Ductile iron (part two)

Moderatore: Moderatore Area Tecnica

Ductile iron (part two)

Sostienici

Sponsor
 

Ductile iron (part two)

Messaggioda Aldebaran » 30/04/2010, 7:59

Mechanical Properties at Elevated Temperatures
The hardness and strength of all standard grades of ductile iron are relatively constant up to about 425 °C (800
°F). It should be noted that the
hot hardness is lower with a lower silicon content.
The resistance to oxidation of ductile irons and other materials at 705 °C (1300 °F) is shown in Table 10. In
ductile iron, resistance to oxidation increases with increasing silicon content. The data indicate that ductile iron
of normal composition has much better oxidation resistance than does cast steel, gray iron, or pearlitic
malleable iron.
The dimensional growth of ductile iron is much less than that of gray iron at elevated temperatures.
. Data on the growth of ductile and gray irons at 540 °C (1000 °F) for 6696 h
, which emphasizes the excellent performance of ASTM grade 60-40-18 ferritic ductile
iron.
The creep strength of ductile iron depends on composition and microstructure ,
ferritic ductile iron has a creep resistance comparable to that of annealed low-carbon cast steel up to 650 °C
(1200 °F).
Hardenability
Ductile iron in various hardened conditions has become accepted for such applications as heavy-duty gears,
spinning mandrels, pump liners, rolls, dies, clutch drums, pistons, brake drums, and agricultural-implement
parts. Ductile iron has limited heat transfer characteristics because the graphite is in a spheroidal shape rather
than the graphite flake form of cast iron. Spheroidal graphite (SG) iron provides an answer to the problem of
heat transfer encountered with compacted graphite and yet SG iron has higher strength than gray cast iron. This
is important in the case of brake drums or rotors, which may develop high braking surface temperatures and
form hard spots of martensite. Alloy ductile iron can develop an acicular structure in the as-cast structure that
may resemble bainite or martensite. However, the latter are structures developed by heat treatment. Most
applications use quenching and tempering or isothermal transformation treatments, which provide ductile iron
with a hard surface overlaying a tough core. The hardenability of ductile irons covers a wide range, but a
ductile iron generally has higher hardenability than a eutectoid steel with comparable alloy content (Ref 17).
Austenitizing temperatures between 845 and 925 °C (1550 and 1700 °F) affect hardenability according to the
amount of combined carbon that is taken into solution. Higher temperature and longer time, up to that required
for saturation of the austenite at the soaking temperature, increase the amount of combined carbon, which
increases hardenability. Graphite has lower solubility in austenite than does pearlite (combined carbon); that is,
it takes a higher temperature and a longer elapse of time to harden ferritic ductile than are required for pearlitic
ductile iron. The time and temperature required for pearlitic ductile iron to reach maximum hardness depend on
the combined carbon content, which is to some degree related to the hardness before heat treatment. An as-cast
pearlitic ductile iron will normally have a higher combined carbon content than will normalized ductile iron of
the same hardness. The silicon content of ductile iron reduces the amount of carbon taken into solution.
Insufficient time or temperature used in hardening ductile iron is made obvious by a microstructure that has the
appearance of bainite in the original ferrite grain boundary and of transformation products (pearlite or
martensite in the center of the grains).
Physical Properties
Cast irons are not homogeneous materials. Certain properties are affected more by the shape, size, and
distribution of the graphite particles than by any other attribute of the structure, a behavior more like that of a
composite material than of a homogeneous metal or alloy. Among the physical properties of ductile iron,
density is affected only by the relative amounts of the microconstituents present, not by their form or
distribution, whereas thermal and electrical conductivity are markedly affected by the form and distribution of
the virous phases, especially graphite, which has properties very different from those of the various metallic
phases.
Density. For most ductile irons, density at room temperature is about 7.1 g/cm3. Density is largely affected by
carbon content and by the degree of graphitization and any amount of microporosity. A low-carbon pearlitic
