50WELDING TECHNOLOGY4.14.7. PROTECTIVE CLOTHING— During arc welding it is not uncommon to experience spatter and sparks flying quite a distance.— Protecting clothing as listed below protect the limbs of the welder against burning by molten-metal, molten flux, spatter and sparks.(/) Apron (ii) Gloves (iii) Sleeves(iv) Cape (v) Shoes etc.— Apron provides protection to the clothes of the welder. Apron may be made up of chrome leather. It offers best protection against hot spattering particles.— Gloves protect the hands of the welder from ultraviolet rays and spattering hot metal. Gloves may be made up of leather or of a combination of cloth and asbestos.— For overhead welding, a jacket or cape is used to protect the shoulders and arms and a cap is employed to protect head and hair.— A welder should wear high topped shoes that go over the ankle. Ordinary shoes will not prevent small globules of molten metal from dropping into shoes.— All clothes, other than the protective ones, also should be of heavy material because thin clothes permit infrared and ultra-violet rays to penetrate to the skin through them.— Open pockets and cuffs of the trousers should be eliminated because they may catch molten particle and burn.4.15. WELDING ELECTRODESAn Electrode is a piece of wire or a rod (of a metal or alloy), with or without flux covering, which carries current for welding. At one end it is gripped in a holder and an arc is set up at the other.4.15.1 TYPES OF WELDING ELECTRODES(See page 151-152.)4.152. ELECTRODE DETAILS.Depending upon the material of the electrode, it may melt and supply filler metal; if it is non-consumable, a separate filler addition generally becomes necessary.The composition of the core wire depends upon the metal to be welded. For example, to weld mild steel, core wire of similar composition will be prepared, in order to get a homogeneous welded joint. The size or diameter of the core wire will depend upon the amount of weld metal to be

ARC WELDING PROCESSES AND EQUIPMENTS1deposited and on the type of joint or the gap to be bridged between the two plates to be welded. Higher currents will be required to weld with bigger diameter electrodes. The length of the core wire is designed after considering

rigidity, electrical resistance, the ease in welding and the diameter of the electrode. Generally thin and larger diameter electrodes are of shorter lengths and medium sized electrodes have bigger lengths. The reason is if thin electrodes are made longer they may bend and welding may not be carried out properly; and if bigger diameter electrodes are made long, their weight may increase too much to make welding operation inconvenient for the operator. In longer electrodes, electrical resistance and thus the heat generated in the electrode body increases, which may spoil the electrode covering. Diameter remaining same, an electrode of higher resistance material is normally made smaller in length.4.153. NON-CONSUMABLE OR REFRACTORY ELECTRODESThey are made up of high melting point materials like carbon (MP 6700°F), pure tungsten (MP 6150°F) or alloy tungsten. These electrodes do not melt away during welding. They maintain the arc which melts the base metal (as in TIG and carbon arc welding). Strictly speaking, these electrodes cannot be called nonconsumable. The electrode length goes on decreasing with the passage of time, because of vaporization and oxidation ol the electrode material during welding. In welding processes uj>ing refractory electrodes, filler metal addition may or may not be needed, depending upon the plate thickness and the type of joint.First amongst the non-consumable electrodes are copper coated carbon or graphite electrodes (Fig. 4.1). Copper coating increases the electrical conductivity or current conducting capacity of the electrodes. A comparison of carbon and graphite electrodes is given below.Carbon Electrodes Graphite Electrodes1. Less expensive. 1. Comparatively costlier.2. Carry less current. 2. Carry larger currents ascompared to carbon electrodes.3. Short life. 3. Long life.4. Simpler arc control. • 4. Arc control is comparat-ively difficult.5. Soft material. 5. Material is hard andbrittle.6. Higher electrical resistance. 6. Lesser electrical resis-tance.Carbon or graphite electrodes ranging from 2 mm to 15 mm are employed for welding purposes.

