In this article we will discuss about:- 1. Meaning of Welding 2. Advantages of Welding over Other Joints 3. Welding Radiation 4. Weldability of Metals 5. Steps in Executing Welding 6. Characteristics 7. Heat Affected Zone (HAZ) in Welding 8. Recent Trends 9. Design 10. Quality Control 11. Inspection of Final Welds and a Few Others.
Contents:
- Meaning of Welding
- Advantages of Welding over Other Joints
- Welding Radiation
- Weldability of Metals
- Steps in Executing Welding
- Characteristics of Welding Process
- Heat Affected Zone (HAZ) in Welding
- Recent Trends in Welding
- Design for Welding
- Quality Control in Welding
- Inspection of Final Welds
- Welding and Its Fields of Applications
- Computerisation of Welding Technology
- Health and Safety in Welding
- Qualification of Welding Procedures, Welders Performance, and Welding Consumables
1. Meaning
of Welding:
The term welding is used to cover a wide range of bonding techniques. Broadly, welding process could be classified as fusion welding and solid-phase welding.
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Fusion Welding is the process of joining two pieces of metal by application of heat. The two parts to be joined are placed together, heated, often with the addition of filler metal, until they melt, and solidify on cooling.
The heat may be developed in several ways viz. combustion of fuel gas with oxygen (oxygen—acetylene gas welding), electric arc, electric resistance heating, plasma arc, electron beams, laser beam etc. Along with the application of heat, in some cases pressure is also applied in order have better action of joining.
For additional strength sometimes filler material is also used. It is a very old art and this started with joining of metals by heating them to very high temperature (which is sufficient to cause cohesion) and then hammering.
The various ways of applying pressure in order to effect welding are hammering and rolling. In welding without the application of pressure, the metals are brought to fluid state and joined by some filler material.
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Solid-phase welds are produced by bringing the clean faces of components into intimate contact to produce a metallic bond with or without application of heat, but application of pressure is essential to induce plastic flow.
Now-a-days many processes of welding have been developed and probably there is no industry which is not using welding process in the fabrication of its products in some form or the other. This is the most rapid and easiest way of fabrication and assembly of metal parts.
The research carried out in this field has given various ways and methods to weld practically all metals. Means have also been found out to weld dissimilar metals. One beauty of welding in comparison to other processes of joining metals is that by this process we can have more than 100% strength of joint and it is very easy process.
We shall be dealing with all the various processes of welding in use these days, the equipment used for each process and the ways of preparation of joint and the various operations necessary.
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Welding is now-a-days extensively used in the following fields:
i. Automobile industry,
ii. Aircraft machine frames,
iii. Structural work,
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iv. Tanks,
v. Machine repair work,
vi. Shipbuilding, pipe-line fabrication in thermal power plants and refineries,
vii. Fabrication of metal structures.
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There is a big competition between welding and casting process now-a-days.
Many of the cast products are now-a-days being fabricated by welding various parts together. Such construction has the advantage that the products are lighter and stronger. Gas cutting is another field of application of welding process which is playing a very important role in industry.
2. Advantages of Welding over Other Joints
:
i. Buildings, bridges and structures can be built lighter and thus higher due to reduction in weight.
ii. These are cheap also due to reduction in weight and material cost. Additional joint strength can be obtained by using considerably smaller structural members. Joints are compact and do not require additional plates as in case of riveted joints.
iii. Welded joints have high corrosion resistance compared to bolted and riveted joints.
iv. Welded joints are fluid tight for tanks and vessels.
v. Welded structures can be altered easily and economically.
vi. Many different types of joints are possible in welded joints.
3. Welding Radiation
:
Welding produces rays of longer wavelength compared to X-rays or gamma rays. These can be broken into visible- light rays, infrared rays and ultraviolet rays. Visible light rays emitted can cause eye strain and general discomfort.
Ultraviolet rays are invisible and can cause burns on unprotected skin. Infrared rays have a longer wavelength and these produce heat when they strike and are absorbed into a surface. Prolonged exposure can cause skin burns.
