The graphite could be in flake (grey) form, or as nodules (ductile iron). These cast irons are divided in following classes:
I. Nickel-alloyed, austenitic graphitic irons (Ni-Resist)- Both grey and nodular forms are used for high corrosion resistance, and for high-temperature service.
II. High silicon (14.5% Si) grey irons are used in applications requiring corrosion-resistance.
III. High silicon (4-6%) grey or nodular irons are used for high temperature applications.
I. Austenitic Graphitic Irons- Ni-Resist Irons:
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These cast irons could be grey or nodular. Due to the addition of austenite stabiliser mainly, nickel (Cu, Mn have very mild effects), these cast irons have austenite phase (stabilised) at room temperature (having a uniform dispersion of carbides). This austenite phase is responsible for several of the properties such as good corrosion resistance, strength at elevated temperatures.
The important properties exhibited by these irons are- good corrosion resistance, wear resistance, high temperature stability and strength, low thermal expansion coefficients, non-magnetic properties, good toughness and strength at low temperatures.
The austenitic graphitic iron could be:
1. Austenitic Grey Iron
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2. Austenitic Ductile Iron
1. Austenitic Grey Irons:
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The composition and properties of flake-graphite austenitic cast irons are given in Table 15.15. Nickel helps to get stable austenite microstructure with good corrosion resistance and strength at elevated temperatures.
The presence of chromium and silicon in these nickel-alloyed grey irons induces good wear resistance and oxidation resistance at elevated temperatures. Grade 1 and 1b are exclusively for corrosion- resistance applications. Grade 2 b, 3 and 5 are mainly for high temperature applications. Grade 4 is for stain-resistance.
Austenitic grey irons have good resistance to corrosion by alkalis, salts, acids, oils, foods; good high temperature oxidation resistance; good abrasive wear-resistance; non-magnetic properties; low but uniform thermal expansion; high electrical resistivity; moderate strength and toughness.
However, these alloys are susceptible to work-hardening during machining. These alloys should be cooled slowly after casting and/or after heat treatment to avoid development of stresses. Grey (austenitic) irons are given similar heat treatments with similar temperatures as austenitic ductile irons.
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2. Austenitic Ductile Irons:
The compositions and properties of austenitic ductile irons are given in Table 15.16, and are similar to austenitic grey irons but magnesium has been added to obtain nodules of graphite. However, austenitic ductile irons have higher strengths and ductility alongwith good desirable properties of grey iron alloys.
Austenitic ductile irons have good frictional wear resistance, corrosion resistance to wide range of corrosive media such as alkalis, salts, acids, oils and foods; good strength and oxidation resistance at high temperatures; non-magnetic properties; some alloys have low thermal expansion coefficient at room temperature. Fig. 15.13 illustrates as-cast microstructure of Ni-resist iron.
Ni-Resist irons are useful in chemical-process industries for compressors, blowers, condenser parts, phosphate furnace parts, pipe, valves, fittings, pots, retorts, pump casings and impellers, etc.; in food- handling equipments as bottling and brewing equipment, canning machinery, distillery equipment, feed screws, meat-grinders, salt filters; in high temperature applications as cylinder liners, exhaust manifolds, valve guides, gas turbine housings, turbocharger housings, nozzle rings, piston-ring carriers in aluminium- silicon pistons.
Heat Treatment of Austenitic Ductile & Grey Irons (Ni-Resist):
1. Stress Relieving:
This process aims to remove residual stresses resulting from casting or machining or both. For most applications, stress-relieving is carried out at 620-675°C for 1 h per 25 mm of section, and then air-cool at 1-2 h per 25 mm section, or do furnace cooling.
About 60% stresses are relieved by holding the casting at 480°C for 1 h per 25 mm section, whereas 95% are relieved at 675°C. Thin-section but large castings may not be stress-relieved if cooled in mould to below 315°C. Tensile strength, hardness and ductility are not affected by stress-relieving.
2. Spheroidize Annealing:
In order to soften the rapidly cooled castings and thin sections, which have excessive carbides resulting in hardnesses above 190 BHN, spheroidize annealing is done. This annealing helps to dissolve the carbides, or at least change their shapes to spheroids. Spheroidize annealing is the process of heating the castings (having up to 4% Cr) to 980-1040°C for 1/2 to 5 hours. It decreases hardness, but strength remains unaffected.
3. High Temperature Stabilization:
The castings, which are used at high temperatures, such as for static, or cyclic service at 480°C or above, except of grade I of table 15.15, are usually given high temperature stabilisation treatment prior to final machining. This treatment consists of heating at 760°C for 4h (min.), or at 870°C for 2 h (min.), followed by furnace cooling to 540°C, and then cooling in air.
