Cast iron is widely used for engine components such as blocks, heads, and manifolds; for drive components such as transmission cases, gearboxes, transfer cases, and differential cases; and for equipment such as pumps, planters, drills, and pulleys. Cast iron is hard and ridged, which makes it ideal for any size casing or frame that must hold its shape even under heavy loads. For example, if a transmission case were to bend under a load, the gears and shafts inside would bind and stop turning. All five types of cast iron have high carbon contents, usually ranging from 1.7% to 4%. The most common grades contain approximately2.5%to3.5% total carbon. The carbon in cast iron can be combined with iron or be in a free state. As more of the carbon atoms in the cast iron combine with iron atoms, the cast iron becomes harder and more brittle.
The five common types of cast iron are as follows:
● Gray cast iron is the most widely used type. It consonantal much free carbon that a fracture surface has a uniform dark gray color. Gray cast iron is easily welded, but because it is somewhat porous it can absorb oils into the surface, which must be baked before welding. Gray cast iron may be fusion welded or braze welded without difficulty if preheating before welding and cooling after welding are controlled. It is low inductility and has moderate tensile strength and high compression strength. Corrosion resistance and tensile strength can be improved by adding nickel, copper, and chromium as alloying materials. Gray cast iron has high machinability.
● White cast iron is the hardest and most brittle of the cast irons because almost all of the carbon atoms are combined with the iron atoms. White cast iron is produced through a process of rapid cooling which causes the carbon to combine with the iron. There is no free carbon as in gray cast iron. This causes white cast iron to be hard, brittle, and very difficult to machine except with special cutting tools. It is so difficult to weld that it is considered unweldable. White cast iron is not generally used for castings. It is the first step in the making of malleable iron. White iron has a fine grain structure and a silvery white appearance when fractured.
● Malleable cast iron is white cast iron that has undergone a transformation as the result of along heat treating process to reduce the brittleness. It forms when white cast iron has been heat treated by a long annealing process that changes the combined iron and carbon structure into iron and free carbon.
The tensile strength, impact strength, ductility, and toughness are higher than that of gray or white cast iron. The material may be bent and formed to a certain degree. In many respects the mechanical properties approach those of low carbon steel. If malleable cast iron is heated above its critical temperature, approximately 1700°F (925°C), the carbon will recombine with the iron, transforming back into white cast iron. Malleable cast iron can easily be welded. To prevent it from reverting back to white cast iron, do not preheat above 1200°F (650°C). Fusion welding destroys the properties of malleable iron in the weld area. Because of fast cooling, the area reverts back to chilled cast iron and must be heat treated. Braze welding is recommended because of the relatively lower temperature (1,500°F) used. If a piece of malleable iron is broken, the fracture will show a white rim and dark center.
● Alloy cast iron has alloying elements such as chromium, copper, manganese, molybdenum, or nickel added to obtain special properties. Various quantities and types of alloys are added to improve alloy cast iron’s tensile strength and heat and corrosion resistance. Almost all grades of alloy cast iron can be easily welded if care is taken to slowly preheat postcolonial the part to prevent changes in the carbon and iron structure.
● Nodular cast iron, sometimes called ductile cast iron, has its carbon formed into nodules or tiny round balls. These nodules are formed by adding an alloy. Nodular cast iron has greater tensile strength bhangra cast iron and some of the corrosion resistance of alloy cast iron. Nodular cast iron is weldable, but-proper preheating and post weld cooling temperatures and rates must be maintained or the nodular properties will be lost.
Nodular iron is also referred to as ductile iron. Amounts of magnesium and/or cerium are added to the iron when it is produced. Without these alloys, the graphite (free carbon) produces a notch effect which lowers the tensile strength, toughness, and ductility of the iron. The alloys change the shape of the graphite particles from flakes to spheroids and so reduce the notch effect. The silicon content of nodular iron is higher than that in other irons. Nodular iron approaches the tensile strength and ductility of steel. It has excellent machinability, shock resistance, thermal shock resistance, wear resistance, and rigidity. Nodular iron is readily fusion welded with a filler rod containing nickel. Both preheating and post heating are necessary, and the weld must be cooled slowly.
Pre-weld and Post-weld Heating of Cast Iron
The major purpose of preheating and post heating of cast iron is to control the rate of temperature change. The level of temperature and the rate of change of temperature affect the hardness, brittleness, ductility, and strength of iron-carbon–based metals such as steel and cast iron.
