by Robert O'Rourke

    The 50^th anniversary of the invention of Ductile Iron has been widely celebrated by metallurgists and foundries that appreciate the impact this engineered metal has had on the cast iron industry. When ductile iron was invented the door opened for enormous growth possibilities for any foundry that focused on converting targeted applications to ductile iron castings.
     Steel parts became the natural conversion metal. Ductile iron is similar to steel in that both are ferrous metals. Strengths in ductile iron approach those of plain carbon steels. and offered free machining characteristics. Ductile iron has excellent castability, is free machining and has better damping properties and wear resistance as compared to carbon steel. Producers of ductile iron continue to have enormous growth potential considering the conversion opportunities that exist in steel castings, forgings and fabrications.
     Over the past several years there has been an increase in the demand for more technical field sales people. Design Engineers are looking at material selection as a way to improve part performance and lower cost of manufacturing. This opens up new opportunities for ductile iron foundries looking to expand their business by focusing on conversions; from steel to ductile iron. The challenge for ductile iron producers is having intimate knowledge of mechanical properties, machinability ratings and a pretty good idea about which grade should work for the targeted application.
     The most common objection by design engineers is that iron is too weak. The objection stems from the basic misunderstanding that cast iron is not one material but is a family of metals with each member having its own unique characteristics. Selecting the material with the best combination of strength, wear resistance and ability to process is the key to lowering cost.
     In the broadest sense, parts fail in one of two modes – either they break or they wear out. So, for all applications, why isn’t the strongest, most wear resistant material always used? The answer is because there are a lot of other factors to consider. Price of the raw material, ease of machinability are obvious ones. Other factors include noise-damping characteristics, which is important in gears and machine tool components. Another way to lower cost is to eliminate the need to heat treat by using a fully pearlitic cast iron instead of carbon steel.
     Comparison of tensile strengths between carbon steel bars and ductile iron can lead to most of the objections for conversion opportunities. Those comparisons can be misleading. (Rarely a component is subjected to uniaxial loading at room temperature at a slow strain rate, as is the case when the tensile test is performed.) Usually, dynamic properties, the applied loads and the most likely mode of failure are the important considerations for design engineers.
     Carbon steel is designated by chemical composition, not mechanical properties, as is the case with ductile iron. Ductile iron is characterized by tensile properties. Published carbon bar strengths are not always typical or even averages and the actual values may vary considerably depending on residual alloys, section size and the internal microstructures. (source: ASM Metals Reference Handbook, 2^nd Edition, 1983, American Society for Metals).
     Ductile iron by definition must conform to specified minimum mechanical properties, not average or even typical values. The minimum tensile and yield strengths can be used for design purposes. Ductile iron is isotropic and the mechanical properties are the same regardless of test bar orientation. Steel forgings have directional properties, which can be an important consideration for applications such as gears.
     The tensile strength of a carbon steel bar will usually be higher than a ductile iron bar having a similar matrix, but in some cases, the yield strength may be lower. 1040 steel for example, in the normalized condition has 85,500 psi tensile strength, 54,300 psi yield strength and 28% elongation. (source: ASM Metals Reference Handbook, 2^nd Edition, 1983, American Society for Metals). 80-55-06 ductile iron with a similar matrix has 80,000 psi tensile strength, 55,000 psi yield strength and 6% elongation. The graphite nodules reduce tensile strength and elongation but most parts are designed to the yield strength, not tensile.
     Cast irons can be produced to a fully pearlitic matrix, which is an advantage over carbon steel. Approximately 1% carbon is required to produce an essentially pearlitic matrix. Most carbon steels commercially produced have between 0.10% to 0.80% so there is a limit to the amount of pearlite that can be obtained in the matrix. In order to achieve maximum wear resistance steel has to be heat-treated. Carbon steel grades having less than 0.35% carbon are usually carburized before they can be heat-treated. A fully pearlitic matrix or even a highly pearlitic one offers suitable wear resistance in the as cast state and can sometimes replace carburized and heat-treated steel, depending on the application.
     Hydrostatic cylinder blocks made with a fully pearlitic ductile iron were shown to have sufficient wear in the as cast condition. Eliminating heat treat significantly reduced the cost of the part compared to steel because of the additional processing required on the steel. Ductile iron is a natural replacement for carbon steels whenever wear resistance is the primary concern.
     When Wells Manufacturing Company introduced the process of continuous casting into North America in 1960, the main focus was on casting conversions. As ductile iron production became more refined, the production of ductile iron continuous cast iron bar stock opened up opportunities for steel bar conversions. Ductile iron bar stock is free machining, has similar mechanical properties to carbon steels, is less dense and available in rounds, squares and simple shapes. By concentrating selling efforts toward steel conversions, the possibilities for new business were limitless.
     Most ductile iron casting conversions from steel are directed at reducing costs by reducing manufacturing time. A complex weldment or forging can be cast to a closer net shape and eliminate the time to fabricate the same part from steel. Ductile iron castings as an alternate to steel castings will usually be an economical alternative because of the reduced cost to melt ductile iron. Lower melting temperatures usually equate to less dross and slag defects as well.
     Dura-Bar reduces machining time. Ductile iron is a free machining grade and parts that are heavily machined are conversion candidates. Bar stock costs may be higher than rolled carbon steel but it is more machinable and easier to debur. A good application candidate starts with a round or rectangle carbon bar and the finished part cost is about 25% material, 75% machining.
     In the process of continuous casting of ductile iron bar, molten iron is held in a refractory lined steel shell. A water-cooled graphite die is mounted on the bottom of the vessel. Molten iron enters the die and a solid skin begins to form that takes the shape of the bar. As the bar is pulled out of the die in a series of strokes, the skin becomes thicker until it can sufficiently support the head pressure of the molten iron inside the bar machine. When the bar exits the die, it consists of a thin outer shell with a molten iron core.

