bainitic high-strength cast iron with spheroidal graphite (review)

BAINITIC HIGH-STRENGTH CAST IRON WITH SPHEROIDAL GRAPHITE

(REVIEW)

B. I. Voronenko and Yu. I. Romatovskii UDC 669.15-196

Bainitic cast iron with spheroidal graphite (BCISG) is a new class of structural materials. The matrix structure of BCISG is bainite and residual austenite. This structure is obtained by isothermal quenching (in this case the BCISG is called isothermally-quenched high-strength cast iron--austempered ductile iron, ADI) or by continuous cooling from the casting or austenization temperature (such BCISG is called cast bainitic cast iron).

The present article is concerned with foreign investigations and inventions in the area of bainitic high-strength cast iron with spheroidal graphite (BCISG) during the past five years.

Producing BCISG by isothermal quenching is a labor- and energy-consuming method which requires special equipment (furnaces and baths), but guarantees attainment of the highest mechanical properties in various sections. The use of continuous cooling from the casting or secondary austenization temperature is a simple technique for obtaining BCISG. However, in this case it is necessary to preselect the composition of the cast iron and control the cooling rate in order to avoid the formation of undesirable structural constituents (pearlite, martensite, and carbides). With the use of isothermal quenching BCISG may be obtained from ordinary cast iron with spheroidal graphite (CISG) without additional alloying. In this case the BCISG has twice the strength of CISG with the same ductility. It is recommended that mechanical working be conducted before isothermal quenching the CISG.

In Table 1 the properties of CISG with matrices of various structures (ferrite, pearlite, bainite, and autenite), and also gray cast iron with flake graphite [i] are compared, and in Fig. 1 the effect of heat treatment on the mechanical properties of CISG [2] is shown. Due to its high mechanical properties BCISG is a prospective low-coat material to substitute for other types of CISG, and also for cast and forged unalloyed or low-alloyed high-strength steels.

BCISG is used for cast toothed and worm gears, crankshafts and other heavily loaded parts. The superior working capabilities of parts made from BCISG (compared to those made from steel) is attributable to its low elastic modulus and coefficient of friction, high crack-resistance and resistance to abrasive wear, sound and vibration damping capacity, good working-in capability, and lower weight. Maximal approximation of the shape of the casting to that of the finished part leads to an approximately 30% decrease in the cost of gears made from BCISG rather than steel. The material-use coefficient is also increased from 25-45 to 60-70%. With available technology the percent of scrap fromBCISGcastings does not exceed 2% [2].

Interest in BCISG has recently appeared in many countries. The worldwide production of BCISG castings steadily increases, but the use of these in industry is shill limited (less than 1% of the production of CISG in general). This is explained by the severe requirements on casting quality, and insufficiency of information concerning the properties and production technology for BCISG. At the same time, two international conferences have already taken place in which questions concerning the production of BCISG in Europe and the USA were considered; its fields of application, structural transformations upon heat treatment, effect of chemical composition and isothermal heat treatment on the mechanical properties of BCISG, and methods of nondestructive control of quality were also considered. Much attention was given to this material in the third international symposium on the metallurgy of cast irons (Stockholm, 1984).

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. I0, pp. 28-34, October, 1991.

762 0026-0673/91/0910-0762512.50 © 1992 Plenum Publishing Corporation

TABLE 1

Cast iron Graphite O" u O" c i 0 0 . 2 I a_ l 6,% a , J/cm 2 N/mm 2

Grey Flake 100--350 I 65--160

J

75--155 I

Spheroidal 180--200 200--280 320--460

High strength: ferritic pearlitic bainitic

10--25 2--7 2--20

350--600 600--850 900--1500

40--150 15--50 20-- I60

E, kN/mm ~

5 0 0 - 1400

600--1200 220--380 1200--1500 380--650 1800--2200 700--1260

-- 180--260

Y, glcm 3 Q, Vlm's

6,8---7,5 40--55

6,9--7,2 40--46 7,0--7,3 34 -42

7,25- 7,35 30--40

7,3--7,45 ~12,6

150 I75 I75--185 160--200

A u s t e n i t i c n i c k e l 380--500 160--200 8--46 2 0 - 50 140-- 160

Cast iron Graphite HB p'106,~'m c~, °C-I'I06 ~/

G r e y F l a k e 160--270 0,45-- 1,2 I 0 12 250-- 1000

0.40--0,50 0,55--0,75 0,60--0,75

High strength: ferritic pearlitic bainitic

130--240 200--300 280 500

140--225

Spheroidal 10--12 11 13 9 --10

5 18,7 Austenitic nickel 1,0-- 1,4

1000---2000 300--I000 300--600

1.01 1,3

Notation. o c) compressive strength; a) impact toughness of unnotched speclmen; E) modulus of elasticity; ¥) density; Q) thermal conductivity; p) specific electrical resistance; ~) coefficient of linear expansion; ~) magnetic susceptibility.

