Module 3

Module 3

• 1.
  Disadvantage of GTAW. 1This is very slow process for weld deposition. 2.It has low tolerance for contamination, hence base, filler wire and electrode have to be very clean before welding. 3. It needs very high level of skill if manually done. Porosity cannot be avoided if contamination exists. 4. Tungsten inclusion- This can occur for following reasons. a. If electrode comes in contact with molten metal or filler metal. b. If hot tip is contaminated with spatter. c. If exceeding current limit for given dia. of electrode or type of electrode. d. Extension of electrode beyond normal distance from collets, this causes over heating of electrode. e. Inadequate tightening of collet. f. Less flow rate of shielding gas or excess wind draft causing electrode oxidation. g. Defective electrodes like splits or cracks. h. Use of improper gas, improper grinding of electrode tip. 32/184 SUBMERGED ARC WELDING This is most efficient method. This uses continuously fed solid wire electrode which provides an arc and it is totally covered by a layer of granular flux. In this process granular flux is distributed ahead of or around the wire electrode to facilitate protection of molten metal. During welding layer of formed slag and still granular flux covering the solidified weld metal. Since electrode and flux are used separately various combinations is available. Two general combinations are well preferred for alloyed weld deposit. 1. An alloy electrode with neutral flux or 2. A mild steel electrode with an alloy flux. AWS designates for both electrode and flux. SAW can be used in fully automatic/mechanized or semi automatic. Most of SAW process uses constant voltage power source. In some applications constant current type is preferred. Similar to GMAW and FCAW wire feeder forces wire through the cable liner to the welding torch. In this process flux needs to be moved to the weld zone. The flux is generally fed through a hopper above welding torch and fed by gravity hence distributed either slightly ahead or around arc from a nozzle surrounding the contact tip. In case of semi automatic SAW flux is forced to the gun using compressed air which fluidizes the granular flux for easy flow. And hopper is connected directly to hand held gun. In other SAW variations choice of AC or DC and either polarity. The type of welding current will affect both penetration and weld bead counter. In some variations multiple electrodes are used with single power source or multiple power source. Due to high rate of weld metal deposit it is very effective in relaying or building up material surfaces. In applications where a surface needs improved corrosion resistance or wear resistance then it is more economical to cover a susceptible base metal with a resistant weld overlay. SAW has biggest advantage in its high deposition rate. It is highly liked by operator as lack of visible arc which allows the operator to control the welding without the need for filter lens or any other protective clothing. It generates less smoke and has higher penetration. Limitations. It can be only used where flux can be deposited over the weld joint when welding in horizontal or flat position; some device is reqd. to hold the flux in place so it can perform the job. Other disadvantage of SAW is that it may need special jigs and fixtures for every work/job. Slag/flux removal is also very important in this welding process. If welding parameters are set incorrectly slag removal becomes difficult in this process. Since flux is covering the entire area operator cannot see where arc is positioned with respect to the joint. If arc is not properly directed incomplete fusion can occur. SAW flux must be stored properly away from moisture in a heated container. If flux is wet porosity and under bead cracking may result. SAW has other problem of solidification cracking. This happens when welding condition provides a weld bead having an extreme width to depth ratio. If beads width is much greater than its depth or vice versa, centre line cracking can occur during solidification.
• 2.
  33/184 PLASMA ARCWELDING Plasma is an ionized gas. Any process using an arc, plasma is created, however PAW is so named because of intensity of this plasma region. PAW and GTAW equipment required is very similar. Both needs same type of power source however difference in heating amount and resulting penetration. Both use Tungsten electrode for creation of arc. However with POW torch there is a copper ___________ within the ceramic nozzle. There is a high velocity of PLASMA gas which is forced through this orifice and past this welding arc, resulting in the constricting of the arc which causes it to be more concentrated and hence more intense. Plasma arcs are of 2 categories- Transferred and Non Transferred. In transferred arc, the arc is created between tungsten electrode and work piece. In non transferred arc it occurs between tungsten electrode and copper orifice. Transferred arc is generally used for both welding and cutting of conductive material, as it results in greatest amount of heating of work piece. Non transferred arc is preferred for cutting of non conductive materials and for welding of materials when amount of heating of the work piece must be minimized. For GTAW AND POW power supply are identical in most respect. Some additional elements are required including Plasma Control Console and Source of Plasma Gas. The torch also differs slightly for both PAW and GTAW. For PAW manual welding torch and separate gases are reqd. Two separate gases are reqd for PAW- Shielding gas and orifice gas. Argon is applied generally for both purposes. For welding with PAW various metals may require Helium or Argon + helium or Argon + Hydrogen. PAW and GTAW applications are similar in use PAW is used for same material and thickness. Where it is required more localized heat source PAW is preferred. For full penetration welding up to ½ inch thickness PAW is used by applying Key hole welding technique. Key hole welding is applied on square butt joint with no root opening. The concentrated heat of the arc penetrates through the material thickness to form a small key hole. As welding progress key hole moves along the joint melting the edges of the base metal which then flow together and solidify after welding arc passes. This creates high quality weld, with no elaborate joint preparation and fast travel speed compared to GTW. 34/184-- Due to localized heat source PAW allows for faster welding speeds and hence less distortion. Space being more between torch