MANUFACTURE APPLICATION OF COMPACTED GRAPHITE CAST IRON

1. MANUFACTURE & APPLICATION OFCOMPACTED GRAPHITE CAST IRON

2. PRESENTED BY

3. SUBRATA CHAKRABARTTI

4. In 1949, a now well-known material called ductile iron was patented. At the same time, a lesser-known material called Compacted Graphite Iron (CGI) was also patented, though it was just considered a curiosity at that time. While ductile iron became a manufacturing staple, CGI never was seriously utilized despite possessing some very interesting properties.

5. WHY COMPACTED GRAPHITE IRON? 

6. The answer lies in the characteristics of CGI. While not quite as strong as Ductile Iron, CGI is 75 percent stronger and up to 75 percent stiffer than Grey Iron. The thermal conductivity and damping characteristics of CGI are 80% and 60% respectively of Grey Iron. It is five times more fatigue resistant than aluminum at elevated temperatures, and twice as resistant to metal fatigue as grey iron.

7. These properties have been found to make CGI ideally suited for engine manufacturing, where lighter and stronger materials are needed which can absorb more power. An assembled automotive engine can be made nine percent lighter with CGI. The engine block weight alone can be reduced by 22 percent. This corresponds to a 15 percent reduction in length and 5 percent reduction in height and width. It can be easily understood why CGI is continually gaining popularity in automobile industry.

8. In 1999, the Bedplate of the new Chrysler 4.7 Liter V-8 Engine was converted to CGI from Grey Cast Iron. When Hyundai Motor Co. unveiled its new Veracruz luxury utility vehicle in 2007, it not only had a new engine design, its engine block was made of Compacted Graphite Iron (CGI). Till date it has replaced grey iron for many important engine parts such as V-Diesel engines for Audi-Volkswagen and FORD-PSA, High volume SOP for Hyundai-Kia V6 2006, Engine components of Caterpillar, Frames and Heads for Rolls-Royce & General Electric, Marine diesel piston rings for Daros, Flywheels and clutch plates for Aston Martin.

9. In September’2009, DCM Engineering has become the first Indian foundry to produce CGI cylinder blocks. It is predicted that the world wide growth of use of CGI in engine components will be 23% by 2010.

10. But its utility in other engineeringsectors have not been seriously explored till now. There can be a number of economically advantageous applications of CGI in non- automobile industries.

11. WHAT IS A COMPACTED GRAPHITE IRON?

12. It can be said that Compacted Graphite Iron is a denigrated or spoiled Ductile Iron and its invention was probably an accident when the base metal for producing Ductile Iron was unintentionally under-treated with magnesium and the resultant metal was found to have certain characteristics very near to that of Ductile Iron and certain characteristics very near to that of Grey Iron.

13. Comparison of typical mechanical properties of Grey Iron, Compacted Graphite Iron and Ductile Iron is shown below

14. But the advantages of CGI in application do not stem from its mechanical properties. If a superior mechanical property compared to Grey Iron is the only criteria, then the natural choice would be Ductile Iron due its ease of manufacturing compared to CG Iron. The most important fact about CG Iron is that, while it has much higher mechanical properties compared to Grey Iron, its thermal and damping characteristics are very near to Grey Iron.

15. Comparison of thermal conductivity of Grey Iron, CGI and Ductile Iron is shown in below:

16. One of the benefits of cast irons is their ability to suppress vibration due to comparatively high internal damping capacity, brought about partly by the micro-structure. A comparison of damping capacity of Grey Iron, CGI and Ductile Iron is shown below:

17. Fatigue problems are the most important problem in industrial applications and machinery parts. Mechanical fatigue as well as Thermal fatigue are the major causes of failure of machinery parts. The combination of greater cracking resistance compared to grey cast irons and better resistance to distortion compared with ductile irons can be a big advantage for CG Irons for applications where mechanical fatigue is predominant

18. S-N Curves for Grey Iron, CGI and Ductile Iron

19. THERMAL FATIGUE RESISTANCE When a material is subjected to a temperature gradient, it tends to expand differentially. During this process, thermal stresses are induced. The source of heat that causes the thermal gradient may be friction as in the case of brake drums, or external media as in the case of cylinder heads. When thermal stresses are generated by changes in temperature, thermal shock or thermal fatigue is the result.