154

WELDING TECHNOLOGYNext amongst non-consumable electrodes are, those of pure tungsten, (1 or 2%) thoriated or (0.3-05%) zirconiated tungsten electrodes. Alloying pure tungsten increases emissivity, resistance to contamination, arc stability, and electrode life. In addition, arc initiation is easier, electrode tip remains cooler (as compared to pure tungsten electrode), electrode consumption is less and there is a gain in current carrying capacity.As compared to carbon electrodes, tungsten electrodes are much more expensive and alloy tungsten electrodes are still more costlier. Tungsten/alloy tungsten electrodes ranging from 0.5 mm to 6 mm diameter are commonly available for welding purposes.Tungsten and alloy tungsten electrodes should preferably be used on DCSP, and where it becomes essential to use the electrode on DCRP (as in the welding of aluminium) electrode overheating is minimized by keeping currents low. In certain cases, with AC welding, zirconiated electrodesgive better results.Contamination of weld metal by tungsten, and consumption of costly tungsten electrode, both are undesirable. The electrode loss and consumption can be minimized by working on straight polarity, using optimum arc current and arc length with an electrode of suitable diameter. Arc should preferably be struck by using a H.F. unit, electrode tip should not touch the molten weld pool, and electrode should remain in the blanket of inert shielding gas till it cools down. Tungsten and alloy tungsten electrodes being expensive need proper care and maintenance for better functioning.Electrode shapes, polarity and its effects. Correct electrode shape and polarity will decidedly produce better welds.EffectsCurrents higher than those in DCRP can be employed. (400 to 500 amps, for 6 mm diameter electrodes). Arc cleaning of the base metal. Normal penetration. Equal heat distribution at electrode and job.Electrode tip is colder as compared to that in DCRP.Average arc voltage in argon atmosphere is 16 V.

3DCRP1. Currents employed are generally less than 125 amps (up to 6 mm diameter electrodes) to avoid over heating.2. 66.66% heat is generated at the electrode and 33.33% at the job.3. Least penetration.4. Average arc voltage in argon atmosphere is 19 V.

5. Chances of electrode overheating, melting and losses.6. Better arc cleaning action.DCSP3. 4.Welding currents up to 1000 Amps can be employed for 6 mm diameter electrodes.33.33% heat is generated at the electrode and 66.66% at the job. Deep penetration.Average arc voltage in argon atmosphere is 12 V.Electrode runs colder as compared to AC or DCRP.No arc cleaning of base metal.4.15.4. CONSUMABLE ELECTRODESThey are low melting point electrodes made up of different metals and their alloys. When the arc between the electrode and job is struck, the end of the electrode starts melting and transfers to the job in the form of droplets. The electrode itself adds filler metal. Droplets transferring (from electrode end and through arc) to the workpiece deposit there most of the heat generated as resistance heating in the electrode and of the arc. Because of this reason a consumable electrode welding system possesses higher thermal efficiency (about 85%) as compared to that of a noncon-Mimable electrode welding arrangement (about 55%). Consumable elec-Irodcs may be of the following types.Bare electrodes. They consist of a metal or alloy wire without any flux coating on them.

4WELDING TECHNOLOGYARC WELDING PROCESSES AND EQUIPMENTS157Lightly coated electrodes. Electrodes with a coating factor* approximately 1.25 are termed as lightly coated electrodes.Example. Citobest electrode of Advani Oerlikon (A.O.)Medium coated electrodes. They are the electrodes with a coating factor about 1.45. Example: Overcord — C, (A.O.).Heavily coated electrodes. The coating factor is between 1.6 and 2.2 for heavily coated electrodes. Example: Choline (A.O.).As compared to lightly coated electrodes, heavily coated ones find applications in severe conditions, they produce deeper penetrations and weld metal of high quality. In heavily coated electrodes, the core wire melts before the flux coating,