ductile iron might have a density as high as 7.4 g/cm3, and a high-carbon ferritic ductile iron may have a density
as low as 6.8 g/cm3. Microporosity, when present, will produce a lower density, depending on the amount
present. High-nickel compositions, such as the Ni-Resist ductile irons, have slightly greater density, about 7.4
to 7.7 g/cm3.
Thermal Properties. Specific heat is relatively unaffected by composition, but of unalloyed ductile iron varies
with temperature.
The melting point varies with silicon content, and the melting range varies with carbon content. The closer the
value of the carbon equivalent is to the eutectic composition, the narrower the melting range. Unalloyed and
low-alloy ductile irons melt in the range of 1120 to 1160 °C (2050 to 2120 °F); austenitic high-nickel ductile
irons melt at about 1230 °C (2250 °F).
The heat of fusion for all ferritic and pearlitic grades of ductile iron is about 210 to 230 kJ/kg (90 to 99
Btu/lb).
The coefficient of linear thermal expansion, like specific heat, is considered to be constant over a given
temperature range even though the coefficient of thermal expansion actually varies with temperature. Because it
varies with temperature, different values of expansion coefficient are needed for different temperature ranges.
The value of the expansion coefficient is ordinarily determined by measuring the change in length of a
standard-size specimen as it is heated and cooled over a specific temperature range. Coefficients of linear
thermal expansion for ferritic and pearlitic ductile irons are given inTable 13. Values for highly alloyed ductile
irons may be considerably different from those given. The coefficient of linear thermal expansion for
austempered ductile iron is related to the proportion of ferrite and retained austenite in the austempered ductile
iron.
The thermal conductivity of ferritic ductile iron is about 36 W/m · K (250 Btu · in./ft2 · h · °F) over the
temperature range of 20 to 500 °C (70 to 930 °F). There is a slight, negative temperature dependence for
thermal conductivity, causing it to drop with increasing temperature. Graphite shape and alloy content have
greater influences on thermal conductivity than does temperature; for instance, increasing the nickel plus silicon
content of ferritic ductile iron reduces thermal conductivity.
Electrical and Thermal Relationship. The electrical and thermal conductivities of cast iron, like those of
many metals, are related to each other in accordance with the Wiedemann-Franz Law, which ascribes both
properties to the mobility of free electrons. The shape of the graphite particles in cast iron greatly effects this
relationship. The thermal conductivity of gray iron containing well-developed flake
graphite is much higher than that of steel, and the electrical conductivity is much lower. As the shape of the
graphite changes from flake to intermediate forms to fully spherical shapes, the difference between the thermal
or electrical conductivity of the cast iron and that of steel becomes less. Accordingly, ductile irons have higher
electrical conductivity and lower thermal conductivity than gray irons.
Electrical Resistivity. The reciprocal function of the electrical conductivity, or electrical resistivity, of ductile
iron increases with temperature. For instance, irons with a specific resistance of 0.5 to 0.55 μΩ· m at room
temperature might have a resistance of 1.25 to 1.30 μΩ· m at 650 °C (1200 °F). Increasing the amount of silicon
in either a pearlite or ferritic matrix also increases electrical resistivity . Increasing the amount of
graphite tends to increase electrical resistivity because graphite has a high resistance, but the graphitization of
pearlite or cementite to produce a ferritic matrix often results in a net decrease in resistance because of the
lower electrical resistivity of ferrite. Alloying elements that dissolve in the matrix normally increase resistance,
but reversals of this tendency may be observed when the relative amounts of various microconstituents are
changed by the presence of the alloying element.
Magnetic properties should be determined separately for each application in which they are important.
Although the properties of cast irons do not approach those of permanent magnet alloys on the one hand, or of
silicon steels on the other, cast irons often are used for parts that require known magnetic properties. Cast iron
can be cast into intricate shapes and sections more easily than most permanent alloys. The magnetic properties