20. Two types of thermal cycles are possible. (i) Low frequency thermal cycle in which the time taken for completion of the cycle is large enough to cool the component. Typical examples are ingot moulds and brake drums of railway coaches.   (ii) High frequency thermal cycle in which the time involved is in mille-seconds and the heating and cooling is influenced by the thermal inertia of the system under consideration. Cylinder heads, piston rings and exhaust manifolds are standing examples of components which experience high frequency thermal cycles.

21. Engine components are subjected to thermal gradient and thermal stresses are induced causing thermal shock or thermal fatigue. In components like brake drums, a low frequency thermal cycle is in action which means the time taken to complete the cycle is large enough to cool the components whereas in components like cylinder heads, piston rings and exhaust manifold a high frequency thermal cycle is in action because the time involved is in mile-seconds and the heating and cooling are influenced by the thermal inertia of the system. 

22. Due to this cracks may appear or mechanical properties may be lowered due to metallurgical variations such as micro structural changes and internal oxidations which can lead to premature failure of the component

23. Metallic components subjected to thermal cycling/thermal fatigue may fail in one of the following ways.

24. Type 1 failure: Cracks appear first on the hot zone of the component (in the heat-checking network) and may eventually propagate through the section. This is the most common type of failure observed in grey cast irons and other brittle materials. Type 2 failure: Severe distortion which ultimately renders the component useless. This type of failure is usually found in ductile iron components. Type 3 failure: Gross cracking through the entire section during the first few cycles. These failures emanate due to the mismatching of materials selected, improper design and random thermal cycling.Type 4 failure: Lowering of mechanical properties of materials due to metallurgical variations such as micro structural changes and internal oxidation, which can lead to premature failure of components.

25. FACTORS EFFECTING THERMAL FATIGUE RESISTANCE

26. To resist thermal fatigue, the material selected for a constrained component should have the following properties (some of the requirements may be relaxed for an unconstrained member such as ingot moulds).

27. (i) High thermal conductivity.(ii) High elastic limit or high tensile strength and a narrow stress/strain hysteresis loop.(iii) Low creep in compression.(iv) High mechanical fatigue strength under conditions of repeated plastic strain.(v) High ductility.(vi) Resistance to internal and external oxidation.(vii) Resistance to micro structural changes.(viii) Elevated temperature properties, especially tensile strength.(ix) Low coefficient of thermal expansion.

28. Mechanical & Physical Properties of 10% Nodularity CGI

30. From the above it can be seen that Compacted Graphite Cast Iron, besides its usefulness to automobile industry, can be a very useful material for applications involving thermal cycle and fatigue in other engineering industries also. 

31. A FEW EXAMPLES OF CGI APPLICATIONS IN ENGINEERING INDUSTRY

32. INGOT MOLDS

35. SOW MOLDS & INGOT MOLDS FOR ALUMINIUM

39. COKE OVEN CASTINGS

43. PALLET CARS FOR SINTER PLANTS

45. MOLDS FOR PIG CASTING MACHINE

47. WHY IS THEN USE OF CG IRON IN NON -AUTOMOBILE INDUSTRIES NOT GROWING?

48. The reason that CGI has not yet been widely adopted for production by foundries is that the stable range of compacted graphite is too narrow to ensure risk-free production which necessitates very strict process control involving heavy investment on control equipments.

49. CHARACTERISTICS OF COMPACTED GRAPHITE IRON

50. The graphite particles in compacted graphite iron appear as individual ‘worm-shaped’ or vermicular particles. The particles are elongated and randomly oriented as in gray iron, however they are shorter and thicker, and have rounded edges. While the compacted graphite particles appear worm-shaped when viewed in two dimensions, deep-etched scanning electron micrographs show that the individual ‘worms’ are connected to their nearest neighbors within the eutectic cell. The complex coral-like graphite morphology, together with the rounded edges and irregular bumpy surfaces of the graphite particles, results in strong adhesion between the graphite and the iron matrix. The compacted graphite morphology inhibits crack initiation and growth and is the source of the improved mechanical properties relative to gray iron.

51. CGI MICROSTRUCTURE CONTAINING 10% NODULARITY

52. Deep Etched SEM Micrograph showing complex coral-like graphite in three dimensions

53. WHY IT IS DIFFICULT TO GET THIS MORPHOLOGY OF GRAPHITE? BECAUSE FOR FLAKE GRAPHITE THERE IS A MAXIMUM LIMIT OF RESIDUAL MAGNESIUM FOR NODULAR GRAPHITE THERE IS A MINIMUM LIMIT OF RESIDUAL MAGNESIUM FOR COMPACTED GRAPHITE THERE IS BOTH MINIMUM & MAXIMUM LIMITS OF RESIDUAL MAGNESIUM AND THE RANGE IS VARY NARROW

54. 