This treatment:
(i) Stabilizes the microstructure, such as, by reducing the carbon content of the matrix by depositing on the already present graphite,
(ii) Allows some growth and distortion. Final machining is done then. The castings, when put in service now, show little change in micro-structure with negligible growth and distortion.
4. Dimensional Stabilization:
Ni-resist cast irons (except grade 1 of table 15.15) used for precision machinery and scientific-instrument-components requiring true dimensional stability, are given dimensional stabilization treatment.
The treatment consists of heating to 870°C for minimum 2 hours; followed by furnace cooling at maximum rate of 50°C/h to 540°C, at which the castings are held for 1 h per 25 mm section, and then cooled in air. After rough machining, reheat to 455-480°c and soak for 1 h per 25 mm of section, and then cool in air. After finish machining, reheat to 260-315°C and then cool in air.
5. Deep-Freezing and Re-Austenitization of D-2 (Table 15.16):
This treatment increases the yield strength of D-2 type ductile irons (Table 15.16), without affecting non-magnetic properties or corrosion resistance in sea water or dil. sulphuric acid. The treatment consists of heating to 925°C, quenching in oil or water, followed by refrigeration at-195°C. It is then reheated to 650-760°C, and cooled.
II. High-Silicon Irons (Corrosion Resistance):
Graphitic cast irons having high silicon content, around 14.5% are unique corrosion-resistant ferritic cast irons; the compositions of three of them are given in table 15.17. Grade 2 and 3 also has 3.25-5% Cr, while the grade 2 also has 0.4-0.6% Mo.
Because of their excellent corrosion resistance, these alloys are widely used in chemical industry for the production and transportation of highly corrosive liquids specially sulphuric and nitric acids; for handling mineral acids in petroleum refining; for sewage-disposal and water-treatment; for the manufacture of fertiliser, textiles and explosives; also pump rotors, agitators, crucibles and pipe- fittings for chemicals.
Heat Treatment:
Because of the brittle nature, the high-silicon castings are given, almost immediately after casting, the stress-relief heat treatment. The process consists of heating the castings to 870-900°C followed by slow furnace-cooling to room temperature This treatment does not effect the corrosion-resistance.
III. High-Silicon Irons (For High Temperature Service):
These are normal grey or ductile, ferritic irons having 4-6% Si, and have stable ferritic matrix structure that does not undergo a phase change up to 815°C (for 4% Si), 871°C (for 5% Si) as A, temperature is raised to these values.
These cheap alloys, thus, find use in many high-temperature applications. But, the presence of higher silicon content, makes them very brittle at room temperature, and raises the impact transition temperature well above the room temperature, improving the ductility when the temperature exceeds 430°C.
The higher silicon content in both grey and ductile irons improves the oxidation resistance at high temperatures, because a dense, adherent film of iron-silicate forms on the surface, which resists oxygen penetration more effectively with increasing silicon content.
‘Silal’ (5.5-7.0% Si), a high silicon grey iron, has high critical (A1) temperature, i.e., has a stable ferrite matrix, with D-type graphite-flakes alongwith good resistance to growth and oxidation- reasonably tough above 260°C. Oxidation resistance is further increased with the addition of chromium up to 2%. Such alloys have been used for high temperature applications such as furnace and stoker parts, burner nozzles, and heat treatment trays.
The high silicon-nodular irons find use in greatest tonnage. The graphite in nodular form further improves the resistance to oxidation and growth. As compared to grey irons, the higher strength and ductility of nodular irons make them useful for more rigorous service. As A1 temperature is raised by silicon, these irons are used to temperatures up to 900°C. Ductility improves when the temperature exceeds 430°C.
Addition of 0.5-1% Mo provides adequate high temperature-strength and creep-resistance. Higher molybdenum (> 1%) results in the formation of stable inter-dendritic carbides of M2C type, which induce maximum high temperature strength, although some reduction in toughness and ductility at room temperature, occurs. High silicon and Si-Mo nodular irons are used for manifolds and turbocharger housings for trucks, etc., and for heat treating racks.
Heat Treatment:
Although high-silicon grey and nodular irons are mainly ferritic (as-cast), but may contain some pearlite and often carbides due to addition of carbide-forming elements, Cr, Mo, which make the already brittle alloys more brittle, and more prone to growth in service.
An annealing treatment of high Si-nodular irons is done to decompose the pearlite, the carbides and to stabilize the casting to avoid growth in service. A graphitizing full-annealing is done when undesirable amounts of carbides are present due to low 4-5% Si in irons.
The process requires heating to higher than 900°C for several hours, followed by slow cooling to below 700°C, and then cooled in air. The carbide free castings, and those having more than 5% Si (carbides easily break-down) are given subcritical annealing, which consists of heating to 720-790°C for 4 h and then air-cooled. These castings have high strength, but lower ductility and toughness as compared to fully- annealed castings.