Welding

 

Welding

Learning Objectives

After completing this topic you will be able to:

• Explain the role of Flux and why the welding arc has to be shielded from the atmosphere and various methods employed to do so
• Recognize different welding processes and their application to shipbuilding and maintenance
• How to prepare the pieces to be welded, and understand the precautions to be taken while welding is carried out.
• Describe different types of welds, and the advantages and applications of each type
• Understand how and why weld faults occur during the welding proces. 
• Understand the effects of the welding process on the welded material and methods of minimizing the same.
• List the Destructive and Non Destructive Testing of welds as required by Classification Societies.
• Understand the theory and practice of Gas Cutting.

Let's start by defining the welding process. It is the fusion by heat of two metal pieces be they ferrous or non ferrous. 

The earliest recorded instances of welding were during the BC period when gold was the metal used and the process called "lap welding". With mans insatiable urge for war, welding came into its own with the discovery of bronze and its use in the manufacture of arms and armor.

As we have said welding is the fusion of two metals by heat. Where does this heat source come from? It come from an external heat source e.g. gas or more commonly  the arc created by touching and separating two base metals (electrode) to form an arc. The temperature of this arc is in the region of 4000⁰ Celsius when subjected to an AC or DC current flow between 20 - 600 Amps. The amperage may be controlled either manually or automatically to get the desired weld e.g. amount of metal etc

The following media explains about the Arc Welding Equipment

The high temperature of the arc melts the metal surfaces and causes them to "fuse" together. Additional metal is provided by consumable electrodes or filler rods when permanent electrodes are used. The welding may be:

• Downhand 
• Vertical 
• Overhead
• Inclined 

What are the various welding processes?

• Gas  or Oxy fuel welding
• Manual arc welding
• Automatic welding processes

1. Electro slag
2. Electro gas
3. Tungsten Inert Gas (TIG)
4. Metal Inert Gas (MIG)
5. Submerged Arc Welding (SAW)
1. Resistance welding

Gas or Oxy fuel welding

Gas cutting of metals 

Gas Welding:

Also known as brazing uses a hand held flame torch and a filler rod. The flame is directed at the parent metal with the metal being provided by the filler rod. This process is used mainly in the jewellery trade; however on board ships, gas welding is frequently used on non ferrous piping for maintenance and repair purposes.

The following media explains about the Gas Welding

Gas / Oxy Fuel Cutting

A torch is used to heat metal to its kindling temperature. A stream of oxygen is then trained on the metal, oxidising it into a metal oxide that flows out as slag

The following media explains about the gas cutting operation

A torch with a dual gas flow system is employed. One set of orifices allows the passage of Acetylene/ Propane/ LNG/ MAPP gas mixture and oxygen; and the other pure Oxygen. A manual or automatic control ensures the selection of gas. Initially the Fuel- Oxygen mixture is used to heat the metal to its oxidising temperature ie “red hot” (700 - 900⁰ Celsius) but not melting point. If the melting point is reached the work piece will not cut it will melt away. Subsequently the “cutting flame" i.e. the oxygen is emitted which oxidizes the red hot  metal which flows away from the heating zone as slag leaving a clean cut. 

Other methods of metal cuttings are 

• Plasma arc cutting
• Gouging
• Erosion or Water jetting

Manual Electric arc welding

Manual Arc Welding

The most commonly used welding process on board a vessel, consists of a transformer and a current regulator that supplies the necessary electricity, with the work piece and the electrode forming the two ends of the circuit. The electrode usually of ductile steel and covered with hydrogen control flux is struck on the work piece and separated. This forms the arc which fuses the metal along with the ductile steel electrode material with the burning flux providing a shield to the arc and the filler metal. As the arc moves on the flux rapidly cools over the hot weld into a hard coating which is easily chipped away once the weld is cold.

The following media explains about the working principle of Arc welding process:

Automatic welding process

Electro -Slag

Electro Slag Welding

Used in the shipyard, this process is most suited for vertical welds of 15 - 300 mm thickness. A copper cup called the "shoe" which is cooled, holds the molten metal at the arc. A continuous feed bare electrode with a consumable cap is inserted into the gap between the vertical plates and the arc is struck. Flux is fed into the gap between the plates until the molten slag reached the arc and extinguishes it. Thereafter the workpiece faces are melted by the resistance process through the slag (see resistance welding). The electrode is fed as the consumable cap moves in the direction of the weld along with the cooled "shoes". The cooling works to cool the welded metal.

