Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Part 2 pptx

The microstructure of cast iron, and especially graphite morphology, greatly influence thermal conductivity, as implied by the data shown in Table 8.. Typical values for the thermal cond

Trang 1

Fig 8 Effect of carbon equivalent on the tensile strength of flake, compacted, and spheroidal graphite irons

cast into 30 mm (1.2 in.) diam bars Source: Ref 10

Although increasing the silicon content decreases the pearlite to ferrite ratio in the as-cast state, both the strength and hardness of as-cast and annealed CG irons improve This is because of the hardening of ferrite by silicon For the same reasons, elongation in the annealed condition decreases, but increases for the as-cast state (Ref 10) Although increasing the phosphorus content slightly improves strength, a maximum of 0.04% P is desirable to avoid lower ductility and impact strength

The pearlite/ferrite ratio, and thus the strength and hardness of CG irons, can be increased by the use of a number of alloying elements such as copper, nickel, molybdenum, tin, manganese, arsenic, vanadium, and aluminum (Ref 6, 14) The effect of copper and molybdenum on the tensile properties of CG irons is shown in Fig 9 After annealing to a fully ferritic structure, it is possible to increase the yield point of CG iron by 24% when using 1.5% Ni (Table 3) This is because of the strengthening of the solid solution by nickel (Ref 6) The reader is cautioned, however, that additions of copper, nickel, and molybdenum may increase nodularity (Ref 6)

Table 3 Effect of heat treatment and alloying with nickel on the tensile properties of CG iron measured on a

Trang 2

(a) F, ferrite; P, pearlite

(b) Annealed, 2 h at 900 °C (1650 °F), cooled in furnace to 690 °C (1275 °F), held 12 h, cooled in air

(c) Normalized, 2 h at 900 °C (1650 °F), cooled in air

Fig 9 Effect of (a) copper and (b) molybdenum on the tensile properties of CG iron Source: Ref 13, 14

In order to compare the quality of different types of irons, several quality indexes can be used, such as the product of tensile strength and elongation (TS × El) or the ratio of tensile strength to Brinell hardness (TS/HB) Higher values of these indexes will characterize a better iron Using the data given in Ref 10, some typical values were calculated for these indexes for unalloyed CG irons Figure 10 compares the TS × El product and the TS/HB ratio for unalloyed and aluminum-alloyed CG irons It can be seen that when 2% Si is replaced by 2% Al, a much better quality CG iron is produced (Ref 11)

Trang 3

Fig 10 Quality indexes for Fe-C-Si and Fe-C-Al irons TS, tensile strength; El, elongation Source: Ref 11

Effect of Structure One of the most important variables influencing the tensile properties of CG irons is

nodularity As nodularity increases, higher strength and elongation are to be expected, as shown in Table 4 and Fig 11, although nodularity must be maintained at levels under 20% for the iron to qualify as CG iron However, spheroidal graphite contents of up to 30% and even more must be expected in th in sections of castings with considerable variation in wall thickness

Table 4 Properties of CG iron as a function of nodularity

Tensile strength Nodularity, %

Trang 4

Fig 11 Correlation between nodularity and elongation for ferritic CG iron Source: Ref 16

As previously discussed, the pearlite/ferrite ratio can be increased by causing alloying elements Another way

of increasing or decreasing this ratio is by using heat treatment The influence of heat treatment on mechanical properties of CG irons with 20% nodularity is given in Table 3

Effect of Section Size Like all other irons, CG irons are rather sensitive to the influence of cooling rate, that

is, to section size, because it affects both the pearlite/ferrite ratio and graphite morphology As mentioned before, a higher cooling rate promotes more pearlite and increased nodularity A typical example of the influence of section size on the microstructure of CG iron is provided in Fig 12 Although CG iron is less section sensitive than FG iron (as shown in Fig 13 for tensile strength), the influence of cooling rate may be

quite significant (see the article "Compacted Graphite Irons" in Casting, Volume 15 of ASM Handbook, formerly 9th Edition Metals Handbook) When the section size decreases until it is below 10 mm (0.4 in.), the

tendency to increased nodularity and for higher chilling must be considered This is particularly true for overtreated irons While it is possible to eliminate the carbides that result from chilling by heat treatment, it is impossible to change the graphite shape, which remains spheroidal, with the associated consequences Other factors influencing the cooling of castings, such as shakeout temperature, can also influence properties

Trang 5

Fig 12 Influence of section size on the microstructure of CG iron produced by inmold Samples from series

E3.3 (a) 3.2 mm (0.125 in.) (b) 6.4 mm (0.250 in.) (c) 12.7 mm (0.500 in.) (d) 25.4 mm (1.000 in.).(e) 50.8 mm (2.000 in.) 100×

Trang 6

Fig 13 Influence of section size on the tensile strength of CG iron Source: Ref 15

Compressive Properties. The stress-strain diagram for compression and tensile tests of CG iron is shown in Fig 14 It can be seen that an elastic behavior occurs up to a compression stress of 200 MPa (30 ksi) Some compressive properties of the 179 HB as-cast ferritic CG iron in Table 2 are compared with those of SG iron in Table 5 It can be seen that the 0.1% proof stress in compression for CG iron is 76 MPa (11 ksi) higher than the 0.1% proof stress in tension, while for SG iron the difference is only 23 MPa (3.3 ksi) Compressive strengths

up to 1400 MPa (203 ksi) have been reported for ferritic annealed CG irons (Ref 8)

Table 5 Comparison of tensile and compressive properties of CG and SG irons

Tensile strength, MPa (ksi) 380 (55) 370 (54) 420 (61)

