Welding of Cast iron

Cast iron includes a variety of iron-based materials such as carbon, silicon manganese, phosphorous, sulfur, etc. It is a versatile metal used for both general and commercial purposes.

Types of Cast iron

Generally, cast irons are defined into four common types

• Gray Cast Iron
•  Malleable Cast Iron
• Nodular Cast Iron.
• White Cast Iron

Gray cast iron

The carbon content in gray cast iron ranges from 2.5 to 3.8 percent while other elements such as silicon can be 1.1-2.8%, manganese 0.4-1.0%, phosphorus 0.15%, silicon 0.4-1.0%, and sulfur 0.10%.

Its surface appears brown when fractured. It is marked by the presence of graphite flakes in a matrix of ferrite or pearlite. It has the lowest melting point of all ferrous alloys (about 1178 °C), is brittle in nature, easy to machine but probably the hardest of all metals to weld economically.

Major areas of application

• Repairing of casting defects in the foundry.
• Repairing of damaged or worn in service.
• For joining together separate parts of weldable cast iron.

Welding characteristics of gray cast iron

During welding, a molten pool of gray cast iron is created at the joint to be welded, forming a weld when it solidifies. If the gray cast iron has an appropriate percentage of silicon, the amount of carbon going into the combined state (as cementite, etc.)or remaining free (as flakes) will depend on the cooling if the cooling rate is fast, most of the carbon will go into the combined state and the result will be a very hard, brittle and unimaginable white cast iron rate of the weld metal. It is possible to the formation of cracks also.

To obtain a soft, stress-free, and easily machined weld, the weld metal coiling rate is retarded by preheating the gray cast iron part prior to welding at a high temperature of 600–700 °C.

It depends on the geometric shape and size of the workpiece whether preheating can be done only at and around the joint or the whole work can be preheated, Sometimes it is preferred to preheat the entire job if practical.

After the weld is complete, the hot part of the component is covered with asbestos, buried in dry sand, and the work is allowed to cool relatively slowly.

Application of post-heat

Even after heat treatment of the entire component or a local area adjacent to the weld, it helps to reduce excessive cooling stress and prevent the formation of white cast iron’.

Other steps (techniques) of post-heat

• Use suitable nonferrous stick electrodes of a lower melting point than gray cast iron.
• Use skip welding to minimize heating of gray iron components.
• In arc welding, direct the arc upon the already deposited weld metal.
• In gas welding, the flame should be directed upon the welding rods.
• The filler material employed for welding also plays an important role in introducing welding stresses and in the formation of cracks in the welded joint.

As far as possible, the filler of material should have the same coefficient of thermal expansion as gray cast iron.

Welding process involved in gray cast iron

• Metal arc welding
• Oxyacetylene welding
• Braze welding
• Brazing
• Thermit welding

Metal arc welding method

• A V joint can be made with an angle of 60 – 90 degrees to the workpiece to be joined by chipping or machining.
• Cleaning of the joint is a must for all dust, dirt, oil, grease, and paint.
• Electrodes of cast iron, mild steel, austenitic stainless steel, nickel alloys, etc, may be employed for welding gray cast iron.

Steps

In the first step, the arc is struck by touching the electrode with the work.

As soon as the molten pool is formed, welding is performed in the usual way.

To reduce established stresses in the workpiece, welds may be placed in short runs (skip welding) and then each allowed to cool. Beating the weld when it is hot also relieves stress.

Skip welding technique

It is a very successful technique in arc welding of cast iron. A short length of weld metal is deposited in one part of the seam then the next length is done some distance away, keeping the sections as far away from each other as possible thus localizing the heat.

• Before welding, preheating (600-700 degrees centigrade)may be carried out and after the welding is over, the job may be covered with an insulating material, by this way it produces good quality welded joints.
• In some situations, post heat treatment is carried out immediately after welding. In that case, there is no need to cover the weld, etc. with an insulating material.
• An ac or dc power source may be employed for welding. The current required to weld with 6 to 10mm cast iron electrodes is approximately 300 -400 Amps respectively.