Welders have to protect themselves against sparks, hot metal, ultraviolet, infrared, and visible light rays, welding fumes, and other hazards. A welder should wear welding jacket or sleeves made of leather or denim, leather leggings, leather welding gloves fitting tightly upto the jacket sleeves, and high-top boots. Clothes should fit tight enough so that no bare skin is exposed to sparks or ultraviolet rays. Safety glasses or glasses with safety lenses fitted with side shields must be worn.
The type of joint is determined by the relative position of the two pieces being joined. There are about five principal types of joints mostly used in welding processes. They include butt joints, lap joints, T-joints, corner joints and edge joints.
The latter four types are also called fillet welds. No edge preparation is normally required and as such these are cheaper to produce than butt welds. Fig. 9.8 (a) shows various types of fillet joints and Fig. 9.9 (b) shows the terminology used for fillet welds.
Fillet welds do not require any edge preparation. It may be noted that single fillet weld is quite strong since fusion takes place throughout the plate thickness.
Weld Parts:
Figs. 9.9 (a) and 9.9 (b) show all parts of the weld and the terminology used.
Welding Symbol:
As per American Welding Society (AWS), standard welding symbol and the important features of welding symbol are shown in Fig. 9.10. It indicates the type and specification of the weld.
Basic weld symbols for different types of welds are:
Fig. 9.11 (b) below shows how to use these welding symbols.
4. Weldability of Metals
:
The term weldability has been defined by the American Welding Society as “the capacity of metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service”.
This means that if a particular metal has good weldability it must be welded readily so as to perform satisfactorily in the fabricated structure, and also it must not require expensive or complicated and exacting procedures in order to produce sound joints.
There are certain similarities and differences among the various welding processes depending upon the weldability of metals. The weldability of any metal can be changed by physical, chemical, thermal and metallurgical properties, i.e., by using a proper welding procedure, shielding atmosphere, fluxing material, filler material and in some cases by proper heat treatment of metal before and after deposition.
The following metals have good weldability in the descending order:
a. Iron,
b. Carbon steel,
c. Cast steel,
d. Cast iron,
e. Low alloy steels and
f. Stainless steels.
5. Steps in Executing Welding
:
i. Identification of welds, type of joint, calculation of weld area by stress analysis, preparation of drawing specifying all important features.
ii. Selection of appropriate welding process depending on availability of equipment, skill of personnel, metallurgical and quality requirements, time available and overall economy.
iii. Welding procedure, viz. welding (cutting, cleaning the plates, edge preparation, etc.) sequence, use of jigs and fixtures, fit up assembly, process planning, testing methods, etc.
iv. Execution of welding with proper supervision and inspection at all the stages.
v. Slag removal, weld dressing.
vi. Stress relieving by proper treatment.
vii. Testing, preferably by non-destructive methods for dimensional, metallurgical, crack detection, etc.
viii. Improvements for future based on feedback from existing systems to avoid defects.
6. Characteristics of Welding Process
:
a. Deposition Rate:
Weight of metal deposited (Kg) in a given period of time (hour).
b. Deposition Efficiency (Also Called Electrode Efficiency in Arc Welding):
It is the ratio of deposited weight to melted weight. It is of order of 60—75% for shielded metal arc welding, 85—90% for flux cored arc welding, 90—95% for gas metal arc welding, 90—100% for gas tungsten arc welding, and around 95% for submerged arc welding.
c. Operation Factor:
It is the ratio of total actual welding time to time the operator spends in executing welding. If is of the order of 20—30% for shielded metal arc welding and gas tungsten arc welding, 50% for gas metal arc welding (manual), and 100% for automatic gas metal arc welding and submerged arc welding.
d. Penetration:
It is an important characteristic of fusion welding and is the ratio of width of the weld to its depth. It is of the order of 1.25 for gas metal arc welding, 2.5 for shielded metal arc welding, 5 for plasma arc welding, and 15 for electron beam welding.
A welding process having greater penetrating power calls for a narrow groove, lesser heat affected zone and distortion, and lesser filler metal consumption.
e. Welding Speed:
Speed at which electrode moves or deposition takes place.
f. Heat Input:
It is expressed as:
It is of the order of 0.1—0.6 for electron beam welding and laser beam welding, 0.3 to 1.5 for gas tungsten arc welding, 0.5 to 3 for gas metal arc welding and shielded metal arc welding, 1—10 for submerged arc welding and 5—50 for electro slag welding.
g. Power Density:
It is the heat intensity expressed in Watts/m2. Penetration of weld is proportional to power density.