55. The actual size and location of the stable CGI plateau is different for each product, it generally spans a range of approximately 0.008% Mg. However, in practice, the usable Mg-range is even smaller. Because active magnesium fades at a rate of approximately 0.001% every five minutes, the initial starting point of the iron must be sufficiently far away from the abrupt CGI-to-gray iron transition. This ‘buffer’ is necessary to ensure that flake-type graphite does not form before the end-of-pouring, which may be as long as fifteen minutes after the initial magnesium addition.Simultaneously, the starting point must not be too close to the right side of the plateau in order to minimize the formation of nodular graphite in the faster-cooling thin sections.

56. A second consideration is that the stable CGI plateau is not stationary. If the active oxygen and/or sulfur contents are high, they will consume the active magnesium and shift the entire plateau toward higher total magnesium values. Conversely, if the oxygen or sulfur levels are relatively low, the CGI plateau will shift toward the left. For these reasons, variations in the composition, cleanliness, oxidation and humidity of the charge material make it impossible to define a fixed chemistry specification for CGI.

57. The sensitivity of CGI to magnesium is illustrated in figure shown below which shows that a flake patch structure in a 25 mm diameter test bar can be converted to a fully compacted microstructure with the addition of only 10 grams of magnesium in a one tonne ladle. The flake patch microstructure provides an ultimate tensile strength of 300 MPa while the tensile strength of the fully compacted microstructure is 450 MPa.

58. The CGI plateau is also sensitive to the addition of inoculant. Higher inoculation levels provide more nuclei which favours the formation of nodular graphite. Therefore, higher inoculation levels shift the CGI plateau toward higher nodularity values while lower inoculation levels cause the plateau to shift downward. Factors such as furnace superheat, holding time, charge composition and type and amount of inoculant therefore influence the location of the stable CGI plateau. The sensitivity to inoculation is illustrated in figure shown below which shows that the addition of 80 grams of inoculant to a one tonne ladle can change the nodularity from 3% to 21% in a 25 mm diameter test bar.

59. PROCESS OF MANUFACTURING CG IRON

60. Several methods have been adopted to produce CGI. The simplest method would have been to under-treat the metal with magnesium so that the residual magnesium is within the specified range.But the treatment process adopted by most of the foundries are not very consistent with respect to Mg recovery, besides Mg recovery is also dependent on the base metal chemistry and thermal conditions.

61. There are two ways to produce Compacted Graphite Iron:1) Assure residual magnesium within the CG Window by very efficient automated treatment method based on online assessment of thermal condition and chemical analysis of the base metal.2) Magnesium addition suitable for producing Ductile Iron and subsequently reducing its nodularizing effect by addition of elements having negative effect onnodularization.

62. The first method is normally adopted by foundries producing automobile components in production line where volume of production and quality requirements justify the investment necessary for installation of equipments for online thermal & chemical analysis of base metal and thereafter automated magnesium and inoculants addition by wire injection method.A schematic process flow-chart is shown in next slide.

64. Majority of casting for engineering industries other than automobile and railways are produced by jobbing foundries where investment for online analysis and automated treatment equipments is not justified. As such the 2nd method i.e. normal magnesium treatment suitable for ductile iron and then reducing the nodulizing effect of magnesium by addition of de-nodulizing elements is most suitable for jobbing foundries intending to manufacture CG Iron castings for engineering industries.

65. TITANIUM PROCESS

66. This method consists of treating the molten metal with both spheroidizing and anti-spheroidizing elements, mainly Ti. This process can provide optimum results in many applications. Indeed, the Ti process appears to be a reliable CGI production method with repetitive results. However, one important factor impedes the general application of this treatment: titanium has a strong carbide, and nitrocarbide formation tendency. TiC and complex Ti nitrocarbides are very hard phases that diminish machining tool lifetime. Whenever possible such phases should be avoided in CGI parts where exacting or heavy machiningprocesses are required.

67. The above figure shows the influence of Titanium on the nodularizing effect of Magnesium.

68. Some proprietary treatment alloy has been developed for CG Iron with Rare Earth instead of Titanium or any other harmful de-nodulaizer and it is claimed that a wider window with respect to Mg content can be used while using these treatment alloys.

