore distributes the weight better.
Often modern day divers have to give up diving after a time because of the weight (which can be around 20+Kgs) causing a strain to the neck muscles.
Contrary to this it does allow the helmet to turn with the head allowing for full vision whereas the older style remains stationary and the head swivels inside the helmet allowing the need for more than one window.
Our second speaker was John Burgess who gave a talk on how to pretreat ferrous materials that had been welded.
Welded materials can leave detrimental fluxes thermal gradients after manufacture and it the removal of these that is necessary to ensure good adhesion for either electroplating or painting.
The theory behind how the heat gradient and the formation of flux residues was shown.
John outlined the different stages of cleaning cycles that are required.
The use of alkali materials and surfactants in cleaning metals was demonstrated followed by the importance of acid cleaning to remove any weld scale that could be present.
One area that particular attention was paid to was to the treatment of high tensile materials. These can be very susceptible to ingress of hydrogen which can result in hydrogen embrittlement leading to cracking of the material.
This was also demonstrated using schematic drawings.
Of course most of the talk was how to treat small to medium sized parts but the treatment of very large areas (eg ships, aircraft) required a different technique and this was touched upon at the end of the discussion.
Our third speaker was Peter Ho, General Manager from Fischer Instrumentation who spoke about the measuring of thermal sprayed aluminium (TSA) on stainless steel using non-destructive instrumentation.
Peter described how thermal spraying was carried out using a wire or powder material together with a gas to spray it onto a substrate to form a coating primarily for corrosion protection.
He described the different types of spraying and equipment used. Prior to this process, painting was used but the discovery of TSA has led to a better protection coating especially where the application is used in the North Sea.
Peter gave an example of the wind turbines which were failing in the supports due to corrosion of the painted metal under the water leading to water ingress and corrosion.
Sending people out to maintain this was very costly so by using TSA at thicknesses of around 200 microns would give protection of up to 30 years.
He then went on to describe the different types of measuring devices from simple magnetic induction on carbon steels (up to 100mms depending upon the probe) to a phase sensitive eddy on stainless steels since most stainless steel is non-magnetic.
Typical thickness using these instruments is up to around 700 microns.
Peter described the importance of the substrate and the effects it can have on the probe to its sensitivity. The conductivity and porosity of the coating also have a major effect.
He described the preparation of calibration standards using similar spraying equipment to that used on the actual job.
Our final speaker was Phil Lever who is a specialist NDT (Non destructive testing) at Lloyds Register.
Phil has worked mainly in the ship industry and his main role is to advise & train surveyors in the field on NDT appreciation and how it is applied.
He described the difference between penetrant and magnetic particle inspection.(MPI)
MPI will only work on magnetic type material and works by inducing a magnetic field into the material and anywhere there is a surface indication it creates a leakage field. By the application of a coloured iron filing type material, the iron filings are attracted to the leakage field and show up the defect.
The process is quick, cheap and inexpensive to examine welds, castings and all manner of things that are ferrous.
Equipment used was explained and he emphasised that it was mainly for surface defects.
The method by which examination is carried out was explained and this starts with visual inspection using a high powered lamp followed (if required) by equipment testing.
When MPI is used then it is important that any paint is removed as anything >50 microns will have an effect upon the sensitivity and may lead to false readings.
Penetrant is more flexible. It can be used on any material that is not porous and widely used on non-ferrous materials. One important part in its use is that the surface must be clean, no presence of paint, as the requirement is for the penetrant to go into the crack and highlight the defect.
Typically red in colour and any defect will suck the penetrant in by capillary action. It is usually allowed to stay on around 20 minutes when the excess is wiped off with a solvent. A developer is then applied which draws out the penetrant out of the crack and spreads it over the surface. The developer is white in colour and highlights the red colour. The size of the indication is measured and reported. Cleanliness is all important.
Eddy current testing is used but is a slow technique although it can be used with coatings of paint up to 2mms thick. The process is very flexible and the probes can be made to any shape. They can be used in threaded holes, gear wheels, slots in the wheels of a plane as examples so it is a very adaptable process.
Phil presented videos describing how various eddy current processes are used and these described the process very well.
At the end of each presentation, questions were taken by the speakers, which led to a good constructive discussion amongst the audience.
The committee of the Southern Branch felt that the evening was very successful and would like to thank John Bevan, John Burgess, Peter Ho and Phil Lever for their time in giving the presentations and to Lloyd Register for hosting the evening
If you would like any further information regarding the IMF please contact Helen Wood at Helen@materialsfinishing.org
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