Ductile Iron
(Heat Treatment)
One reason for the phenomenal growth in the use of Ductile Iron castings is the high ratio of performance to cost that they offer the designer and end user. This high value results from many factors, one of which is the control of microstructure and properties that can be achieved in the as-cast condition, enabling a high percentage of ferritic and pearlitic Ductile Iron castings to be produced without the extra cost of heat treatment. To obtain the advantage of producing high quality castings as-cast requires the use of consistent charge materials and the implementation of consistent and effective practices for melting, holding, treating, inoculation and cooling in the mold. By following these practices, especially the use of high purity charges and good inoculation, castings can be produced as-cast essentially free of carbides and with pearlite contents less than 10%, in section sizes as low as 0.150 in. (3.8 mm).
However heat treatment is a valuable and versatile tool for extending both the consistency and range of properties of Ductile Iron castings beyond the limits of those produced in the as-cast condition. Thus, to fully utilize the potential of Ductile Iron castings, the designer should be aware of the wide range of heat treatments available for Ductile Iron, and its response to these heat treatments.
Ductile Iron castings may be heat treated to:
This Section deals with heat treating conventional Ductile Iron. Austempering heat treatments, and the heat treatment of alloy Ductile Irons, are discussed in Sections IV, and Section V.
Although Ductile Iron and steel are superficially similar metallurgically, the high carbon and silicon levels in Ductile Iron result in important differences in their response to heat treatment. The higher carbon levels in Ductile Iron increase hardenability, permitting heavier sections to be heat treated with lower requirements for expensive alloying or severe quenching media. These higher carbon levels can also cause quench cracking due to the formation of higher carbon martensite, and/or the retention of metastable austenite. These undesirable phenomena make the control of composition, austenitizing temperature and quenching conditions more critical in Ductile Iron. Silicon also exerts a strong influence on the response of Ductile Iron to heat treatment. The higher the silicon content, the lower the solubility of carbon in austenite and the more readily carbon is precipitated as graphite during slow cooling to produce a ferritic matrix.
Although remaining unchanged in shape, the graphite spheroids in Ductile Iron play a critical role in heat treatment, acting as both a source and sink for carbon. When heated into the austenite temperature range, carbon readily diffuses from the spheroids to saturate the austenite matrix. On slow cooling the carbon returns to the graphite "sinks", reducing the carbon content of the austenite. This availability of excess carbon and the ability to transfer it between the matrix and the nodules makes Ductile Iron easier to heat treat and increases the range of properties that can be obtained by heat treatment.
All Ductile Iron heat treatments, apart from stress relief, tempering and subcritical annealing, involve heating the casting to a temperature above the critical temperature range (Figure 7.1). In ferrous heat treatment, the critical temperature (Al) is the temperature above which the austenite phase is stable. Unlike steels, which have a constant critical temperature (eutectoid temperature), Figure 7.2, Ductile Irons are ternary, iron-carbon-silicon alloys in which the critical temperature varies with both carbon and silicon contents. Figure 7.3 shows the effect of carbon on this ternary phase diagram at the 2% silicon level. Figure 7.4 shows the effect of silicon on the critical temperatures for typical cast irons. This relationship, the desired carbon content in the austenite and the need to dissolve carbides, are the primary determinants of the correct austenitizing temperature for Ductile Iron.
The most simple and economic form of heat treatment is the controlled shakeout of the castings from the mold. By removing the castings from the mold above the critical temperature, the rate of cooling can be increased, favoring the formation of pearlite with a resultant increase in casting hardness and strength (Figure 7.5). If the alloy content is sufficiently high, castings with bainitic structures can also be produced by this method. Hardening castings through early shakeout requires extremely close control of shakeout times and casting composition and immediate stress relief of complex castings to avoid the detrimental effects of internal stresses.
Austenitizing is the process of holding the Ductile Iron casting above the critical temperature for a sufficient period of time to ensure that the matrix is fully transformed to austenite. The austenitizing temperature, along with the silicon content, determines the carbon content of the austenite. Both austenitizing time and temperature depend on the microstructure and composition of the as-cast material. In order to break down primary carbides, austenitizing temperatures in the range 1650-1750oF (900-940oC) are normally used, with times ranging from one to three hours. High silicon content and high nodule count reduce breakdown times, while the presence of carbide stabilizers such as chromium, vanadium and molybdenum require substantially longer times. Pearlite decomposition occurs much more rapidly and at lower temperatures than carbide breakdown. This breakdown is enhanced by high silicon and high modularity and retarded by pearlite stabilizing elements such as manganese, copper, tin, antimony and arsenic. The segregation of manganese and chromium to cell boundaries can result in the incomplete dissolution of both pearlite and carbides and the resulting impairment of mechanical properties.