o u N/ram 2)

MOO

300 o ~ e ~2 16 4%

Fig . 1. Dependence of the mechanica l properties of CISG on the type of heat treatment: i) isothermal quenching to bainite; 2) quenching to martensite and tempering; 3) normalizing; 4) original (cast) state; 5) annealed.

During the past five years 27% of the publications on C!SG have been devoted to BCISG, i.e., the study of cast irons has been evolving in exactly this direction. At the same time, out of 80 published works and 40 inventions concerning BCISG only four articles and one in- vention are attributable to Soviet authors, according to data from RZh "Metallurgiya." More- over, while GOST 7293-70 "High Strength Cast Irons with Spheroidal Graphite for Castings" specified two grades of isothermally quenched cast iron (VCh 100-4 and VCh 120-4), in the same GOST from 1985, introduced by Minenergomash, irons of this type are no longer specified (however, they are not yet standardized in other countries).

STRUCTURE AND PHASE TRANSFORMATIONS IN BCISG

The structural matrix of isothermally quenched BCISG consists of bainite (acicular), ferrite, and high-carbon stabilized austenite. Upper or lower bainite is formed, depending upon the isothermal reaction temperature. Lower bainite is harder and stronger, but less ductile than upper. The presence of silicon in BCISG results in two decomposition reactions

763

in the supercooled austenite at constant temperature within the bainite range. The first reaction leads to the formation of a bainite--austenite structure with high strength and plasticity; the second to the formation of ferrite and carbide with decreased toughness and ductility. Hence, there is a limited annealing-time interval for the attainment of high strength and plasticity. Alloying elements may delay the second reaction, and these broaden the time and temperature intervals permitting the attainment of high properties in CISG. The segregation of some elements decreases the effectiveness of isothermal quenching [3].

In the process of isothermal quenching to upper bainite, for example at 400°C, a structure of ferrite + austenite with high toughness is formed in 20 min. In four hours the austenite decomposes to a ferrite--carbide mixture with decreased toughness. In the lower part of the bainite range (300°C), during the initial stages of the transformation carbides precipitate from the ferrite, since the austenite cannot hold all of the precipitating carbon in solution. In the final stage austenite transforms to ferrite and carbide. This reaction proceeds slowly [4]. Isothermal quenching to lower bainite is used for high-strength parts operating under abrasive conditions, and upper bainite for parts which require high toughness.

Segregation of elements at the cell boundaries during solidification influences the kinetics of the bainite reaction. After isothermal holding at 350-450°C a two-stage reaction (¥ + (~ + ~) + ~ + silicocarbides) occurs in the homogeneous matrix, dependent upon the chemical composition of the matrix, dimensions of the solidification cells, austenitizing conditions and the segregation of dissolved elements, particularly silicon. The bainite reaction does not go to completion in the neighborhood of the solidification cell boundaries. Upon final cooling to room temperature this leads to the formation of martensite in those regions. In the regions of austenite situated along its interface with graphite the bainite reaction proceeds much quicker. Therefore, it is recommended that the size of the solidification cells be controlled, and a maximal density of the graphite inclusions be obtained by modification and optimization of the solidification conditions [5].

Investigation of the chemical and structural inhomogeneities in low.-alloyed BCISG re- vealed that cast iron in which liquation of the carbide-forming elements at the boundaries of the eutectic cells occurred possesses low mechanical properties [6]. In order to mini- mize chemical and structural inhomogeneity in BCISG the introduction of predominantly graphite-forming elements is recommended.

The strength of BCISG increases with time at room temperature. This is connected with the formation of martensite as a result of the transformation of residual austenite upon relaxation of the stresses induced by the bainite reaction. The extent of the change in properties at room temperature is considerably influenced by the austenite grain size, isothermal annealing temperature, and alloying elements. The greatest change of structure with time occurs when the residual austenite transforms to the greatest extent. There- fore, it is important to correctly determine the moment at which the first stage of the bainite reaction ends during isothermal quenching, and obtain a minimal amount of residual austenite in order that the structure of the BCISG should remain stable [7]. To establish the moment at which the second (embrittling) stage of austenite decomposition begins electrical resistance methods are used, since dilatrometric methods, in some cases, do not yield results [8].

