Handbook Heat Treating (1991) WW Part 11 doc

Fig. 15 Influence of austenitizing te mperature on hardness of ductile iron. Each value represents the average of three hardness readings. Specimens (13 mm, or 1 2 in., cubes) were heated in air for 1 h and water quenched. Castings should be tempered immediately after quenching to relieve quenching stresses. Tempered hardness depends on asquenched hardness level, alloy content, and tempering temperature, as well as time. Tempering in the range from 425 to 600 °C (800 to 1100 °F) results in a decrease in hardness, the magnitude of which depends upon alloy content, initial hardness, and time. Figure 16 shows the change in the Vickers hardness of two quenched ductile iron alloys with tempering temperature and time (Ref 9). Tempering ductile iron in this temperature range is a two-stage process. The first involves the precipitation of carbides similar to the process in steels. The second stage (shown in Fig. 16 by the drop in hardness at longer times) involves nucleation and the growth of small, secondary graphite nodules at the expense of the carbides. The drop in hardness accompanying secondary graphitization produces a corresponding reduction in tensile and fatigue strength as well. Because alloy content affects the rate of secondary graphitization, each alloy will have a unique range of useful tempering temperatures. The influence of tempering temperature between 425 and 700 °C (800 and 1300 °F) for tensile and hardness specimens tempered for 2 h is shown in Fig. 17. Fig. 16 Vickers hardness (10 kg load) versus tempering time at several tempering temperatures for (a) an alloy with 3.61% C, 3.11% Si, 0.04% Mo and (b) an alloy with 3.64% C, 2.57% Si, 0.49% Mo. Source: Ref 9 Fig. 17 Influence of tempering temperature on mechanical properties of ductile iron quenched from 870 °C (1600 °F) and tempered 2 h. Data represent irons from four heats with comp osition ranges of: 3.52 to 3.68% C, 2.28 to 2.35% Si, 0.02 to 0.04% P, 0.22 to 0.41% Mn, 0.69 to 0.99% Ni, and 0.045 to 0.065% Mg. Data for tensile strength, tensile yield strength, and elongation are for irons (from two of these heats) that contained 0.91 and 0.99% Ni. Reference cited in this section 9. K.B. Rundman and T.N. Rouns, On the Effects of Molybdenum on the Kinetics of Secondary Graphitization in Quenched and Tempered Ductile Irons, Trans. AFS, Vol 90, 1982, p 487-497 Austempering Ductile Iron When optimum strength and ductility are required, the heat treater has the opportunity to produce an austempered structure of austenite and ferrite. As shown previously in Fig. 4, the austempered matrix is responsible for a significantly better tensile strength-to-ductility ratio than is possible with any other grade of ductile cast iron. The production of these desirable properties requires careful attention to section size and the time-temperature exposure during austenitizing and austempering. Section Size and Alloying. As section size increases, the rate of temperature change between the austenitizing temperature and austempering temperature decreases. Quenching and austempering techniques include the hot-oil quench ( ≤ 240 °C, or 460 °F, only); nitrate/nitrite salt quenches; fluidizedbed method (for thin, small parts only); and, in tool-type applications, lead baths. In order to avoid high-temperature reaction products (such as pearlite in larger section sizes), salt bath quench severities can be increased with water additions (Ref 10) or with alloying elements (such as copper, nickel, manganese, or molybdenum) that enhance pearlite hardenability (Ref 3). It is important to understand that these alloying elements tend to segregate during solidification so that a nonuniform distribution exists throughout the matrix. This has a potentially detrimental effect on the austempering reaction and therefore on mechanical properties. Ductility and impact toughness are the most severely affected. Manganese and molybdenum have the most powerful effect upon pearlite hardenability but will also segregate and freeze into intercellular regions of the casting to promote iron or alloy carbides. While nickel and copper do not affect hardenability nearly as much, they segregate to graphite nodule sites and do not form detrimental carbides. Combinations of these elements, which segregate in opposite fashions, are selected for their synergistic effect on hardenability. Austenitizing Temperature and Time. The schematic phase diagram of Fig. 7 shows that as austenitizing temperature increases, so does the matrix carbon content; the actual matrix carbon content depends in a complex way on the alloy elements present, their amount, and their location (segregation) within the matrix. The most important determinant of matrix carbon content in ductile irons is the silicon content; as silicon content increases for a given austenitizing temperature, the carbon content in the matrix decreases. Austenitizing temperatures