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BASIC GUIDE TO WELDING Welding is a process of joining two metal elements by the application of heat. This heat may be applied different form (by Gas flame or by electric arc or by electric resistance or by gas plazma). Welding are widely used in industries for manufacturing/fabrication of pipe works, structures, vessels, tanks and various repairing works. Welding has become a very important process in any industry. Welding of Critical components should be done by competent welders according to various standards (e.g. IBR approved or AWS-American Welding Society). The AWS has defined master charts of welding procedures for different materials and applications. These are called WPS(Welding procedure specifications).WPS covers the design of the joint, base metals, filler wire or electrodes and their domestic name with AWS numbers, amount of current, technique of welding, heat treatment if any, pre- heating post heating etc. Once the WPS is selected it should be followed in toto for the particular welding system. For different materials different WPS shall be required. Proper welding electrodes or filler metal wires shall be used for specific base metal. Size (Diameter) of the electrode or filler wire is very important as the welding current and OCVs are accordingly selected. If the correct welding rod or filler metal wire is not selected then welding joint produce between two elements may not of full strength and quality, which may fail in the destructive testing. Welding joints are designed for 100% equivalent to the base metals.Non-destructive testing of welding joints is done by Radiography, Die Penetrating Test and Ultra Sonic flaw detective techniques. According to criticality of the service welding joint quality is ensured. This is called QAP or quality assurance plan. QAP gives the total requirement of the NDTs and DTs for the particular job. Destructive tests are performed on the samples or production coupens.  There are many welding process in applications in industries. e.g. Metal Arc Welding,(MMAW and AMAW) Oxy-acetylene gas Welding, Tungsten Inert gas Arc Welding (TIGAW), Metal inert gas Arc Welding (MIGAW), electreon beam welding, Electric resistance welding, Plasma Welding etc. In any welding process basically the metal parts at the joint are heated over a range of fusion temperature, which cause moten metal to flow in presence of flux or inert gas to prevent of oxidation, and the two molten metal elements get fused along with the filler metal if any. The heating and cooling rate of the metal parts affects the metallurgical and mechanical properties of the base metals as well as the weld met. In critical application where alloying elements are present in the metal to be welded, to re-gain the properties at par with the parent metal, welding joints passes through pre-heat treatment or post weld heat treatments so that detrimental effects of welding procedure are nullify and to restore the original properties of the base metal. Distortion and residual stresses developed during welding procedures and their magnitude depends upon the metal and welding procedures. Due to

welding heat and followed by cooling, internal stresses get developed within the weld met and the heat affected zones (HAZ). These stresses are required to be relieved by  proper heat treatment after completion of the welding. In very thick sections, some times for half filled joints, intermediate stress relieving is recommended depending on the presence of alloying elements.The weld metal when it is in molten state, can dissolved many gases and when it gets cooled dissolved gases tends to get escape from the metal matrix which left many gas pockets and porosity in the weld met. To avoid this, inert gases are used to envelop the weld met and the surrounding area while in molten stage. Welding of forging (Carbon steel or Alloy Steel) and Cast iron are very critical in nature and special care is required on job basis. Welding shall be done strictly as per approved procedure.To meet the stringent requirement various grades of electrodes have been desined and manufactured as per recommendations of AWS. In general supiriour quality electrodes are preferred over the normal grade if choice is available, however required properties shall always be kept in mind. Proper grade carbon steel welding rods shall be used for specific carbon steel. Similarly suitable Stainless steel(Austenite)welding electrodes  shall be used for specific Stainless Steel grade. Special welding electrode shall be used for welding two dissimilar metal. There are various grade and size (cross sectional dia of welding/filler rod) of welding electrodes available in market. Instructions of the manufacturer shall always be read and understand before usage. Manual Metal Arc Welding (MMAW) : In manual metal arc welding the heat is generated from electric arc formed between the work (base metal)and the metal welding electrode through flow of electric current. The welding rod is coated with special flux as a shield (Non oxidizing gas or molten slag) to protect the metal. Metal arc welding can be of AC or DC type depending on the source of current. DC current can be straight polarity (electrode negative) or reversed polarity (electrode positive). In case of DC reversed the weld penetration will be deep and in case of DC straight the weld will have a faster deposit rate with medium penetration. Low carbon steel and Austenite steels are suitable for Metal Arc welding. Current in amperes plays an important role in Metal Arc Welding and flow of current is controlled to maintain the quality of welding. 

