Welding Complete PPT with Question PDF

10/9/2011 By S K Mondal Welding Definition Requirement for a high quality welding y Welding...

10/9/2011 By S K Mondal Welding Definition Requirement for a high quality welding y Welding is a process by which two materials, usually 1. A source of satisfactory heat and/or pressure, metals, are permanently joined together by coalescence, which is induced by a combination of 2. A means of protecting or cleaning the metal, and temperature, pressure, and metallurgical conditions. 3 Caution to avoid, 3. avoid or compensate for, for harmful y The particular combination of these variables can range from high temperature with no pressure to high metallurgical effects. pressure with no increase in temperature. y Welding (positive process) y Machining (negative process) y Forming, casting (zero process) Classification of welding processes Weldability / Fabrication Processes ¾ Oxy fuel gas welding (OFW) ¾ Arc welding (Aw) y The weldability of a material will depend on the ¾ Resistance welding specific welding or joining process being considered. ¾ Solid state welding (friction welding, ultrasonic welding, y For resistance welding of consistent quality, it is forge welding etc.) usually necessary to remove the oxide immediately y Unique process before welding. ¾ Thermit welding y Fabrication weldability test is used to determine ¾ Laser beam welding mechanical properties required for satisfactory ¾ Electroslag welding performance of welded joint. ¾ Flash welding y The correct sequence of the given materials in ¾ Induction welding ascending order of their weldability is ¾ Electron beam welding Aluminum < copper < cast iron < MS Contd… 1  10/9/2011 Case of Aluminium Case of Cast Iron y The oxide coating on aluminum alloys causes some y Cast iron is more difficult to weld because of its high difficulty in relation to its weldability. carbon content and brittleness (poor ductility) y It also has high thermal conductivity and a very short y Massive carbon deposits have a tendency to form in temperature range between liquidus and solidus and when the areas adjacent to the weld, and high‐carbon liquid its viscosity is very low. low martensite tends to form in the heat affected zones. heat‐affected zones y Aluminium is poor absorber of laser light. These microstructures are very brittle and may crack spontaneously while welding is in progress or later y During fusion welding, the aluminum would oxidize so when load is applied to the workpiece. readily that special fluxes or protective inert‐gas y Cast iron can be joined by the oxyacetylene brazing atmospheres must be employed. process and shielded metal‐arc welding (stick) y Friction welding and TIG welding is good for aluminium. process. y For aluminium AC current plus high frequency is must. y Some cases preheating and/or post heating is required. Case of Stainless Steel Case of Stainless Steel y Stainless steel is a difficult metal to weld because it y The ferritic stainless steels are generally less weldable contains both nickel and chromium. than the austenitic stainless steel and require both y The best method for welding stainless steel is TIG preheating and postweld heat treatments. welding. y Welds of ferritic stainless steel can be by y The electric arc is also preferred for welding stainless (i) autogenously (i.e. (i e without the addition of filler steels. A heavily coated welding rod, which produce a metal) shielded arc, is employed. (ii) with an austenitic stainless steel y You must do a better job of pre‐cleaning. (iii) using a high nickel filler alloy. y Using a low arc current setting with faster travel (iv) Type 405 filler (low 11% Cr, low carbon and small speeds is important when welding stainless steel, 0.2% Al) because some stainless steels are subject to carbide y Welding process: TIG, MIG, Shielded‐metal arc precipitation. welding and Plasma arc welding Contd….. IES 2010 IES‐2006 Assertion (A): It is generally difficult to weld Aluminum parts by normal arc welding process. Assertion (A): Aluminium has poor weldability. Reason (R): Hard and brittle Aluminum‐oxide film Reason (R): Aluminium has high thermal is formed at the welded joints. conductivity and high affinity to oxygen. (a) Both A and R are individually true and R is the (a) Both A and R are individually true and R is the correct explanation of A correct explanation of A (b) Both A and R are individually true but R is NOT the (b) Both A and R are individually true but R is not the correct explanation of A correct explanation of A (c) A is true but R is false (c) A is true but R is false (d) A is false but R is true (d) A is false but R is true 2  10/9/2011 IES 2011 IES 2011 During plasma arc welding of aluminium, improved Consider the following statements. removal of the surface oxide from the base metal is Cast iron is difficult to weld, because of obtained with typical polarity of : 1. Low ductility (a) DC Straight 2. Poor fusion (b) DC reverse 3. Tendency to crack on cooling (c) AC potential Which of these statements are correct ? (d) Reverse polarity of phase of AC potential (a) 1, 2 and 3 (b) 1 and 2 only (c) 2 and 3 only (d) 1 and 3 only IES‐2006 IES‐1999 Fabrication weldability test is used to determine The correct sequence of the given materials in (a) Mechanical properties required for satisfactory ascending order of their weldability is performance of welded joint (a) MS, copper, cast iron, aluminium (b) Susceptibility of welded joint for cracking (b) Cast iron, iron MS MS, aluminium copper (c) Suitability for joint design (c) Copper, cast iron, MS, aluminium (d) Appropriate machining process (d) Aluminium, copper, cast iron, MS IES 2010 IES 2010 Weldability of ferritic stainless steel used in Consider the following statements regarding automotive exhaust system is improved by welded joints: selecting stainless steel electrode having low 1. It is a permanent type of joint. content of 2. It is reliable and economical for pressure vessel construction. construction (a) Carbon (b) Nitrogen 3. It is free from fabricational residual stresses. (c) Chromium (d) Carbon and Nitrogen 4. Such joints are suitable