Annealing softens Ductile Iron by producing a carbide-free, fully ferritic matrix. Table 7.1 describes recommended practices for annealing Ductile Iron. These procedures range from a low temperature or sub-critical anneal used to ferritize carbide-free castings, to two-stage and high temperature anneals designed to break down carbides. The primary purpose of annealing, or ferritizing, Ductile Iron is the production of castings with maximum ductility and toughness, reduced strength and hardness, improved machinability and uniform properties. Figure 3.17 (Section 3) shows that annealing castings with different levels of copper and tin has reduced strength and hardness, increased elongation, and generally eliminated the variations in as-cast properties produced by the different alloy levels (Figure 3.16). Figures 3.44, 3.51 and Table 3.4 illustrate the effects of both standard and subcritical annealing on the fracture toughness of Ductile Iron.
Recommended practices for annealing Ductile Iron castings.
Type of Anneal Purpose Temperature (a) Time Cooling Cycle (b) Low temperature (Ferritizing) In absence of carbides.(a) Temperature of castings.
(b) Slow cooling from 1000° to 650°F (540° to 315°C) is to mimimize residual stresses.
Normalizing involves the austenitizing of a Ductile Iron casting, followed by cooling in air through the critical temperature. An as-cast Ductile Iron casting is normalized in order to: break down carbides, increase hardness and strength, and produce more uniform properties (see Figures 3.16 and 3.18). Normalizing should be carried out at an austenitizing temperature approximately 100°C (212°F) above the critical temperature range. Typically, austenitizing temperatures in the range 1600-1650°F (875-900°C) and holding times of one hour, plus one hour per inch of casting thickness, are adequate to produce a fully austenitic structure in unalloyed castings relatively free of carbide. The cooling rate should be sufficiently rapid to suppress ferrite formation and produce a fully pearlitic structure. Depending on casting section size and alloy content, adequate cooling rates can be achieved in still air, or large fans may be required. If fan cooling cannot produce the desired pearlitic structure, the castings should be alloyed with pearlite stabilizing elements such as copper, tin, nickel or antimony. Figure 7.6 illustrates the effect of alloy content and section size on the hardness of normalized Ductile Iron. Step normalizing, which employs a second, lower temperature stage prior to air cooling, can be used to provide the improved matrix control required for the production the pearlitic/ferritic grades of Ductile Iron.
Maximum hardness in Ductile Iron castings is obtained by austenitizing, followed by quenching sufficiently rapidly to suppress the formation of both ferrite and pearlite, to produce a metastable austenite which transforms to martensite at lower temperature. As-quenched hardness depends on the carbon content of the martensite and the volume fraction of martensite in the matrix. In conjunction with the silicon content, the austenitizing temperature determines the carbon content of the austenite. For a silicon content of approximately 2.5%, an austenitizing temperature of 1650°F (900°C) will result in the optimum carbon content and maximum hardness (Figure 7.7). Lower temperatures, 1475-1550°F (800-845°C), will produce a low carbon austenite which, on cooling, will transform to a softer martensite.
The formation of low carbon martensite will cause reduced distortion and cracking in complex castings during quenching and, when tempered , low carbon martensite has toughness superior to both tempered high carbon martensite and normalized microstructures (see Figure 3.44, Section III). Higher austenitizing temperatures increase the carbon content of the austenite but the bulk hardness is reduced due to retained austenite and a lower resultant martensite content. Regardless of the austenitizing and quenching conditions, quenched Ductile Iron castings must be tempered before use to eliminate internal stresses, control strength and hardness and provide adequate ductility.