giving rise to a cavity, hence producing arc constriction and arc heat con-centration on the workpiece.Because of unstable arc, irregular metal transfer and atmospheric contamination, Bare Electrodes do not produce sound and satisfactory welds but still they find application where weld strength is not a primary consideration and it is difficult to carry postcleaning of the joint.Covered Electrodes produce very good weld appearances, weld metal propertiesand defect free joints.Certain electrodes give good results on DCRP and others work equally well on AC or DC, for example(a) AWS E 6012, BS E 206, IS M 206253 electrodes work well on bothAC and DCSP.(b) AWS E 6010, BS E 100, IS M 100264 are operated on DCRP only.(c) AWS E 7018, BS E 601 JH, IS M 601389 JH are used on AC or DCRP.Some of the welding electrodes e.g., ~AWS E 6013,6010, BS E 316, E 614, IS M 616478 HJ, M 216251 weldj well in all positions whereas others give best results only when welding i carried out in certain positions e.g. AWS E 6027, BS E 922 P, IS M 922 XXXP are better employed for welding in flat and horizontal positionsonly.Diameter of the electrode

Covered electrodes may also be categorized as follows :(a) Electrodes which have cellulose content impart deep penetration and increased electrode burn off rate. Welding may be carried out in all positions. Examples of electrodes having cellulose content:\AWS E 6010, 6011, 7010, 7011, BS E 110, IS (814-1970) E 10022 A, M 100264. j(b) Autile (titania) electrodes give a smooth and quiet arc, negligible spatter, and easily removable slag. The electrode will work on both AC and DC. Addition of other ingredients can make the slag fluid or viscous with the result that the electrode can be used for welding in different positions. Examples of such electrodes :AWS E 6012,6013, 7013,8013 B2, BS E 317, IS M 317275, E 31432 Cetc.

(c) Electrodes containing iron oxide produce fluid slag, good weld appearance and mechanical properties. Electrode can be operated on AC and DC both. Slag is easily detachable. These electrodes (because of slag fluidity) are generally employed for flat welding. Examples of electrodes :E 6030,7030, BS E 432, IS M 422275 etc.(d) Electrodes with iron power content have stable arc and large metal deposition rates.Examples of electrodes : E 6027, 6024, BS E 922 P, IS M 722 XXXP,(e) Low hydrogen electrodes contain low moisture content, flourspar, limestone etc. They produce weld metal highly resistant to cracking and lor maximum notch ductility. Such electrodes need some heating before use; they work in all positions and can be operated on AC and DCRP. Examples of electrodes:AWS E 6016,7016, BS E 616 H, IS M 616489 H, etc.4.15.5. ELECTRODE COATING INGREDIENTS AND THEIR FUNCTIONSThe covering/coating on the core wire consists of many materials which perform a number of functions as listed below:

1. Slag forming ingredients, like silicates of sodium*, potassium, m.i^nesium, aluminium, iron oxide, china clay,'mica, etc,:, produce a slag winch because of its light weight forms a layer on the in'olten metal and jtmiects the same from atmospheric contamination!

2. Gas shielding ingredients, like cellulose, wood, wood flour, starch, calcium carbonate etc., form a protective gas shield around the electrodeend, arc and weld pool.3. Deoxidizing elements like ferro-manganese, and ferrosilicon, refinethe molten metal,4. Are stabilizing constituents like calcium carbonate, potassium silicate, titanates, magnesium silicates, etc. add to arc stability and ease ofstriking the same.5. Alloying elements like ferro alloys of manganese, molybdenum etc. may be added to impart suitable properties and strength to the weld metal and to make good the loss of some of the elements, which vaporize while welding.6. Iron powder in the coating improves arc behaviour, bead appearance; helps increase metal deposition rate and arc travel speed.In addition* the electrode covering may perform the following functions:7. The covering improves penetration and surface finish.8. Core wire melts faster than the covering, thus forming a sleeve (Fig. 4.71) of the coating which constricts and produces an arc with high concentrated heat.9. It limits spatter, produces a quiet arc and easily removable slag.