for static loading only. 5. Welding is a versatile and flexible metal joining process. Which of the above statements are correct? (a) 1, 2 and 3 only (b) 2, 3 and 4 only (c) 1, 2, 3, 4 and 5 (d) 1, 2 and 5 only 3  10/9/2011 Gas Flame Processes: y Combustion of oxygen and acetylene (C2H2) in a Welding, Cutting and Straightening welding torch produces a temp. in a two stage reaction. y Oxy‐fuel gas Welding (OFW): Heat source is the y In the first stage flame produced by the combustion of a fuel gas and C2 H2 + O2 → 2CO + H2 + Heat oxygen. This reaction occurs near the tip of the torch. y In the second stage combustion of the CO and H2 and y OFW has largely been replaced by other processes but occurs just beyond the first combustion zone. it is still popular because of its portability and the low 2CO + O2 → 2CO2 + Heat 1 capital investment. H2 + 2 O2 → H2O + Heat Oxygen for secondary reactions is obtained from the y Acetylene is the principal fuel gas employed. atmosphere. Three types of flames can be obtained by varying y A higher ratio, such as 1.5 : 1, produces an oxidizing the oxygen/acetylene (or oxygen/fuel gas) ratio. flame, hotter than the neutral flame (about 3300oC) y If the ratio is about 1 : 1 to 1.15 : 1, all reactions are but similar in appearance. carried to completion and a neutral flame is produced. y Used when welding copper and copper alloys but y Most welding is done with a neutral flame, since it will harmful when welding steel because the excess oxygen have h the th least l t chemical h i l effect ff t on the th heated h t d metal.t l reacts with ith the carbon, carbon decarburizing decarburi ing the region around the weld. Oxy‐acetylene gas welding Oxidising flame Oxy‐acetylene gas welding neutral flame 4  10/9/2011 y Excess fuel, on the other hand, produces a carburizing flame. y The excess fuel decomposes to carbon and hydrogen, Metal Flame and the flame temperature is not as great (about MS N 3000oC). High carbon steel R y Flames of this type are used in welding Monel (a Grey cast iron N, slightly oxidizing Alloy steel N nickel‐copper alloy), high‐carbon steels, and some Aluminium Slightly carburizing alloy steels, and for applying some types of hard‐facing Brass Slightly oxidizing material. Copper, Bronze N, slightly oxidizing Nickel alloys Slightly carburizing Lead N Oxy‐acetylene gas welding Carburizing flame IES 2009 Conventional Explain the three types of oxy‐acetylene flames. Indicate with the help of sketches the various zones, respective temperature ranges and applications of each type of flame. [ 20 – Marks] Uses, Advantages, and Limitations y OFW is fusion welding. y No pressure is involved. y Filler metal can be added in the form of a wire or rod. Diagram y Fluxes may be used to clean the surfaces and remove contaminating oxide. The gaseous shield produced by vaporizing flux can prevent oxidation during welding, and the slag produced by solidifying flux can protect the weld pool. Flux can be added as a powder, the welding rod can be dipped in a flux paste, or the rods can be pre‐coated. Contd… 5  10/9/2011 y Exposer of the heated and molten metal to the various Oxy acetylene welding equipment gases in the flame and atmosphere makes it difficult to y Oxygen is stored in a cylinder at a pressure ranging from 13.8 MPa to 18.2 MPa. prevent contamination. y Due to high explosiveness of free acetylene it is stored y Heat source is not concentrated, a large area of the in a cylinder y with 80‐85% p porous calcium silicate and then filled with acetone which absorb upto 420 times metal is heated and distortion is likely to occur. by its volume at a pressure 1.75 MPa. y Flame welding is still quite common in field work, in y At the time of acetylene release if acetone comes with acetylene the flame would give a purple colour. maintenance and repairs, and in fabricating small y Another option is acetylene generator. quantities of specialized products. CaC2 + 2 H 2O → C2 H 2 + Ca (OH )2 IES 2010 Pressure Gas Welding The ratio between Oxygen and Acetylene y Pressure gas welding (PGW) or Oxyacetylene gases for neutral flame in gas welding is Pressure Welding is a process used to make butt (a) 2 : 1 (b) 1 : 2 joints between the ends of objects such as pipe aand‐railroad d a oad rail. a. ((c)) 1 : 1 ((d)) 4 : 1 y The ends are heated with a gas flame to a temperature below the melting point, and the soft metal is then forced together under considerable pressure. y This process, therefore, is actually a 'form of solid‐ state welding. GATE‐1994 GATE‐2003 In Oxyacetylene gas welding, temperature at the The ratio of acetylene to oxygen is inner cone of the flame is around approximately………. for a neutral flames used in (a) 3500°C gas welding. (b) 3200 3200°C C (a) ( ) 1:1 (c) 2900°C (b) 1 : 2 (d) 2550°C (c) 1 : 3 (d) 1.5 : 1 6  10/9/2011 IES 2010 GATE‐2002 Assertion (A): Oxidizing flame is used in gas welding to join medium carbon steels having high The temperature of a carburising flame in gas melting point. welding is that of a neutral or an oxidising flame. Reason (R): In gas welding, oxidizing flame (a) Lower than produces the maximum temperature compared to (b) Higher than neutral and reducing flame. (c) Equal to (a) Both A and R are individually true and R is the (d) Unrelated to correct explanation of A (b) Both A and R are individually true but R is NOT the correct explanation of A (c) A is true but R is false (d) A is false but R is true IES‐2009 IES‐1998 By which one of the following methods gray cast In oxy‐acetylene gas welding, for complete iron is usually welded? combustion, the volume of oxygen required per (a) TIG welding (b) MIG welding unit of acetylene is (c) Gas welding (d) Arc welding ((a)) 1 (b) 1.5 (c) 2 (d) 2.5 IAS 1994 IAS‐1995 In gas welding of mild steel using an oxy‐ Assertion (A): If neutral flame is used in oxy‐ acetylene welding, both oxygen and acetylene acetylene flame. the total amount of acetylene cylinders of same capacity will be emptied at the same consumed was 10 litre. The oxygen consumption time. from the cylinder is Reason (R): Neutral flame uses equal amounts of oxygen and d acetylene. l (a) 5 litre (a) Both A and R are individually true and R is the correct (b) 10 litre explanation of A (c) 15litre (b) Both A and R are individually true but R is not the correct explanation of A (d) 20 litre (c) A is true but R is false (d) A is false but R is true 7  10/9/2011 Oxygen Torch Cutting (Gas Cutting) y For thicker plates with specified contour, shearing cannot be used and oxy‐fuel gas cutting (OFC) is y Iron and steel oxidize (burn) when heated to a useful. temperature between 8000C to 10000C. y Gas‐cutting is similar to gas welding except torch tip. y High‐pressure oxygen jet (300 KPa) is directed against a heated steel p plate, the oxygen yg jjet burns the metal and blows it away causing the cut (kerf ). y For cutting metallic plates shears are used. These are useful for straight‐line cuts and also for cuts up to 40 mm thickness. Fig‐ differences in torch tips for gas welding and gas cutting Contd… Contd… y Larger size orifice produces kerf width wider and larger oxygen consumed. y At kindling temperature (about 870oC), iron form iron oxide. y Reaction: 3Fe + 2O2 → Fe3O4 +6.67 +6 67 MJ/kg of iron The other reactions: 2Fe + O2 → 2FeO + 3.18 MJ/kg of iron 4Fe + 3O2 → 2Fe2O3 + 4.9 MJ/kg of iron y All exothermic reactions preheat the steel. Contd… y The drag lines shows the characteristics of the movement y For complete oxidation 0.287 m3 oxygen/kg of iron is of the oxygen stream. required y Due to unoxidized metal blown away the actual requirement is much less. y Torch tipp held verticallyy or slightly g y inclined in the direction of travel. y Torch position is about 1.5 to 3 mm vertical from plate. Fig‐ positioning of cutting torch in oxy‐ fuel gas cutting y Drag is the amount by which the lower edge of the drag line trails from the top edge. y Good cut means negligible drag. Contd… Contd… 8  10/9/2011 y If torch moved too rapidly, the bottom does not get y Gas cutting is more useful with thick plates. sufficient heat and produces large drag so very rough and irregular‐shaped‐cut edges. y For thin sheets (less than 3 mm thick) tip size should y If torch moved slowly a large amount of slag is be small. If small tips are not available then the tip is generated and produces irregular cut. inclined at an angle of 15 to 20 degrees. Fig. Recommended torch position for cutting thin steel Contd… Application y Useful only for materials which readily get oxidized and the oxides have lower melting points than the metals. y Widely used for ferrous materials. y Cannot be used for aluminum, bronze, stainless steel and like metals since they resist oxidation. Difficulties y For high carbon steel material around the cut should y Metal temperature goes beyond lower critical be preheated (about 250 to 300oC) and may post heat temperature and structural transformations occur. also necessary. y Final microstructure depends on cooling rate. y Cutting CI is difficult, since its melting temp. is lower than iron oxide. y Steels with less than 0.3 % carbon cause no problem. y If chromium and nickel etc are present in ferrous alloys oxidation and cutting is difficult. Contd… 9  10/9/2011 IES‐1992 The edge of a steel plate cut by oxygen cutting will get hardened when the carbon content is (a) Less than 0.1 percent (b) Less than 0.3 0 3 percent (c) More than 0.3 percent (d) Anywhere between 0.1 to 1.0 percent IES 2007 IES‐2001 Consider the following statements in respect of oxy‐ Oxyacetylene reducing flame is used while acetylene welding: carrying out the welding on 1. The joint is not heated to a state of fusion. 2. No pressure is used. (a) Mild steel (b) High carbon steel 3. 3 Oxygen is stored in steel cylinder at a pressure of 14 (c) Grey cast iron (d) Alloy steels MPa. 4. When there is an excess of acetylene used, there is a decided change in the appearance of flame. Which of the statements given above are correct? (a) 1, 2 and 3 (b) 2, 3 and 4 (c) 1, 3 and 4 (d) 1, 2 and 4 IES‐1992 IES‐2005 Thick steel plate cut with oxygen normally shows Consider the following statements: signs of cracking. This tendency for cracking can 1. In gas welding, the torch should be held at an angle of be minimised by 30° to 45° from the horizontal plane. 2. In gas welding, the Size of the torch depends upon the ((a)) Slow speed p g cutting thickness thi k off metal t l tto b be fformed. d (b) Cutting in two or more stages 3. Drag in gas cutting is the time difference between (c) Preheating the plate heating of the plate and starting the oxygen gas for cutting. (d) Using oxy‐acetylene flame Which of the statements given above are correct? (a) 1, 2 and 3 (b) 1 and 2 (c) 2 and 3 (d) 1 and 3 10  10/9/2011 Plasma Cutting y Uses ionized gas jet (plasma) to cut materials resistant to oxy‐fuel cutting, y High velocity electrons generated by the arc impact gas molecules, and ionize them. y The ionized g g nozzle ((upto gas is forced through p 5500 m/s), / ), and the jet heats the metal, and blasts the molten metal away. y More economical, more versatile and much faster (5 to 8 times) than oxyfuel cutting, produces narrow kerfs and smooth surfaces. y HAZ is 1/3 to ¼ th than oxyfuel cutting. y Maximum plate thickness = 200 mm Electric Arc Welding Electric Arc Welding Fig. Basic circuit for arc welding Principle of Arc y Work is negative and electrode is positive is reverse y An arc is generated between cathode and anode when polarity (RPDC). they are touched to establish the flow of current and y SPDC conditions are preferred. then separated p byy a small distance. y DC arc‐welding ldi maintain i t i a stable t bl arc and d preferred f d for f y 65% to 75% heat is generated at the anode. difficult tasks such as overhead welding. y If DC is used and the work is positive (the anode of the y For a stable arc, the gap should be maintained. circuit), the condition is known as straight polarity (SPDC). Contd… Contd… 11  10/9/2011 y Manual arc welding is done with shielded (covered) Three modes of metal transfer during arc welding electrodes y Bare‐metal wire used in automatic or semiautomatic machines. y Non consumable electrodes (e.g tungsten) is not consumed d by b the h arc andd a separate metall wire is used d as