69. RE-SULFURIZATION PROCESS

70. This process appears to be most appropriate for jobbing foundries intending to manufacture CG Iron castings for engineering industries. This process can be adopted by gas fired Cupola based C.I foundries very easily.

71. Using a 0.015 to 0.025 percent sulfur addition (after magnesium additions) in the form of iron sulfide or iron pyrites, which contained nominally 49 percent sulfur and is approximately 100 mesh by down to denodulize magnesium treated iron, it is possible to consistently produce acceptable CG irons with less than 20 percent nodularity. The amount of sulfur addition in magnesium-treated iron needed to obtain a compacted graphite iron depends on the residual magnesium content after magnesium treatment as well as holding time prior to pouring. Other important factors are casting wall thickness, mold type and thermal gradient effects.

72. Three steps are necessary to produce compacted graphite irons by using controlled sulfur additions after magnesium treatment. Base iron production controls should maintain sulfur contents at 0.03 percent and lower. II. Magnesium treatment by an appropriate technique for each foundry, targets a lower Mg range than for ductile iron, typically at a level of 0.025 to 0.04 percent Mg residual (Mg res) for compacted graphite production. III. The sulfur addition after magnesium treatment is to reduce the effective residual magnesium to the appropriate range for compacted graphite formation and for the casting weight and common section size (typically 0.010 to 0.025 percent final residual Mg after sulfur has been added).

73. There are three important control parameters for producing CG Iron by this process:

74. 1. Final Magnesium Residual Level (Mg(fin)) : Final residual magnesium levels (Mg(fin)), obtained after a sulfur addition during the post-inoculation stage is an important parameter that defines the graphite phase. Mg(fin) ranged between 0.010 to 0.019% will produce CG Iron with less than 20% nodularity while CG Iron with higher nodularity will be produced when the Mg(fin) is greater than 0.02%

75. 2. The Ratio of Final Magnesium and Final Sulfur (Mg(fin) / S(fin) This ratio also influences the final structure of the production castings. The higher this ratio becomes, lower levels of compacted graphite form and correspondingly, higher nodularity compacted irons tend to form. When this ratio falls in the range of 1.0 to 2.0, CGI iron forming tendencies are very high.

76. 3. The Ratio of Magnesium Added and Initial Sulfur (Mg(Add) / S(In) The final structure of compacted graphite irons is very dependent on the treatment method of the ductile base iron. The level of magnesium addition in relationship to initial sulfur level is extremely important, because the nodular graphite structure stability is not just determined by residual magnesium level but also by that initial sulfur relationship. As the ratio of Mg(add) / S(in) becomes greater than 3.2, high nodule compacted graphite formation is favored. However, when this ratio of Mg(add) / S(in) lies in the range of 2.0 to 3.2, both types of compacted graphite structures can result. It is preferable to have a initial sulfur level greater than 0.018% to get compacted graphite structure with less than 20% nodularity.

77. An example of relationship between Magnesium and Sulfur in CG Iron having a C.E. of 4.0 and 0.015 initial Sulfur and 0.055 residual Magnesium tested from 25mm thick standard Y-Block is shown below:

85. CONCLUSIONS With its wonderful characteristics ,having the advantages of both Grey Iron and Ductile Iron, Compacted Graphite Iron is the dream of any design engineer, provided foundries can produce with consistency. It has already charmed the automobile industry and has the capability to make significant impact on other engineering industries also. The only restrain is the strict & difficult process control which not many foundries can achieve. But this is not the end of the road. Research still continuing to find a simpler way to produce CG Iron, as was the case for Ductile Iron in the beginning.

86. REFERENCES Dr Steve Dawson – “Compacted Graphite Iron – A Material Solution for Modern Diesel Engine Cylinder Blocks and Heads” M. Bazdar, H.R. Abbasi*, A.H. Yaghtin, J. Rassizadehghani – “Effect of sulfur on graphite aspect ratio and tensile properties in compacted graphite irons “ S Y Buni, N Raman and S Sesha – “The role of graphite morphology and matrix structure on low frequency thermal cycling of cast irons” 4. I. Riposan, M. Chisamera, R. Kelley, M. Barstow, R. L. Naro – “Magnesium- Sulfur Relationships in Ductile and Compacted Graphite Cast Irons as Influenced by Late Sulfur Additions” 5. C.M.Ecob and C.Hartung - “An Alternative Route for the Production of Compacted Graphite Irons” 6. Stephen Karsay – “Ductile Iron Production” ------------------------------------

87. THANK YOU