between 845 and 925 °C (1550 and 1700 °F) are normal, and austenitizing times of approximately 2 h have been shown to be sufficient to recarburize the matrix fully. Figure 10 shows that the austenitizing temperature, through its effect upon matrix carbon, has a significant effect on hardenability. The higher austenitizing temperature with its higher carbon content promotes increased hardenability, which causes a slower rate of isothermal austenite transformation. This reduced rate of austenite reaction to all transformation products is shown in the beginning portions of the IT diagrams in Fig. 18. Fig. 18 Time to 5% transformed for a low- alloy ductile iron austenitized at 870 and 925 °C (1600 and 1700 °F). Source: Ref 6 Austempering Temperature and Time. The austempering temperature is the primary determinant of the final microstructure and therefore the hardness and strength of the austempered product. As the austempering temperature increases, the strength and impact toughness vary as shown in Fig. 19 for irons with two levels of manganese. The attainment of maximum ductility at any given austempering temperature is a sensitive function of time, as shown in Fig. 20 for a number of ductile cast iron alloys (Ref 12). The initial increase in elongation occurs as stage I (Fig. 2) and elongation progresses to completion, at which point the fraction of austenite is a maximum. Further austempering merely serves to reduce ductility as the stage II reaction causes decomposition to the equilibrium bainite product. Typical austempering times vary from 1 to 4 h. Micrographs of ASTM ADI grades 5 and 2 (Table 1) are shown in Fig. 6(a) and 6(b), respectively. Fig. 19 Effect of austempering temperature on properties of ductile iron. (a) Yield strength and tensile strength versus austempering temperature. (b) Impact strength versus austempering temperature. Source: Ref 11 Fig. 20 Elongation versus austempering time for a group of ductile iron alloys. Source: Ref 12 References cited in this section 3. E. Dorazil, B. Barta, E. Munsterova, L. Stransky, and A. Huvar, High Strength Bainitic Ductile Iron, Int. Cast. Met. J., June 1982, p 52-62 6. D.J. Moore, B.S. Shugart, K.L. Hayrynen, and K.B. Rundman, A Microstructural Determination of Isothermal Transformation Diagrams in a Low Alloy Ductile Iron, Trans. AFS, 1990, in press 10. J.A. Lincoln, Austempered Ductile Iron, in First International Conference on Austempered Ductile Iron: Your Means to Improved Performance, Productivity and Cast, American Society for Metals, 1984, p 167- 184 11. B.V. Kovacs, Austempered Ductile Iron: Fact and Fiction, Mod. Cast., March 1990, p 38-41 12. R.B. Gundlach and J.F. Janowak, Austempered Ductile Iron Combines Strength with Toughness and Ductility, Met. Prog., July 1985, p 19-26 Surface Hardening of Ductile Iron Ductile iron responds readily to surface hardening by flame, induction, or laser heating. Because of the short heating cycle in these processes, the pearlitic types of ductile iron ASTM 80-60-03 and 100-70-03 are preferred. Irons without free ferrite in their microstructure respond almost instantly to flame or induction heating and require very little holding time at the austenitizing temperature in order to be fully hardened. With a moderate amount of free ferrite, the response may be satisfactory, but an entirely ferritic matrix, typical of the grades with high ductility, requires several minutes at 870 °C (1600 °F) to be fully hardened by subsequent cooling. A matrix microstructure of fine pearlite, readily obtained by normalizing, has a rapid response to surface hardening and provides excellent core support for the hardened case. With proper technique and the control of temperature between 845 and 900 °C (1550 and 1650 °F), the ranges of surface hardness for ductile iron with different matrices expected in commercial production are: • Ductile iron, fully annealed (ferritic), water quenched behind the flame or induction coil, 35 to 45 HRC • Ductile iron, predominantly ferritic (partly pearlitic), stress relieved prior to heating , self quenched, 40 to 45 HRC • Ductile iron, predominantly ferritic (partly pearlitic), stress relieved prior to heating, water quenched, 50 to 55 HRC • Ductile iron, mostly pearlitic, stress relieved before heating, water quenched, 58 to 62 HRC Heating time and temperature, amount of dissolved carbon, section size, and rate of quench help to determine final hardness values. Often soluble-oil or polymer quench media are used to minimize quench cracking where the casting section changes. Flame or induction-hardened ductile iron castings have been used for heavy-duty applications such as foils for cold working titanium, ring gears for paper-mill drives, crankshafts, and large sprockets for chain drives. Induction hardening is discussed below; flame hardening is discussed in a separate so-named article in this Volume. The response of ductile iron to induction hardening is dependent