0.1% proof stress, MPa (ksi) 246 (35.7) 224 (32.5) 261 (37.8)

0.2% proof stress, MPa (ksi) 350 (50) 250 (36.3) 287 (41.6)

Source: Ref 6

Trang 7

Fig 14 Stress-strain curves in compression and tension for CG iron with 4.35 carbon equivalent Source: Ref 9

Shear Properties. For a pearlitic CG iron, the shear strength on 20 mm (0.8 in.) diam specimens was measured at 365 MPa (53 ksi), with a shear-to-tensile strength ratio of 0.97 (Ref 17) Ratios of 0.90 for SG iron and of 1.1 to 1.2 for FG iron have been reported Materials exhibiting some ductility have ratios lower than 1.0 (Ref 8)

Modulus of Elasticity. As is evident from Fig 5 and 14, CG irons exhibit a clear zone of proportionality, both

in tension and in compression Typical values for both static and dynamic (resonance frequency method) measurements are given in Table 2 Dynamic tests give slightly higher numbers In general, the moduli of elasticity for CG iron are similar to those of high-strength FG irons and can even be higher as nodularity increases

The elasticity modulus measured by the tangent method depends on the level of stress, as shown in Fig 15 A comparison of the stress dependency of the elasticity modulus for different types of cast irons is shown in Fig

16 Poisson's ratios of 0.27 to 0.28 have been reported for CG irons (Ref 8)

Trang 8

Fig 15 Stress dependency by E-modulus for two heat-treated CG irons Source: Ref 6

Fig 16 Influence of stress level on the elasticity modulus (a) in tension and (b) in compression for pearlitic FG,

CG, and SG irons Source: Ref 6

Impact Properties. While SG iron exhibits substantially greater toughness at low pearlite contents, pearlitic

CG irons have impact strengths equivalent to those of SG irons (Fig 17) Charpy impact energy measurements

at 21 °C (70 °F) and -41 °C (-42 °F) showed that CG irons produced from an SG-base iron absorbed greater energy than those made from gray iron-base iron (Ref 10) This is attributed to the solute hardening effects of tramp elements in the gray iron

Trang 9

Fig 17 Effect of pearlite content on the 21 °C (70 °F) Charpy V-notch impact strength of as-cast CG irons

compared to that of SG iron Source: Ref 10

The results from dynamic tear tests were similar, although greater temperature dependence was observed A comparison of the dynamic tear energies of CG cast irons is presented in Fig 18 It is noted that significant differences in the values obtained occur in the ferritic condition, but that equivalent values are obtained when the matrix structure is primarily pearlitic

Trang 10

Fig 18 Dynamic tear energy versus temperature for (a) CG and (b) SG irons Source: Ref 10

Studies on crack initiation and growth under impact loading conditions showed that, in general, the initiation of matrix cracking was preceded by graphite fracture at the graphite-matrix interface, or through the graphite, or both The most dominant form of graphite fracture appeared to be that occurring along the boundaries between graphite crystallites (Ref 18) Matrix cracks were usually initiated in the ferrite by transgranular cleavage (graphite was nearly always surrounded by ferrite), although in some instances intergranular ferrite fracture appeared to be the initiating mechanism Matrix crack propagation generally occurred by a brittle cleavage mechanism, transgranular in ferrite, and interlamellar in pearlite In general, the impact resistance of CG irons increases with carbon equivalent and decreases with phosphorous or increasing pearlite

As may be seen in Table 6, cerium-treated CG irons seem to exhibit a higher impact energy than titanium-treated irons It is thought that this may be attributed to TiC and TiCN inclusions present in the matrix

magnesium-of magnesium-titanium-treated CG irons (Ref 6)

Trang 11

Table 6 Impact toughness of a cerium-treated CG cast iron and two magnesium-titanium-treated CG cast irons

Test temperature

Impact bend toughness (a)

Notched- bar(b)impact

(a) Unnotched 10 × 10 mm (Charpy) testpiece

(b) V-notched 10 × 10 mm (Charpy) testpiece

Fatigue Strength. Because the notching effect of graphite in CG iron is considerably lower than that in FG irons, it is expected that CG iron will have higher fatigue strengths than FG iron (Table 1) Table 7 lists the fatigue strengths of five CG irons from Table 2 The as-cast ferritic (>95% ferrite) CG with a hardness of 150

HB had the highest fatigue strength and the highest fatigue-endurance ratio (fatigue strength/tensile strength) Fatigue properties for three of these CG irons with comparable endurance ratios are shown in Fig 19 It is evident that pearlitic structures, higher nodularity, and unnotched samples resulted in better fatigue strength The fatigue-endurance ratio was 0.46 for a ferritic matrix, 0.45 for a pearlitic matrix, and 0.44 for a pearlitic higher-nodularity CG iron (Ref 17) With fatigue notch factors (ratio of unnotched to notched fatigue strength)

of 1.71 to 1.79, CG iron is almost as notch sensitive as SG iron (>1.85) Gray iron is considerably less notch sensitive, with a notch factor of less than 1.5 (Ref 6)

Table 7 Fatigue strengths and endurance ratios for five CG irons from rotating bending tests

Matrix structure Graphite type Tensile

strength

Fatigue strength

Fatigue- endurance ratio

Hardness,

HB

Trang 12

MPa ksi MPa ksi

Trang 13

Fig 19 Fatigue curves in rotating bending tests for ferritic, pearlitic, and higher nodularity CG irons from Table

7 Source: Ref 17

Statistical analysis of a number of experimental data allowed the calculation of a relationship between fatigue strength (FS) and tensile strength (TS) of CG irons (Ref 6):