The effect of the austenization time and temperature, and time of isothermal annealing in the range 270-420°C on the amount of residual austenite in unalloyed BCISG has been investigated [9]. Isothermal annealing at 370°C for 30 min results in an increase in the amount of residual austenite from 22 to 48% with increase in the austenizing temperature from 850 to 1000°C (.holding time = 2 h). In the case of austenization for 2 h at 900°C and isothermal annealing for I h in the range 270-420°C the amount of residual austenite at 420 and 370°C is 30% at 320 and 270°C it is 20 and 17%, respectively. After isothermal annealing for 133 h at 420 and 370°C residual austenite is not detected, but at 320 and 270°C the amounts are 15 and 9%, respectively. Homogenization of the austenite decreases the micro- segregation of silicon at the matrix--graphite interface and gradient in the distribution of residual austenite in BCISG.

764

EFFECT OF ALLOYING ON THE MECHANICAL PROPERTIES OF BCISG

Optimal properties in isothermally quenched BCISG are attained at the alloy composition ~1.5% Ni, 0.5% Cu, and 0.32% Mo. Manganese, segregating at the grain boundaries, retards the bainite transformation, promotes the formation of martensite even after prolonged annealing and, by stabilizing residual austenite [i0], lowers the strength and ductility of BCISG. The best strength properties in BCISG are found at 0.02% Mn.

Manganese concentrations above 0.35% in isothermally quenched BCISG are not desirable, since the optimal ductility is obtained with the smallest amount of untransformed austenite [ii]. However, for austenitizing temperatures below 870°C the manganese concentration may be increased.

The presence of silicon leads to the formation of free ferrite in isothermally quenched BCISG. With increasing silicon concentration o u decreases, but o0. 2 and 6 increase. Addi-

tion of 22-25% Si to BCISG (optimal concentration) improves its properties; higher Si concentrations have a deleterious effect.

The effect of alloying with Cu and Mo on the thermokinetic transformation diagrams of supercooled austenite and the properties of BCISG after isothermal quenching have been investigated [12]. Copper, and especially manganese, increase the incubation time for the pearlite transformation, which permits the supercooling of austenite into the bainite range at a decreased rate. BCISG possesses good hardenability, especially when alloyed with Ni, Mo or Cu. A linear dependence of the hardenability of BCISG on the concentrations of Cu and Mo is described in [12].

Alloying with Mo decreases the amount of pearlite formed, but the greatest effect is obtained by alloying with Mo and Cu in combination.

Increasing the carbon concentration from 3.6 to 4% produces a decrease in hardness, but does not affect ductility. Increasing Si from 2 to 2.9% improves the fatigue strength of BCISG [13]. Copper concentrations up to 1.5% and Ni to 1% have no influence on the mechanical properties of cast iron. The presence of 0.2-0.63% Mo decreases the strength, hardness, and ductility of BCISG. However, according to [3] Mo increases strength and ductility. Maximal strength is found in BCISG alloyed with Mo, or Ni and Mo.

The effect of Cu, Ni, Mn, and Mo on the structure and hardenability of BCISG has been investigated [14]. Satisfactory hardenability in castings up to 25 mm in cross section is produced by isothermal quenching BCISG in the absence of Mn. In castings with cross sections up to 50 mm the required hardenability is attained in Cu--Ni BCISG with 0.4% Mn, or in Ni--Mo BCISG with 0.2% Mn. Not one of the melts investigated provided hardenability in castings 75 mm in cross section.

Isothermal transformation diagrams for austenite, and the structure and mechanical properties of bainitic--austenitic CISG of the Swiss firm Sulzer (grades Sulzer GGG-100 and GGG-120) are given in [15].