of cast irons depend largely on structure; the influence of alloying elements is indirect, derived mainly from the
influence exerted on structure. A ferritic structure exhibits low hysteresis loss and high permeability, whereas a
pearlitic structure exhibits high hysteresis loss and low permeability. Free cementite produces an iron with low
permeability, magnetic induction, and remanence, together with high coercive force and hysteresis loss.
Spheroidal graphite slightly increases hysteresis loss in pearlitic irons, but considerably reduces hysteresis loss
in ferritic irons. Low-phosphorus ferritic ductile iron that is essentially free of cementite and very low in
combined carbon has the highest permeability and lowest hysteresis loss of all cast irons.
In alloyed ductile irons, copper and nickel decrease permeability and increase hysteresis loss. When nickel is
present in sufficient quantities to produce an austenitic matrix, ductile iron becomes paramagnetic. Manganese
and chromium reduce magnetic induction, permeability, and remanent magnetism and increase hysteresis loss.
Silicon has little effect on magnetic properties in pearlitic ductile iron, but slightly increases maximum
permeability and reduces hysteresis loss in ferritic ductile iron.
References cited in this section
14. C.F. Walton, Ed., Gray and Ductile Iron Castings Handbook, Gray and Ductile Founders' Society, 1971
18. H.T. Angus, Cast Iron: Physical and Engineering Properties, 2nd ed., Butterworths, 1976
Ductile Iron
Revised by Lyle R. Jenkins, Ductile Iron Society; and R.D. Forrest, Pechiney Electrometallurgie, Division Fonderie
Machinability
Ductile iron has been chosen in many instances on the basis of significantly lower machining costs, which
resulted in lower overall cost of the part. Ductile iron has approximately the same machinability as gray iron of
similar hardness. At low hardnesses (ductile iron with tensile strength up to about 550 MPa, or 80 ksi, the
machinability of ductile iron is better than that of cast mild steel. At higher hardnesses, the difference in
machinability between ductile iron and cast steel is less pronounced.
Experience has shown that when converting from malleable iron or pearlitic malleable iron castings to ductile
iron, surface speeds should be increased by about 20%. This allows the chip to roll faster and break easier,
improving tool life and thereby providing an increase in productivity. The higher elongation of ductile iron
causes the chips to roll and produce more abrasion on the tool. This added abrasion is reduced if the chip breaks
instead of rolling. Also, as-cast ductile iron does not have the surface decarburized layer normally found in
malleable irons that decreases tool life.
Work hardening is produced when machining ductile iron, making it important to avoid light cuts. Coolants
improve high-speed tool life, but are not as effective for carbide tool life until high-surface speeds are used.
A comparison can be made between the machinability of ASTM class 40 gray iron and that of ASTM grade 60-
40-18 ductile iron. The gray iron has a tensile strength of about 275 MPa (40 ksi) and a hardness of 190 to 220
HBN. The ductile irons have a yield strength of about 275 MPa (40 ksi) and a hardness of about 187 HBN. The
higher silicon content of ductile iron compared to malleable iron or gray cast iron causes the ferrite to be harder.
The recommended cutting speed for rough turning using a single-point high speed steel tool at a feed of about
0.38 mm/rev (0.015 in./rev) is 20 m/min (70 sfm) for ASTM class 40 gray iron and 43 m/min (140 sfm) for
ASTM grade 60-40-18 ductile iron. For a similar rough turning operation, cast mild steel with a tensile strength
of about 415 to 485 MPa (60 to 70 ksi) requires a cutting speed of 33 m/min (110 sfm). Replacing the high
speed steel tool with one made of sintered carbides allows increases in cutting speed for all three materials, but
does not change the ratios of cutting speeds among the three materials. For ductile iron, the cutting speed, using
carbide tools dry, increases to about 82 m/min (270 sfm).
Example 1: Comparison of the Machinability of Ferritic Ductile Iron With That of
Selected Grades of Gray Cast Iron.
For this test program, test castings representing a range of cooling conditions were produced (see Fig. 33). The
five sections were then cut apart and centered, and the surfaces were turned off to depths of about 1.6 to 3.2 mm
( 1
16
to 1
8
in.). The resulting diameters were 95, 70, 60, and 45 mm (3.75, 2.75, 2.37, and 1.75 in.). Test turning
was done by making successive 0.508 mm (0.200 in.) cuts at a cutting speed of 76 m/min (250 sfm). Three cuts