Hardenability is a measure of how rapidly the Ductile Iron casting must be cooled in order to suppress the ferrite and pearlite transformations and produce a martensitic, bainitic or austempered matrix. Hardenability is an important property of any casting that is to be quench hardened because it determines the depth to which a fully or partially martensitic matrix can be produced and the severity of quench required to harden castings of different section size. The effects of various alloying elements on the hardenability of Ductile Iron are illustrated in Figure 7.8. To calculate the hardenability of a casting the absolute hardenability (DA), based on the carbon content, is first determined. The ideal critical diameter (DI) is then calculated by multiplying DA by the multiplying factors determined from Figure 7.8 for each alloying element. For example, a Ductile Iron of the composition:
Total Carbon, % 3.60 DA = 2.00 Silicon, % 2.50 MF = 1.50 Manganese, % 0.35 MF = 1.15 Phosphorous, % 0.07 MF = 0.80 Nickel, % 1.00 MF = 1.25 the ideal critical diameter would be calculated as follows: DI = DA x (MFSi) x (MFMN) x (MFP) x (MFNi) = 3.45 inches (88 mm).Total Carbon, % 3.60 DA = 2.00 Silicon, % 2.50 MF = 1.50 Manganese, % 0.35 MF = 1.15 Phosphorous, % 0.07 MF = 0.80 Nickel, % 1.00 MF = 1.25 the ideal critical diameter would be calculated as follows: DI = DA x (MFSi) x (MFMN) x (MFP) x (MFNi) = 3.45 inches (88 mm).
Thus, for the composition used in this example, a 3.45 in. (88 mm) diameter bar, when quenched in water, will have a matrix containing 50% martensite at the bar center.
Alloying elements for quenched and tempered Ductile Iron should not be selected on the basis of hardenability alone. Chromium, which is extremely effective in promoting hardenability, is very detrimental to Ductile Iron quality because it increases the formation of carbides in the as-cast state. Manganese not only promotes the formation of carbides but also retards the tempering process. Thus, for both metallurgical and economic reasons, alloying elements should be selected carefully and used at the lowest levels which provide the desired hardenability.
TTT (time, temperature, transformation) diagrams are also useful in selecting heat treatment practices for Ductile Irons. Figure 7.9 shows a typical TTT diagram for a low silicon gray iron. Each cooling path in this Figure defines the time-temperature cooling relationship required to produce a specific microstructure. The position of the transformation zone on the TTT diagram, defined by start and finish curves, determines the rate and extent of cooling required to avoid certain transformations and promote others. To ensure that a quenched component is entirely martensitic, the slowest cooling rate must be sufficiently fast to avoid the "nose" of the transformation zone.
Each composition of iron has a unique TTT diagram, with the location of the transformation zone controlled by the composition (Figure 7. 10). In this Figure the influence of molybdenum on the various transformations reveals why it has a high hardenability multiplying factor (Figure 7.8). Increasing molybdenum content shifts the transformation zones to the right, allowing complete transformation to martensite at the slower cooling rates found in larger casting section sizes. Knowledge of the many TTT diagrams published for Ductile Iron enables the foundry and heat treater to select appropriate alloy contents and quenching conditions to produce suitably hardened castings.
The quenching medium and the degree of agitation in the quench bath are important variables that can be used to ensure that a suitable microstructure is produced by the quenching process. Common quench media, in order of increasing severity are oil, water and brine. Agitation of the quenching bath may be required to increase both quench severity and the uniformity of cooling in complex castings or batches of castings. To minimize internal stresses, distortion and cracking, especially in complex castings, the least severe quenching medium that produces the desired microstructure should be selected. As the required severity of quenching increases, it becomes increasingly important to temper the castings immediately after quenching.
Tempering reduces the strength and hardness and increases the ductility, toughness and machinability of quenched or normalized Ductile Iron. In addition, tempering quenched castings also reduces residual stresses, decreases the amount of retained austenite, and reduces the probability of cracking. These changes in properties are achieved by holding the castings at a temperature that is below the critical temperature. Tempering is a diffusional process and thus is time and temperature dependent. Tempering conditions are influenced strongly by the desired change in properties, the alloy content, the microstructure being tempered and the nodule count. Low alloy content, martensitic structures and high nodule count reduce tempering temperatures and/or times, while high alloy content, a normalized (pearlitic) structure and low nodule count increase tempering times.
Castings may be tempered after normalizing to provide an optimum combination of high strength and toughness. This process also provides the additional advantage of improving the control of properties through selection of tempering temperature and time.
Quenching and tempering are the standard heat treatments applied to Ductile Iron castings requiring maximum strength and wear resistance. In addition to maximizing strength, these treatments can provide close control of casting properties over a wide range of strength and ductility, and optimum combinations of strength and toughness (see Figure 3.44). Figure 7.11 illustrates the wide range of properties of quenched and tempered Ductile Iron castings that can be obtained through selection of the appropriate tempering temperature (Figure 7.12).
