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Spot Welding Underwater
Speciality Welds Ltd (now Weldcraftpro) have developed solutions to the long standing problems associated with underwater wet welding and in particular, the skills required to produce high quality welds in poor or nil visibility. The system, which has been named Hammerhead in keeping with the companies 'fish' brandnames, addresses problems in obtaining high quality wet welds without the need for experienced (skilled) welderÂdivers.
By removing the individual welding skills from the operation, there's no need for the diver-welder to control parameters that affect quality, such as travel speed, electrode angle, arc length, accurate deposition, etc. Because the operator no longer needs to control these parameters, it's not essential to have good visibility. So, even in nil visibility conditions high quality repeatable welds can be produced time after time.
How the system works:
In removing the skills necessary to carry out underwater wet welding, Keats has modified the fundamental approach to how stick welding is carried out. The system allows the operator a far more simplified role. This is achieved, in simple terms, by creating a spot/plug weld, rather than having to despoit a fillet weld in a specific joint. By removing the need for a fillet weld deposit Keats has also simplified the joint configeration (simple lap joint) and all the extensive cleaning and preparation necessary, chipping off meters of slag prior to additional
passes, etc. In fact there's no need for additional passes, as the process is designed as a 'one-shot' approach, i.e. one electrode produces one spot/plug weld.
Other than the control system/electrodes, all other equipment is exactly as conventional 'stick' welding. The control unit is connected to the welding power source via the remote control facility and is powered by 110V supply. All welding leads pass through the 400 amps safety switch before going to the diver.
The role of the control unit:
The control system manages and manipulates the following:
First peak/high current setting
Second background/low current setting
The first high current setting allows the electrode to pierce through the materials therby, creating a hole through which both materials are joined together. The role of the timer is to limit the depths of the penetration so as to avoid bursting through the base (back) material. After the first weld cycle is completed and depth of penetration acheived, the second lower current is automatically initiated and its this current that then fills the whole, creating a spot/plug weld that has penetrated both sections of material, creating the weld nugget. During the operation the diver or indeed robot need onh to apply sufficient pressure to the electrode to push it through the material while welding. The guidelines shown below provide basic benchmark settings for selecting current and timer. The operator can then make any minor adjustments as are seen necesÂsary to ensure adequate weld quality. Presently only one size of electrode is available, namely 3.2 mm (1/8") but this covers a wide range of material thickness.
Weld strength properties:
The size of a given weld and therefore the number of welds required is based on the following principle. (Area of a circle with 'd' as the diameter of the weld).
pi d squared over 4
Therefore, a single spot weld can offer the following theoretical strength properties (neglecting any bending moment).
Max load = pi d squared over 4 x shear strength.
Basic benchmark settings for selecting current and timer (table not shown).
Typically the shear strength for plain carbon steel is generally assumed to be 4/ 5 the ultiÂmate tensile strength. The Hammerhead electrode offers a tensile strength of 650 N/ mm2 (94 ksi) and therefore, will offer a shear strength of approx. 520 1\.I/ mm2 (75 ksi). Therefore, a 10.0 mm (3/ 8") diameter weld nugget w ill produce a max load capability of 40.840 kN (9,181 Ib/f) per spot. The Hammerhead electrode offers a tensile strength of 650N/mm2 (94ksi) and therfore wioll offer a shear strength of 520Nmm2 (75ksi). Therefore a 10.0mm dia (3/8") nugget weld will produce a max load capability of 40.840kN (9,181 lb/f) per spot weld.
Using the following principle to calculate the overall stress acting on weld (G2.2) see pdf article for photo) i.e. Stress (UTS) = Force/area = 606Nmm2 = 39.8kN/Area 65.62mm2 .
The spot weld shown took 40 kN to fracture and had a total area of 65mm2. The UTS was 606N/mm2.
Compare this to a defect free multipass fillet weld (D2.2) fracturing at 259kN, but having a total area of 746 mm2, with a UTS of 347N/mm2. The spot weld offers nearly twice the UTS capability for a fraction of the deposited weld metal. It also offers strength improvements over normal ferritic steel electrodes and generally the heat affected zone (HAZ) hardness is improved. The specially formulated electrodes, which has a 27.5 Cr and 14.45 Ni equivalent, thereby, allowing for high percentage dilutions, of up to 50%.
The process clearly remains a manual welding operation and is not defect free. Trials were limited to flat bar/plate stock, welded in the flat and vertical positions. However, the welding skills necessary for a diver to produce a weld have been removed, as have much of the material preparations and need for good visibility. Of course, the operator will need to familarise himself with the process, but this can be achieved in a very short period. A good diver would be expected to produce an acceptable weld within a matter of minutes rather than hours or days. The process does offer solutions to welding in poor visibility, without the need for skilled welders. So all in all, this process offers a serious alternative to this long-standing problem. As an added bonus, the system also works very well for welding above water. A full and detailed report has been published following the DTi Smart Award trials and is available to download from the website at www.weldcraftpro.com. The report covers the welding of wet and dry fillet welds and wet and dry spot welds, and offers a direct comparison for weld quality and strength, whilst providing for the welding variables used. It also shows the results provided by non-welders and welders alike.
David J. Keats, proprietor of Speciality Enterprises (T/A Weldcraftpro) United Kingdom.
Published in Welding and Cutting 4 (2005) NO. 6