Electro-Gas

Electro Gas Welding

Similar to the Electro slag process, with the main difference that the shield is made by CO₂ gas which is supplied through the cooled shoe. Applications of this process are also similar to the Electro slag process.

TIG

Tungsten Inert Gas

This is an arc welding process that operates at high temperature (over 3000 Celsius) to fuse the metal parts. While it is more expensive than stick welding, it is cleaner and more versatile and may be used with non ferrous metals like aluminium and brass. Though expensive it produces weld of very high quality.

It uses a non consumable cooled titanium electrode and argon gas as a shield. The arc is struck by passing a high frequency discharge through the electrode/ work piece gap. A filler rod may be used to supply additional metal to the weld.

The following media explains about the TIG Welding process:

MIG

Metal Inert Gas

Uses a consumable electrode wire which is continuously fed through a feeder unit with shielding from an external CO₂ gas or mixed gas supply. It normally uses a constant voltage DC supply.

The following media explains about the MIG Welding process:

This process can be used in all types of welding and is increasingly being used in shipbuilding. Advantages which are common to both TIG and MIG being  

• Higher welding speeds  with Greater deposition rates 
• Less post welding cleaning (e.g. no slag to chip off weld) 
• Better weld. 

The main disadvantage being higher cost and the fact that it (MIG) cannot be used in vertical or overhead welds.

SAW

Submerged Arc Welding (SAW)

A process whereby the arc is submerged in granular flux. A copper backing strip is used under the weld, and granular flux is fed through a feeder until, and the excess flux is recovered as the arc moves in the direction of the weld. AC and DC may both be used depending on the process being automatic or semi automatic. The electrode consists of a continuous feed consumable wire with no flux coating. 

Advantages of this process are

• High quality weld with very high speed 
• Smooth, uniform finished weld with no spatter. 
• Little or no smoke with no arc flash 
• Easy process with no requirement of high operator skills

Resistance Welding

Resistance Welding

It is a process whereby two overlapping plates are subject to pressure and then a current is fed to the compressed joint. This causes resistive heating that causes the metal to melt, coalesce and fuse. There is no arc and filler metal. The weld so produced is called a "nugget".

Various methods exist, the main ones are:

• Flash welding
• High frequency
• Percussion welding
• Projection welding
• Resistance seam welding
• Resistance spot welding (most common process), involves the use of water cooled copper electrodes which are clamped with the sheets into place. The electrical current is then applied to the electrodes causing the weld nugget to form.
• Upset welding

This process is used heavily in the automotive industry and in the manufacture of pipes for specialised uses. 

Other welding processes not dealt with here include:

• Thermit welding

The following media explains about the Thermit welding process:

Butt, Lap and Fillet Welds

Types of Weld

• The Butt or Groove weld
• The Fillet weld
• Tack weld

Butt Weld

Usually used for joining two horizontal plates, this weld is very strong and gives excellent strength under tensile forces.

• Single V: Used for plate thicknesses of up to 20 mm.
• Square: No edge preparation is necessary. Usually used for thinner plate in the region of below 6mm.
• Double V: Used on thicker plate usually above 20 mm.
• Double U: They are usually used on thick base metals over 20mm where a V-groove would be at such an extreme angle, that it would cost too much to fill.

FILLET

Used for joining right angled plates or lapped joints. Two important values of a fillet weld are its leg length and the throat length. The throat should be 70% of the leg length.

• Lap: Two horizontal plates are overlapped and a fillet weld is made at each end. Used to join thin plates and diaphragms. They have a lower tensile strength than an equivalent butt joint.
• Staggered/ Intermittent: While continuous weld are used for watertight joints, staggered or intermittent welds may be used at certain sections of the joints where high strength is not a requirement. Staggered weld are used to prevent deformation during the welding process.

Preparation of plate edge for welding

Weld Preparation

Some types of weld require that the faces of the plates being joined be machined. This allows the weld metal to achieve maximum penetration and fusion which leads to a weld of high quality. The adjoining diagrams show how the edges are prepared. They are usually machined to achieve the desired shape, but is case of maintenance welds the may be achieved through grinding or gas cutting.