It is of the order of 5 x 106 to 5 x 108 W/m2 for shielded metal arc welding and gas metal arc welding, 5 x 106— 5 x 1010 W/m2 for plasma arc welding, 1010 to 1012 W/m2 for electron beam and laser beam welding processes.
7. Heat Affected Zone (HAZ) in Welding:
The HAZ of a steel is the region heated from AC1 temperature to the temperature just below the melting temperature. During fusion welding, material close to the weld, experiences a large thermal fluctuation.
Some metallurgical changes in the HAZ do occur. These may be major matrix phase changes or precipitation process. Even in materials showing no phase change or precipitation during welding, recrystallization and grain growth may occur.
The HAZ has an important role to determine the weld cold cracking, notch toughness, hydrogen embrittlement, stress corrosion cracking etc. in severe environmental conditions of service. Therefor a detailed study of HAZ is desirable.
The width of the HAZ can be estimated by the peak temperatures obtained at discrete points from the weld centre line by experiment. The variation of the micro- structure at different zones of welding can be examined from photo macro-and micro-graphs.
The thermal cycles associated with arc welding and submerged arc welding generate the heat affected zone (HAZ). Some cases of reheat cracking in these zones have been observed. These are thus considered to the detrimental to the component integrity.
However with improved base metals and welding procedures (low heat input of 20 kJ/cm, low angle of attack, high overlap, and use of temper bead technique at the weld shoe), it is possible to ensure a predominant fine-grain microstructure in the butt weld HAZ. This fine structure under the severe quenching of the weld thermal cycle and the tempering of the post-weld heat treatment (PWHT) reaches a high level of notch toughness.
8. Recent Trends in Welding:
A model of the gas metal arc welding process is being developed that will relate weld pool geometry to current, voltage, wire speed and weld speed. A laser enhanced electro- optical camera is being developed to provide an image of the weld pool and electrode wire with almost complete suppression of arc light.
An ultrasonic transducer is being developed which will be placed adjacent to the welding torch to provide direct measurements of side-wall fusion and weld pool penetration. It will also detect porosity, lack of fusion, and cracks in welds on pass-by-pass basis.
9. Design for Welding:
For obtaining best results, the following points should be considered while designing the welding of any joint.
1. The surfaces which are to be joined by any welding process must be sufficiently clean to permit clean metallic surfaces to come in contact.
2. Fluxes are to be used in welding all types of metals except mild steel, so that the oxide formed during heating is dissolved and sound welded joints can be obtained.
3. The selection of the welded joints should be such as to satisfy the requirements of design, cost and practical welding.
Of course the best joint is one that is least expensive and satisfies the following points:
(а) Intensity of loading and its characteristics i.e. whether the load is produced by tension, compression or combination of both and to what degree the bending, fatigue or impact stresses are playing a role.
(b) The effect of warping in cooling and the ease of welding, both of which affect the appearance of the joint.
(c) The cost of joint preparation and the actual cost of welding.
(d) The workmanship and the type of skill required.
4. Provided that the physical properties of weld metal are equal or superior to those of base metal, which is usually true, properly made grooved welds should not be reinforced beyond the minimum depth of throat.
5. The amount of welding specified for a welded structure should be minimum, i.e. consistent with the stresses permissible in the component parts such as base metal, bolts and other fasteners.
6. Since welding in the flat position is generally faster and causes less fatigue than welding in other positions, the structure should be designed or positioned accordingly during welding, wherever practicable.
7. All welds should be easily accessible to facilitate manufacturing, testing and repairing with minimum of handling.
10. Quality Control in Welding:
For success of welding joints, quality control deserves greater attention. Quality control manual, describing in detail every step in quality monitoring before, during, and after fabrication of the product, must be prepared to ensure that no short cuts are made and proper equipment and techniques are used to produce quality welds.