filler. y There are three modes of metal transfer (globular, spray and short‐circuit). Major Forces take part in Metal Transfer (i) gravity force (ii) Surface tension g (iii) electromagnetic interaction (iv) hydrodynamic action of plasma JWM 2010 GATE‐1993 Assertion (A) : Bead is the metal added during In d.c. welding, the straight polarity (electrode single pass of welding. negative) results in Reason (R) : Bead material is same as base metal. (a) Lower penetration (a) Both A and R are individuallyy true and R is the (b) Lower deposition rate correct explanation of A (c) Less heating of work piece (b) Both A and R are individually true but R is NOT the (d) Smaller weld pod correct explanation of A (c) A is true but R is false (d) A is false but R is true 12  10/9/2011 Arc welding equipments 1. Droopers: Constant current welding machines ¾ Good for manual welding 2. Constant voltage machines ¾ Good for automatic welding Fig. Machine with different settings Contd… Fig. Characteristic curve of a constant voltage arc‐welding machine Formula y Requires a large current (150 to 1000 A), voltage is between 30 and 40 V, actual voltage across the arc varying from 12 to 30 V. V I + =1 y To initiate a weld, ld the h operator strike k the h electrode l d and d OCV SCC start arc. IES 2010 In arc welding, the arc length should be equal to (a) 4.5 times the rod diameter (b) 3 times the rod diameter (c) 1.5 times the rod diameter (d) Rod diameter 13  10/9/2011 IES‐2005 IES‐2001 Consider the following statements: 1. In arc welding, 65% to 75% heat is generated at the In manual arc welding, the equipment should anode. have drooping characteristics in order to maintain 2. Duty cycle in case of arc welding is the cycle of complete welding of work piece from the (a) Voltage constant when arc length changes beginning. 3. Arc blow is more common with DC welding. (b) Current constant when arc length changes Which of the statements given above are (c) Temperature in the are constant correct? (d) Weld pool red‐hot (a) 1, 2 and 3 (b) 1 and 2 (c) 2 and 3 (d) 1 and 3 IES‐2001 IES‐1998 In arc welding, d.c. reverse polarity is used to bear The voltage‐current characteristics of a dc greater advantage in generator for arc welding is a straight line (a) Overhead welding between an open‐circuit voltage of 80 V and short‐ (b) Flat welding of lap joints circuit current of 300 A. The generator settings for maximum arc power will be (c) Edge welding (a) 0 V and 150 A (b) 40 V and 300 A (d) Flat welding of butt joints (c) 40 V and 150 A (d) 80 V and 300 A IAS‐1999 IAS‐1998 Open‐circuit voltage of 60 V and current of 160A Assuming a straight line V‐I characteristics for a were the welding conditions for arc welding of a dc welding generator, short circuit current as 400A certain class of steel strip of thickness 10 mm. For and open circuit voltage as 400 which one of the arc welding of 5mm thick strip of the same steel, following is the correct voltage and current setting the welding voltage and current would be for maximum arc power? (a) 60 V and 80 A (a) 400 A and 100 V (b) 200 A and 200 V (b) 120 V and 160 A (c) 400 A and 50 V (d) 200 A and 50 V (c) 60 V and 40 A (d) 120 V and 40 A 14  10/9/2011 Duty Cycle y The percentage of time in a 5 min period that a welding machine can be used at its rated output 2 without overloading. ⎛ I⎞ Required duty cycle, Ta = ⎜ ⎟ T y Time is spent p in setting g up, p metal chipping, pp g cleaning g ⎝ Ia ⎠ and inspection. y For manual welding a 60% duty cycle is suggested and Where ,T = rated duty cycle for automatic welding 100% duty cycle. I = rated current at the rated duty cycle Io = Maximum current at the rated duty cycle Contd… Electrode Consumable Electrodes 1. Non‐consumable Electrodes ¾Provides filler materials. 2. Consumable Electrodes ¾Same composition. ¾This requires that the electrode be moved toward or away from the work to maintain the arc and Non‐consumable Electrodes satisfactory welding conditions. ¾Made of carbon, Graphite or Tungsten. ¾Carbon and Graphite are used for D.C. ¾ Electrode is not consumed, the arc length remains constant, arc is stable and easy to maintain. Contd… Contd… Consumable electrodes are three kinds: Electrode coating characteristic (a) Bare 1. Provide a protective atmosphere. (b) Fluxed or lightly coated 2. Stabilize the arc. (c) Coated or extruded / shielded 3. Provide a protective slag coating to accumulate y For automatic welding, welding bare electrode is in the form of impurities, prevent oxidation, and slow the cooling of continuous wire (coil). the weld metal. 4. Reduce spatter. 5. Add alloying elements. 6. Affect arc penetration 7. Influence the shape of the weld bead. 8. Add additional filler metal. 15  10/9/2011 GATE‐1994 Electrode coatings The electrodes used in arc welding are coated. l. Slag Forming Ingredients. asbestos, mica, silica, This coating is not expected to fluorspar, titanium dioxide, Iron oxide, magnesium (a) Provide protective atmosphere to weld carbonate, Calcium carbonate and aluminium oxide. (b) Stabilize the are (c) Add alloying elements 2. Arc Stabilizing Ingredients. or ionizing agents: (d) Prevents electrode from contamination potassium silicate, TiO2 + ZrO2 (Rutile), Mica, Calcium oxide, sodium oxide, magnesium oxide, feldspar (KAI Si3 O8) Contd… 3. Deoxidizing Ingredients. Cellulose, Calcium carbonate, dolo‐ mite, starch, dextrin, wood flour, graphite, aluminium, ferromanganese. 4. Binding Materials Sodium silicate, potassium silicate, asbestos. 5. Alloying Constituents to Improve Strength of Weld 6. TiO2 and potassium compounds increase the melting rate of the base metal for better penetration. 7. Iron powder provides higher deposition rate. Contd… Contd… y The slag is then easily chipped. Binders y AC arc welding used potassium silicate binders. y Coatings are designed to melt more slowly than the filler wire. y DC arc welding used sodium silicate binders. y Potassium otass u has as a lower o e ionization o at o pote potential t a as co compared pa ed with sodium. 