Preheating the casting before welding reduces the internal stresses caused by the rapid or localized heating resulting from welding.
Welding stresses occur because, as metal is heated and cooled, it expands and contracts. Unless the heating and cooling cycles are slow and uniform, stresses within brittle materials will cause them to crack. In some aspects, the brittleness of cast iron is much like that of glass. Both cast iron and glass will crack if they are heated or cooled unevenly or too quickly.
Post weld heating changes the rate of cooling.
Rapid cooling of a metal from a high temperature is called quenching. The faster an iron-carbon metal is quenched,the harder, more brittle, less ductile, and higher in strength metal will become.
The slow cooling of an iron-carbon metal from a high temperature is called annealing. The slower an iron- carbon metal is cooled from a high temperature,the softer, less brittle, more ductile, and lower in strength the metal will become.
To reduce welding stresses, maintain the casting at the same temperature used for preheat or higher for 30 minutes following welding. The casting should cool slowly over the next 24 hours. Cover the casting tomprevent the part from being cooled too rapidly by the surrounding air following welding. A firebrick ormheavy metal box can be used to keep cool air away from the casting.
Welding Cast Irons:
Cast iron contains between 1.7% and 4% carbon, which makes the metal brittle. All cast irons are difficult to weld because they contain a great deal of carbon. Welding temperatures and an uncontrolled cooling rate can produce undesirable micro structures in the metal. Carbides tend to form at the weld boundaries, and high-carbon In a martensite also tends to form. Both of these are brittle and have very low strength. Welding also produces high stresses in this brittle metal. Ductile and malleable cast irons are easier to weld. Gray cast iron is cooled slowly and forms graphite in a flake form as it cools. It is brittle and lacks ductility. when grey cast iron is broken, the exposed metal is gray in appearance. Gray cast iron can be arc welded using a nickel or nickel alloy electrode such as ENi-CI, ENiFe-CI, ENiFeMn -CI, ERNi-CI, or ERNiFeMn-CI. The nickel content in the filler metal permits the weld to move or “creep” after welding, relieving stresses and preventing cracks. Nickel-bearing filler metals, being less brittle than the cast iron base metal, also make the completed weld easier to machine. Pure iron filler metal, which contains very low-carbon content, can also be used. Carbon from the base metal is diluted in the weld area by the pure iron filler metal and creates a less brittle weld.
Specification AWS A5.15 describes cast-iron electrodes and their recommended uses. Gray cast iron can also be braze welded successfully. White cast iron forms when cast iron cools very rapidly. The high carbon content remains throughout the micro structure. As a result, the cast iron is brittle and has virtually no ductility. It is considered unweldable. The fractured end of white cast iron has a white appearance. Preheating of cast iron produces slower cooling rates. Post-veld heating slows the cooling rate and reduces the amount of hardness in the weld and heat affected zone. See Figure 21-15 for recommend preheat and inter-pass temperatures for welding cast iron.
Backing strips can be used to support the joint area during welding or brazing. See Figure 21-19.
Oxy-fuel Gas Welding Cast Iron
The oxyfuel torch tip size used for welding cast iron should be similar to the tip size used for welding steel of the same thickness. A neutral flame should be used along with a cast-iron welding rod of the proper size. RCI and RCI-A are two filler rods used for oxyfuel welding cast iron. A flux is also required.
The backhand weld technique is usually used for welding cast iron. See Figure 21-20. Backhand welding tends to lo slow the cooling rate of the weld and reduce stresses. The torch should be held at a 60° angle to the plate. The inner cone should not touch the metal. The welding rod is coated with flux. The flux enters the weld as the welding rod is consumed. The flux must have the correct constituents and be fresh, clean, and moisture-free.
In cast-iron welding, the molten pool is not very fluid.It is important that gas pockets and oxides be worked to the surface of the weld. This can be done by stirring the molten pool with the filler rod. The oxides or gases are then removed by the flux. The weld must have thorough penetration, and a slight crown is preferred. An oscillating torch and filler rod motion are usually used. The oxyfuel gas process is also used to braze and braze weld cast iron.
Characteristics of Cast Iron
You will recall that cast irons are classified as gray iron, white iron, nodular iron, and malleable iron. Both gray cast iron and malleable iron are used commercially. We will be concerned here with the fusion welding of gray cast iron.
Malleable iron cannot be fusion welded with the oxyacetylene process and must be braze welded.
