Malleable cast iron is capable of being bent, extended or shaped to some extent. For example, if a malleable casting is placed in a vise and an attempt is made to break it, it will bend before breaking. If castings are subjected to great strains and rough usage, malleable cast iron is required. Malleable cast iron can be distinguished from gray cast iron if a fracture is broken clear through the casting. The malleable iron has a clearly discernible white, steely skin extended slightly from the surface of the casting toward the center. A fracture showing the interior of a gray iron casting lacks the bright skin at the surface.
Annealing- Malleable iron castings are not used in the as-cast condition. They must be annealed to make them malleable. The castings are packed for annealing in different ways, depending on the product desired. For example, black heart castings are made by tightly packing the white iron castings in the annealing boxes, surrounding them with mill scale (oxide of iron), and covering them. They are then placed in the annealing furnaces and maintained at 730 to 815°C (1350 to
1500°F) for approximately 60 hours. During this time, the carbon in the iron changes from the chemically combined form, separating from the iron and becoming interspersed among the grains of the iron as very fine particles of a coke-like carbon called temper curbon. While this change is occurring, the mill scale, or iron oxide, surrounding the castings in the annealing boxes combines with the carbon in the surface of the castings, decarburizing the surface metal, thus changing it into a grade of steel. If a black heart malleable casting is broken by blows from a sledge hammer, the fracture shows a bright steel skin for a slight depth around the surface of the casting, and a black interior.
Pipe Fittings– Many pipe fittings, and some other types of castings which do not require great strength, are made in all black form, and are called all-black malleable castings. These castings are packed in the annealing boxes in an inert medium like sand. The packing prevents warping of the castings while they are heated at red heat in the annealing boxes. In some cases, no packing at all is used.
White Heart Castings- Another grade of malleable castings is known as white heart castings, and are more common in Europe than in the United States. They are packed in mill scale or iron ore, and are decarburized throughout the casting so that the fracture is white.
Shrinkage in Malleable Castings- The shrinkage of malleable castings is important to welders. Gray iron shrinks about 10.4 mm/m (1/8 in./ft) during solidification and cooling. White iron shrinks about 21 mm/m (1/4 in./ft). Cast steel shrinks about 26 d m (5/16 in./ ft). An interesting observation is that during the annealing of white cast iron, it regains about half of its shrinkage, growing in size during the annealing process so that the net shrinkage of malleable castings is
about the same (10.4 mm/m [1/8 in./ft]) as that of gray iron castings.
Welding Malleable Castings
Considering the changes in the iron which take place during the annealing process, it follows that if the torch or the arc is applied to the malleable iron and the temperature of the section to be welded is actually raised to the point of fusion, the entire structure of the casting is altered, and the weld section reverts approximately to white iron. Therefore, an ordinary fusion
weld on a malleable casting produces a weld area which does not have the strength or the toughness of the rest of the casting, may not even be as strong as a gray iron casting, and would also be hard and brittle. Any weld which requires actual fusion of the base metal of the casting might be counter to the objective of annealed castings. This is usually what takes place, and the fusion welding process is not recommended.
Arc Welding, Malleable Iron
When arc welding is used for malleable cast iron, the procedure is generally very much the same as for cast iron, however, special precautions are necessary because malleable iron and cast iron have very different characteristics. Malleable iron is affected by the heat of fusion to a greater extent. If the casting must have malleable characteristics when the weld is completed, there is no alternative but to heat treat it again, as it was first treated to make it a malleable casting.
Joint Preparation– In preparing the joint for arc welding, the joint opening should be chipped out, or ground to either a single-or double-V opening, depending on the thickness of the metal. A single-V should extend to within about 1.6 mm (1/16 in.) from the bottom, and when the two pieces are placed together, there should be about 1.6 mm (1/16 in.) between the edges at the point where the bevels meet. The groove angle should be approximately 90″.
The surfaces of the V should be thoroughly wire brushed, or sandblasted if equipment is available. If the casting is covered with grease or oil, a solvent should used to clean it.
Temperature Control- It is important that the casting be kept as cool as possible: first, to prevent change in the form of the carbon, and second, to prevent strains in the casting itself. The arc should never, at any time, be held on the casting long enough to heat the metal to
a red heat.