16  10/9/2011 IES 2007 IES‐1997 The coating material of an arc welding electrode Assertion (A): The electrodes of ac arc welding are coated with sodium silicate, whereas electrodes used contains which of the following? for dc arc welding are coated with potassium silicate 1. Deoxidising agent binders. Reason (R): Potassium has a lower ionization 2. 2 Arc stabilizing agent potential i l than h sodium. di 3. Slag forming agent (a) Both A and R are individually true and R is the correct explanation of A Select the correct answer using the code given below: (b) Both A and R are individually true but R is not the (a) 1, 2 and 3 (b) 1 and 2 only correct explanation of A (c) 2 and 3 only (d) 1 and 3 only (c) A is true but R is false (d) A is false but R is true IES‐2002 Welding Flux Match List I with List II and select the correct answer: Available in three forms List I (Ingredients) List II (Welding functions) y Granular A. Silica 1. Arc stabilizer y Electrode wire coating B. Potassium i oxalate l 2. De‐oxidizer idi y Electrode core C. Ferro silicon 3. Fluxing agent D. Cellulose 4. Gas forming material Codes:A B C D A B C D (a) 3 4 2 1 (b) 2 1 3 4 (c) 3 1 2 4 (d) 2 4 3 1 Welding Positions Welding Current y Welding current depends upon: the thickness of the welded metal, type of joint, welding speed, position of the weld, the thickness and type of the coating on the electrode and its working length. Fig. The position of electrode for horizontal welding y Welding current, I = k. d, amperes; d is dia. (mm) Fig. Positioning of electrode for welding in vertically upward position 17  10/9/2011 Welding Voltage Arc Length y The arc voltage depends only upon the arc length y For good welds, a short arc length is necessary, because: V = k1 + k2l Volts 1. Heat is concentrated. 2. More stable Where l is the arc length in mm and k1 and k2 are 3. More protective atmosphere. constants, k1 = 10 to 12; and k2 = 2 to 3 The minimum Arc voltage is given by Vmin = (20 + 0.04 l) Volt Contd… A long arc results in y Large heat loss into atmosphere. y Unstable arc. y Weld pool is not protected. y Weld has low strength, strength less ductility, ductility poor fusion and excessive spatter. Fig. Arc Power Vs Arc Length Arc length should be equal to the diameter of the electrode size GATE‐2002, Conventional The arc length‐voltage characteristic of a DC arc is given by the equation: V = 24 + 4L, where V is voltage in volts and L is arc length in mm. The static volt‐ampere characteristic of the power source is approximated by a Bead width should be equal to three diameter of the electrode size straight line with a no load voltage of 80 V and a short circuit current of 600A. Determine the optimum arc length for maximum power. 18  10/9/2011 Arc blow in DC arc welding y Arc blow occurs during the welding of magnetic materials with DC. y The effect of arc blow is maximum when welding corners where magnetic field concentration is maximum. y The effect is particularly noticeable when welding with bare electrodes or when using currents below or above y Again the problem of arc blow gets magnified when welding highly magnetic materials such as Ni alloys, because of the strong magnetic fields set up by these metals. y Cause: Unbalanced magnetic forces. Contd… Contd… Effect of arc blow The effects of arc blow can be minimized with D.C. welding by y Low heat penetration. y Shortening the arc. y Excessive weld spatter. y Reduce current y Pinch effect in welding is the result of electromagnetic y Reducing weld speed. forces y Balance magnetic field by placing one ground lead at y Weld spatter occurs due to each end of the work piece. ¾High welding current y Wrapping the electrode cable a few turns around the ¾Too small an electrode arc work piece. Contd… IES‐2001 IES‐2001 Arc blow is more common in Pinch effect in welding is the result of (a) A.C. welding (a) Expansion of gases in the arc (b) D.C. welding with straight polarity (b) Electromagnetic forces (c) ( ) D.C. D C welding ldi with ith bare b electrodes l t d (c) ( ) Electric El t i force f (d) A.C. welding with bare electrodes (d) Surface tension of the molten metal 19  10/9/2011 GATE‐1992 Gas shields A low carbon steel plate is to be welded by the manual y An inert gas is blown into the weld zone to drive away metal arc welding process using a linear V ‐ I characteristic DC Power source. The following data are other atmospheric gases. available : y Gases are argon, argon helium, helium nitrogen, nitrogen carbon dioxide and OCV of Power source = 62 V Short circuit current = 130 A a mixture of the above gases. Arc length, L = 4 mm y Argon ionizes easily requiring smaller arc voltages.It is Traverse speed of welding = 15 cm/s good for welding thin sheets. Efficiency of heat input = 85% Voltage is given as V = 20 + 1.5 L Calculate the heat input into the workprice Contd… y Helium, most expensive, has a better thermal Carbon Arc welding conductivity, is useful for thicker sheets, copper and y Arc is produced between a carbon electrode and the aluminium welding, higher deposition rate. work. y The arc in carbon dioxide shielding g g gas is unstable, y Shielding is not used. used least expensive, deoxidizers needed. y No pressure y It is a heavy gas and therefore covers the weld zone y With or without filler metal very well. y May be used in "twin arc method", that is, between two carbon (graphite) electrodes. IES 2010 Tungsten Inert Gas welding (TIG) Assertion (A): Straight polarity is always recommended for Carbon‐electrode welding. y Arc is established between a non‐consumable tungsten electrode and the workpiece. Reason (R): Carbon arc is stable in straight polarity. y Tungsten is alloyed with thorium or zirconium for (a) Both A and R are individually true and R is the better current‐carryingy g and electron‐emission correct explanation of A characteristics. (b) Both A and R are individually true but R is NOT the y Arc length is constant, arc is stable and easy to correct explanation of A maintain. (c) A is true but R is false y With or without filler. (d) A is false but R is true Contd… 20  10/9/2011 y Very clean welds. y All metals and alloys can be welded. (Al, Mg also) y Straight polarity is used. y Weld voltage 20 to 40 V and weld current 125 A for RPDC to 1000 A for