Welding is one of the most commonly used fabrication techniques for manufacturing engineering components for power, fertilizer, petro-chemical, automotive, food processing, and many other sectors. Welding generally uses localized heating during common fusion welding processes (shielded metal arc, submerged arc, gas metal arc welding etc.) for melting the faying surfaces and filler metal. However, localized and differential heating & cooling experienced by the metal during welding makes it significantly different from other manufacturing techniques:
1) Residual stresses are induced in welded components (development of tensile residual stresses adversely affects the tensile and fatigue properties of work piece).
2) Simple shape components to be joined are partially melted.
3) Temperature of the base metal during welding in and around the weld varies as function of time.
4) Chemical, metallurgical and mechanical properties of the weld are generally anisotropic.
5) Process capabilities of the welding in terms of dimensional accuracy, precision and finish are poor.
6) Weld joints for critical applications generally need post weld treatment such as heat treatment or mechanical working to get desired properties or reline residual stress.
Advantages and Limitation of Welding as a Fabrication Technique :-
Welding is mainly used for the production of comparatively simple shape components. It is the process of joining the metallic components with or without application of heat, pressure and filler metal. Application of welding in fabrication offers many advantages, however; it suffers from few limitations also. Some of the advantage and limitations are given below.
Advantages of welding are enlisted below:
1) Permanent joint is produced, which becomes an integral part of work piece.
2) Joints can be stronger than the base metal if good quality filler metal is used.
3) Economical method of joining.
4) It is not restricted to the factory environment.
Disadvantages of welding are enlisted also below:
1. Labour cost is high as only skilled welder can produce sound and quality weld joint.
2. It produces a permanent joint which in turn creates the problem in dissembling if of sub-component required.
3. Hazardous fumes and vapours are generated during welding. This demands proper ventilation of welding area.
Types of welding processes :-
1) Solid state welding processes.
2) Liquid state welding processes Liquid state welding processes.
3) Solid / liquid state bonding processes.
Manual metal arc welding (MMAW) or shielded metal arc welding (SMAW) is the oldest and most widely used process being used for fabrication. The arc is struck between a flux covered stick electrode and the work pieces. The work pieces are made part of an electric circuit, known as welding circuit. It includes welding power source, welding cables, electrode holder, earth clamp and the consumable coated electrode. Welding power sources used may be transformer or rectifier for AC or DC supply. The requirement depends on the type of electrode coating and sometimes on the material to be welded.The constant-current or drooping type of power source is preferred for manual metal arc welding since it is difficult to hold a constant arc length. The changing arc length causes arc voltage to increase or decrease, which in turn produces a change in welding current. The steeper the slope of the volt-ampere curve within the welding range, the smaller the current change for a given change in arc voltage. This results into stable arc, uniform penetration and better weld seam inspite of fluctuations of arc length.
A manual welding power source is never loaded continuously because of operations such as, electrode changing, slag removal etc. Most MMA welding equipment has a duty cycle of around 40% at maximum welding current. Coated Electrodes are specified based on core wire diameter. Commonly used electrode diameters are 2, 2.5, 3.18, 4, 5 and 6 mm. Length of electrodes may depend on diameter of core wire ranging from 250 to 450 mm i.e. larger the core diameter larger the length. However, special electrodes may be of 8-10 mm diameter. Thin coated electrodes have very good bridge ability at the joint gap but weld bead has coarse ripples and penetration is also poor. Medium coated electrodes lead to reasonably good bridge ability, medium ripples in weld bead and modest penetration. Thick coated electrodes have poor bridge ability, however, bead appearance is excellent with fine ripples and also excellent penetration. The ingress of oxygen and nitrogen from the atmosphere to the weld pool and arc environment would cause embrittlement and porosity in the weld metal and this must be prevented. The Actual method of arc shielding from atmospheric nitrogen and oxygen attack varies with different type of electrodes which are in two main categories.
1) Bulk of covering material converts to a gas by the heat of the arc, only a small amount of slag is produced. Protection depends largely upon a gaseous shield to prevent atmospheric contamination as in case of cellulosic electrode.
2) Bulk of covering material converts to a slag, only a small volume of shielding gas produced as in the case of rutile and basic coated electrodes.
3) Electrode coating performs many functions depending upon coating constituents, during welding to improve weld metal properties.
Various constituents of electrode coating are cellulose, calcium fluoride, calcium carbonate, titanium dioxide, clay, talc, iron oxide, asbestos, potassium / sodium silicate, iron powder, ferro-maganese, powdered alloys, silica etc. Each constituent performs either one or more than one functions. Coating consists of high cellulosic content more than 30% and TiO2 up to 20%. These are all position electrodes and produce deep penetration because of extra heat generated during burning of cellulosic materials. However, high spatter losses are associated with these electrodes. Coating consists of TiO 2 up to 45% and SiO2 around 20%. These electrodes are widely used for general work and are called general purpose electrodes. Coating consists of iron oxide more than 20%. Sometimes it may be up to 40%, other constituents may be TiO2 10% and CaCO3 10%. Such electrodes produce self detaching slag and smooth weld finish and are used normally in flat position. Coating consist of CaCO3 around 40% and CaF2 15-20%. These electrodes normally require baking at temperature of approximately 250 ° C for 1-2 hrs or as per manufacturer's instructions. Such electrodes produce high quality weld deposits which has high resistance to cracking. This is because hydrogen is removed from weld metal by the action of fluorine i.e. forming HF acid as CaF2 generates fluorine on dissociation in the heat of arc.