on the amount of pearlite in the matrix of as- cast, normalized, and normalized and tempered prior structures (Ref 13). In quenched and tempered iron, the secondary graphite nodules formed during tempering are close enough together to supply sufficient carbon to the matrix by re- solution during induction heating. In the as-cast condition, a minimum of 50% pearlite is considered necessary for satisfactory hardening with induction heating cycles of 3.5 s and longer and hardening temperatures of 955 to 980 °C (1750 to 1800 °F). Structures containing less pearlite can be hardened by using higher temperatures, but at the risk of retaining austenite, forming ledeburite, and damaging the surface. With more than 50% pearlite, hardening temperatures may be reduced to within the range of 900 to 925 °C (1650 to 1700 °F). In the Normalized Condition. For heating cycles of 3.5 s and longer, at temperatures of 955 to 980 °C (1750 to 1800 °F), 50% pearlite in a prior structure would be considered a minimum. Normalized and tempered irons exhibit a poor response with lower pearlite content because of the depletion of the matrix carbon. In the tempering operation, the carbon migrates from the pearlite matrix to the graphite nodules. In the heating cycle, carbon is reabsorbed in the matrix from the nodule; however, there is insufficient time for it to migrate throughout the ferritic areas. Another factor in the response of ductile iron is the graphite nodule count; the greater the number of nodules per unit area, the deeper the hardening for any given heat cycle. This effect is more evident as the percentage of ferrite increases (Fig. 21). Fig. 21 Relationship between depth of induction hardening to 50 HRC and graphite nodule count in normalized and tempered ductile (nodular) iron Quenched and Tempered. The response of quenched and tempered nodular iron to induction hardening is excellent over a wide range of microstructures containing up to 95% ferrite. As a prior treatment, quenching and tempering has the advantage of permitting a lower prior hardness; there is a risk of distortion and quench cracking, however. Example 1: Response of a Quenched and Tempered Ductile Iron to Induction Surface Hardening. A quenched and tempered structure that provided good response to induction hardening was obtained by oil quenching from 900 °C (1650 °F) and tempering at 620 °C (1150 °F) for 1 h. This treatment produced a hardness of 262 HB, which could have been lowered, if necessary, by increasing the tempering temperature to 675 °C (1250 °F). By induction heating to a depth of 4.7 mm (0.184 in.), a surface hardness of 54 to 56 HRC was developed, and a depth of hardness to 50 HRC of 4.2 mm (0.164 in.) was obtained. Nitriding is a case-hardening process that involves the diffusion of nitrogen into the surface at a temperature of about 550 to 600 °C (1020 to 1110 °F). Usually the source of nitrogen is ammonia, and the process produces a surface layer about 0.1 mm (0.004 in.) deep with a surface hardness approaching 1100 HV. The surface layer is typically white and featureless in an etched microstructure, but nitride needles can be found just below it. Alloying elements can be used to increase case hardness, and 0.5 to 1% Al, Ni, and Mo have been reported to achieve useful results. Nitrided cases provide, in addition to very high hardness, increased wear resistance and antiscuffing properties, improved fatigue life, and improved corrosion resistance. Typical applications are for cylinder liners, bearing pins, and small shafts. Nitriding can also be carried out in liquid salt baths based on cyanide salts. Such processes have lower temperatures of treatment, although case depth may be decreased. More recently, processes for nitriding in a plasma have been developed and applied with success to ductile iron, but the process may be more restricted because of the special equipment and cost likely to be involved. Remelt Hardening. With the very high local heating achievable with plasma torches or lasers, it is possible to produce a very small melted area on the surface of a ductile iron component. This area then rapidly resolidifies because of the self- quenching effect of the casting mass. The remelted and resolidified region has a structure of white iron that is substantially graphite free and therefore has high hardness and wear resistance. The area that is remelted by a 2 kW laser is very small, typically 1.5 mm (0.06 in.) in diameter and 0.5 to 2 mm (0.02 to 0.08 in.) in depth, and having a hardness of about 900 HV without cracking. By traversing the casting surface, the area hardened by this method can be of useful size and is likely to find application in tappets, cams, and other small components subjected to sliding wear. Figure 22 shows the microstructure of a pearlitic iron traversed by a 1.5 kW laser at 456 mm/s (18.25 in./s). Fig. 22 Remelt- hardened and transition zones in a pearlitic iron after treatment with a 1.6 kW, 1.5 mm (0.06 in.) diam laser beam of 4.56 mm/s (0.18 in./s). Etched in picral. 