Gray cast iron is an alloy of iron, carbon, and silicon. The carbon in gray cast iron may be present in two forms: in a carbon and iron solution and as a free carbon in the form of graphite. When it is broken, the fractured surface has a gray look due to the presence of the graphite particles. Gray cast iron is easy to machine because graphite is a fine lubricant. The presence of graphite also causes gray cast iron to have low ductility and tensile strength.
Gray cast iron may be braze welded or fusion welded. For the most part, cast iron is welded in maintenance and repair work. Welding is seldom used as a fabricating process. Braze welding is preferred since it can be applied at a low temperature, and the bronze weld is highly ductile.
Fusion welding is used when the color of the base metal must be retained and when the welded part is to be subjected to service temperatures over 500°F. Table 9-1 summarizes the various cast iron welding procedures.
Control of expansion and contraction is very important in cast iron welding. The bulk and shape of the casting and whether or not light sections join heavy sections affect preheating and welding technique.
When fusion welding cast iron, all parts of the casting must be able to expand equally to prevent cracking and locked-in stress in the job. If the torch is applied directly to the cold casting and the joint to be welded is raised to the melting point, the expansion of the heated metal will cause a break or crack in the relatively cold casting surrounding the weld. If it does not break, severe internal stresses are locked in that may later cause a failure under service. Small castings can be preheated with the oxyacetylene flame during the welding operation if the entire casting is heated evenly. Large castings may have to be preheated in a firebrick furnace built around the casting. Heating is usually done with gas- or oil-fired burners. The furnace is covered with heat-resistant material to retain the heat and keep out cold drafts. The casting is welded through a large hole in the heat-resistant material. When the weld is completed, the casting is again raised to an even heat all over. Then it is buried in heat-resistant material and allowed to cool very slowly.
Fusion welding requires the use of a good grade of cast iron filler rod that matches the material being welded. The rod must contain enough silicon to replace the silicon that tends to burn out during welding. Silicon assists in the flow of the molten metal during welding and retards oxidation that may lead to the formation of slag inclusions and blowholes. Good cast iron welding rods contain 3 to 4 percent silicon.
Cast iron filler rods are supplied in diameters of 3/16, 1/4, 3/8, and 1/2 inch and in lengths of 12 to 18 inches. They carry the AWS-ASTM classification of RCI, RCI-A, and RCI-B. They may be round, square, or hexagonal in shape. If the welder needs a longer or heavier rod, he or she can weld two or more rods together.
The problem in welding cast iron, as in welding other metals, is to prevent oxide from forming and, when it does, to remove it from the weld. The flux dissolves the oxide, floats off other impurities such as sand, scale, and dirt, and increases the fluidity of the molten metal.
The student welder must learn to apply flux properly. Too much flux can cause as much trouble as too little. Excessive flux becomes entrapped in the molten metal and causes blowholes and porosity. Also, the molten iron will combine with certain elements in the flux if it is applied in excess. You will learn by experience the right amount to use. The amount that adheres to the hot end of the welding rod when it is dipped in the flux is usually enough. Do not throw additional quantities into the weld as you are welding.
Problems associated with welding cast Irons:
1) Cast iron is that the metal is filled with impurities. When you melt the metal the embedded dirt and sand contaminated the weld and tend to not mix well with the cast iron filler rod. What you end up with is not a fused intermixed metal bead but a porosity filled weld that can be easy to chip away with a chisel because it has not bonded to the base metal.
2) Pre heat & Post Heat required. And different as per its composition. Pre heat to stress relieve & post heat to control rate of cooling.
3) The large amount of carbon in the base metal tends to segregate itself along grain boundaries as the weld puddle solidifies and cools. What this does is lower the tensile strength even lower so that in the weld fusion boundary the shrinkage force pulls the metal apart as it cools, ie it tends to crack along the weld when cools.
4) Cracking. If it cools down too quickly you will get cracks along the weld.
5) Proper selection of filler rod & flux required as per the grade and composition of the cast iron.
6) To improve weld performance can be achieved by application of several special techniques. These include:
- Joint design modifications.
- Groove face grooving Studding.
- Special deposition sequences and
- electrode manipulation.
7) Some cast irons cannot be fusion welded.
8) Some Cast irons cannot be effectively welded. (white Cast Iron)
9) Some Cast iron you shall have to utilize special Ni coated Electrodes.
10) When welding a large casting expansion and contraction of the casting should be allowed for, maybe removal/ loosening of foundation etc could be required.
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