Miscellaneous

Steel Repairs

Welding

Electric Arc

Coating



The coating on coated electrodes serves several purposes

Coated electrodes are normally supplied in a water proof container. Once opened effort should be made to prevent as far as possible moisture ingress. Some coatings have a high affinity to moisture and must be kept in a heated cupboard. It is a good idea to keep all electrodes in this cupboard as moisture in the coating can severely effect the quality of the weld

Some hints and tips

Take a welding course

Rule of thumb when starting with an unknown set is 40amps per mm of rod e.g 2.5mm=100amps.

Increase the current slightly when using long extension cables. Try not to have these cables in a coil when welding

The quality of the weld is directly related to the surface preparation. Take a little extra time to prepare and shape the area to weld

Ensure all slag is removed , if necessary by using an angle grinder to gouge out any pin holes, before welding over a run

If your welding rod sticks then clean with an angle grinder the mark especially on critical welds such as pressure vessels. Check the condition of the coating on the rod. If the coating is damaged say with one side of the wire exposed discard the rod or burn it back to good on a non-essential piece

After finishing a run the end of the rod invariably gets a slag coating. When you come to strike again it is difficult so the tendency is to strike harder and harder. The consequence of this is that not only does the slag come off but a good piece of the coating leading to an erratic arc. Penetrate the slag cap by gently rubbing it on a rough surface, or use your fingers to snap it off (as I do)

Keep you glass clean- the fumes tend to coat the glass with a layer of dust. Keep wiping this off. If you cannot see the weld arc properly you have too dark a tint grade of glass. These tend to be supplied suitable for welding maximum amps ( say 300 Amps)

Where the shape of the weld is critical then use two hands. One to hold the holder the other grip the rod a couple of inches back from where the arc is. ( I used to do all boiler tube welding in this way and as I get older most of the vertical welding as well)

Position the piece to avoid as much as possible any other type of weld except horizontal- this is not a cheat but is common sense. Watch a coded welded how he works

Brace yourself against something before starting if possible. Loop the cable over your body so that the weight of the cable does not fall onto you hands

When doing a multi run weld into a narrow groove or right angle join use a small diameter gp rod for the root weld as this is critical and any slag inclusion will cause porosity in the rest of the weld runs

If you get porosity dont mess about putting a thousand runs over the top of it, grind the bugger out and start again

Don't be afraid to use rods. For awkward jobs bend the rod half way down if it helps. I welded an economiser using only the final inch of each rod which was bent to suit due to the position of the hole ( I also had to use a small inspection mirror to see it). Don't be tempted to use old part used rods for critical jobs as they inevitably have a high moisture content in the coating and make for a very porous and/or brittle weld.

When lighting an oxy-acet flame light the acetylene and increase slowly until you see the carbon smoke just disapear. The put on you oxygen.

When using oxy-acet gas cutters make sure you always have a bucket of water nearby incase of blow back.

If you are going to gas weld aluminium - don't. Reach for the TIG welder or if you must MIG. Or if you really, really must, stick weld. If you do not have these and have to gas weld make sure you well remove the oxide layer ( using a non-ferrous wire brush) and weld as soon as possible. Try to make up only the flux you need for the job that day. Get loads of practice.

PRACTICE-that is the main difference between a good and bad welder



Welding faults


Root Faults For deep vee multi run welds the first run or root weld is critical to the quality of the welds laying on top. Typical faults may be caused by too high or low a current of too large a rod .
Fusions Faults The three main causes of this is too low current for rod, too high a travel rate or when too small a rod is used on a cold surface
Bead Edge Defects normally in the form of under cutting or edge craters. The main cause for this is incorrect current setting. Too high will lead to undercutting, too low to edge craters. Similar effects may occur at the correct current due to incorrect arc length. Edge faults are particularly common in vertical welding or 'weave' welding. The general cause for the latter being a failure to pause at the extremes of the weave. Edge defects are stress raisers and lead to premature weld failure.
Porosity May have many causes the most common being moisture in the rod coating or in the weld joint. Poor rod material selection is also a factor
Heat Cracks this is a destructive fault caused generally due to incompatibility of the Weld material and weld Rod. Indeed in some cases the material may be deemed unweldable. Heat cracks occur during or just after the cooling off period and are caused by impurities in the base metal segregating to form layers in the middle of the weld. The layers prevent fusion of the crystals. The two main substances causing this are Carbon and Sulphur. A switch to 'basic' electrodes may help.
Another cause is tension across the weld which , even without segregation in the weld, cause a crack. This occurs during a narrow critical temperature range as the bead coagulates. During this period the deformation property is small, if the shrinkage of the base material is greater than the allowed stretch of the weld then a crack will result. One method of preventing this is to clamp the piece inducing a compressive force on the weld during the cooling period
Shrinkage Cracks These form due to similar effect of allowed weld deformation being less than base metal shrinkage although it is not associated with the critical temperature rang above and therefore cannot be elevated by compression. The use of 'basic' electrodes can help
Hydrogen cracks This is generally associated with either hardened material or material hardened during the welding process. The hydrogen source can be moisture, oil, grease etc. Ensuring that the rod is dry is essential and preheating the weld joint to 50'C will help. The cracking occurs adjacent to the weld pool and allied to the tension created during the welding process will generate a through weld crack.
Slag Inclusion This common fault is caused by insufficient cleaning of the weld between runs. If necessary as well as using a chipping hammer and brush grind back each weld run with an angle grinder. Once the slag is in the weld it is near impossible to removed it by welding only