10. With proper constituents, the slag may have quick freezing property and thus make overhead and vertical welding easy.11. Coating saves the welder from the radiations otherwise emitted from a bare electrode while the current flows through it during welding.12. Suitable coating will improve metal deposition rates.13. Proper coating ingredients produce weld metals resistant to hot and cold cracking.4.15.6. MANUFACTURE OF ELECTRODESWires of different chemical compositions and sizes are obtained from different steel manufacturers. In electrode making plant, they are chemically cleaned, cut to different lengths (300, 350, 450 mm etc.) and straightened.There are two methods of applying flux coating on the core wire, (a) by dipping, (b) by extrusion. Extrusion method is very fast and economical; produces strong uniform and concentric coatings and has largely replaced the dipping process.

(a) Dipping method. Number of core wires cut to definite length are clamped vertically in a fixture and are dipped in a bath of molten flux.

When a suitable thickness of the flux gets adhered to the core wire, the fixture is raised and the flux is allowed to dry.(b) Extrusion process. Coating ingredients as discussed earlier are mixed up in desired quantities, binder (oftenly sodium silicate) is added and the resultant mass is brought in the form of a thick, viscous, stiff paste. This paste is shaped in the form of a cylinder which is fed into the extrusion press. Core wire and thick paste of flux simultaneously under pressure pass through a die, thus attaching the flux coating on the core wire. The coating thickness depends upon the die opening and can be varied. As a next step the flux from the gripping end of the electrode is removed by an electrically rotated wire brush; after which the electrodes are fed to ovens where they are dried and baked to remove excess moisture.The electrodes are thereafter sorted, wrapped in polythene paper, put into packets, and bulk is boxed into wooden cases. Packets and boxes generally have mentioned on them electrode coding, electrode size, nature of current and polarity, batch number, name of manufacturer, date of manufacture, etc. etc.4,15.7. SELECTION OF ELECTRODESSelection of a right kind of electrode for a particular application is very important to achieve desired properties in the welded joints.The choice of an electrode depends on the following factors :(a) Chemical composition of the base metal. In order to keep weld metal homogeneity I he electrode should have chemical composition more or less similar to that of the plate to be welded.

(b) Thickness of work piece. Fig. 4.72 suggests the values of electrode diameters for welding steel plates of different thicknesses. Preferably, electronic diameter should be less than (he plate thickness.(c) Nature of electrode {Hutting (cellulose, rutile, low hydrogen etc.), arc behaviour and metal tm*es due to volatilisation and spatter.

(d) Positions (flat, horizontal, vertical, overhead), in which welding isto be carried out.(e) Type of joint (lap, butt, fillet, etc.) and number of runs.if) Type of power source (AC or DC) available.(g) Type of polarity (DCSP, DCRP).(h) Weld bead geometry and the shape of the weld bead surface (flat, convex etc.).(i) Amount of weld metal to be deposited and deposition efficiency ofthe electrode.(/) Surface finish and quality of weld metal.(k) Mechanical and other properties required in the welded joint.(/) Cost of the electrode.4.15.8. CARE AND STORAGE OF ELECTRODESUtmost care is required in handling and storage of electrodes. Electrode coating should neither get damped nor be damaged or broken.1. Electrodes with damp coating will produce a violent arc, porosityand cracks in the joint.Electrodes with damaged coating will produce joints of poor mechanical properties.2. To avoid damage to coating, (a) electrodes during storage should neither bend nor deflect, (b) electrode packets should not be thrown orpiled over each other.3. Electrodes should be stored in dry and well-ventilated store rooms. Storage temperature should be about 12°C above that of external airtemperature with 0-60% humidity. Cellulose electrodes are not so critical but they should be protected against condensation and stored in a humi--dityofO-90%. j4. Before use the electrodes may be dried as per manufacturer'srecommendations e.g. BS : E 616 H or IS : M 616478H electrodes may hidried at 150°C for 1 hour before use. \5. All electrodes, and especially costlier ones, should be used till theyare left hardly 40-50 mm. &