The effects of Mo, Cu, and Ni on the structure and hardenability of specimens 13-230 mm in diameter in the cast state, and after annealing at 700°C followed by cooling in the fur- nace or in an air stream, have been studied [16]. It was shown that Mo increases the amount of pearlite in BCISG, but Cu and Ni suppress the formation of ferrite. At 1% Cu and 0.5% Ni the BCISG was pearlitic in specimens of all sizes. Molybdenum in combination with Ni and Cu promotes the formation of a bainitic--martensitic structure, and the addition of Mo alone increases the amount of the bainite--martensite constituent in BCISG after quenching and tempering. The hardness of BCISG in the cast state depends strongly on specimen size, but this dependence is reduced by alloying. Annealing reduces the hardness of BCISG independent of alloy content. Alloying with Mo raises the hardness in the normalized state, and has a large effect on the hardenability of BCISG. In BCISG with 0.4% Mo the hardness after quenching is 53 HRC, and increases to 56 HRC upon addition of 0.5 Cu and 0.5% Ni.

Cast BCISG with o u up to i000 N/mm 2 at 6 = 3.5% [17] has been obtained. However, its structure is strongly dependent upon the section-size of the specimen, and to obtain 100% bainite in parts of differing section-size it is necessary to suitably control the composition. Excessive alloying slowed the bainite transformation and caused interdendritic segregation.

765

The effect of combined alloying with Ni, Mo, and Cu on the extent of transformation of austenite into a bainitic--martensitic structure in the cast state has been investigated using castings with varying section thicknesses (10-175 mm) [18]. The relationship obtained has been presented in the form of a nomogaph which permits the selection of the composition of BCSIG required to attain given properties in castings with a specified wall thickness.

In many inventions, mainly Japanese, the compositions and heat treatment schedules of BCISG are given: for example, the isothermal quenching regimen for that composition of BCISG which does not require preliminary annealing. This composition of BCISG includes 0.7- 1.5% Cu, and no more than 0.1% Cr + V + Co + B [19]. A composition of BCISG which after isothermal quenching to bainite attains o u = 1120 N/mm 2 and 6 = 13% without supplementary alloying with scarce elements is proposed. This is accomplished by decreasing the concentrations to P < 0.03% and N < 0.008% [20]. A composition and heat treatment regimen for BCISG with 0.4% Cu, which permits the attainment of o u > i000 N/mm 2, 6 > 7%, 300 HB, and 22-27%

of retained austenite in the structure is proposed [21]. In order to increase the stre~gth and toughness of bainitic cast iron 1-5% Ni and 0.5-3% Cu are added to CISG of ordinary composition. This permits the austenization temperature to be lowered. After solidification, Ni, Cu, and Mn form microsegregations at the graphite--matrix interfaces and within the eutectic cells. Regaining microsegregations in the bainitic structure after isothermal quenching promotes increased strength and toughness in BCISG [22]. BCISG containing Ni, Cr, and Mo, and having a bainitic structure with uniformly dispersed carbides, Shor hardness e 70 HS, ~u ~ 500 N/mm 2, and Kic > 900 N/mm 3/2 is recommended for the outer surface of composite rolls

for hot rolling [23]. A heat treatment regimen for cast BCISG crankshafts with added Mo and Cr, possessing a high endurance limit o_ l = 380-460 N/mm 2 and elastic modulus E = 165" 175 kN/mm 2 is proposed [24]. Cast iron compositions for the outer surfaces of composite rolls for hot rolling, and the heat-treatment regimen providing high wear resistance and resistance to cracking are examined. The cast iron contains Cr, Ni, Mo, and Ti after isothermal quenching possesses a surface hardness of Shor 75-85 HS, and may be used in the finishing stands of hot rolling mills [25]. Cast BCISG with additions of Ni, Cr, Mo, Cu, and Ce possessing o u = 970-1125 N/mm ~, 325-402 HB, and resistance to thermal cycling of 1850-2780 cycles

(600°C ~ 20°C, water cooling) is recommended for rolling mill rolls [26].

HEAT-TREATMENT OF BCISG

Isothermal quenching of CISG has advantages over quenching and tempering: the insignifi- cant volume changes and thermal stresses decrease the danger of crack formation and warping of the castings; the temperature range of isothermal annealing is 270-450°C (this is well below the tempering temperature used in quenching and tempering, which removes the danger of ferritization and the formation of secondary graphite); the general length of the casting heat-treatment process is shortened in continuous production and castings with a high density of spherical graphite inclusions.