50× Reference cited in this section 13. T.L. Burkland and A.H. Rauch, Prior Structure Effect on Ductile Iron Response to Induction Hardening, Trans. AFS, Vol 70, 1962, p 896-908 Stress Relieving of Ductile Iron When not otherwise heat treated, complex engineering castings of ductile iron may be stress relieved at 510 to 675 °C (950 to 1250 °F). Temperatures at the lower end of this range are satisfactory for many applications; temperatures at the higher end will eliminate virtually all residual stress (Fig. 23) but will also effect some reduction in hardness and tensile strength. Recommended ranges of stress-relieving temperature for various types of ductile iron are as follows: • Unalloyed: 510 to 565 °C (950 to 1050 °F) • Low-alloy: 565 to 595 °C (1050 to 1100 °F) • High-alloy: 595 to 650 °C (1100 to 1200 °F) • Austenitic: 620 to 675 °C (1150 to 1250 °F) The required time at temperature will depend on the temperature used, the complexity of the casting, and the completeness of stress relief desired (Fig. 22), but 1 h plus 1 h per inch of section thickness is recommended general practice. Fig. 23 Stress relief obtained in ductile iron held at three temperatures for 1 2 to 8 h. Initial hardness was 102 to 103 HRB. Hardness after holding at 540, 595, and 650 °C (1000, 1100, and 1200 °F) for 8 h was 102 to 104, 101 to 103, and 90 to 93 HRB. Cooling should be uniform to avoid reintroducing stresses. Castings should be furnace cooled to 290 °C (550 °F), after which they can be air cooled. In most instances, however, austenitic iron can be uniformly air cooled from the stress- relieving temperature. Effect of Heat Treatment on Fatigue Strength In heat treating to improve fatigue properties, the proper composition and temperature must be selected to ensure the greatest improvement, and it is essential to have an optimum-quality cast microstructure (that is, high nodule count, excellent nodularity, and freedom from defects). For example, one cause of low fatigue strength in quenched and tempered ductile irons results from the precipitation of secondary graphite throughout the matrix upon tempering. The softening that accompanies this event results in reduced fatigue strength as well (see rapid hardness decrease in Fig. 16). The amount of secondary graphite can be controlled by composition (primarily carbon and silicon) and tempering temperature (the incidence of secondary graphite increases with temperature). Naturally as the amount of carbon increases so will the quantity of secondary graphite. An increased silicon content will increase the rate of secondary graphitization (Ref 9). The fatigue properties of as-cast ductile irons can be improved significantly by heat treatment, but not in the same proportion as can the static tensile properties. Fatigue strength at 20 × 10 6 cycles (rotary bending) has been shown to increase with matrix hardness as a result of heat treatment (Ref 14), with fatigue strengths ranging from 170 to 200 MPa (25 to 30 ksi) in annealed irons and from 310 to 345 MPa (45 to 50 ksi) in austempered or quenched and tempered irons. Recently rotary bending fatigue strengths (at 20 × 10 6 cycles) in austempered irons of approximately 480 MPa (70 ksi) have been reported (Ref 15), and fatigue strengths on the order of 690 MPa (100 ksi) can be attained with rolling or peening after austempering. Of course, fatigue strength will be optimal when the matrix structure is homogeneous throughout and no defects are present. The nature of ductile cast iron is such that there are many microstructural sources that can reduce fatigue strength; these include increasing nodule size, microporosity, eutectic carbides in intercellular regions, and slag or other inclusions. All of these problems tend to be exaggerated in heavy-section castings. References cited in this section 9. K.B. Rundman and T.N. Rouns, On the Effects of Molybdenum on the Kinetics of Seco ndary Graphitization in Quenched and Tempered Ductile Irons, Trans. AFS, Vol 90, 1982, p 487-497 14. M. Sofue, S. Okada, and T. Sasaki, High Quality Ductile Cast Iron with Improved Fatigue Strength, Trans. AFS, Vol 86, 1978, p 173-182 15. D. Krishnaraj, K. Rao, and S. Seshan, Influence of Matrix Structure on the Fatigue Behavior of Ductile Iron, Trans. AFS, Vol 97, 1989, p 345-350 Introduction FERRITIC AND PEARLITIC malleable irons are both produced by annealing white iron of controlled composition. Thus, annealing is an essential part of the manufacturing process for these irons and, as such, is discussed in detail in the article entitled "Malleable Iron" in Properties and Selection: Irons, Steels, and High-Performance Alloys, Volume 1 of ASM Handbook Malleable irons have largely been replaced by ductile iron in many applications. This is due in part to the necessity of lengthy heat treatments for malleable iron and the difficulty in cooling thick sections rapidly enough to produce white iron. Malleable iron is still often preferred for thin-section castings and parts that require maximum machinability and wear resistance. Figure 1 compares mechanical strengths of nodular and malleable irons. [...]