and work piece, welder has better visibility. The tungsten electrode being recessed with torch, welder may not get tungsten in contact with molten metal and hence tungsten inclusion can be avoided. Due to key hole process it is positive indication that complete penetration is achieved and weld uniformity is available. The weld uniformity is arrived at due to the fact that plasma arc welding is less sensitive to change in arc length. PAW is limited to effective joining of materials less than 1 inch thickness. Initial cost of PAW is higher than GTAW. PAW needs higher operator skill. PROBLEMS Two types of metal inclusions are expected. Tungsten inclusion may result from the high current level. Since tungsten is recessed inside this problem may less likely to occur, too high current can result in copper orifice melting and getting deposited on weld metal. Another problem due to key hole welding is tunneling. This occurs when key hole is not completely filled at the end of the weld, leaving a cylindrical void which may extend entirely through throat of the weld. It is possible to have incomplete fusion since arc and joint are so narrow. 35/184 ELECTRO SLAG WELDING ( ESW ) It is not commonly used. It exhibits highest deposition rates. This process is characterized by joining of members which are placed edge to edge so that joint is vertical. Welding is done in single pass such that progression is from bottom to top of the joint without interruption. Even though welding progresses vertically up the joint, position of welding is considered flat due to the location of electrode with respect to puddle. During welding the molten metal is separated on two sides by water cooled shoes. This process is not considered arc welding process. It relies on heating from elec. Resistance of molten flux to melt the base and filler metals. The process uses arc to initiate the operation, however arc is extinguished once there is sufficient flux melted to provide heating to maintain the welding operation as it progresses upward along the joint. This is used for joining very heavy sections. It is essentially limited to welding carbon steels in thickness greater than ¾” It has a high deposition rate. Multiple electrodes can be used if one is not enough. Even metal strip can be used instead of wire to increase deposition rate to higher values. No special joint preparation is reqd.
• 3.
  Even a roughflame cut surface is enough satisfactory for this method. Since entire thickness of joint is fused in a single pass there is no tendency for any regular distortion to occur during or after welding. Hence alignment is easily maintained. LIMITATIONS Extensive time reqd to set up and get ready for weld. Hence it is economical for large sections only. It has few problems which if occurs can be of major proportion. Gross porosity can occur due to wet flux or the presence of leak in one of the water cooled shoes. There is possibility of getting center line cracks due to weld metal shrinkage. Grain growth can occur due to tremendous heating which can degrade weldment’s mechanical properties. 35/184 OXYACETYLENE WELDING. (OAW) Acetylene is the only fuel gas capable of producing high enough temp for effective welding. In this process energy for welding is created by flame hence this process is considered as chemical welding method. Heat is provided by chemical reaction shielding for this welding is accomplished by this flame only. Hence no flux or external shielding is reqd. Oxygen is cylinder is a HP container capable of of with standing pressure of approx. 2200 PSIG. Acetylene cylinder is filled with porous material similar to cement. Acetylene exists in cylinder dissolved in Acetone. Gaseous acetylene is extremely unstable at pressure exceeding 15 PSI. and explosion can occur even without presence of oxygen. Since it contains liquid the cylinder must be kept vertical to prevent drawing off liquid. The regulator reduces high internal pressure to working pressure. The torch has a mixing section where oxygen and acetylene combine to provide desired mixture. For carbon steel welding gas mixture ratio is adjusted to provide neutral flame. Increase of oxygen shall provide oxidized flame and increase of acetylene shall make it carburized flame. The filler metal used for OAW has simple identification system. Like RG-45 and RG- 60. R is for rod, G is for gas and digits indicate minimum tensile strength of weld deposit in thousands of PSI. It is primarily used for welding of thin steel sheet and small dia pipes. It is relatively in expensive and portable. No electrical input involved. Whenever cylinders are to be moved, regulators are removed and valves are covered with special screw on caps for protection from impacts. LIMITATIONS It does not provide concentrated heat like given by arc. Hence for groove weld joint preparation must be exhibiting a thin feather edge to ensure complete fusion at root of the weld joint. This lower concentration of heat results in relatively slow process. This needs very high skill. Since heat source is not concentrated flame must be directed to assure proper adequate fusion. If during welding carburizing or oxidizing flame is produced weld metal property would be degraded. 36/184 STUD WELDING (SW) This is considered an arc welding process as heat is generated by arc between stud and base metal. The process is controlled by a mechanical gun connected to power supply through control panel. Welding is done easily and repetitively. Process is done in 4 cycles properly timed and sequenced by control box. Once stud is positioned and trigger is pulled then stud is welded in 4 steps. When properly done stud weld should exhibit complete fusion throughout cross section as well as reinforcing fillet or flash entire circumference of the stud base. The equipment consists of Dc power supply source, control unit, stud welding gun, gas shielding can be added for welding AL studs. Building and bridge industry use SW. It needs very less operator skill. It is very economical. Its use eliminates need for hole drilling or manual welding. During inspection by hammering good stud shall ring and poor joint could produce dull thud. Any elec. Or mech. malfunctioning could produce poor quality of welds.. Stud shape is limited to some configuration which can be held in gun’s chuck. It can have 2 discontinuities. Lack of 360 degree flash and incomplete fusion at interface. These are caused by improper machine setting . Presence of water or rust or mill scale on base metal surface could effect resulting weld quality.