CISG has a hardness > 400 HB after isothermal quenching to lower bainite, and is used to produce components which experience high contact stresses. Upon quenching to upper bainite CISG acquires high toughness, ductility, wear resistance and resistance to thermal cycling at a hardness of 250-360 HB and may be used, for example, for the production of crankshafts. BCISG acquires the optimal strength and plasticity after isothermal quenching to 375-400°C from an austenitizing temperature of 900°C: o u = 900-1100 N/mm 2, 00. 2 = 600-800

N/mm 2, 8 = 7-10%, HB = 270-300. In this case the amount of residual austenite is 30% [27].

In BCISG with additions of Mo, Ni, and Cu the maximum strength and ductility are ob- served after isothermal treatment at 300-320°C. Increasing the holding temperature from 350 to 500°C decreases the amount of residual austenite and subsequently it completely disappears, accompanied by a decrease in ductility, 8 to 1-2%. At an isothermal holding temperature of 500-600% C, 6 ~ 6%, but o u < i000 N/mm 2 [28].

A regression relationship connecting the minimal time (min) of isothermal holding of CISG with alloy composition has been obtained [29, 30]:

lg~= -- 0,8+ 1,455. (% si) +0,9. (% Mn) +0,55 × x (% ?,4o) +0,33. (%Cu) -i-0,t5. (%Ni).

CISG austenitized at 930 and 870°C for 2 h and isothermally treated at 370°C to upper bainite, and at 315-°C to lower bainite, had o u = i060 N/mm 2 and 1400 N/mm 2, respectively [31].

766

The ductility was above that of CISG quenched to lower bainite. In order to obtain the maximum ductility in CISG with Mn < 0.35% it is necessary to establish the optimal holding time

at 370°C.

The isothermal quenching of CISG with various concentrations of Mn, Ni, Mo, and Cu after austenization at 875-900°C has been investigated [32]. At temperatures above 370°C needles or plates of ferrite form in the austenite. Interruption of the isothermal (above 370°C) reaction by quenching leads to the formation of martensite, which raises the hardness and lowers the toughness of the CISG. At holding temperatures below 370°C the austenite decomposition products are finer, and are reminiscent of upper bainite in steel. Alloy- ing CISG with Ni and Mo broadens the possibilities for isothermal quenching. Mechanical properties characteristic of isothermally treated CISG may be obtained by means of the controlled or interrupted cooling of castings to the bainite range of austenite decomposition temperatures. The following heat-treatment regimen is proposed: air cooling from 870 to 370°C, hold 1 h at 370--345°C for completion of the first stage of the bainite reaction, and cool in air to room temperature.

Increasing the holding time at 245°C from 1 to 6 h of cast irons with added Ni and Mo increases o u from 965 to 1500 N/hIM 2 , 00. 2 from 800 to 1220 N/MN 2 and 6 from 1 to 2%, with

a simultaneous decrease of hardness from 450 to 400 HB [33].

The mechanical properties of cast irons with various forms of graphite (spheroidal, vermicular, temper graphite) subjected to isothermal quenching at 230-430°C were compared. The strength after isothermal quenching was highest in CISG, lower in cast iron with vermicular graphite, and lowest in ductile case irons [34].

Results of investigations by BCIRA (British Cast Iron Research Association) on the influence of technical parameters on the properties of BCISG showed that, depending on composition and heat-treatment, the values of o u varied from 900 to i000 N/mm 2, and 6 from i to 14%.

The best combination of properties was secured by austenitizing at 900-925°C. Maximum strength was obtained by isothermally quenching at 275-325°C, and maximum ductility and impact toughness at 375°C [13].

The length of the incubation period for the decomposition of supercooled austenite is smaller, the lower the austenization temperature and the lower the density of graphite inclusions in CISG. The duration of the reaction in specimens with a low density of graphite inclusions, however, is greater. The stability of supercooled austenite is greatest at 415°C. At 300--200°C austenite in CISG transforms to bainite, and at 170°C to martensite, the needles of which increase in size with increase in the austenitizing temperature [35].

The effects of the austenitizing and isothermal decomposition temperatures, and also of deformation, on the kinetics of formation of bainitic ferrite have been studied [36]. The CISG was isothermally annealed at 315-430°C. In the process of forming ferrite plate- lets the hardness, amount of carbon in the austenite, and the volume fraction of the latter rapidly varied. At the end of the first stage of the bainite reaction these quantities be- come constant. The rate of the process sharply increases after deformation of the austenite prior to the start of decomposition, because of a multifold increase in the number of ferrite nuclei. The rate is slightly decreased by the addition of Mn and Mo, and increased by re- ducing the austenizing temperature from 930 to 870°C. With increase in the bainite transformation rate the density of ferrite plates and uniformity of the CISG structure increase.