FS (in MPa) = (0.63 - 0.00041 · TS)

Values calculated with this equation fit well between those of FG and SG irons The intermediate position of

CG irons from this standpoint is also shown in Fig 20

Trang 14

Fig 20 Ratio of alternating bend fatigue strength/tensile strength of FG, CG, and SG irons Source: Ref 6

A 36% decrease of the alternating bending fatigue strength was observed on unnotched bars with casting skin compared to machined bars This compares with a 50% reduction for comparable strength FG iron and a 32% reduction for ferritic SG iron

References cited in this section

1 E Nechtelberger, H Puhr, J.B von Nesselrode, and A Nakayasu, Paper 1 presented at the 49th International Foundry Congress, International Committee of Foundry Technical Associations, Chicago,

1982

6 E Nechtelberger, The Properties of Cast Iron up to 500 °C, Technicopy Ltd., 1980

7 J Sissener, W Thury, R Hummer, and E Nechtelberger, AFS Cast Met Res J., 1972, p 178

8 C.F Walton and T.J Opar, Ed., Iron Castings Handbook, Iron Casting Society Inc., 1981

9 G.F Sergeant and E.R Evans, The British Foundryman, May 1978, p 115

10 K.P Cooper and C.R Loper, Jr., Trans AFS, Vol 86, 1978, p 241

11 F Martinez and D.M Stefanescu, Trans AFS, Vol 91, 1983, p 593

13 J Fowler, D.M Stefanescu, and T Prucha, Trans AFS, Vol 92, 1984, p 361

14 R.B Gundlach, Trans AFS, Vol 86, 1978, p 551

15 Spravotchnik po Tchugunomu Ljitiu (Cast Iron Handbook), 3rd ed., Mashinostrojenie, 1978

16 K.H Riemer, Giesserei, Vol 63 (No 10), 1976, p 285

17 K.B Palmer, BCIRA J., Report 1213, Jan 1976, p 31

18 A.F Heiber, Trans AFS, Vol 87, 1979, p 569

Compacted Graphite Iron

Doru M Stefanescu, The University of Alabama

Elevated-Temperature Properties

Tensile Properties. The variation of tensile properties with temperature for CG iron produced with mischmetal treatment alloys is similar to that typical for SG iron (Fig 21), but the values are somewhat lower (Ref 19) Similar results are reported for CG irons produced with Mg-Ti-ferrosilicon alloys shown in Fig 22

cerium-As expected, a slight increase in nodularity led to higher tensile strength values at all temperatures

Trang 15

Fig 21 Variation of tensile properties of Ce-treated CG and SG irons with temperature Source: Ref 19

Fig 22 Variation of tensile properties of Mg+Ti-treated CG irons Source: Ref 20

Growth and Scaling. Tests conducted for 32 weeks in air have shown that at 500 °C (930 °F) the growth and scaling of CG iron was not significantly different from that exhibited by FG irons of similar composition However, at 600 °C (1110 °F), the growth of CG irons was less than that of FG iron, and scaling resistance was superior (Fig 23)

Trang 16

Fig 23 Scaling and growth of heavy section flake and compacted graphite cast irons at 600 °C (1110 °F)

Source: Ref 9

In other oxidation studies of cast irons conducted at 600 °C (1110 °F), it was concluded that weight gains due to oxidation are 10 to 15% higher for CG irons than for SG irons, but 30 to 60% lower for CG irons than for FG irons (Ref 21)

Thermal Fatigue. When castings are used in an environment where frequent changes in temperature occur, or where temperature differences are imposed on a part, thermal stresses occur in castings and may result in elastic and plastic strains and finally in crack formation The casting can thus be destroyed as a result of thermal fatigue Changes in microstructure, associated with stress-including volume changes, as well as surface and internal oxidation, may also be associated with temperature difference induced stresses

The interpretation of thermal fatigue tests is complicated by the many different test methods employed by various investigators The two widely accepted methods are constrained thermal fatigue and finned-disk thermal shock tests (Ref 22, 23)

In the constrained thermal fatigue test, a specimen (see Fig 24 (a) for dimensions) is mounted between two stationary plates that are held rigid by two columns, heated by high frequency (450 kHz) induction current, and cooled by conduction of heat to water-cooled grips (Fig 24(b)) The thermal stress that develops in the test specimen is monitored by a load cell installed in one of the grips holding the specimen During thermal cycling, compressive stresses develop upon heating, and tensile stresses develop upon cooling As thermal cycling continues, the specimen accumulates fatigue damage in a fashion similar to that in mechanical fatigue testing; ultimately, the specimen fails by fatigue Initially the specimen develops compressive stress upon heating due

to constrained thermal expansion (Fig 25) Some yielding and stress relaxation occur during holding at 540 °C (1000 °F), and upon subsequent cooling the specimen develops residual tensile stress During subsequent

Trang 17

thermal cycling, the maximum compressive stress that has developed upon heating decreases continuously, and the maximum tensile stress upon cooling increases, as shown for six different irons in Fig 26

Fig 24 (a) Dimensions of constrained fatigue test specimen (b) Schematic of apparatus for constrained

fatigue tests Dimensions given in millimeters Source: Ref 23

Fig 25 Typical thermal stress cycles at the beginning of the test for FG and CG irons Source: Ref 23

Trang 18

Fig 26 The shift in thermal stress versus the number of cycles for six irons cycled between 100 and 540 °C