 Metal Innert Gas Welding(MIG): Metal inert gas welding process uses an arc between a metal electrode and the work piece just like Manual Metal arc welding but, in this case the electrode is in form

of continuous wire fed from a spool and an externally inert shielding gas (Argon,N2 or CO2) is introduced over the metal electrode and hot metal work piece. This welding process is also called semi-automatic welding. There are three main principles for metal transfer in MIG process from the metal electrode and the metal work piece-Short-circuiting-Droplet/spray-Pulsed Short circuiting or pulsed metal transfer are used for low current operation while spray/droplet(<200A) metal transfer is only used  with high welding current (>200A)The inert gas or shielding gas performs a number of important function in MIG welding i.e. It forms the arc plasma, It stabilizes the arc root on the material surface and It ensure smooth transfer of molten metal droplets from the filler wire to the weld pool. Thus the shielding gas have a substantial effect on the stability of the welding quality. This process is used where heavy deposits are needed. This faster than MMAW.        

   Tungsten Inert Gas Welding(TIG):In this welding process the arc is formed between a

non consuming tungsten electrode and the work piece and a shield inert gas  of Argon or Helium is introduced over the electrode and the weld pool. The small intense arc provided by the pointed tungsten electrode  and is ideal for high quality and precision welding. Filler wire is fed separately between the arc and inert gas. In case of MIG welding the electrode itself is the filler wire but in case of TIG welding electrode is of Tungsten tip which is non-consuming and filler wire is fed separately. Inert gas or shield gas is introduced in both the cases. TIG is operated on constant current either DC or AC. In case of DC welding electrodes are normally pure tungsten and in case of AC tungsten with Zirconia electrode are used as in case of AC welding much higher temperatures are produced.            Gas Welding:In case of gas welding temperature is generated with Oxyacetylene gas torch flame and filler wire is fed separately.Oxyacetylene welding is commonly referred as gas welding in which hot flame is produced of about 3200 degree Cent. There are three type of flame setting as neutral, oxidizing and carburising which can be set by varying the proportion of Oxygen and Acetylene. The oxidizing flame is obtained by increasing the oxygen flow and carburizing flame is set through in acetylene flow. The action of the oxyacetylene flame on the surface of the material to be welded can be adjusted to produce a soft, harsh or violent reaction by varying the gas flows. Welding of Different Material: Welding of Cast Iron:Cast Iron is suppose to be difficult to weld material as the carbon content in cast iron is high as 2~4%. Generally cast iron parts involves repair (cracks) and rarely joining of two cast iron parts. The widely used grade of cast iron is grey cast iron which is present in most of the machine parts. The high carbon contents(roughly 10 times as compare to most steel

grades ) of the material tends to form flakes of graphite in place of alloying with iron. The critical temperature of most cast iron is 1450 degree Fahrenheit. The welding arc will heat the casting to temperature above this limit and hence dissociation takes place. That is the reason why castings should  not to be held at and above this temperaure for long period of time. Special procedure and care is required for welding of cast irons. Pre-heating the cast iron will slow the cooling rate of the weld and thus stablises the weld joint. High nickel content electrodes are used for welding of cast iron due to low shrinkage allowance. It is always preferred to pre-heat cast iron to 500~1200 degree F which is near to critical temperature. Perform the welding at high temperature and after welding allow the part to cool down slowly by wrapping it with blanket or sand which will reduce cracking tendencies. Sealing cracks: Because of the nature and structure of the cast iron, tiny cracks tend to appear next to the weld even when good procedures are followed. Some times in pressure containing parts these cracks are required to be sealed. There are several sealing procedures available in the market. One generally followed is a kind of soldering. Solder flux is applied and after heating, special filler solder in tiny pieces is spread over the area to be sealed and flame heating is done. Solder material fills the micro cracks in the cast iron structure.  Studding Method: This procedure is known as Casto-Guna method of welding of cast iron. This is generally employed in the repair of the bigger parts or castings. In this procedure, Small holes are drilled and taped over the surface that have to receive repair weld metal. Un-hardened Steel studs are screwed into the threaded holes leaving 5~6 mm studs above the surface. Then welding is done over these exposed studs to cover the entire surface of the crack with metal deposit. Welding of Steels:Almost 85% of the total metal produced and used in the world are steel. Steel is made principally of iron and carbon mainly with other alloying elements such as manganese, silicon, phosphorous, nickel, chromium, vanadium, cobalt etc. are added according to the special requirements. Steel are given name on their principal alloying elements such as carbon steel, Nickel-chrome steel, chrome-moly steel, chrome-manganese steel etc. Different grades of steel requires different procedure of weldings and depending on the alloying elements, welding procedure and electrodes are made and selected. Low carbon steel: In these grades of steels carbon content ranges from 0.10~0.25% and manganese ranges from 0.25~1.5%. These steels are widely used in industrial fabrication and construction and can be easily welded with any procedure like arc welding, gas welding and Electric resistance welding etc. If the carbon content is not more than 0.15% then pre-heat is not necessary except for extremely heavy section. If the carbon exceeds