The use of alloyed BCISG with a structure of acicular ferrite and austenite stabilized by a high carbon content decreases the possibility of embrittlement during heat-treatment. Addition of 1.5% Ni and 0.3% Mo to the cast iron delays the time of onset of embrittlement during isothermal holding to at least 4 h, whereas for unalloyed cast iron this begins after 1 h. High strength and toughness may be obtained also by regulating the cooling rate after knocking the hot casting out of the flask, and holding in the bainite transformation range. Isothermal quenching followed by tempering can lead to the formation of a structure consisting entirely of bainite ferrite. It is desirable to obtain such a structure in castings used at high temperatures, for example chill molds [32].

Thermokinetic diagams for the decomposition of supercooled austenite in CISG with 0.i- 1.4% Cu and 0-0.5%Mo during cooling from 900°C at the rate of 0.4-18°C/sec are given in [12].

In order to improve the fracture toughness of CISG a heat-treatment has been proposed, consisting of holding in the two-phase (~ + 7) region for a time dependent upon the rate of

767

redistribution of the elements Ni, Cu, Mn, Mo, Cr, and Ti [37]. Following this, isothermal

quenching to bainite is carried out.

The structure and properties of types 320S2 and 310S2N4 BCISG have been investigated following two regimens of heat-treatment: i) oil quenching from the y-region + isothermal quenching from the (~+ y)-region with 3 h holding time at 350°C; 2) isothermal quenching

from .... (~+ y)-region to .... C r~ ] ~u L~8 "- increase in the resistance to crack propagation D J U L I I ~

_ n .... max im~i a~L~[ treatment /~ ~ .... ~ ~ "' =~--- <x) was ,uteu. For cast iron oiuo2m~ a~Le[ heat-treatment /'~ ~'-- k l ] L n ~

l l -- 1 . . . . . . . . . . . . . g U ~ [~taln~u LeSL Lemperature -±u~7 uu~pouu to a concentration of ~ .... ~--J austenite ~ ~ ~ 1 ~ o ~ )

--zJ t o - - ± U U

m . . . . . • ] t ~ L ~ are many inventions, p£- ..................................... ± ~ , c ± p a ± ± y Japau~, WIL[t optimization of .... LU~

I I ~ L - UE~dLn1~Ht U± Du±oG. For ....... ' - a a[tu ~xamp±~, process for obtaining u±gu strength

~uuLx±zty uy LUe ~se of CHill . . . . . . . . . . . . . . . . . . . . uiOC~ , WttiUtt help to increase the density of spheroidal graph- $ L g ~ t E L l g l ~ 5 ~ U £ ' I I I ~ ~ U l l U i ± i U ~ L ± U I I , [ t ~ U ~ I ! ~ i l U p U ~ U L J ~ , ~ U J . A I I ~ L - I - E ~ L | ~ I I L r e men

for ~ T O O o f l - L i ' a U K ---1-z -I - - 1- - - u . . . . . . . . . . . ] 1-z - - u±o~ castings for ±~," .......... Le~meu~-~-laL~ gears in - = ..... ~- v~llisi~3 ,a~ ue~u prupo~eu, wu±uh

~ItULL~II~ LI~ treatment time --= conserves ltl@tai [4£], also a ' ............... auu heaL- L~atm~ut method for

spring ...... ~'~-- ~- a~emu±~es ±u truck and railroad car suspensions which must have a high endurance

limit and resistance to fatigue [42]. A regimen of heat-treatment which guarantees attainment of a bainitic structure over the entire section of a CISG casting has been recon~n~ended [43]; a heat-treatment regimen for CISG with additions of Mo, Cu, Ni, Cu and A1 guaranteeing o u = 1020-1100 N/mln ~, 6 = 6-11% and 285-300 HB [44]; a heat-treatment regimen for a CISG an-

alogue which yields o u = 1050-1150 N/mm ~, with u - 6-8% [45]. A method of heat-treatment

has been proposed for CISG molds for casting under pressure, with additions of Cu and Mo, which guarantees high mechanical properties, wear resistance, durability, corrosion resistance, and decrease in weight of the mold [46]. A high-productivity method for obtaining CISG containing 0.5% Cu with increased strength (o u = 1160 N/mm 2, 00. 2 = 840 N/mm 2, 420 HB)