SPDC. y Shielded Gas: Argon y Torch is water or air cooled. Fig. TIG GATE 2011 Which one among the following welding processes used non – consumable electrode? (a) Gas metal arc welding (b) Submerged arc welding (c) Gas tungsten arc welding (d) Flux coated arc welding IES 2010 GATE‐2002 In an inert gas welding process, the commonly used gas is Which of the following arc welding processes does not use consumable electrodes? (a) Hydrogen (a) GMAW (b) Oxygen (b) GTAW (c) ( ) Helium H li or Argon A (c) Submerged Arc Welding (d) Krypton (d) None of these 21  10/9/2011 IES‐1994 IES‐2000 Which one of the following welding processes Which one of the following statements is correct? uses non‐ consumable electrodes? (a) No flux is used in gas welding of mild steel (a) TIG welding (b) Borax is the commonly used flux coating on (b) MIG welding welding electrodes (c) Manual arc welding (c) Laser beam welding employs a vacuum chamber (d) Submerged arc welding. and thus avoids use of a shielding method (d) AC can be used for GTAW process Gas Metal Arc Welding (GMAW) or MIG y A consumable electrode in a gas shield. y Fast and economical. y Arc is between workpiece and an automatically fed bare‐wire electrode. y A reverse‐polarity dc arc is generally used. y Argon, helium, and mixtures of the two can be used. y Any metal can be welded but are used primarily with the non‐ferrous metals. y When welding steel, some O2 or CO2 is usually added to improve the arc stability and reduce weld spatter. Contd… Fig. MIG 22  10/9/2011 IES 2007 IES‐1997 In MIG welding, the metal is transferred into the Consider the following statements: form of which one of the following? MIG welding process uses (a) A fine spray of metal 1. Consumable electrode 2. non‐consumable electrode (b) Molten drops 3. D.C. power supply 4. A.C. power supply (c) Weld pool Of these statements (d) Molecules (a) 2 and 4 are correct (b) 2 and 3 are correct (c) 1 and 4 are correct (d) 1 and 3 are correct IES 2010 Submerged Arc welding (SAW) Assertion (A): Inert gas and bare electrode instead of flux coated electrode is used in the case of y A thick layer of granular flux is deposited just ahead of automatic TIG and MIG welding processes. a bare wire consumable electrode, and an arc is Reason (R): Better protection is provided by a cloud of inert gas than the cover created by the flux. maintained beneath the blanket of flux with onlyy a few (a) Both A and R are individually true and R is the small flames being visible. correct explanation of A (b) Both A and R are individually true but R is NOT the y A portion of the flux melts. Molten flux and flux correct explanation of A provides thermal insulation, slows cooling rate and (c) A is true but R is false produce soft, ductile welds. (d) A is false but R is true Contd… y Most suitable for flat butt or fillet welds in low carbon steel (< 0.3% carbon). y The process is not recommended for high‐carbon g steels, tool steels, aluminum, magnesium, titanium, lead, or zinc. 23  10/9/2011 Characteristic of submerged arc welding Advantages y High speeds, y Wire electrodes are inexpensive. y High deposition rates, y No weld spatter. y Deep penetration, penetration y Nearly 100% deposition efficiency. efficiency y High cleanliness (due to the flux action). y Lesser electrode consumption. Limitations IES 2011 The welding process in which bare wire is used as y Extensive flux handling, electrode, granular flux is used and the process is y Contamination of the flux by moisture. characterized by its high speed welding, is known as: (a) Shielded arc welding y Large‐grain‐size Large grain size structures. structures (b) Plasma arc welding y Welding is restricted to the horizontal position. (c) Submerged arc welding (d) Gas metal arc welding y Chemical control is important 24  10/9/2011 IES‐2006 IES‐2005 In which of the following welding processes, flux Which of the following are the major is used in the form of granules? characteristics of submerged arc welding? (a) AC arc welding 1. High welding speeds. (b) Submerged arc welding 2. High deposition rates. 3. Low penetration. (c) Argon arc welding 4. Low cleanliness. (d) DC arc welding Select the correct answer using the code given below: (a) 2 and 3 (b) 1, 2 and 3 (c) 3 and 4 (d) 1 and 2 IES‐2008 GATE‐1999 Assertion (A): Submerged arc welding is not For butt ‐welding 40 mm thick steel plates, when recommended for high carbon steels, tool steels, the expected quantity of such jobs is 5000 per aluminium, magnesium etc. Reason (R): This is because of unavailability of month over a period of 10 year, choose the best suitable fluxes, reactivity at high temperatures and suitable welding process out of the following low l sublimation bli i temperatures. available l bl alternatives. l (a) Both A and R are true and R is the correct explanation of A (a) Submerged arc welding (b) Both A and R are true but R is NOT the correct (b) Oxy‐acetylene welding explanation of A (c) Electron beam welding (c) A is true but R is false (d) A is false but R is true (d) MIG welding Atomic Hydrogen welding (AHW) y Temperature of about 3700oC. y An a.c. arc is formed between two tungsten electrodes along which streams of hydrogen are fed to the y Hydrogen acts as shielding also. welding zone. The molecules of hydrogen are dissociated by the high heat of the arc in the gap y Used for very thin sheets or small diameter wires. between the electrodes. electrodes The formation of atomic hydrogen proceeds with the absorption of heat: y Lower thermal efficiency than Arc welding. H2 = 2H ‐ 421.2 k J / mol y Ceramics may be arc welded. y This atomic hydrogen recombines to form molecular hydrogen outside the arc, particularly on the relatively y AC used. cold surface of the work being welded, releasing the heat gained previously: 2H = H2 + 421.2 k J / mol. Contd… 25  10/9/2011 IES‐2005 In atomic hydrogen welding, hydrogen acts as (a) A heating agent (b) One of the gases to generate the flame (c) ( ) An A effective ff ti shielding hi ldi gas protecting t ti the th weldld (d) A lubricant to increase the flow characteristics of weld metal 26  8/26/2011 Resistance Welding Principle y