Welding Electrodes of cast iron

The electrodes made up of the following materials may be used for gray cast iron welding.

• Cast iron with high silicon and low sulfur content.
• Mild steel or soft iron
• Phosphor bronze
• Monel metal and other nickel alloys
• Copper steel

Cast iron electrodes are ideally used in the repair welding of moderate size castings. They produce deposits of strength and colo color approaching that of the casting. Preheating of the job is necessary.

Mild steel electrodes with special coverings are used for repair welding of small pits and cracks in castings where subsequent machining is not done. Mild steel deposit absorbs carbon from the base metal such as cast iron and hardens. To avoid it sometimes, a layer of phosphor bonze is deposited on the sides of the joint and then filled with meld steel; preheating of the job is desirable.

Phosphor bronze electrodes are preferred for nice machinability and wear resistance. Monel metal and other nickel alloys produce ductile and machinable welds.

• Excellent welding results are obtained with commercially pure(99%) nickel flux coated electrodes.
• Preheating of the job is not essential unless thicker or complex sections are involved.
• Aluminium-bronze electrodes are useful for joining cast iron to nonferrous metals.

Oxy-acetylene welding of gray cast iron

Gray cast iron can be successfully welded by oxy acetylene welding, but this requires a high heat input, during preheating and welding operations. high heat input in irons can cause distortion or makes dimensional changes in the equipment. However, the slow cooling rate reduces the tendency for heat-affected zone hardening.

Joint preparation

A 60-90 degrees Vee grove should be ground or chipped out or cut with a cutting torch or cutting electrode. This grove should not pass completely through the casting as otherwise, the alignment would be difficult. For this section, a 75 to 90 degrees vee joint is generally used. For very heavy sections of 25mm and above, a 90-degree 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.

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.

Pre-heating the job

The job, before welding, is pre-heated at 620 degrees centigrade 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 a gas flame. Thin sections may be preheated locally, whereas heavy sections should be preheated in their entirety in a furnace.

Gas welding rod

For welding grey cast iron, filer rods having the same composition as the base metal and containing a minimum of sulfur and phosphorus are used.

Welding rodes should contain enough carbon and silicon to allow for losses of these elements during welding. 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 % and S 0.10 max.

Welding flux

The 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 sulfate, iron oxide, etc.

Welding procedure(steps)

• Prepare the edges to be welded as described earlier
• Ensure that the plate edges and surroundings are cleaned to remove foreign matter from the surface.
• Take a suitable filler wire and flux
• Selecta a welding tip one size larger than for steel of the same thickness
• Adjust the welding torch for a neutral flame
• Move the gas flame along the sides of the vee groove until the entire joint is preheated to dull rec.
• 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 mm from the metal surface.
• As the bottom of the vee is thoroughly fused, move the flame from side o 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 lead the porosity.

Once the molten pools form heat the filler rod end by bringing it into the outer envelope of the flame, Dip the heated rod end into the flux.

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 the backhand technique.

More careful temperature control is possible with the backhand technique. Since it directs the flame back upon the weld metal and filler rod. Maintain flame tip about 6.4 to 19mm from the surface of the 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 order words when one section 25mm 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 are skimmed from the weld with the help of a filler rod which is then tapped against the workable to remove them from the filler rod.

As the weld is complete, cover it with asbestos or any other insulating material.(blankets) 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.

Welding should be done as fast as possible.

Braze welding of gray cast iron

Braze welding methods are used for making field repairs. New castings are generally not repaired by braze welding because of poor color matches.

• Filler rod materials for braze welding may be Naval brass, manganese bronze, or nickel bronze.
• Flux may be added manually by dipping the filler rod’s heated end into it or the filler rod itself may be flux-covered.
• Preheating is not necessary unless the casting is heavy or complicated, in which case preheating between 316 and 400 degrees is sufficient.