(212 and 1000 °F) Source: Ref 23

Experimental results (Fig 27) point to higher thermal fatigue for CG iron than for FG iron and also indicate the beneficial effect of molybdenum In fact, regression analysis of experimental results indicates that the main factors influencing thermal fatigue are tensile strength (TS) and molybdenum content:

where N is the number of thermal cycles to failure, tensile strength is in kps per square inch (ksi), and

molybdenum is in percent

Trang 19

Fig 27 Results of constrained thermal fatigue tests conducted between 100 and 540 °C (212 and 1000 °F)

Source: Ref 23

In the finned-disk thermal shock test, the specimen (see Fig 28a for dimensions) is cycled between a temperature environment and a high-temperature environment, which causes thermal expansion and contraction The thermal shock test apparatus is shown in Fig 28(b) Because in this type of test thermal conductivity plays a significant role, FG iron showed much greater resistance to cracking than did CG iron Major cracking occurred in less than 200 cycles in all CG iron specimens, while the unalloyed FG iron developed minor cracking after 500 cycles and major cracking after 775 cycles The alloyed FG iron, because

moderate-of its higher elevated-temperature strength, did not show any sign moderate-of cracking even after 2000 cycles (Ref 23) The CG iron containing more ferrite had a slightly better thermal fatigue resistance than the CG iron with less ferrite

Trang 20

Fig 28 (a) Dimensions (in millimeters) of finned-disk specimen (b) Schematic of apparatus for finned-disk

thermal shock test Source: Ref 23

In general, for good resistance to thermal fatigue, cast irons must have high thermal conductivity; low modulus

of elasticity; high strength at room and elevated temperatures; and, for use above 500 to 550 °C (930 to 1020

°F), resistance to oxidation and structural change The relative ranking of irons varies with test conditions When high cooling rates are encountered, experimental data and commercial experience show that thermal conductivity and a low modulus of elasticity are most important Consequently, gray irons of high carbon content (3.6 to 4%) are superior (Ref 22, 23) When intermediate cooling rates exist, ferritic SG and CG irons have the highest resistance to cracking, but are subject to distortion When low cooling rates exist, high-strength pearlitic SG irons or SG irons alloyed with silicon and molybdenum are best with regard to cracking and distortion (Fig 29)

Trang 21

Fig 29 Results of thermal fatigue tests on various cast irons; specimens cycled between 650 and 20 °C (1200

and 70 °F) Source: Ref 22

A rather detailed analysis of the behavior of various irons at elevated temperatures is given in Ref 6 Extensive experimental work on cylinder heads is reviewed A critical analysis of most of the accepted criteria for assessing the quality of irons for castings used at elevated temperatures is also included

Trang 22

19 K Hutterbraucker, O Vohringer, and E Macherauch, Giessereiforschung, No 2, 1978, p 39

20 D.M Stefanescu and G Niculescu, unpublished research

21 I Riposan, M Chisamera, and L Sofroni, Trans AFS, Vol 93, 1985, p 35

22 K Roehrig, Trans AFS, Vol 86, 1978, p 75

23 Y.J Park, R.B Gundlach, R.G Thomas, and J.F Janowak, Trans AFS, Vol 93, 1985, p 415

Physical Properties

Thermal conductivity plays a significant role in structural components subjected to thermal stress The higher the thermal conductivity, the lower the thermal gradients throughout the casting, and therefore the lower the thermal stresses The microstructure of cast iron, and especially graphite morphology, greatly influence thermal conductivity, as implied by the data shown in Table 8 Graphite exhibits the highest thermal conductivity of all the metallographic constituents The conductivity of graphite parallel to the basal plane is about four times higher than that perpendicular to its basal plane (Ref 24) Consequently, FG has higher thermal conductivity than SG (Fig 30) It is therefore expected that FG iron will have higher thermal conductivity than SG iron, which in turn will be better than that of steel, as shown in Fig 31 (Ref 26) Not unexpectedly, as the amount of graphite increases, thermal conductivity is also improved

Table 8 Thermal conductivity of structural constituents in iron-base alloys

Thermal conductivity, W/(cm · K) Structural

constituents

0-100 °C (32-212 °F)

500 °C (930 °F)

1000 °C

(1830 °F)

Parallel to basal plane 2.93-4.19 0.84-1.26 0.42-0.63

Perpendicular to basal plane 0.2

Trang 23

Cementite 0.071-0.084

Source: Ref 1

Fig 30 Mechanism of heat conduction in various Fe-C alloys Source: Ref 25

Fig 31 Influence of graphite shape on the relative thermal and electrical conductivities of Fe-C alloys Source:

Ref 26

Trang 24

The thermal conductivity of ferrite is reduced by dissolved alloying elements For steel, the conductivity of the matrix can be calculated by the equation (Ref 6):

where λis the thermal conductivity of alloyed steel, λ0 is the thermal conductivity of unalloyed steel, and ΣC is the sum of alloying elements in % This equation can also be used to estimate the influence of various alloying additions on the conductivity of cast irons

Typical values for the thermal conductivity of CG iron at room temperature are given in Table 2, and various cast irons are compared in Table 9 From this last table it can be seen that the thermal conductivity of CG iron

is very close to that of gray cast iron and considerably higher than that of SG iron (Ref 1, 9) This behavior is explained by the fact that much like flake graphite, compacted graphite is interconnected As for FG irons, increasing the carbon equivalent results in higher thermal conductivity for CG iron As the temperature is increased, the thermal conductivity reaches a maximum at about 200 °C (390 °F), an effect also shown by SG irons, but not by FG iron (Fig 32) The thermal conductivity of a typical CG iron mold is compared in Fig 33 with results for an ingot mold and bottom plate made from FG iron and a sample of ferritic SG iron (Ref 27)