Both heat and pressure are used. Resistance Welding y Heat is generated by the electrical resistance of the work pieces and the interface between them. y Pressure is supplied externally and is varied throughout the weld cycle. y Due to pressure, a lower temperature needed than oxy‐fuel or arc welding. By S K Mondal Contd… y Overall resistance very low. y They are not officially classified as solid‐state welding by the American Welding Society. y Very high‐current (up to 100,000 A) y Very rapid and economical. y Very low‐voltage (0.5 to 10 V) is used. y Extremely l well ll suited d to automated d manufacturing. f y No filler metal, no flux, no shielding gases. Contd… FIG. The fundamental resistance‐welding circuit Fig. The desired temperature Fig. Typical current and distribution across the pressure cycle for resistance electrodes and the work welding. The cycle includes pieces in lap resistance forging and post heating welding. operations. Fig. The arrangement of the electrodes and the work in spot welding, showing design for replaceable electrode tips. 1  8/26/2011 Advantages Limitations 1. Very rapid. 1. High initial cost. 2. Fully automation possible. 2. Limitations to the type of joints (mostly lap joints). 3. 3 Conserve material; no filler metal, metal shielding gases, gases or 3 Skilled maintenance personne1 are required: 3. flux is required. 4. special surface treatment needed. 4. Skilled operators are not required. 5.Dissimilar metals can be easily joined. 6. High reliability and High reproducibility. Application Different types y The resistance welding processes are among the 1. Resistance spot welding most common technique for high volume 2. Resistance seam welding joining. 3 Projection welding 3. 4. Upset welding 5. Flash welding 6. Percussion welding Resistance spot welding Heat input and Efficiency Calculations y The process description given so far is called resistance spot welding (RSW) or simply spot welding. y This is essentially done to join two sheet‐metal jobs in a lap p jjoint, forming g a small nugget gg at the interface of the two plates. Contd… 2  8/26/2011 Electric Resistance Welding Example‐1 Calculate the melting efficiency in the case of arc Joule’s law applicable welding of steel with a potential of 20 V and current of Q = I2 Rt, Joules 200 A. The travel speed is 5 mm/s and the cross‐ sectional area of the joint is 20 mm2. Heat required to melt steel may be taken as 10 J/ and the heat transfer efficiency as 0.85. [PTU ‐2004] Example‐2 Example‐3 Calculate the melting efficiency in the case of Two steel plates each 1 mm thick are spot arc‐welding of steel with a potential of 20 V and welded at a current of 5000 A. The current flow a current of 200 A. The travel speed is 5 mm/s time is 0.1 s. The electrodes used are 5 mm in and.the the cross cross‐sectional sectional area of the joint is 20 diameter Determine the heat generated and diameter. mm2. Heat required to melt steel may be taken its distribution in the weld zone. The effective as 10 J/mm3 and the heat transfer efficiency as resistance in the operation is 200 μΩ. 0.85. Example‐4 Example‐5 Two steel sheets of 1.0mm thickness are resistance welded in a lap joint with a current of How much heat would be generated in the spot 10 000 A for 0.1 second. The effective resistance welding of two sheets of 1 mm thick steel that of the joint can be taken as 100 micro ohms. ohms The required q a current of 10000 A for 0.1 seconds? joint can be considered as a cylinder of 5 mm An effective resistance of 100 μΩ. is assumed. diameter and 1.5mm height. The density of steel is 0.00786 g/mm3 and heat required for melting steel is 10 J/mm3. 3  8/26/2011 Example‐6 Example‐7 Two 1.2 mm thick, flat copper sheets are being spot Two steel sheets of 1.0‐mm thickness are resistance welded using a current of 6000 A and a current flow welded in a projection welding with a current of 30 time of t = 0.18 s. The electrodes are 5 mm in diameter. 000 A for 0.005 second. The effective resistance of the Estimate the heat generated in the weld zone. Take joint can be taken as 100 micro ohms. The joint can be effective resistance as 150 μΩ. considered d d as a cylinder l d off 5 mm diameter d and d 1.5 mm height. The density of steel is 0.00786 g/mm3 and heat required for melting steel is 10 J/mm3. Resistance seam welding y Welding current is a bit higher than spot welding, to compensate short circuit of the adjacent weld. y Weld is made between overlapping sheets of metal. y In other process a continuous seam is produced by The seam is a series of overlapping spot welds. passing a continuous current through the rotating electrodes with a speed of 1.5 m/min for thin sheet. y The basic equipment is the same as for spot welding. welding except that the electrodes are now in the form of rotating disks. y Timed pulses of current pass to form the overlapping welds. Contd… Contd… Projection welding y Limitations of spot welding. 1. Electrode condition must be maintained continually, and only one spot weld at a time. 2. For additional strength multiple welds needed. y Projection welding (RPW) overcomes above limitations. Fig. Resistance seam welding Contd… 4  8/26/2011 y Dimples are embossed on work pieces at the weld locations and then placed between large‐area y Projections are press‐formed in any shape. electrodes, and pressure and current applied like spot y Multiple welds at a time. welding. y Current flows through the dimples and heats them y No indentation mark on the surface. and pressure causes the dimples to flatten and form a weld. y Bolts and nuts can be attached to other metal parts. Fig. Principle of projection welding, (a) prior to application of current and pressure (b) and after formation of welds Contd… Upset welding y Made butt joint compared to lap joint. y Pieces are held tightly and current is applied. y Due to pressure joints get slightly upset and hence its name. y Useful for joining rods or similar pieces. Contd… Contd… y This is the process used for making electric resistance‐ Flash Welding welded (ERW) pipes starting from a metal plate of suitable thickness. y It is similar to upset welding