Cleaning Weld Beads- Before welding a second bead over one already made, the first bead should be thoroughly cleaned: wire-brushed until the steel is bright and free from any particles of the coating or of the slag. Cleanliness is an absolute necessity, otherwise foreign particles may become embedded, causing a pocket or hole.
When using a steel electrode, the weld is a likely to be hard, too hard for machining. However, this is not important when making an average repair on a malleable casting, because machining is not usually required. If machining is required, it is better to use a bronze electrode.
If the casting is heavy and the section is thick, it may be better to drill and tap holes at various points and insert threaded studs.
Electrodes for SMAW
Electrodes made of phosphor-bronze and heavily coated to shield the molten metal from the air can be used in making malleable iron welds. The work should be prepared and cleaned in exactly the same way as for other types of welding, and the electrodes should be used with DCEP. A 4 mm (5/32 in.) electrode can be used with a current from 70 to 170 amp, and 24 to 28
arc volts. If a 4.8mm (3/16 in.) electrode is used, current should be from 90 to 220 amp, with voltage from 24 to 28 volts. It is desirable to hold the electrode at approximately 90″ to the work. It may be necessary to reduce the current somewhat as the work progresses and the heat increases. It should be remembered that with these rods, as with others, the casting must not be
heated too hot: the lowest current required to achieve fusion should be used.
Monel alloy electrodes may also be used to produce machinable welds of high strength. Because of the higher melting point of Monel, about 1360°C (2480”F), higher currents are needed to produce proper flow and fusion. Each layer or bead is moderately peened to diminish stresses, and thoroughly cleaned between beads.
Oxyacetylene Torch Brazing
To make a repair weld in a malleable casting, the crack or break must be chipped or ground out to form a V to an included angle of approximately 60”. The surfaces of the V should be vigorously wire-brushed to clean them thoroughly. If the casting is completely broken, the parts should be placed in correct alignment, and supported on the welding table to prevent
sagging or warping out of shape. It is a good practice to use a bronze rod with a rather low melting point. Heating malleable iron to the point of fusion will change its characteristics, so the temperature should be kept as low as possible to avoid overheating.
Temperature Control- The melting temperature of a bronze welding rod is about 885°C (1626°F); a manganese-bronze welding rod will melt at approximately 925°C (1700°F). Therefore, it is necessary to heat the casting only to the melting temperature of the particular rod being used, which is about a red heat, in order to flow the bronze over the metal. It will adhere to the surface, or “tin” the metal.
Flux- As in most brazing, a suitable flux is needed. The flux should be a malleable iron flux, so designated on the label, and not the type of flux used for brazing cast iron. The object of the flux is to clean the surface of the casting, so that the metal will “tin” readily.
Tinning- The most important part of any braze is the initial tinning, or coating, of the base metal with the bronze. If this coating readily flows over the metal at the right temperature, the balance of the weld can be built up to the top of the V. In this respect, malleable iron does not differ from gray iron castings, except that it is critical not to overheat the malleable castings. Gray castings can be placed in a preheating furnace and preheated without any danger of changing the
structure of the metal, but greater attention must be given to preventing such changes when heating malleable castings.
When braze welding malleable castings, a short section of the V should be tinned as described, and this section should be built up to completion as rapidly as possible. The next section should then be tinned and completed, continuing with short increments until the
entire braze is completed.
Penetration- When braze welding a small casting with the oxyacetylene torch, the welder is often
tempted to fit the two parts together in alignment, and to flow the bronze on the surface, believing that the braze metal will penetrate the depth of the break and adhere to it. This procedure may work in a furnace brazing process in which castings are heated for a long time until the heat penetrates evenly to the center of the casting. The bronze spelter used for this purpose penetrates entirely to the bottom of the casting, and such repairs are very strong when properly made. It is
difficult, however, to heat the center of a small malleable casting sufficiently with the torch without over heating the surface. As a rule, any attempt to torch-braze in this manner will result in fusing only to the surface skin of the casting, with no appreciable fusing at the center. It is usually a better practice to form a V in the casting and tin a weld from the bottom up to make sure that all portions of the weld actually adhere to the iron surface.
As a general rule, whether an arc or a torch is used to weld malleable iron, bronze welding rods and electrodes are preferred because of the lower temperature at which they can be applied, to avoid fusion of the malleable iron during the process.