The following media explains about the Edge preparation and Weld Joints:

Butt welds that require edge preparation are:

• Single V butt joint
• Double V butt joint
• Double U butt joint

Fillet welds that require edge preparation are:

• Full penetration joints

Precautions While Welding

Welding safety starts with an understanding of the risks which include electric shock, injuries related in inhalation of toxic fumes, eye injury and skin burns. To start, protective clothing (PPE) in the form of aprons, gloves and shoes must worn during all welding operations.

For arc welding, the electric arc is a very powerful source of light. During all electric welding processes, use of goggles and a hand shield or helmet equipped with a suitable filter glass to protect against the intense ultraviolet and infrared rays is a must. The area is to be screened so that others in the vicinity are not exposed to the bright light. All this in addition to the fire hazards associated with hot work require that the welding process when carried out ashore or onboard be carried out under the guidance of a responsible person.

Use of tack welding

Tack weld

Short or "Bullet" welds used to temporarily hold the work piece together until the final weld is made. They are ground or burnt off before the final weld i.e. the tack is not welded over.

Weld faults - lack of fusion, penetration, reinforcement, root penetration, slag incursion, porosity, overlap, undercut

Defects in the weld

As with all processes, welding too has its share of defects which if allowed to occur or pass undetected can weaken the joint and cause serious faults to develop in the structure with catastrophic results. Amongst the defects which can occur are:

Lack of fusion

Incomplete fusion of the weld metal (filler) with the base metal sides. Usually caused due to incorrect current settings.

Lack of penetration/ root penetration

The weld does not penetrat fully and evenly from one edge of the work piece to the other. Usually caused due to incorrect current settings.

Lack of reinforcement

Reinforcement is the additional weld metal that is present in the joint over and above both the joint surfaces. This additional metal gives added strength. Inadequate reinforcement weakens the joint.

Porosity

The incursion of air/ gas into the weld. Caused by the arc being too long causing overheating. Also caused by high current.

Slag incursion

The incursion of impurities into the weld metal. This is caused by improper slag removal during a multi pass weld. May also be from the by products of burning flux.

Overlap

Weld metal overflowing over the base metal adjacent to the joint.

Lamellar tearing

Lamellar tearing is a cracking phenomenon which occurs beneath welds, and is principally found in rolled steel plate fabrications. The tearing always lies within the parent plate, often outside the transformed (visible) heat-affected zone (HAZ), and is generally parallel to the weld fusion boundary.

Undercut

Base metal beyond the joint being melted with a resultant reduction in the profile of the plate. Caused by too high current settings. Common defect when manual welding is carried out by unskilled personnel.

Various Test for welds

The competent authority, which in the case of the ship construction are the IACS members have specific rules that require the testing of weld during construction and repair of ships. The IACS document “Requirements concerning materials and welding” lays out the various procedures that are recommended for the numerous varied usage of welding in ship building.  The shipyard makes out a “Welding Procedure Specification” and submits the same to the Classification Society. These include the qualifications of the human element involved in the process, be it the manual welder or the operator of the automated process, and the tests required to be carried out before the procedure is approved.

Tests may be:

Visual testing, Surface crack detection (dye penetrant testing or magnetic particle testing), Radiographic or Ultrasonic testing, Transverse & Longitudinal tensile test, Transverse bend test Charpy V-notch impact test Macro examination and Hardness test.

Weld tests may be divided into three groups:

• Visual inspection
• Non Destructive Testing (NDT)
• Destructive Testing

Visual inspection:

Can reveal faults like Undercut, Overlap and Lack of penetration. The inspection is carried out by sounding the weld, inspecting it with a magnifying glass and sometimes with a magnet. The slag should be completely removed and the weld cleaned before the inspection.

The following media explains about the Visual Inspection of Welds:

NDT

As the name signifies, they are a set of methods that reveal defects that cannot be detected by sight without destroying the welded joint. Main amongst them are:

1. Dye Penetrant

A luminous dye is sprayed over the joint and wiped off after a fixed time period. After this the weld is observed under UV light. Any cracks would have absorbed the dye and would show up under the light. Used for detection of surface cracks.

The following media explains about the Dye penetrant testing:

2. Radiographic

X Rays or Gamma Rays are used to determine the internal soundness of welds. Whether in the shop or in the field, the reliability and interpretive value of radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with the background. To be sure that a radiographic exposure produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on the part so that its image will be produced on the radiograph. Used to detect internal faults in the welded joint.