     The heat from the molten iron core reheats the outer skin that was rapidly chilled inside the die. The matrix in the rim is transformed to austenite and cools in still air as the bar moves horizontally along a series of rollers. The center of the bar is allowed to solidify and cool in still air. The resulting microstructure in the continuous cast bar is a homogenized matrix of pearlite, ferrite, or a ratio of the two, depending on the grade being produced.
     Solidification and cooling rates are consistent for each bar size and the different grades of ductile iron produced are controlled with the addition of pearlite stabilizing alloys added to the transfer ladles. This practice produces microstructures that are stable to temperatures up to 1000F.
     Molten iron is continuously added to the bar machine crucible during the production run to maintain head pressure and a sufficient distance between the die opening and the top of the molten metal bath. Impurities float to the top of the bath, well away from the die opening which eliminates slag, dross and other tool wearing inclusions.

     Consistency in the matrix structure and elimination of impurities is an essential part of reducing machining cost. A wide range of microstructures within a particular grade of ductile iron will cause variations in machinability. Consistency in chemistry matrix structures is the key to consistent machinability. Inclusions can cause catastrophic failure of the tool insert and must be eliminated.
     Understanding material properties and knowing the property requirements for an application is very important in selecting the best grade of ductile iron for an application. Carbon steels are designated by chemical composition. Ductile iron is designated by the minimum tensile strength, yield strength and elongation. Besides tension properties, torsion strength, shear strength, modulus of elasticity, impact properties and heat treat response are just a few material characteristics that may also need to be considered.
     The chemical composition of carbon steel affects mechanical properties. High carbon steels will have higher tensile strengths, lower elongation, decreased machinability and better response to heat-treat than low carbon steels. Additions of sulfur, manganese, phosphorus and lead are commonly used to improve machinability, usually at the expense, to some degree, of strength.
     The amount of carbon dissolved in iron determines the amount of pearlite in the matrix which influences most of the mechanical properties, heat-treat response and machinability. Elements such as sulfur and phosphorus form sulfides and phosphides that do not dissolve in iron and make carbon steel "free machining" which means the chip formed during cutting is discontinuous. Holes drilled in free machining steels will usually require less deburing time.
     Ductile iron has carbon levels that exceed the solubility limit in iron. At 2800F, approximately 6% carbon is soluble in iron. At the eutectic point, only 2% can remain in solution and the excess carbon is precipitated into a graphite nodule. The precipitated graphite is a solid phase, which promotes the same benefits as the inclusions deliberately put in carbon steels. Ductile iron bars are free machining and drilled holes require less deburing.
     If there was no way to control the amount of carbon that remains in solution, there would not be any way to control mechanical properties, but fortunately that is not the case. In fact, the amount of carbon dissolved in the matrix is very controllable and so are the mechanical properties and the machinability of the ductile iron grade being produced.