Welding inspector should take care of the following points:
i. Verify that all work is in accordance with the applicable codes or standards and that no deviations are permitted.
ii. Verify that base metals and filler metals (electrodes, wires, etc.) conform to specification, and are properly maintained.
iii. Verify that welding machines and equipment are in suitable condition to produce acceptable welds.
iv. Verify that welders have adequate experience and qualification to do the job.
v. Verify that joint preparation and fit up are as specified on drawings and within tolerances.
vi. Inspect, evaluate, and mark all weld joints with a minimum of visual inspection.
vii. Review and evaluate destructive and non-destructive tests.
viii. Verify that welders are using specified techniques for given applications, positions, or electrodes.
ix. Maintain required records and reports.
11. Inspection of Final Welds:
Following features need inspection:
a. For fillet welds:
i. Leg length (difference in leg length on equal-leg fillets should not exceed 3 mm).
ii. Convexity of weld crown (usually a flush to 2.5 mm convexity is permitted).
iii. Weld length.
b. For groove welds:
i. Penetration at the root for complete fusion (no factory edge or un-fused edge should be visible at the root).
ii. Convexity of the weld crown (3 mm normally allowed). Additionally both fillet and groove welds should be checked for following welding flaws.
iii. Cracks—undercut—excessive spatter—porosity— under fill.
Weld flaws are evaluated with following three determining factors: type, (slag inclusion, cracks, etc.), size (small slag inclusion of 2 mm is permitted but rejected beyond that) and location (discontinuities on corners or ends of welds are serious).
12. Welding and Its Fields of Applications:
Originally, the economic importance of welding was realised mainly for repairing and salvaging of all kinds of worn and damaged metal equipment and parts.
The economies and improvements brought about by the more recent techniques of the cutting and the welding processes have placed them as an outstanding tool for manufacturing, construction and maintenance purposes.
Some of its applications are listed below:
i. Replacing Casting:
A wide variety of machine parts, which were manufactured by casting, are now being designed and fabricated as weldments. Machinery base, frames and brackets are made up of standard steel shapes and rolled plates and joined by any one of the welding processes.
ii. Replacing Riveting and Bolting:
Welding is gaining importance day by day in the joining of metals as it gives speedy and sound joints and at the same time, the joined structure is lighter in weight.
iii. Welding as the Only Means of Fabrication:
Welding is the only solution in cases where the equipment is to be constructed of steel plates, the thickness of which is greater than those joined by means of riveting and caulking.
Practical Applications of Welding in Manufacturing, Construction and Maintenance:
Welding has been successfully adopted by the aeronautical industry in the construction and maintenance of aeroplane engines and accessories, boiler shells, pressure vessels and tanks, bridges, manufacture of cranes, building construction, cutting tools and dies, earth moving equipment, furnaces and boilers.
13. Computerisation of Welding Technology:
Before selecting the desired welding method and the welding variables, a welding engineer has to do lot of time consuming work. Computer aided selection of welding variables will surely help him to improve the quality, select optimum parameters, cut down the cost, improve reliability of calculation. The information and case histories can be stored in a more compact form and thus best use can be made of the past experience.
The welding engineer at the outset calculates the joint volume to be filled up with weld metal as this controls welding costs. Any preheat temperature that should be used to prevent weld metal or HAZ cracking can be calculated if the chemical composition, joint geometry and hydrogen potential etc. are known.
Other conditions being equal, the necessity or otherwise of preheat has a bearing on the cost. It may be mentioned that a program to predict the preheat required to avoid hydrogen induced cracking is very important. The hydrogen induced cracking is the most dangerous defect because this defect occurs several days after welding and thus can escape detection soon after welding.
It is also possible to arrive at welding procedure with the help of a computer knowing the carbon equivalent of the material being welded and the allowable hardness level in HAZ or weld metal.
The estimation of the consumable required for a given joint configuration is also important. The quantity of consumables to be procured is dictated by the quantity of weld metal to be deposited.
In calculating the joint volume, joint details such as thickness of material, bevel angle, root gap, nose, radius of curvature etc. are required.
The inputs required for estimation of consumables are the material code, welding process code, type of joint code, thickness of material and joint length. The computer provides information like size of electrodes, size of filler wire, number of passes, and the total amount of consumables.