As previously implied, increased nodularity results in lower thermal conductivity (Ref 1, 28)

Table 9 Thermal conductivities of FG, CG, and SG irons at various temperatures

Thermal conductivity, W/m · K (Btu/ft · h · °F) Graphite shape Carbon

Trang 25

Fig 32 Thermal conductivities of various cast irons Source: Ref 1

Fig 33 Thermal conductivities of different materials for ingot molds Source: Ref 27

Trang 26

Thermal Expansion. For irons of similar chemical composition, there seems to be no difference in expansion regardless of graphite shape (Ref 28) However, when different compositions are used in order for an iron to fall in a typical range for a given type of cast iron, the linear expansion of CG iron is between that of SG and FG irons (Fig 34) (Ref 1)

total-Fig 34 Linear thermal expansion of various cast irons Source: Ref 1

Sonic and Ultrasonic Properties. Resonant frequency (sonic testing) and ultrasonic velocity measurements provide reliable methods for verifying the structure and properties of castings As shown in Fig 35, ultrasonic velocity is directly related to nodularity Unfortunately it is rather difficult to distinguish between CG and low-nodularity SG irons Better results seem to be obtained when ultrasonic velocity is related to tensile strength Figure 36 shows the correlation between tensile strength and ultrasonic velocity or resonant frequency for test bars of 30 mm (1.2 in.) diameter

Trang 27

Fig 35 Correlation between ultrasonic velocity and nodularity Source: Ref 13

Trang 29

Fig 36 Tensile strength related to (a) ultrasonic velocity and (b) resonant frequency for cast irons of varying

graphite structures Source: Ref 9

When these tests are applied to castings, the ultrasonic velocity for CG structures is independent of the shape of the casting, but should be calibrated for the section thickness Thus, for 30 mm (1.2 in.) diam bars, the range associated with CG is between 5.2 and 5.45 km/s, but for very large castings such as ingot molds, the ultrasonic velocity for good CG structures lies between 4.85 and 5.10 km/s Sonic testing, on the other hand, must be calibrated for a particular design of casting, for which examples of satisfactory and unsatisfactory structures must be previously checked to provide a calibration range (Ref 9)

1982

24 E Mayer-Rassler, Giesserei, Vol 54 (No 13), 1967, p 348

25 H Kempers, Giesserei, Vol 53 (No 1), 1966, p 15

26 K Lohberg and J Motz, Giesserei, Vol 44 (No 11), 1957, p 305

27 P.A Green and A.J Thomas, Trans AFS, Vol 87, 1979, p 569

28 R.W Monroe and C.E Bates, Trans AFS, Vol 93, 1985, p 615

Other Properties

Corrosion Resistance. At room temperature, the corrosion rate of CG iron in 5% sulfuric acid is nearly half that of FG iron but higher than that of SG iron (Fig 37) With increasing temperature, the difference becomes smaller The pearlitic matrix has higher corrosion resistance than the ferritic one As expected, corrosion accelerates when stress is applied (Ref 29) Detailed information on the corrosion resistance of cast irons is

available in the article "Corrosion of Cast Irons" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Trang 31

Fig 37 Influence of (a) temperature and(b) tensile stress on the corrosion behavior of various cast irons in 5%

sulfuric acid Source: Ref 29

Machinability. Standardized machinability tests comparing CG irons with other castings are difficult to find in the literature The results of drill tests on castings shown in Fig 38 seem to indicate that the machinability is similar to that of ductile iron and that the wear of the drill is greater than for FG iron

Trang 32

Fig 38 Drill wear on various cast irons(drill speed, 780 rpm; rate of feed, 72 mm/min) Source: Ref 9

Nevertheless, in general, from both experimental data and practical experience in machine shops, it can be concluded that for a given matrix the machinability of CG iron is between that of gray and ductile iron (Ref 1,

9, 21) The CG morphology makes the iron sufficiently brittle for machine swarf to break into small chips, yet strong enough to prevent the swarf from forming powdery chips Neither large swarf nor fine, powdery swarf is ideal for high machinability (Ref 10)

Damping Capacity. The relative damping capacity of various irons, obtained by measuring the relative rates at which the amplitude of an imposed vibration decreases with time, Ref 17, is:

Apparently, changes in the carbon equivalent or matrix do not significantly influence the damping capacity, but heavier sections will produce higher damping capacities (Ref 9)

1982

29 A.E Krivosheev, B.V Marintchenkov, and N.M Fettisov, Russ Casting Prod., 1973, p 86

Trang 33

• Higher tensile strength to hardness ratio

• Much higher ductility and toughness, which result in a higher safety margin against fracture

• Lower oxidation and growth at high temperatures

• Less section sensitivity for heavy sections

Compared to SG irons, certain advantages can be claimed for CG irons:

• Lower coefficient of thermal expansion

• Higher thermal conductivity

• Better resistance to thermal shock

• Higher damping capacity

• Better castability, leading to higher casting yield, and the capability for pouring more intricate castings

• Improved machinability

CG iron can be substituted for FG iron in all cases in which the strength of FG iron has become insufficient, but

in which a change to SG iron is undesirable because of the less favorable casting properties of the latter Examples include bed plates for large diesel engines, crankcases, gearbox housings, turbocharger housings, connecting forks, bearing brackets, pulleys for truck servodrives, sprocket wheels, and eccentric gears

Because the thermal conductivity of CG iron is higher than that of SG iron, CG iron is preferred for castings operating at elevated temperature and/or under thermal fatigue conditions Applications include ingot molds, crankcases, cylinder heads, exhaust manifolds, and brake disks