were made and timed for each test section.
Pearlitic Ductile Irons Compared With Pearlitic Gray Irons. The test described in Example 1 compared
annealed ferritic ductile iron with pearlitic gray iron of slightly higher hardness. Other data comparing pearlitic
gray irons with pearlitic ductile irons show no great machinability differences when the irons are of similar
hardness. The comparison of tool life in the machining of ductile and gray irons is shown in Fig. 33(b).
Ductile Irons Compared With Malleable Cast Irons. In another investigation, a comparison was made
between ductile iron castings and malleable iron castings. The machining operations included facing, boring,
threading, and drilling. There was no significant difference in machining costs between these two materials
when the work was done on a production basis.
References cited in this section
19. Machining Data Handbook, Vol 1, 3rd ed., Metcut Research Associates, 1980
20. "Machinability Report," U.S. Air Force, 1950
Ductile Iron
Revised by Lyle R. Jenkins, Ductile Iron Society; and R.D. Forrest, Pechiney Electrometallurgie, Division Fonderie
Welding
Special materials and techniques are available for the repair welding of ductile iron castings, or for joining
ductile iron to itself or to other ferrous materials such as steel, gray iron, or malleable iron. Like the welding of
other cast irons, the welding of ductile iron requires special precautions to obtain optimum properties in the
weld metal and adjacent heat-affected zone. The main objective is to avoid the formation of cementite in the
matrix material, which makes the welded region brittle; but in ductile iron an additional objective, that of
retaining a nodular form of graphite, is of almost equal importance. The formation of martensite or fine pearlite
can be removed by tempering.
A technique developed and patented by Oil City Iron Works uses a special ductile iron filler metal and a special
welding flux that is introduced through a powder spray-type oxyacetylene welding torch. In this technique,
which is used predominantly for the cosmetic repair of ductile iron castings, parent metal is puddled under a
neutral region of slightly reducing flame, and filler metal is added as the special flux is sprayed into the puddle
through the torch. A properly executed weld will be essentially free of eutectic carbides in the weld metal and
will have a pearlitic matrix with bull's-eye ferrite surrounding the particles of spheroidal graphite.
Typically, the mechanical properties of the weld metal are very similar to those of the parent metal for all heattreated
conditions: as-welded, annealed, normalized, or quenched and tempered. The only possible exception is
that the ductility of the weld metal may be slightly lower than that of equivalent parent metal at the lower
hardnesses, such as those typical of the as-welded or annealed conditions.
Among other advantages, this process obtains a perfect color match with the cast metal; yields weld metal with
composition, microstructure, and properties very close to those of the original casting; and minimizes the
transition zone, the heat-affected zone, and any residual stresses.
As an alternative to the Oil City process, ductile iron can be welded with a high-nickel alloy, using the fluxcored
arc welding (FCAW) process. In flux-cored arc welding, a hollow wire with the composition 50Ni-44Fe-
4.25Mn-1.0C-0.6Si and containing a special flux is used as the electrode in standard FCAW equipment. This
method is used to join ductile iron to itself or to steel or other types of cast iron more often than it is used to
effect the cosmetic repair of castings. The mechanical properties of the high-nickel weld metal and of the
adjacent heat-affected zone are usually equivalent to the properties of ASTM grade 65-45-12 ductile iron. A
major disadvantage of welding with the high-nickel alloy is that it does not respond to heat treatment, and thus
weldments made with this alloy cannot be heat treated to obtain uniformly high strength levels, as can
weldments made using the Oil City process.
Low-temperature welding rods and wire that have high wetting properties on cast iron base metals, are
available. This effects the joining of metals at such low temperatures that the base metal does not melt. The
composition of the weld metal is such that it has dimensional changes with temperature similar to those of
ductile iron, thereby reducing stresses. The color match is perfect, the hardness is low, the tensile strength is