     Ductile irons with relatively low levels of combined carbon will have a matrix consisting primarily of ferrite. They have the lowest tensile strength and wear resistance, highest elongation and usually will be the easiest to machine. The level of combined carbon can be increased to produce a matrix that is fully pearlitic which results in higher tensile strengths and wear resistance, lowest elongation and will be more difficult to machine. The ratio of pearlite to ferrite in the matrix will produce ductile iron grades that have properties somewhere between those with a fully pearlitic or fully ferritic matrix.

     The most common question for any design engineer or field salesperson that is looking to replace a carbon steel part with ductile iron is "which grade matches the one being replaced?". That question is difficult to answer without knowledge of the application and knowing which properties are important to its function.
     Since parts usually fail because they break or they wear out the best alternate grade is the one that more closely matches the mechanical properties and matrix of the one being replaced. With an equivalent matrix, ductile iron will usually exhibit better wear resistance because of the graphite nodules. The surface of a ductile iron part will retain lubricant better than a steel part with the same matrix, which can also improve wear resistance.
     Vibration damping is important in gears and in applications where harmonic vibrations cause failure. In an automotive balance shaft, gear noise reduction resulted directly from the conversion of 1144 steel to 80-55-06 ductile iron without any change in how the part was being manufactured. The 4140 pistons in an impact hammer were cracking prematurely because of harmonic vibrations. The failures stopped when the part was converted to austempered ductile iron. Although the ductile iron had lower tensile strengths, the vibration damping characteristics reduced harmonic vibrations and the conversion was a success.
     The best way to select a ductile iron grade to be used in place of carbon steel is to pick one that has a similar matrix and hardness. The best way to do that is to match the matrix structure as close as possible. Usually a ferritic ductile will be the best candidate to replace carbon steels having up to .35% carbon. Partially pearlitic ductile irons such as an 80-55-06 are the best candidates for the medium carbon steels. Fully pearlitic ductile irons are best for replacing carbon steels that require heat treat to improve wear resistance. The machinability of a fully pearlitic ductile may be less than the carbon steel but the savings from eliminating all the steps associated with heat treat can offset the additional cost of machining.
     It would be naïve to assume that all carbon bar applications can be directly replaced by ductile iron and that is certainly not the case. However, there is an enormous opportunity for ductile iron in applications that can benefit from lower machining and processing costs, the possible elimination in heat treat, improved vibration damping properties and the domestic availability of ductile iron bars and castings.
     The challenge for the ductile iron producer is to generate solid engineering data on properties other than tensile, yield and elongation. Machinability ratings and current recommendations for speeds, feeds, depth of cut and type of inserts need to be established for modern machine tools. Engineering data including fatigue properties and other strength characteristics must be readily available for the design engineer. Most importantly, the field sales person must have the technical knowledge required to answer the questions and concerns a designer may have regarding a ductile iron candidate.
     What is the future for ductile iron? Definitely it is more steel conversions. Looking for and developing conversion opportunities makes the market potential for this engineered metal limitless.

Ductile Home   •   Officers & Directors   •   Back Issues   •   Contact Us   •   Legal