The largest industrial application by weight of CG iron produced is for ingot molds weighing up to 54 Mg (60 tons) According to a number of reports summarized in Ref 30, the life of ingot molds made of CG iron is 20 to 70% longer than the life of those made of FG iron

In the case of cylinder heads, it was possible to increase engine output by 50% by changing from alloyed FG iron to ferritic CG iron (Ref 1) The specified minimum values for cylinder heads are 300 MPa (43 ksi) tensile strength, 240 MPa (35 ksi) yield strength, and 2% elongation

Modern car and truck engines require that manifolds work at temperature ranges of 500 °C (930 °F) At this temperature, FG iron manifolds are prone to cracking, while SG iron manifolds tend to warp CG iron manifolds warp and oxidize less and thus have a longer life Other engineering applications are summarized in Ref 1 and 30

Trang 34

1982

30 D.M Stefanescu and C.R Loper, Jr., Giesserei-Prax., No 5, 1981, p 74

References

1982

2 H.H Cornell and C.R Loper, Jr., Trans AFS, Vol 93, 1985, p 435

3 R Elliott, Cast Iron Technology, Butterworths, 1988

4 D.M Stefanescu, I Dinescu, S Craciun, and M Popescu, "Production of Vermicular Graphite Cast Irons

by Operative Control and Correction of Graphite Shape," Paper 37 presented at the 46th International Foundry Congress, Madrid, 1979

5 D.M Stefanescu, F Martinez, and I.G Chen, Trans AFS, Vol 91, 1983, p 205

6 E Nechtelberger, The Properties of Cast Iron up to 500 °C, Technicopy Ltd., 1980

7 J Sissener, W Thury, R Hummer, and E Nechtelberger, AFS Cast Met Res J., 1972, p 178

8 C.F Walton and T.J Opar, Ed., Iron Castings Handbook, Iron Casting Society Inc., 1981

11 F Martinez and D.M Stefanescu, Trans AFS, Vol 91, 1983, p 593

12 K.R Ziegler and J.F Wallace, Trans AFS, Vol 92, 1984, p 735

14 R.B Gundlach, Trans AFS, Vol 86, 1978, p 551

15 Spravotchnik po Tchugunomu Ljitiu (Cast Iron Handbook), 3rd ed., Mashinostrojenie, 1978

16 K.H Riemer, Giesserei, Vol 63 (No 10), 1976, p 285

18 A.F Heiber, Trans AFS, Vol 87, 1979, p 569

19 K Hutterbraucker, O Vohringer, and E Macherauch, Giessereiforschung, No 2, 1978, p 39

20 D.M Stefanescu and G Niculescu, unpublished research

22 K Roehrig, Trans AFS, Vol 86, 1978, p 75

23 Y.J Park, R.B Gundlach, R.G Thomas, and J.F Janowak, Trans AFS, Vol 93, 1985, p 415

24 E Mayer-Rassler, Giesserei, Vol 54 (No 13), 1967, p 348

25 H Kempers, Giesserei, Vol 53 (No 1), 1966, p 15

26 K Lohberg and J Motz, Giesserei, Vol 44 (No 11), 1957, p 305

27 P.A Green and A.J Thomas, Trans AFS, Vol 87, 1979, p 569

28 R.W Monroe and C.E Bates, Trans AFS, Vol 93, 1985, p 615

29 A.E Krivosheev, B.V Marintchenkov, and N.M Fettisov, Russ Casting Prod., 1973, p 86

30 D.M Stefanescu and C.R Loper, Jr., Giesserei-Prax., No 5, 1981, p 74

Trang 35

Malleable Iron

Introduction

MALLEABLE IRON is a type of cast iron that has most of its carbon in the form of irregularly shaped graphite nodules instead of flakes, as in gray iron, or small graphite spherulites, as in ductile iron Malleable iron is produced by first casting the iron as a white iron and then heat treating the white cast iron to convert the iron carbide into the irregularly shaped nodules of graphite This form of graphite in malleable iron is called temper carbon because it is formed in the solid state during heat treatment

Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of nodular graphite and a low-carbon metallic matrix Consequently, malleable iron and ductile iron are suitable for some of the same applications requiring good ductility and toughness, with the choice between malleable and ductile iron based on economy and availability rather than properties However, because solidification of white iron throughout a section is essential in the production of malleable iron, ductile iron has a clear advantage when the section is too thick to permit solidification as white iron Malleable iron castings are produced in section thicknesses ranging from about 1.5 to 100 mm ( 1

16 to 4 in.) and in weights from less than 0.03 to 180 kg ( 1

16 to 400 lb) or more

Ductile iron also has clear advantages over malleable iron when low solidification shrinkage is needed In other applications, however, malleable iron has a distinct advantage over ductile iron Malleable iron is preferred in the following applications:

• Thin-section casting

• Parts that are to be pierced, coined, or cold formed

• Parts requiring maximum machinability

• Parts that must retain good impact resistance at low temperatures

• Parts requiring wear resistance (martensitic malleable iron only)

Malleable iron (and ductile iron as well) also exhibits high resistance to corrosion, excellent machinability, good magnetic permeability, and low magnetic retention for magnetic clutches and brakes The good fatigue strength and damping capacity of malleable iron are also useful for long service in highly stressed parts

Malleable Iron

Metallurgical Factors

Although variations in heat treatment can produce malleable irons with different matrix microstructures (that is, ferritic, tempered pearlitic, tempered martensitic, or bainitic microstructures), the common feature of all malleable irons is the presence of uniformly dispersed and irregularly shaped graphite nodules in a given matrix microstructure These graphite nodules, known as temper carbon, are formed by annealing white cast iron at temperatures that allow the decomposition of cementite (iron carbide) and the subsequent precipitation of temper carbon