6. Electrodes should preferably be retained in original (manufacturer's) packing for identification. Loss of identity of electrodes can wastej a lot of time in recognising them correctly. j4.15.9. CLASSIFICATION AND CODING OF MILD STEEL AND \ LOW ALLOY STEELS ELECTRODES jIt seems essential that electrodes should have along with them some information about their properties and main features. Instead, evet|

electrode manufacturer may give separate identification marks for his electrodes, it is much better if one classification and coding scheme is adhered to by all electrode manufacturers (at least in one country). With this logic in view, different societies developed and framed coding schemes in their countries.Electrode classification provides information on constituents of flux coating, nature of slag, current and polarity, arc behaviour, welding position, appearance of weld deposit, quality of weld (if radiographic) etc. and thus helps selecting most suitable electrode.Electrode packets are marked with a code number which identifies an electrode and its main features (Fig. 4.73).Welding Positions F, V, If, O. Polarity and Voltage

D ±, A 70i

c AWS-ASTM E 6012 Current

oD1-* iNG

Range95-125 Amps

IS E2164II

i BS E216

Conforms :o IS SI4-Part I and II

Fig. 4.73. Electrode coding.Three systems of electrode classification and coding, namely American, British and Indian have been briefly discussed below:(I) American (AWS-ASTM) System

E XX XX or E 60 1 2 EXXX XX or E 100 1 5Letter E signifies that electrode is suitable for metal (electric) arc welding.First two or three digits indicate minimum tensile strength of weld metal in thousands of pounds per sq. inch, e.g. 60,000 and 100,000 lbs/sq. inch. Other values of XX and XXX are 45,70,80,90 and 120.Last but one digit indicates the welding position. It can be represented by numbers like 1, 2 and 3 which indicate that welding can be carried out in any position, flat and horizontal positions, and flat position respectively.

X ] Last digit which may be 0, 1, 2, 3, 5 or 6 tells about power (2) : supply, type of covering, type of arc, penetration charac-(5) teristics, etc. (Table 4.8).Example: E 60 J 2 means(a) Electrode is meant for metal (electric) arc welding,(b) It possesses a minimum tensile strength of 60,000 pounds persquare inch,(c) It can weld satisfactorily in all positions,(d) Electrode covering has a high titania (rutile) content, is bounded with sodium silicate, can be operated on DCSP or AC, produces medium penetration, heavy slag, a convex weld bead appearance and a medium quality weld deposition.(2) British (BS) System :L X X X L First 1st 2nd 3rd Last Letter Digit Digit Digit Letter Examples: E 317 ME145PVarious letters and digits indicate the following: 1st Letter— It can be E, R or D. E indicates that it is a solid extrudedelectrode, R means reinforced electrode and D indicates 'Dipped electrode', i.e. an electrode manufactured by dipping process.1st Digit - It indicates the class of covering. It can be 1,2,3,4,5,6 or 9.1 means high cellulose content.2 means high titania content resulting in fairly viscous slag.3 means appreciable titania content resulting in a fluid slag.4 means high iron and/or Mn oxides and/or silicates content! resulting in inflated slag.5 means high iron oxides and/or silicates content resulting in a heavy solid slag. 16 means high calcium carbonate and fluoride content. i 9 any other type of covering not mentioned above. 12nd Digit — It indicates the position in which electrode can weld satisfactorily. Second digit can be represented by 1, 2, 3, 4, 5,6 or 9.

1. indicates welding positions like flat, horizontal, inclined! vertical, overhead (i.e. electrode is suitable for welding in all positions).2. flat, horizontal. 3. flat only.