Temper embrittlement, a type of embrittlement found in certain quenched and tempered steels, may also occur in similarly treated Ductile Irons with susceptible compositions. This form of embrittlement, which does not affect normal tensile properties but causes significant reductions in fracture toughness, can occur in Ductile Irons containing high levels of silicon and phosphorus which have been tempered in the range 650-1100°F (350-600°C) and cooled slowly after tempering. Although normally associated with tempered martensite, temper embrittlement can also occur if the matrix is tempered to the fully ferritic condition. Temper embrittlement can be prevented by keeping silicon and phosphorus levels as low as possible, adding up to 0.15% molybdenum and avoiding the embrittling heat treating conditions.
The formation of secondary graphite during the tempering of martensitic Ductile Iron can be responsible for both the degradation and increased variability of mechanical properties. Secondary graphitization is favoured by high austenitizing and tempering temperatures and high levels of silicon, copper and nickel. Like temper embrittlement, the use of small additions of molybdenum can eliminate this problem. To further prevent its occurrence, the tempering of martensitic Ductile Irons to hardnesses below 270 BHN, which require high temperature tempering, should be avoided.
Ductile Iron can be surface hardened by flame or induction heating of the casting surface layer to about 1650oF (900oC), followed by a quenching spray. Hardness levels as high as HRC 60 can be achieved by these procedures, producing a highly wear resistant surface backed by a tough, ductile core. Pearlitic grades of Ductile Iron, which have an intimate mixture of lamellar carbide and ferrite, respond most effectively to surface hardening due to their reduced diffusion distances.
The presence of residual stresses can be detrimental to both the production and performance of Ductile Iron castings. If sufficiently severe, residual stresses can cause castings to distort and crack even during normal handling. Lower residual stresses can cause the casting to distort during subsequent heat treatment or machining. Residual stresses can also result in premature yielding or fracture when the casting is used in an applied stress environment that should have ensured safe operation.
Both the occurrence and the effects of residual stresses in castings vary according to the design of the casting, production procedures, and the end use of the casting. Large, heavy section, or "chunky" (all dimensions approximately equal) Ductile Iron castings are usually stress free as-cast and require no subsequent stress relief. Complex castings with large variations in section size or constrained thin castings are more likely to contain residual stresses requiring stress relief. Sand molds are good insulators and even complex castings may cool sufficiently slowly to prevent the development of significant residual stresses. However, the premature "shakeout" of castings from molds can cause severe residual stresses, in addition to variations in hardness.
Rigid molds and cores may prevent normal metal contraction during cooling and result in residual casting stresses. Subsequent processing such as shot peening, welding, heat treatment or surface hardening, if not performed properly, can induce significant residual stresses that may become evident during machining or subsequent use of the casting.
Stress relief is achieved by heating the casting to a sufficiently high temperature that its strength is reduced to the extent that the residual stress can be relieved by plastic deformation. The extent to which stresses will be relieved or eliminated is dependent on several factors, including the initial severity of the residual stresses, the stress relieving time and temperature, the heating-cooling cycle, and the composition and microstructure of the casting. Figure 7.13 shows that stress relief is proportional to the level of initial stress, and that the degree of stress relief is strongly temperature dependent. After stress relief a uniform rate of cooling must be maintained throughout the casting to prevent the reintroduction of stresses. This is normally accomplished by cooling in the furnace from the stress relieving temperature to approximately 800°F (430°C). For complex castings, and where the greatest degree of stress relief is desired, furnace cooling to 300°F (150°C) is recommended. The heating rate may be as important as the cooling rate in the prevention of internal stresses, especially for complex or highly stressed castings. Placing such castings in a hot furnace will result in differential thermal stresses that could cause distortion during the subsequent heat treatment.
Scaling, growth and distortion of castings during heat treatment should be considered in order to minimize the detrimental effects of these phenomena. Scaling, which increases with time and temperature, can be eliminated by the use of a controlled atmosphere furnace. An overall increase in casting dimensions may occur during heat treatment due to the graphitization of eutectic carbides and the conversion of pearlite to ferrite. At austenitizing temperatures Ductile Iron castings have very low strength and will easily sag and distort if not properly supported. To reduce the risk of distortion, austenitizing time and temperature should be kept to the minimum required to ensure complete carbide breakdown and austenitization of the matrix.
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