0.15% then pre-heat is required up to 250 degree C depending on the thickness and alloying elements. Medium carbon steel: In these grades of steels carbon content is in range of 0.25~0.50% and manganese 0.60~1.60%. With moderate carbon and manganese the low hydrogen type electrodes are recommended for welding. Pre-heating and post heating are generally required depending upon the thickness. Low hydrogen electrodes are generally backed in the ovens before using. This required to eliminate any moisture in the electrode flux.  High carbon steel: In these grades of steels here carbon content ranges from 0.30~1.00%. Special precautions must be taken when welding these classes of steels and low hydrogen type electrodes are mandatory to be used. Pre-heating from 250~320 degree C is necessary followed by post heating. During welding also in thick sections the inter pass temperature is required to be monitored and maintained.Low alloy or Low Nickel chrome steel: These grades contains carbon ranges from 0.14~0.34%, manganese from 0.40~0.90%, silicon 0.20~0.35%, nickel 1.10~3.70%, chromium 0.55~0.75% and Molybdenum from 0.5 to 1.0 %. Thin Section of these steel in the lower carbon ranges can be welded without pre-heat but if the carbon content is more than 0.20 then low pre-heat is required. The weld-met must be stress relieved by post weld heating. In thick sections, inter pass temperatures shall also be monitored and maintained. Electrodes shall be suitable for the particular grade of steel. In case of dissimilar grades electrodes suitable to superior grade shall be selected. Low manganese steel: These are hardened steels and carbon content ranges from 0.18~0.48% manganese from 1.60~1.90% and silicon 0.20~0.35%. Extra care is required for welding of these steels. These are not crack resistance type of steels. In these steels pre-heat is required but cooling rate is more important. Job should never be quenched. Cooling rate depends on the cross section area of the job. Alloy steels or Chromium steel: In these steels carbon content is 0.12~1.10%, manganese 0.30~1.00%, CHROMIUM 0.20~2.25%, and silicon 0.20~0.30%. Thin sections with carbon content on lower side, these steels can be welded without special pre-heat condition. As the carbon and chromium increases high harden ability results and pre-heat of as high as 400 degree C is required. In these steels if a low alloy high strength steels is to be welded to lower strength grades, the electrodes should be selected should preferably be suitable to high strength steel. Also the welding procedure shall also be suitable to high strength steel.In high alloy steels, pre-heat is required according to the alloying element content and thickness of the section. Post-weld-heat-treatment is required to relieve the internal residual stresses developed during the welding. The composition of the welding electrode should be designed to produce weld

deposit similar or better then the base metal and to responds to post-heat treatment in some manner as the base metal. Some common welding electrodes AWS (American welding Society) numbers are given below for ready reference: AWS E 27: For welding cast iron non machineableAWS E 223,224,2240,2242, 242: For welding cast iron machineableAWS E 6013, 7018, 8016:For welding carbon steelAWS E 7010:For welding carbon steel high-pressure pipeAWS E 7016:For welding medium tensile strength carbon steel AWS E 308L, 316L, 310H, 347H : For welding SSAWS E Ni Cr Fe-2, 3: For welding incoalloy