Welding Fumes

Welding fumes are generated during the welding operation and consist of a mixture of the filler material and the base material gasses and dust.
The best method of preventing inhalation of these fumes is by forced air extraction. Where this is not possible then the personnel in the area should place themselves away from the general air flow.



 

  

   

  

   

 



The following lists the types of fumes and there potential risk   

Fumes from low alloyed or unalloyed steels- no heat treatment  

Generally considered a low health risk

Fumes from low alloyed or unalloyed steels- no heat treatment  

The surface treatment may cause harmful substances to be present and released during the welding process.


Galvanised or surface treated with zinc

The fumes given off may contain zinc oxide inhalation of which can lead to zinc fume poisoning. This is an unpleasant effect lasting for a couple of hours but it is not believed permenant. I have had this and it takes the form of shortness of breath and can be very frightening at its worst

Cadmium treated surfaces

creates cadmium oxide inhalation of which causes very harmful lung damage

Lead or mercury coating

Found only as a surface treatment on very old plate

Stainless steels

Fumes contain Nickel and Chrome gasses inhalation of which causes severe respiratory damage

Gas Welding or burning

Nitrous oxides can rapidly build up in enclosed areas when using larger nozzles. This becomes apparent by irritiation to the eyes and throat. where the torch has been incorrectly adjusted there is the risk of carbon monoxide being formed which is very dangerous and will lead to suffocation  

Note: When welding metals containing or having as a surface treatment Cadmium, Chrome, Lead, Mercury or Nickel then either adequate forced air extraction should be used or the welder should be supplied with an air fed mask- if this is you make sure you do it as the damage will only come apparent later on

Metal Inert Gas (MIG)

Also called Gas Metal Arc Welding (GMAW). Where CO2 is used as the shielding gas the system may also be known as Metal Active Gas (MAG). Generically the term MIG is applied to the welding sets.


The shield for the arc is formed from a supply of inert gas. Gas stored in a bottle is led via a flow regulator through a tube to the welding torch. When the trigger on th torch is depressed firstly the gas valve is opened and the shield gas emiited from the nozzle. Further depressing the trigger makes an electrical switch and the wire feed is activated and the metal wire electrified.
To start the welding operation the torch is held a set distance-sat 10-15mm, from the work piece, the trigger is pressed and the arc established. Note that the arc is not 'struck' in the same way as stick welding. To improve the arc creation is it advisable to sharpen the wire to a point before starting
Wire Stickout - The amount of wire sticking out of the holder at startup should be controlled. Too long and the weld arc is cool and may not be properly shielded by the gas. Too short and the holder tip can be overheated and weld spatter may enter the nozzle and cause turbulence in the gas flow.


There are then two basic techniques.


With the current set above a threshold (about 150amps for 0.8mm wire) the metal transfer is in the form of a continuous spray of molten metal. This gives good penetration and is suitable for thicker material ( greater than 3mm). With the current reduced the drop size increases until the arc cannot be maintained and the wire torches the material ( thus the technique is also known as 'Short Circuit transfer' but more generally as 'Short Arc'. The metal in contact melts and is replaced by the wire as it feeds through the tip. This is particularly suitable for positional welding and thinner material.
Two adjustments are available for control of the arc. One is for current, the other is for wire feed rate. The flow of gas can be adjusted to allow for such as environmental conditions e.g wind blowing.