The following media explains about the Radiography testing:

 

3. Magnetic particle

A solution of iron filings are spread over the welded joint. The joint is then magnetised, and distortion in the magnetic field caused by weld defects can be observed in the pattern of the filings under the magnetic field. Used to detect surface cracks.

The following media explains about the Magnetic Particle Testing:

4. Ultrasonic

A ultra high frequency sound generator passes the sound waves through a probe attached to the joint with a coupling paste. The image generated by the ultrasound pulses shows up on a monitor.  Very effective in the detection of penetration faults.

The following media explains about the Ultrasonic Testing:

5. Destructive testing

As the name suggests the test piece is subject to material strength tests eg Tensile, Hardness, yield etc.

Classification societies also require destructive testing on sample pieces to determine yield strength and breaking stress. The joints and electrodes are subjected to various tests to determine their material properties.

The Society may also require material strength tests to be conducted on the finished test piece before approving a welding procedure.

Purpose of flux

The Arc and its Shield

As the arc fuses the metal, the molten pool has to be shielded from the atmosphere to prevent oxidation. This shield is provided by gas. The gas may be provided separately from an external source (Automatic welding processes) or might be produced by the burning of Flux (Manual & automatic processes) .The constituents of the flux are very important since the byproducts of the combusted flux cannot be allowed to contaminate the weld. The flux is also required to improve the weld quality by:

1. Smoothening weld metal surface with even edges. 
2. Causing minimum spatter adjacent to the weld. 
3. Stabilising the welding arc. 
4. Penetration control. 
5. Provide a strong, tough coating when cold. 
6. Easier slag removal. 
7. Improved deposition rate. 

Flux consists of powdered metal alloys, Ferro alloys, deoxidizers (mineral silicates, fluorides, basic carbonates). The type of flux used determines the type of electrode and are given in the Bureau of Indian Standards (BIS) / International Standards Organization (ISO). Electrodes are of the stick type (SMAW or Shielded Metal Arc Welding) that are consumed and become a part of the weld, and the non consumable ones as used in the TIG process.

The two main classes of electrode are

• Rutile
  The flux coating consists of nearly pure mineral oxides of titanium. This allows easier slag removal, reduces spatter and increases the viscosity of slag.
• Hydrogen controlled
  The flux mainly contains fluorides and carbonates.

Single pass, multi pass and Back run

Single pass

A single progression or "run" of welding along a joint. It forms a layer of weld.

Multi pass

When thick plates are joined multiple passes may be required to achieve the desired weld. It is often employed in the submerged arc process. The runs or passes are carefully arranged to minimise distortion.

Back run

A single pass made to the back of a butt weld.

Full penetration fillet weld

Full penetration: It is the strongest of the various types of fillet weld. The abutting plate has to have its edge prepared with the throat extending in a diagonal from one surface of the plate to the other.

Distortion due to welding and measures to minimise them

Structural defects caused by welding

Welding is a thermal process which generates a large amount of heat which is accompanied by rapid expansion and cooling. Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process.

Distortion may be in the form of longitudinal and transverse shrinkage causing the structure to distort or change shape.

Causes of distortion may be due to the following:

• Intense local heating will cause rapid expansion and subsequent contraction thereby setting up stresses within the welded structure.
• Restrained joints, owing to the rigidity of the structure or external clamping while the welding sequence is in progress limits stress relief provided by shrinkage. Residual stresses are locked up in the weld which might later cause the weld to fail through cracking.

Prevention

• Minimise welded joints by good design. Reduce the number of runs in multi pass welds.
• Improve accessibility of the welded joint to allow down hand welding.
• Exact edge preparation and proper sequence of runs in multi pass joints.
• Use of restraints (Clamps) along with proper welding sequence to prevent distortion of the section/ structure.
• Welding a double sided joint from both sides
• Weld direction and continuity. The direction of the weld should be toward the open side thereby allowing the joint to shrink without distortion. Back step/ Skip welding techniques which alternate the weld sequence after starting at the centre of the joint allow shrinkage thereby minimising locked in stresses and distortion.

         

Welding induced distortion

Avoiding distortion in a Butt weld

Classification societies requires tests on weld materials and electrodes before approval

Please refer the topic ‘Various Tests for Welds’.