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Oxy
Acetylene
Welding
of
Gray
Cast
Iron |
Oxy
Acetylene
Welding
of
Gray
Cast
Iron - Gray cast iron is successfully welded by gas welding but it requires massive inputs of heat, both in preheating and during the welding operation. This large heat input may cause distortion or dimensional changes of the components. The slower cooling rate resulting from gas welding, however, lessens the tendency for hardening of the heat affected zone.
Joint Preparation A 60 to 90° Vee groove should be ground or chipped out or cut with a cutting torch or cutting electrode. This groove should not pass completely through the casting as otherwise alignment would be difficult. For thin sections, a 75 to 90° Vee joint is generally used.
For very heavy sections of 25 mm and above, a 90° double Vee joint is often recommended. When welding can be made from one side only, the groove angle should be increased to about 120 degrees.
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When the groove extends through the casting, backing up with a graphite backing plate should be provided. When repairing cracks, a hole should be drilled at each end of the crack prior to welding to prevent further propagation of the crack.
Preheating the job The job, before welding, is preheated at 620°C in a furnace and then covered with asbestos cloth, exposing only the cavity to be welded. If a furnace is not available, the casting can be covered with asbestos cloth and locally heated by gas flame. Thin sections may be preheated locally, whereas heavy sections should be preheated in their entirety in a furnace.
Gas Welding Rods For welding gray cast iron, cast iron filler rods having the same composition as the base metal and containing a minimum of sulphur and phosphorus are used. Welding rods should contain enough carbon and silicon to allow for losses of these elements during welding.
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Special rods containing titanium and with a high silicon content are also employed sometimes. The welding rods are in the form of square or round cast bars A typical cast iron welding rod composition (%) may be
C 3.25-3.50
Si 2.75-3.00
Mn 0.60-0.75
P 0.50-0.75
S 0.10 Max
Super silicon, cast iron, IS 1278 type 5.1 rod (square) of Indian Oxygen Limited may be used for welding high grade castings where subsequent machining is necessary such as lathe beds, cylinder blocks etc.
Welding Flux Flux increases the fluidity of the fusible iron-silicate slag, as well as aids in the removal of the slag. Fluxes for gray iron filler rods are usually composed of borates, soda ash and small amounts of ammonium sulphate, iron oxide etc.
Welding Procedure Prepare the edges to be welded as described earlier. Clean the plate edges and around to remove foreign matter from the surface. Take a suitable filler rod and flux. Select a welding tip one size larger than for steel of the same thickness. Adjust the welding torch for neutral flame.
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Move the gas flame along the sides of the Vee groove until the entire joint is preheated to dull red. Concentrate the flame at the bottom of the Vee at the starting point. Keep the tip of the inner cone about 3.0 to 6.0 mm from the metal surface.
As the bottom of the Vee is thoroughly fused, move the flame from side to side, melting down the sides gradually so that the liquid metal runs down and combines with the molten metal at the bottom of the Vee. Rotate the torch in a circular motion so that the sides and bottom of the Vee are kept in molten condition.
If the metal gets too hot and tends to run away, raise the flame slightly. If the weld metal gets overheated, excessive gas will generate which is liable to become trapped in the weld metal causing porosity. Once the molten pool forms, heat the filler rod end by bringing it into the outer envelope of the flame. Dip the heated rod end into the flux.
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The flux stricken filler rod is introduced into the molten pool. The flame is applied at the tip of the filler rod and welding is carried out using backhand technique. More careful temperature control is possible with backhand technique, since it directs the flame back upon the weld metal and filler rod.
Maintain flame tip about 6.4 to 19 mm from the surface of weld pool as per cross section of the material. Never go on dipping the filler rod in and out of the puddle, it may, get contaminated (oxidized) by the atmosphere. The rod should always be held inside the flame/weld. As the filler rod melts, the molten metal will rise in the groove.
As the section of the Vee is built slightly above the level of the rest of the casting, move the puddle forward. In other words when one section 25 mm or so long is built up with filler metal, the bottom of the Vee adjacent to it is next melted and the operation is repeated.
When white spots or gas bubbles appear in the puddle or at the edges of the seam add more flux and play the flame around the specks until the impurities float to the top. These impurities arc skimmed from the weld with the help of filler rod which is then tapped against the worktable to remove them from the filler rod.
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As the weld is complete, cover it with asbestos or any other insulating material. The job is allowed to cool slowly in order to minimize the tendency to harden and crack formation. All slag and oxide must be filed (and not hammered) away from the weld before it is cold. Cast iron should be welded as fast as possible. To strengthen welds in cast iron:
(i) Insert studs in the edges to be joined, which when welded, add materially to the strength of the weld.
(ii) Cut notches in the cast iron surfaces to be welded. Gray cast iron welding of long jobs should be done intermittently using skip welding etc.
Post weld stress relieving is recommended particularly for complex castings or where accurate machining will follow. The casting should be placed in a furnace, that is, at the same temperature as the casting, immediately upon completion of welding and gradually heated to 593 to 650°C for one hour per 25 mm of thickness; it may then be cooled to 260°C or below, at a rate no faster that 28°C per hour.
Though gas welding of gray cast iron is highly successful, yet it is replaced by electric arc welding when overhead welding is required.
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