The angle of the torch will affect the degree of penetration. Too small an angle will also reduce the effectiveness of the shield gas. Weaving the torch may be used for increasing the size of the weld lay. It is important if doing this to pause at the extremes of each weave. Failure to do this leads to undercut and weakness of the weld. When starting on a cold work piece penetration is reduced. To improve this the arc should be made on a separate piece attached to the work piece. Another technique is to add heat to the start area by starting the arc at an increased distance ( say 2 to 3 cm), the torch is then brought quickly back to the more normal 1-2cm. When finishing the weld the torch should be quickly brought out and the travel rate increase to reduce the heat in the weld pool before the trigger is released


Other gasses and gas mixes are in use but the above are the most common. For smaller sets flux cored wire is used thereby negating the need for a seperate gas supply



Tungsten Inert Gas (TIG)

Also known as Gas Tungsten Arc Welding


Frequently used in the welding of Aluminium brasses, Cunifer, and stainless steels. This is a particularly effective weld process particularly for the aluminium brasses such as yorcalbro often found in sea water systesm. The small heat effected zone is particularly important as super granulation causes a softening of yorcalbro leading to bulging and failure under pressure


Technique-The basic technique is a cross between Stick welding and gas welding. The arc is struck against the surface, the shield gas is ionised and a stable arc is formed. The electrode must be quickly brought up to the weld height to prevent contamination of the weld pool. The tungsten electrode does not melt. A filler rod of correct material is introduced in a similar fashion to gas welding. The electrode holder is held at a 75' angle and the filler rod held at 30' in the direction of the forehand travel.For TIG the shield gas has the added requirement of preventing oxidation and cooling the tungsten electrode.


Electrode


The electrode is tungsten or tungsten alloy ( with thorium or zirconium) which has a higher melting point. The electrode is grind, using the technique shown above to a point. Too fine a point and the tip can melt and contaminate the weld. Too steep and the arc is unstable and penetration poor. About 3 - 6 mm extends from the nozzle whose throat size is governed by the welding current.

Gouging

This refers to the technique of shaping or cutting metal using specialised electric arc rods. The arc is struck with the rod in the perpendicular position. Oncew the arc is struck the angle is reduced to about 20' ( do not point the rod into the weld. This allows for clean displacement of material. For vertical pieces the rod travel is down


Air Arc Gouging

This is a system similar in manner to standard gouging but using copper coated graphite rods through which compressed air is pumped. The rod melts the metal and the compressed air displaces it. This system is seen in commmon use for underwater repairs.

Friction Stir Welding

This very modern practice is now becoming increasingly used in shipbuilding particularly for joining aluminium.


Two pieces of material are butted together. The FSW head, consisting of a profiled probe rotating at high speed is brought into contact with the join. Heat is generated and the metal softened and forced around the the probe to the rear. In this way material from both pieces are merged and thus the join is main. The weld is made in the semi solid state. There is no sparking, fumes and a reduction in noise. Weld speeds are increase by about 10% over conventional means


Structural Repairs - minimising distortion

Distortion

Distortion occurs during electric arc welding process due to localised heating of the material and by the hot infill material cooling and shrinking.

These processes are well known and proper selection of preparation and weld technique have largely eliminated this as an area of concern. However faults can still occur and this is due mainly to poor workmanship.

Typical causes of defects are;


This shows typical angular distortion of a plate. Distortion may be transverse or longitudinal and may also cause bowing or dishing or some combination of all three


The amount of shrinkage is dependent on the following factors;


Although there is a risk of cracking due to the locked in stresses the reality is that this is very rare. A greater risk comes from the removal of the strongbacks after completion. These should be ground off and not 'hammered' as this tends to tear the parent material. Should a tear occur this has to be ground out, filled with weld and ground back.

Weld Procedures

There has to be a balance with the desire for a perfect repair against the practical and commercial viability of achieving this.

Distortion can be minimised by the use of double sided welds. However this may prove difficult to achieve in practice. instead single sided welds may be preferred with the use of backing strips.

It should be noted that many yards now prefer MIG welding due to its speed, reduced cleaning requirements and simplicity. The down side of MIG is lack of penetration and great care must be taken to ensure that welds using this techniques have been properly prepared.

To reduce distortion the following may be adopted;


The Cap on the weld is weld filler that does not contribute to the load bearing structure of the weld.

The use of double sided welding can reduce the amount of weld used however this is at the cost of time. Proper edge preparation can help to reduce the weld material required especially for single sided welding.

Bevelling one edge of a butt weld can help with compensating for single sided welds


Repair of Distortion

Should distortion occur there are two methods of restoring the shape.

Mechanical - This involves the use of direct force on the material. Typically this will take the form of strong back and jack. As these are considered point loads there is rela risk of further damage and car should be taken.

Thermal - This involves the application f localised heat. The amount of heat is governed by the material but generally the temperature should not exceed 650'C although this may be increased to 1000'C for low carbon and high tensile steels. Care should be taken with water quenching not to change the structure of the material

The plate is bent by applying localised heat, this causes a hot spot were the material expands but is constricted by the cold surrounding material. When the heated material cools it tends t bunch up pulling the plate inwards towards the original heat source


Where there is a large area area of distortion suggest as plate dishing, then straightening should be my several point heating rather than a single large one