... temperatures above 260 °C (500 °F) and have been used successfully for furnace and stoker parts, burner nozzles, and for heat treatment trays The high-silicon and Si-Mo nodular irons are currently produced as manifolds and turbocharger housings for trucks and some automotive applications They are also used in heat treating racks Heat Treatment of High-Silicon Irons for High-Temperature Service The high-silicon... Ni-Hard 4, was performed by supercritical heat treatment when as-cast hardness was insufficient An austenitizing heat treatment usually comprised heating at temperatures between 750 and 790 °C (1380 and 1450 °F) with a soak time of 8 h Air or furnace cooling, not over 30 °C/h (50 °F/h), was conducted followed by a tempering/stress-relief heat treatment Refrigeration heat treatment is the more commonly practiced... temperature This heat treatment does not reduce hardness or abrasion resistance In the heat treatment of any white cast iron, care must be taken to avoid cracking by thermal shock; never place the castings in a hot furnace or otherwise subject them to rapid heating or cooling The risk of cracking increases with the complexity of the casting shape and section thickness High-Temperature Heat Treatment... C-19% Cr-2.4% Mo-0.9% Cu iron subjected to various heat treatments Source: Ref 2 Typical mechanical properties for three white irons of widely varying compositions are shown as bar graphs in Fig 3 The properties for the austenitic matrix were obtained with as-cast irons; the martensitic properties were obtained by heat treatment In all irons, heat treating to achieve a martensitic matrix resulted in... controlled by the rate of heating and by the temperature developed at the surface of the part being hardened In induction hardening, this is accomplished by the close regulation of power output, operating frequency, heating time, and alloy content of the iron The maximum hardness obtainable in the matrix of a properly hardened part is 67 HRC; however, conventional hardness measurements show less than the true... jaws of these tools as originally designed were inserts made of hardened tool steel Shell mold casting these jaws and heat treating them to pearlitic malleable iron made it possible for the jaws to be cast integrally to the desired intricate contour and to be hardened by induction heating and water quenching in order to provide required wear resistance Hardening was thus restricted to the jaws, and... cast iron materials having good toughness at low temperatures The procedures and temperatures of the heat treatments for these ductile irons with nodular graphite are similar to those for gray (flake-graphite), corrosion-resistant austenitic cast irons Heat treatment is discussed in the next section "Heat Treatment of Austenitic Ductile Irons." Austenitic Gray Irons These cast irons exhibit properties... hardness, strength; and heat resistance are desired, and where increased expansivity can be tolerated, Cr may be increased to 2.5 to 3.0% (f) Type 6 also contains 1.0% Mo Table 2 Typical mechanical properties of flake-graphite austenitic cast irons per ASTM 436-84 Type Tensile strength(a) Hardness, HB(b) MPa ksi 1 170 25 131-183 1b 205 30 149-212 2 170 25 118 -174 2b 205 30 171-248 3 170 25 118 -159 4 170 25... alloys are susceptible to work hardening during machining and require careful cooling from the casting operation and/or subsequent heat- treating operations to minimize the initial stresses and the rate of work hardening during metalremoval operations Castings that have not been heat treated may cause "chattering" during machining ASTM Specification A 436 defines eight grades of austenitic gray iron alloys,... 148- 211 D-3 380 55 205 30 6 139-202 D-4 415 60 202-273 D-5 380 55 205 30 20 131-185 (a) In 50 mm (2 in.) Fig 1 Photomicrograph of a D5S Ni-Resist ductile iron casting showing nodular graphite structure 400× Applications The nickel-alloyed irons, or Ni-Resist irons, have found wide application in chemical process-related equipment such as compressors and blowers, condenser parts, phosphate furnace parts, . predominantly ferritic (partly pearlitic), stress relieved prior to heating , self quenched, 40 to 45 HRC • Ductile iron, predominantly ferritic (partly pearlitic), stress relieved prior to heating, water. 600 °C (1020 to 111 0 °F). Usually the source of nitrogen is ammonia, and the process produces a surface layer about 0.1 mm (0.004 in.) deep with a surface hardness approaching 110 0 HV. The surface. (950 to 1050 °F) • Low-alloy: 565 to 595 °C (1050 to 110 0 °F) • High-alloy: 595 to 650 °C (110 0 to 1200 °F) • Austenitic: 620 to 675 °C (115 0 to 1250 °F) The required time at temperature will