4. flat, horizontal, inclined.5. flat, horizontal, vertical, overhead.6. vertical, overhead. 9. not classified above.3rd Digit — It gives an idea of current, polarity and open circuit voltage of the welding power source. Any number like 0,1, 2,3,4, 5,6,7 or 9 can be the third digit.0 indicates D +, i.e. DCRP1 D +, A95, i.e. DCRP or AC with OC voltage over95 volts.2 D-, A70, i.e. DCSP or AC with OCV over 70 V.3 D-. A45, i.e. DCSP or AC with OCV over 45 V.4 D +, A70, i.e. DCRP or AC with OCV over 70 V.5 D±, A95, i.e. DCSP, DCRP, AC with OCV over95 V.6 D±, A70, i.e.DCSP, DCRP, AC with OCV over70 V.7 D±, A45, i.e. DCSP, DCRP, AC with OCV over45 V9 Not classified above. Last letter -P indicates deep penetration electrode, and M means a molybdenum bearing electrode. Example: E 145 P means(a) It is a solid extruded electrode, '(b) It has a high cellulose content, j(c) It can weld in flat, horizontal and inclined positions, j(d) It can be operated on DCRP, DCSP or AC with a power source having OCV above 95 volts, and(e) It is a deep penetration electrode.(3) Indian (IS) System:X X X X X L 2nd 3rd 4th 5th 6th Last DIGITS LetterE307411Various digits and letters indicate the following: 1st Letter-It can be E or R. E indicates that electrode is solid extruded |and R means an electrode extruded with reinforcement. 1st Digit - It indicates the class of covering. It can be 1, 2,3, 4, 5, 6 of and has the meaning same as that of the first digit of Britiflj system, discussed earlier.

2nd Digit — It indicates the positions in which electrode can weld satisfactorily. Second digit may be 0, 1, 2, 3, 4, or 9. 0 and 1 signify that the electrode can be used for welding in all positions, and in flat, horizontal, overhead and vertical positions respectively. 4 indicates flat and horizontal fillet positions. 2, 3 and 9 have the same meaning as in British standard. 3rd Digit — It has the same meaning as that of the third digit of British standard, except that the open circuit voltage is 90 in place of 95 volts, and 50 instead of 45. 4th and — They indicate range of tensile strength and value of minimum 5th Digit yield stress., e.g. 41 (fourth and fifth digits) and 51 mean that tensile strength ranges from 410-510 and 510-610 N/mm2 and minimum yield stress is 330 and 360 N/mm2 respectively. 6th Digit — It tells percentage elongation and impact value. Last — P indicates a deep penetration electrode, H hydrogen control-Letter led electrode, and J, K, L indicate electrodes with iron powder coating and metal recovery 110-130%, 130-150% and above 150%, respectively. Example: E 307411 means(a) It is a solid extruded electrode.(b) Its covering contains appreciable amount of titania; a fluid slag. {c) It is aii position eiecLrode,(d) It can be operated on DCRP, DCSP or AC with a power source having, open circuit voltage 50 volts,(e) Weld metal tensile strength ranges between 410 and 510 N/mm2 and minimum yield stress is 330 N/mm2. (10 N/mm2 = 1.02 kgf/mm2).(/) Minimum percentage elongation of weld metal (in tension) is 20% of 5.65 v^ and impact value of weld metal at 27°C is 4.8 kgf m (or 47 J). Where S0 is the cross-section area of the specimen being tested.4.15.10. TYPICAL APPLICATIONSSome of the applications of mild steel and alloy (medium to high I ensile) steel electrodes are given below:Klectrode coding ApplicationsAWS : E6010 Pipe lines, pressure vessels, storage tanks and field US : E 100 work.IS : M 100264

AWS : E 6013 Pipe lines, truck bodies, steel window frame, farm BS : E 307,316,317 machinery, bridges, railway wagon, ships, trailers,IS : M 307264 boilers, tanks and coaches, building construction.