except the arc rather than y The plate is first formed into the shape of the pipe with the resistance heating. help of the three roll set as shown in Fig. above. The ends of the p plate would then be forming g the butt jjoint. y One pieces is clamped with cam controlled movable y The two rotating copper disc electrodes are made to contact the two ends of the plate through which the platen and other with is fixed platen. current is passed. The ends get heated and then forge‐ welded under the pressure of the rolls. y The ends of the pieces to be upset welded must be perfectly parallel. Any high spots if present on the ends would get melted first before the two ends are completely joined. Contd… 5  8/26/2011 y Two pieces are brought together and the power supply is Percussion Welding switched on. Momentarily the two pieces are separated to create the arc to melt the ends of the two pieces. y Similar to flash welding except arc power by a rapid Then again the pieces are brought together and the discharge of stored electrical energy. power switched off while the two ends are fused under force. Most of the metal melted would flash out y The arc duration is only y 1 to 10 ms, heat is intense and through the joint and forms like a fin around the joint. highly concentrated. y Faster than upset welding. y Small weld metal is produced, little or no upsetting, and low HAZ. y Application: Butt welding of bar or tube where heat damage is a major concern. Contd… Other Welding h i Technique Thermit Welding y Temp. 2750°C produced in 30 seconds, superheating y Heating and coalescence is by superheated molten the molten iron which provide both heat and filler metal obtained from a chemical reaction between a metal. metal oxide and a metallic reducing g agent. g y Runners and risers are provided like casting. y Used mixture one part aluminum and three parts iron y Copper, brass, and bronze can be welded using a oxide and ignited by a magnesium fuse. (1150°C). different starting mixture. 8Al+ 3Fe3O4 → 9Fe + 4Al2O3 + heat y Used to joint thick sections, in remote locations. Contd… 6  8/26/2011 Electro Slag Welding y A 65‐mm deep layer of molten slag, protect and y Very effective for welding thick sections. cleanse the molten metal. y Heat is derived from the passage of electrical current y Water‐cooled copper molding plates confined the through a liquid slag and temp. temp 1760 1760°C C liquid and moved upward. y Multiple electrodes are used to provide an adequate supply of filler. Contd… Contd… y Applications: Shipbuilding, machine manufacture, heavy pressure vessels, and the joining of large castings and forgings. y Slow cooling gpproduces a coarse g grain structure. y Large HAZ. Contd… Electron Beam Welding y A beam of electrons is magnetically focused on the work piece in a vacuum chamber. y Heat of fusion is produced by electrons decelerate. decelerate y Allows precise beam control and deep weld penetration. y No shield gas (vacuum chamber used) 7  8/26/2011 Laser Beam Welding y Very thin HAZ and little thermal distortion. y Used a focused laser beam provides power intensities y Filler metal and inert gas shield may or may not used. in excess of 10kW/cm2 y Deep penetration. y The high high‐intensity intensity beam produces a very thin column y No N vacuum needed. d d of vaporized metal with a surrounding liquid pool. y No direct contact needed. y Depth‐to‐width ratio greater than 4: 1. Contd… Contd… y Heat input is very low, often in the range 0.1 to 10 J. Forge Welding y Adopted by the electronics industry. y Blacksmith do this. y Possible to weld wires without removing the y Borax is used as a flux. polyurethane insulation. insulation y The ends to be joined were then overlapped on the anvil and hammered to the degree necessary to produce an acceptable weld. y Quality depends on the skill of the worker and not used by industry. Contd… Friction Welding y Machine is similar to a centre lathe. y Heat is obtained by the friction between the ends of y Power requirements 25 kVA to 175 kVA. y The axial pressure depends on the strength and the two parts to be joined. hardness of the metals being joined. y One part is rotated at a high speed and other part is y Pressure 4 40 MPa for low‐carbon steels to as high g as 45 450 MPa for alloy steels. axially aligned and pressed tightly against it. y Friction raises the temperature of both the ends. Then rotation is stopped abruptly and the pressure is increased to join. Contd… Contd… 8  8/26/2011 y Very efficient. y Wide variety of metals or combinations of metals can be joined such as aluminium to steel. y Grain size is refined y Strength is same as base metal. metal y Only round bars or tubes of the same size, or connecting bars or tubes to flat surfaces can join. y One of the components must be ductile. y Friction welding is a solid state welding. Fig‐ friction welding process Contd… Ultrasonic Welding (USW) Restricted to the lap joint Weld thin materials‐sheet, foil, and wire‐‐or the USW is a solid‐state welding. attaching thin sheets to heavier structural members. High‐frequency (10 to 200, KHz) is applied. Maximum thickness 2.5 mm for aluminum and 1.0 Surfaces are held together under light normal mm for harder metals. pressure. Number N b off metals t l andd dissimilar di i il metal t l combinations bi ti Temp. do not exceed one‐half of the melting point. and non metals can be joined such as aluminum to The ultrasonic transducer is same as ultrasonic ceramics or glass. machining. Equipment is simple and reliable. Less surface preparation and less energy is needed. Contd… Contd… Explosion Welding Applications y Done at room temperature in air, water or vacuum. y Joining the dissimilar metals in bimetallics y Making microcircuit electrical contacts. y Surface contaminants tend to be blown off the surface. y Welding ld refractory f or reactive metals l y Typical impact pressures are millions of psi. psi y Bonding ultrathin metal. y Well suited to metals that is prone to brittle joints when heat welded, such as, y Aluminum on steel y Titanium on steel Contd… 9  8/26/2011 y Typically the detonation velocity should not exceed 120% of the