greater than 390 MPa (57 ksi), and the elongation is 25 to 30%. The weld metal is suitable for tungsten inert gas
welding operations. The welding rods or wire are produced by Shichiho Metal Industrial Company Ltd.
Other methods for welding ductile iron include submerged arc welding and the use of austenitic consumable
materials (see Ref 21, 22, 23 and the article "Welding of Cast Irons," in Welding, Volume 6 of ASM Handbook,
formerly 9th Edition Metals Handbook.
References cited in this section
21. D.L. Olson, "Investigation of the MnO-SiO2-Oxides and MnO-SiO2-Fluorides Welding Flux Systems,"
DAAG29-77-G-0097, U.S. Army Research Office, June 1978
22. M.A. Davila, D.L. Olson, and T.A. Freese, Submerged Arc Welding of Ductile Iron, Trans. AFS, 1977
23. M.A. Davila and D.L. Olson, The Development of Austenitic Filler Materials for Welding Ductile Iron,
Paper 23, Welding Institute Reprint, Welding Institute, 1978
Ductile Iron
Revised by Lyle R. Jenkins, Ductile Iron Society; and R.D. Forrest, Pechiney Electrometallurgie, Division Fonderie
References
1. R.B. Gundlach and J.F. Janowak, Approaching Austempered Ductile Iron Properties by Controlled
Cooling in the Foundry, in Proceedings of the First International Conference on Austempered Ductile
Iron: Your Means to Improved Performance, Productivity, and Cost, American Society for Metals, 1984
2. Lyle Jenkins, Ductile Iron--An Engineering Asset, in Proceedings of the First International Conference on
Austempered Ductile Iron: Your Means to Improved Performance, Productivity, and Cost, American
Society for Metals, 1984
3. R.B. Gundlach and J.F. Janowak, A Review of Austempered Ductile Iron Metallurgy, in Proceedings of
the First International Conference on Austempered Ductile Iron: Your Means to Improved Performance,
Productivity, and Cost, American Society for Metals, 1984
4. P.A. Blackmore and R.A. Harding, The Effects of Metallurgical Process Variables on the Properties of
Austempered Ductile Irons, in Proceedings of the Fist International Conference on Austempered Ductile
Irons: Your Means to Improved Performance, Productivity, and Cost, American Society for Metals, 1984
5. C.R. Loper, P. Banerjee, and R.W. Heine, Risering Requirements for Ductile Iron Castings in Greensand
Moulds, Gray Iron News, May 1964, p 5-16
6. "The Fatigue Life of Cast Surface of Malleable and Nodular Iron," Bulletin 177, Metals Research and
Development Foundation
7. D.L. Sponseller, W.G. Scholz, and D.F. Rundle, Development of Low-Alloy Ductile Irons for Service at
1200-1500 F, AFS Trans., Vol 76, 1968, p 353-368
8. W.S. Pellini, G. Sandoz, and H.F. Bishop, Notch Ductility of Nodular Irons, Trans. ASM, Vol 46, 1954, p
418-445
9. C. Vishnevsky and J.F. Wallace, The Effect of Heat Treatment on the Impact Properties of Ductile Iron,
Gray Iron News, July 1962, p 5-10
10. R.K. Nanstad, F.J. Worzala, and C.R. Loper, Jr., Static and Dynamic Toughness of Ductile Cast Iron, AFS
Trans., Vol 83, 1975
11. G.N.J. Gilbert, Tensile and Fatigue Tests on Normalized Pearlitic Nodular Irons, J. Res., Vol 6 (No. 10),
Feb 1957, p 498-504
12. R.C. Haverstraw and J.F. Wallace, Fatigue Properties of Ductile Iron, Gray Ductile Iron News, Aug 1966,
p 5-19
13. H.D. Merchant and M.H. Moulton, Hot Hardness and Structure of Cast Irons, Br. Foundryman, Vol 57
(Part 2), Feb 1964, p 62-73
14. C.F. Walton, Ed., Gray and Ductile Iron Castings Handbook, Gray and Ductile Founders' Society, 1971
15. C.R. Wilks, N.A. Matthews, and R.W. Kraft, Jr., Elevated Temperature Properties of Ductile Cast Irons,
Trans. ASM, Vol 47, 1954
16. F.B. Foley, Mechanical Properties at Elevated Temperatures of Ductile Cast Iron, Trans. ASME, Vol 78,
1956, p 1435-1438
17. C.C. Reynolds, W.T. Whittington, and H.F. Taylor, Hardenability of Ductile Iron, AFS Trans., Vol 63,
1955, p 116-122
18. H.T. Angus, Cast Iron: Physical and Engineering Properties, 2nd ed., Butterworths, 1976
19. Machining Data Handbook, Vol 1, 3rd ed., Metcut Research Associates, 1980
20. "Machinability Report," U.S. Air Force, 1950
21. D.L. Olson, "Investigation of the MnO-SiO2-Oxides and MnO-SiO2-Fluorides Welding Flux Systems,"
DAAG29-77-G-0097, U.S. Army Research Office, June 1978
22. M.A. Davila, D.L. Olson, and T.A. Freese, Submerged Arc Welding of Ductile Iron, Trans. AFS, 1977
23. M.A. Davila and D.L. Olson, The Development of Austenitic Filler Materials for Welding Ductile Iron,
Paper 23, Welding Institute Reprint, Welding Institute, 1978
Avatar utente
Aldebaran
 
Messaggi: 5402
Images: 4925
Iscritto il: 31/03/2010, 19:26
Località: Milano - Chieti

Torna a Iron & Steel

Chi c’è in linea

Visitano il forum: Nessuno e 1 ospite