Is cast iron just cast iron?
Cast iron comes in many different types with different properties. Not all can be welded, cut, or machined in the same way, and some types are better suited for specific applications than others. This article discusses the most common types and how to use them.
My company, Kanawha Mfg. Co., which has been in business since 1902, primarily as a job shop, once had a foundry that was clearly visible from the West Virginia state capital in Charleston. When this foundry was built, there was no such thing as the Environmental Protection Agency (EPA). The facility could produce smoke and ash without repercussion from anyone. In those days if a company provided jobs, no one griped. At that time we had more than 300 employees.
As the populace and the government became more environmentally conscious, it became impossible to comply with the emission standards in such a highly populated community. In 1983 the foundry was closed and about 200 employees lost their jobs. Some of the workers were already past retirement age, but loved their jobs so much that they kept working.
We produced all the Gravely® tractor engine castings for several years. We also made manway covers for several cities. We have a patent for a reciprocating coal feeder that requires the use of several shapes and forms of castings (Figure 1 and Figure 2). Today we provide the patterns and hire other foundries to do the pouring. We still do the final grinding and machining.
We've had a lot of experience with cast iron, and I'm here to tell you that there are many types with different properties. Cast iron isn't just cast iron.
The most commonly produced types of cast iron are alloyed, white, gray, malleable, and ductile/nodular.
A good foundry can produce several different types and shapes of castings and may also prepare the molds for most patterns. The primary reason for the use of castings is that they are ideal for making multiple parts of the same size, shape, and composition.
Alloyed Cast Iron
This patented cast iron is referred to as Meehanite®. Our company uses this product extensively in the coal feeders that we manufacture. It can be altered as necessary for different parts of the equipment.
The elements commonly used in Meehanite are silicon (Si), copper (Cu), manganese (Mn), aluminum (Al), zirconium (Zr), and titanium (Ti). The beauty of alloyed cast iron is that it can be produced and heat-treated in many different forms and for specific hardnesses.
White Cast Iron
White cast iron often is used for its good compression strength (for machine bases, etc.), but it also has excellent hardness and wear resistance, even at elevated temperatures. This material is used for conveyor rolls and other equipment that only needs to be wear-resistant or heat-resistant and does not require good ductility. It cannot be a part of a fabrication or weldment.
White cast iron can be alloyed with chromium and/or molybdenum to increase the hardness. The alloying produces hardnesses of 65 to 70 Rockwell C.
The term white cast ironprobably is related to the reduced appearance of graphite, which gives the other cast irons a gray color. White cast iron's rapid solidification causes the graphite to remain combined with the iron in the form of large chunks of carbon.
Welding white cast iron is not recommended, but some foundries have been able to perform repair welds on castings made from the material with varying success. This material is usually used in the as-castform because it is relatively difficult to machine, cut, or weld.
Gray Cast Iron
Gray cast iron is quite different from white cast iron. It appears gray due to the graphite flakes that do not combine with the iron to form carbides, but combines with carbon. A slower cooling rate causes the carbon to separate out and form the graphite flakes. The type and amount of alloying material also has a bearing on the coloring and flaking. It is sometimes referred to as graphitic carbon. (See ASTM A247 for seven different types.) The large flakes are called kish. The smaller flakes are referred to as eutecticflakes.
Gray cast iron usually is readily machinable even at hardnesses nearing 40 Rockwell. It is also resistant to galling, unlike some wrought iron. It often is used for oil sumps, crankcases, and axle housings. The gray cast material sometimes is used in an acidic environment, and in these cases there is a requirement for higher silicon content, usually in the 3.5 percent to 4 percent range. Gray cast iron has poor impact resistance but good compressive strength.
This type of cast iron is weldable unless it contains an excessive amount of sulfur and phosphorus. Repair welding of gray cast iron is not uncommon, but precautions must be taken when welding in uneven thicknesses and areas of high restraint. Our company has been successful in brazing the broken feet of electric motors when other methods failed.
Malleable Cast Iron
This product basically is a form of heat-treated white iron. However, the malleable cast iron is treated to become ferritic rather than allowing the formation of cementite (formed by rapid cooling). Malleable primarily is used in pipe fittings, connectors for fencing, and similar applications. It is readily machinable and threading results are excellent.
This material is weldable with careful welding procedures and the proper welding materials. The welding material should be NiCrFeMn-CI when performing repair welds or welding malleable cast iron to malleable cast iron. The joining of this material to low-carbon steel is more difficult (Figure 3). The difference in ductility creates a problem, especially when the use of preheat is prohibitive, such as on equipment already in place with parts that cannot be subjected to heat.
We recently attempted to weld a threaded malleable cast iron fitting to a very thick piece of low-carbon steel without preheating. The reason for omitting preheating was that the weld had to be made on a nearly completed piece of equipment. We also used a welding wire that had only 2 percent nickel. As you can see in Figure 3, the weld was beautiful, but the end result was not. Had we used preheat and the proper welding material, we probably would have had no problem, but that wouldn't have been a new experience! Our welders love to laugh at old man Smith!
Nodular/Ductile Cast Iron
Nodular cast iron often is referred to as ductileiron because of its higher ductility than other cast irons. This material is produced at a lower cost than many of the other cast irons, in part because it requires a shorter heat-treating cycle. In most cases, it costs less than carbon steel.
Nodular cast iron is readily machinable and may be welded with nearly all the common welding processes. Buildup can be successfully performed on this material with automatic and semiautomatic equipment. Among the most common work that our company performs with this method is the restoration of bores in coal mining equipment for bearing fits. The NiFeMn-CI welding materials may be used. Although some experts say that no preheat is required, we prefer to be on the safe side and apply some preheat.
We have only one of our old-time foundrymen left, and he is going to retire in 2010. He can look at a casting and determine whether it is good or bad by the mold marks and the heads. Very few of these people left.