• The use of a salt bath is best for cleaning any of the cast irons prior to brazing welding. If a salt bath is not available, then the ground grove surface of cast iron is heated with a slightly oxidizing flame to dull red color cooled and wire brushed. This removes graphitic smear from the groove surface.

• Slightly oxidizing flame is used for braze welding gray cast iron

• After welding job should be covered with thermal protection so it cools slowly.

Advantge of braze welding

• Low thermal stresses reduce the possibility of cracking.
• Low peak temperatures avoid the formation of brittle transformation products as seen in other welding processes.
• The copper alloy weld metal is sufficiently ductile to absorb most shrinkage stresses without chacking.

Disadvantages of braze welding

• There is a color mismatch between the weld metal and the base metal
• The strength of a braze weld falls off rapidly with increasing temperature, so this process is not used where service temperatures exceed 260 degrees centigrade.
• Galvanic corrosion, due to dissimilar metals may be a problem( bad result)
• The corrosion resistance of weld metal is low when it is exposed to strong alkalies.

Brazing of gray cast iron

Brazing of gray cast iron is done to repair casting defects where strength and color match are not of primary importance.

• Brazing for this material requires special precleaning methods to remove graphite from the surface of iron because the presence of graphite on the cast-iron surface would prevent welting and adhesion of the brazing alloy.
• It is carried out at a temperature a low a feasible in order to avoid a reduction in the strength of iron.

Filler rod( bare electrode)

A 6% tin bronze brazing rod, melting at about 925 degrees centigrade can be successfully employed for brazing gray castiron.

Silver brazing alloys are frequently used as filler rods. Typical compositions of such alloys are:

Ag 44-46% ,Cu 14-16%, Zn 14-18%, Cd 23-25% Brazing temperature 625-760 degrees centigrade.

Silver brazing rods containing nickel produce greater bond strength s.

Joint preparation

For brazing, lap or sleeve joints may be used.

process

Brazing is generally done with an oxyacetylene torch a neutral or slightly carburizing flame. Other methods such as furnace brazing, resistance brazing, induction brazing, etc, are also commercially used for (soft weld) production of small parts.

Preheating between 205-427 degrees centigrade before torch or induction brazing may produce better results.

Applications

• To repair defective equipment or damaged castings.
• to join assemblies at a lower cost than possible by another surrounding process.

Thermit welding of gray cast iron

Heavy structures such as machinery basis or frames are thermit welded.

Since thermit metal shrinks as much as cast iron any weld longer than eight times the sectional thickness may develop minute hairline cracks. Thus the designer must make suitable allowance for contraction colling.

Welding of Malleable Cast iron

During the welding of malleable iron, if the temperature exceeds 760 degrees centigrade, the metallurgical structure in part of the heat-affected zone ( Haz)is changed back to white cast iron.

Since this is undesirable, it is important that either that welding process should be used which does not heat malleable iron above 760 degrees centigrades or that a malleableizing great treatment of the casting is carried out. If such a treatment is not practical from economy and other viewpoints the following heat treatment may be used for obtaining a ductile weld.

• Heat the casting to 843 degrees centigrade, hold one hour for each 25 mm of thickness, cool in the furnace to 726 degrees centigrades, and then hold at this temperature for six hours and cool.

Welding process

Most malleable iron casting are repair welded by the bronze welding process. Cast iron rods are being used only for building up worn parts.

Oxyacetylene and electric welding processes present difficulties because the temperature goes above 760 degrees centigrade and therefore after welding, the casting requires a post-heat treatment.

Welding of nodular cast iron

As regards welding behavior, nodular or ductile iron and malleable iron are sufficiently similar.

The nodular iron can be brazed but could not be successfully gas welded.

Welding of alloy cast iron

Alloy cast iron contains various elements such as chromium, copper, and molybdenum are used as alloying elements. It is also welded in a similar manner to gray cast iron. The filler rods employed are of similar composition as the parent metal, where this is not possible, gray iron rods may be used.

Conclusion:

For welding of cast iron, it is required to clean the job properly and maintain the heat input method as well as post heat treatment.