The desired formation of temper carbon in malleable irons has two basic requirements First, graphite should not form during the solidification of the white cast iron, and second, graphite must also be readily formed during the annealing heat treatment These two metallurgical requirements influence the useful compositions of

Trang 36

malleable irons and the melting, solidification, and annealing procedures (see the article"Classification and Basic Metallurgy of Cast Iron" in this Volume for an introduction to the metallurgy of malleable iron) Metallurgical control is based on the following criteria:

• Produce solidified white iron throughout the section thickness

• Anneal on an established time-temperature cycle set to minimum values in the interest of economy

• Produce the desired graphite distribution (nodule count) upon annealing

Changes in melting practice or composition that would satisfy the first requirement listed above are generally opposed to satisfaction of the second and third, while attempts to improve annealability beyond a certain point may result in an unacceptable tendency for the as-cast iron to be mottled instead of white

Composition. Because of the two metallurgical requirements described above, malleable irons involve a limited range of chemical composition and the restricted use of alloys The chemical composition of malleable iron generally conforms to the ranges given in Table 1 Small amounts of chromium (0.01 to 0.03%), boron (0.0020%), copper ( 1.0%), nickel (0.5 to 0.8%), and molybdenum (0.35 to 0.5%) are also sometimes present

Table 1 Typical compositions for malleable iron

Trang 37

A mixture of gray iron and white iron in variable proportions that produces a mottled (speckled) appearance is particularly damaging to the mechanical properties of the annealed casting, whether ferritic or pearlitic malleable iron Primary control of mottle is achieved by maintaining a balance of carbon and silicon contents

Because economy and castability are enhanced when the carbon and silicon contents of the base iron are in the higher proportions of their respective ranges, some malleable iron foundries produce iron with carbon and silicon contents at levels that might produce mottle and then add a balanced, mild carbide stabilizer to prevent mottle during casting Bismuth and boron in balanced amounts accomplish this control A typical addition is 0.01% Bi (as metal) and 0.001% B (as ferroboron) Bismuth retards graphitization during solidification; small amounts of boron have little effect on graphitizing tendency during solidification, but accelerate carbide decomposition during annealing The balanced addition of bismuth and boron permits the production of heavier sections for a given base iron or the utilization of a higher-carbon higher-silicon base iron for a given section thickness

Tellurium can be added in amounts from 0.0005 to 0.001% to suppress mottle Tellurium is a much stronger carbide stabilizer than bismuth during solidification, but also strongly retards annealing if the residual exceeds 0.003% Less than 0.003% residual tellurium has little effect on annealing, but has a significant influence on mottle control Tellurium is more effective if added together with copper or bismuth

Residual boron should not exceed 0.0035% in order to avoid module alignment and carbide formation Also, the addition of 0.005% Al to the pouring ladle significantly improves annealability without promoting mottle

Melting Practices. (Ref 2) The iron for most present-day malleable iron is melted in coreless induction furnaces rather than the previous air furnace, cupola-air furnace, or cupola-electric furnace systems The sulfur and nitrogen contents of the charge carbon used in melting must be high enough to provide 0.07 to 0.09% S and

80 to 120 ppm N in the iron The sulfur reduces the surface tension and improves fluidity The nitrogen increases the tensile strength without impairing elongation and toughness Long holding periods in the molten state in the furnace and excessive superheating temperatures should be avoided, because they give rise to an unsatisfactory solidification structure, which in turn results in unsatisfactory heat-treated structures (Ref 2)

Melting can be accomplished by batch cold melting or by duplexing Cold melting is done in coreless or channel-type induction furnaces, electric arc furnaces, or cupola furnaces In duplexing, the iron is melted in a cupola or electric arc furnace, and the molten metal is transferred to a coreless or channel-type induction furnace for holding and pouring Charge materials (foundry returns, steel scrap, ferroalloys, and, except in cupola melting, carbon) are carefully selected, and the melting operation is well controlled to produce metal having the desired composition and properties Minor corrections in composition and pouring temperature are made in the second stage of duplex melting, but most of the process control is done in the primary melting furnace (Ref 2)

Molds are produced in green sand, silicate CO2 bonded sand, or resin-bonded sand (shell molds) on equipment ranging from highly mechanized or automated machines to that required for floor or hand molding methods, depending on the size and number of castings to be produced In general, the technology of molding and pouring malleable iron is similar to that used to produce gray iron

Solidification. Molten iron produced under properly controlled melting conditions solidifies with all carbon in the combined form, producing the white iron structure fundamental to the manufacture of either ferritic or pearlitic malleable iron (Fig 1) The base iron must contain balanced quantities of carbon and silicon to simultaneously provide castability, white iron in even the thickest sections of the castings, and annealability; therefore, precise metallurgical control is necessary for quality production Thick metal sections cool slowly during solidification and tend to graphitize, producing mottled or gray iron This is undesirable, because the graphite formed in mottled iron or rapidly cooled gray iron is generally of the type D configuration, a flake form in a dense, lacy structure, which is particularly damaging to the strength, ductility, and stiffness characteristics of both ferritic and pearlitic malleable iron

Trang 38

Fig 1 Structure of as-cast malleable white iron showing a mixture of pearlite and eutectic carbides 400×