The program for calculation of preheat temperature is written in interactive mode so that computer can guide the fresher with instructions. The inputs for calculation of preheat temperature are—chemical composition of material, hydrogen potential of the process, individual joint thicknesses, and the arc energy.
Alternatively if the preheat can be fixed for a given combined thickness, the program can depict limits of arc energy. Thus various combinations of preheat and arc energy can be tried out on the computer to arrive at a safe and cost effective welding procedure.
14. Health and Safety in Welding:
Every welder should be aware of health hazards like fires, explosion, electrocution, burns, welder flush, oxygen depletion, toxic fumes/gases/particles/vapours, radiation, trips and falls, and take adequate steps and measures to safeguard themselves against these hazards.
Exposure to welding smoke could cause irritation to eyes, chest, respiratory tract, inflammation of lungs. Gases and particles in welding fume may be toxic or non-toxic. While particles of size greater than 5 µm are filtered by nose, and less than 0.1 µm are breathed out, particles between size 0.1 and 5 µm are retained in lung. Maximum acceptable concentration (MAC) in general is 6 mg/m. Welding fume extraction equipment helps to reduce concentration. Use material safety data sheets to identify hazardous material used in welding e.g. use cadmium free silver solders, asbestos free electrodes.
Ultraviolet radiation given off by welding reacts with oxygen and nitrogen in the air to form ozone and nitrogen oxides. Even 0.2 mg/m3 of concentration is harmful and causes irritation of nose and throat and serious lung diseases.
Electrical hazards also exist in welding even though welding source operates at low voltage. To keep insulation of electrode holder and cable high, these should be kept dry and in good condition. Machines should conform to safety standards. All machines with moving part must be guarded for safety of workers. Keep welding area clean of equipment’s, cables, hoses etc. to prevent trips and falls.
Intense light and radiation (visible, UV and IR) can damage retina/cornea of eye. Use auto darkening helmet, welding curtain and sound protection curtain for safety of welding operators and others. All welding processes require protective measures. Use extraction hood over the workpiece to avoid exposure of fumes and gases to workers. Use proper ventilation and welding helmet with overpressure.
Safety Practices to be observed in Welding:
i. Use personal protective equipment for safety of eyes, ears, lungs and all body parts. Never use oil on welding equipment. Never weld or cut containers containing flammable material. Never weld on painted/coated parts. Fasten cylinder properly and open valves correctly.
ii. Set the operating pressure carefully and light the flame with approved lighter. Control flash back and back fires. Handle hot metals with pliers/tongs. Check connections for leaking gases and never smoke near cylinders. Never leave the work area without closing cylinder valves.
iii. Provide adequate ventilation for working in confined spaces. Gas cylinders/welding power source be located outside the confined space in secured position.
iv. Use air respirator while welding in confined space.
v. Use welding curtain and sound insulating partition walls and protection curtains.
vi. Welding booths should be painted with a dull finish that does not reflect UV light.
vii. Remove all flammable or combustible material before striking arc or lighting a flame.
viii. Do not work in one position for long time and use a foot rest when standing for long periods.
15. Qualification of Welding Procedures, Welders Performance, and Welding Consumables:
The aspect of qualification of welding procedure specification (WPS), welding procedure qualification record (PQR), welders performance qualification and qualification of welding consumables is very essential to ensure requisite quality of welding. No compromise can be made on this as consequences can be disastrous.
Welding procedure specification (WPS) is a written procedure prepared to provide direction for making production welds as per national codes and its purpose is to determine that the weldment proposed for construction is capable of providing the required properties for its intended application. Of course the welder has to be skilled workman and no compromise can be made in selection. WPS includes both essential and non-essential variables with acceptable ranges.
Welding procedure qualification record (PQR) is a record of welding data used to weld a test coupon and includes variables recorded during welding as also the results of various tests carried out.
Welders performance qualification ensures that the qualified welders using approved welding procedures are capable of developing the minimum requirement specified for acceptable weldment.
Welders are tested under the full supervision and control of manufacturer. The welders qualification is limited by essential variables given for each welding process for each type of weld and position.
The performance qualification tests are intended to determine the ability of welders to make sound welds.
Qualification of welding consumables like welding electrodes and filler materials is done as per ASME-Section II-Part-C.