M 316263 M 317274AWS : E 6015 For welding high carbon or low alloy steels, highIS : E 6102XX sulphur bearing steels and armour plate.AWS : E 6020 Bridges, cranes, locomotive fire boxes, truck chassisBS : E 422,426 frame, boilers and pressure vessels, fabricated plateIS : M 422275 girders, columns etc. Produce radiographic qualityM 426274 welds.AWS : E 6027 A deep penetration electrode, used for welding heavyBS : E 922P deck plates, plate girders for bridge work, locofireIS : M 92241 IP box, rotary kilns and driers.AWS : E 7010, High pressure boiler plates or tubes containing 1%7014 Cr, 0.5% Mo, carbon moly. piping, pipelines for oilBS : E 100, E 317 and gas transmission. IS: E 100411 E 307512AWS : E 7013 Bridges, dams, ships, tanks and wagons, cranes, rail-BS : E 317 way bogies. IS : M 317375AWS : E 7016 Concrete reinforcement rods, rails, wagons, crank-BS : E 616H shaft rebuilding, earth moving machinery, bufferIS: M 616479H layer prior to hard facing, pressure vessels, buttwelding of rail ends.AWS : E 7016 High carbon steel to mild steel, high carbon steelBS : E 601H parts, cast steel, steel rich in sulphur, steel of un-IS : M 601279H known composition.AWS : E 7018 Bridges, penstocks, pressure vessels, atomic reactorBS : E 601JH shell and pipe work, blast furnace steel work, earthIS : M 601389 JH moving machinery.AWS : E 7018 Pressure vessels, air receivers, boilers, bridges andBS : E 614HJ penstocks, buffer layer prior to hardfacing. ProduceIS : M 614479 radiographic quality welds.AWS : E 8013B2 For welding steels; ASTM: \ Cr, \ Mo; l\ Cr, \ Mo, ISBS : E lCrMoR gteel. 07 Cr 90 Mo 55} german steels: 13 Cr Mo 44,IS:E31432C 13 Cr MoV 42 etc.

AWS : E 8016 Loco butt welding, buffer layer before hard facing.BS: E 614IS : E 614512HBS:E611H High temperature high pressure boiler plates andIS : E 611512H tubes operating upto 575°C.AWS : E 8018G High tensile steel machinery parts, cold worked alloy BS : E 611JH steel, fine grained steel containing Cu-Ni or Ni.IS: M 611469JHAWS : E 9016 Chemical plants, superheated steam boilers, and refi-BS : E 611 nery parts operating up to 600°CIS.-E611512H

AWS : E 9018-B3 For welding steels containing Cr and Mo and being BS : E2Cr MoBH used in boilers, oil refineries, chemical plants, and IS : E 61131D in power plants as structures and pipes operatingupto 610°C.AWS : E 11018 Bridge and penstock construction, surfacing and BS : E 601JH repair of road building machinery and ore dressingIS : M 601XXXJH plants, concrete reinforcement rods, earth movingmachinery.AWS : E 13018G High tensile steel, penstocks, earth moving equip-BS : E 601JH ment, heavy steel fabrications.IS : 601XXXJH4.16. SHIELDING GASES AND ASSOCIATED MIXTURES 4.16.1. INTRODUCTION— Shielding gas serves to keep harmful atmospheric gases, such as oxygen, nitrogen and hydrogen, and moisture away from the weld as the molten metal solidifies. These elements, when not effectively kept away from the liquid metal as it solidifies, produce in ferrous metals the majority of weld defects, such as porosity, pinholes and weld brittleness. Therefore, the type of shielding gas used can have a profound effect upon many of the weld and base metal properties as well as the finished weld bead appearance.— Shielding gases are used to surround the arc area in order to prohibit air from coming in contact with the molten metal.— Air contains oxygen, nitrogen and hydrogen, which, of all the elements found in the atmosphere, cause the most difficulty in the welding of ferrous materials.