After it solidifies and cools, the metal is in a white iron state, and gates, sprues, and feeders can be removed easily from the castings by impact, This operation, called spruing, is generally performed manually with a hammer because the diversity of castings produced in the foundry makes the mechanization or automation of spruing very difficult After spruing, the castings proceed to heat treatment, while gates and risers are returned

to the melting department for reprocessing

First-Stage Anneal. Malleable iron castings are produced from the white iron by an annealing process that converts primary carbides into temper carbon This initial anneal is then followed by additional heat treatments that produce the desired matrix microstructures This section focuses on the initial (first-stage) anneal that produces the temper carbon in blackheart malleable iron The additional heat treatments used to produce the desired matrix microstructure are discussed in the sections relating to ferritic, pearlitic, or martensitic microstructures

During the first-stage annealing cycle, the carbon that exists in combined form, either as massive carbides or as

a microconstituent in pearlite, is converted into nodules of graphite (temper carbon) The rate of annealing of a hard iron casting depends on chemical composition, nucleation tendency (discussed in the section "Control of Nodule Count" in this article), and annealing temperature With the proper balance of boron content and graphitic materials in the charge, the optimum number and distribution of graphite nuclei are developed in the early portions of first-stage annealing, and growth of the temper carbon particles proceeds rapidly at any annealing temperature An optimum iron will anneal completely through the first-stage reaction in approximately 31

2 h at 940 °C (1720 °F) Irons with lower silicon contents or less-than-optimum nodule counts may require as much as 20 h for completion of first-stage annealing

The temperature of first-stage annealing exercises considerable influence on the rate of annealing and the number of graphite particles produced Increasing the annealing temperature accelerates the rate of decomposition of primary carbide and produces more graphite particles per unit volume However, high first-stage annealing temperatures can result in excessive distortion of castings during annealing, which leads to straightening of the casting after heat treatment Annealing temperatures are adjusted to provide maximum practical annealing rates and minimum distortion and are therefore controlled within the range of 900 to 970 °C (1650 to 1780 °F) Lower temperatures result in excessively long annealing times, while higher temperatures produce excessive distortion

Annealing is done in high-production controlled-atmosphere continuous furnaces or batch-type furnaces, depending on production requirements The furnace atmosphere for producing malleable iron in continuous furnaces is controlled so that the ratio of CO to CO2 is between 1:1 and 20:1 In addition, any sources of water

Trang 39

vapor or hydrogen are eliminated; the presence of hydrogen is thought to retard annealing, and it produces excessive decarburization of casting surfaces Proper control of the gas atmosphere is important for avoiding an undesirable surface structure A high ratio of CO to CO2 retains a high level of combined carbon on the surface

of the casting and produces a pearlitic rim, or picture frame, on a ferritic malleable iron part A low ratio of CO

to CO2 permits excessive decarburization, which forms a ferritic skin on the casting with an underlying rim of pearlite The latter condition is produced when a significant portion of the subsurface metal is decarburized to the degree that no temper carbon nodules can be developed during first-stage annealing When this occurs, the dissolved carbon cannot precipitate from the austenite, except as the cementite plates in pearlite

Control of Nodule Count. Proper annealing in short-term cycles and the attainment of high levels of casting quality require that controlled distribution of graphite particles be obtained during first-stage heat treatment With low nodule count (few graphite particles per unit area or volume), mechanical properties are reduced from optimum, and second-stage annealing time is unnecessarily long because of long diffusion distances Excessive nodule count is also undesirable, because graphite particles may become aligned in a configuration corresponding to the boundaries of the original primary cementite In martensitic malleable iron, very high nodule counts are sometimes associated with low hardenability and nonuniform tempering Generally, a nodule count of 80 to 150 discrete graphites particles per square millimeter of a photomicrograph magnified at 100× appears to be optimum This produces random particle distribution, with short distances between the graphite particles

Temper carbon is formed predominantly at the interface between primary carbide and saturated austenite at the first-stage annealing temperature, with growth around the nuclei taking place by a reaction involving diffusion and carbide decomposition Although new nuclei undoubtedly form at the interfaces during holding at the first-stage annealing temperature, nucleation and graphitization are accelerated by the presence of nuclei that are created by appropriate melting practice High silicon and carbon contents promote nucleation and graphitization, but these elements must be restricted to certain maximum levels because of the necessity that the iron solidify white

1 C.F Walton and T.J Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981, p 297-321

2 L Jenkins, Malleable Cast Iron, in Encyclopedia of Materials Science and Engineering, Vol 4, M.B Bever,

Ed., MIT Press, 1986, p 2725-2729

Malleable Iron

Types and Properties of Malleable Iron

There are two basic types of malleable iron: blackheart and whiteheart Blackheart malleable iron is the only type produced in North America and is the most widely used throughout the world Whiteheart malleable iron is the older type and is essentially decarburized throughout in an extended heat treatment of white iron This article considers only the blackheart type

Malleable iron, like medium-carbon steel, can be heat treated to produce a wide variety of mechanical properties (Table 2) The different grades and mechanical properties are essentially the result of the matrix microstructure, which may be a matrix of ferrite, pearlite, tempered pearlite, bainite, tempered martensite, or a combination of these (all containing nodules of temper carbon) This matrix microstructure is the dominant factor influencing the mechanical properties Other less significant factors include nodular count and the amount and compactness of the graphite (Ref 1) A higher nodular count may slightly decrease the tensile and

Trang 40

yield strengths (Ref 3) as well as ductility (Ref 4) More graphite or a less compact form of graphite also tends

to decrease strength (Ref 1)

Table 2 Properties of malleable iron castings

Microstructures and typical applications are given in Tables 3 and 4

Tensile strength

Yield strength

Định dạng
Số trang	160
Dung lượng	5,23 MB