and impact toughness (a = 26 J/cm 2) has been worked out [47]. An energy saving heat-treatment method for CISG castings with high toughness and resistance to fatigue is proposed in Japanese inventions [48, 49]. An energy-saving regimen for the heat treatment of BCISG by cooling from the austenitizing temperature to a temperature above M S with the help of a jet

of oil or water, or in a boiling layer, i.e., without the use of a salt bath, has been de- veloped which secures the attainment of high and uniform mechanical properties in castings of complex shape and abrupt changes in section thickness [50, 52]. Finally, a heat-treatment regimen for CISG trimming dies used in working castings under pressure which results in high strength, toughness, and wear resistance at a hardness of 45-48 HRC [53] is proposed, and a regimen for isothermally quenching CISG at 350°C with yields o u = 1330 N/[~ 2 ,

o0. ~ = 1070 N/mm 2, 6 = 7%, 398 HB; a~ = 96 J/cm 2 and E = 190 kN/mm ~ [54].

PROPERTIES OF BCISG

BCISG is close to high-strength low-alloyed steel in its properties. It has higher strength, ductility, and wear resistance than pearl±tic cast iron. Castings of alloyed BCISG with 20-40% residual austenite have approximately the same properties at wall thicknesses of 30-70 mm; o u = i000-1200 N/mm 2, ~0.2 = 700-950 N/~f~n 2, 6 = 2%, 280-390 HB, o_~ =

350 N/mm 2, based on i0 ~ cycles. BCISG possesses high fracture toughness. At the same strength level Klc of isothermally quenched BCISG is twice that of ordinary CISG. Thus, at

o u = i000 N/mm 2, Klc of CISG with bainitic--austenitic structure is 1200 N/mm ~/~ while that

of ordinary CISG is about 700 N/mm ~/2 Isothermally quenched CISG possesses enhanced im-

pact toughness down to --100°C [15].

Grades GGG-80, GGG-100, and GGG-120 BCISG possess higher properties in dynamic and fatigue loading than ordinary CISG, according to DIN 1693 FRG. Having a high fracture toughness they, at the same time, possess good workability. BCISG grade GGG-80 BAF has

maximum fracture toughness (Klc> 2000 N/mm ~/2) and workability. Its structure is stable

down to --160°C [15]. BCISG grade GGG-10 B/A with 60-80% bainite and 40--20% residual = a - 4 0 austenite has o u 850-1300 N/mm ~ 6 = 6-20%, 278-336 HB, =+20 = 110-160 J/cm 2, , ~ 0 . 2 5 0 . 2 5 =

60-80 J/cm 2, and weight loss during dry friction 2.5 times less than that of steel with

the same hardness [55].

The effect of the isothermal quenching regimen on the mechanical properties of CISG with additions of Cu and Mo has been investigated [56]. In the initial state, CSIG with additions of CU possesses minimum impact toughness, and after isothermal quenching at

768

385°C (2 h), maximum. Copper also raises the fracture toughness of BCISG. A general re- quirement for obtaining BCISG with high strength and toughness is the absence of primary carbides in the matrix.

With decreasing temperature of isothermal holding from 380 to 235°C the values of 6, a, and o_ l gradually decrease; o u and o0. 2 increase linearly with decreasing temperature

down to 275°C and then sharply decrease to their initial values. The strength properties of BCISG toothed gears are higher than those of heat treated steel gears [57].

Maximum impact toughness in BCISG is obtained after austenitization at 950°C for 15 min. Increased holding time at the austenizing temperatures 850 to 950°C leads to decreased impact toughness at all isothermal holding temperatures. Decreasing the Si concentration from 2.9 to 1.6% appreciably decreases the impact toughness after isothermal quenching to equivalent hardness. The best combination of mechanical properties in CISG is associated with a structure of upper bainite and residual austenite, containing a small amount of lower bainite [58].

With suitable alloying it is possible to obtain cast CISG with a bainitic matrix having

o u = 900 N/~ 2, 6 = 3%, a~ 4°.2s = 4.6 J/cm 2, and Klc = 1750 N/mm 3/2. Tempering at 325°C in-

creases Klc by 20% [59]~

The fracture toughness of BCISG after isothermal quenching at 300-400°C with holding times from 2 min to 33 h has been investigated [60]. With increasing temperature of isothermal holding the fracture strength increases when a small quantity of bainite is present, and decreases when a large quantity is present. The largest fracture strength is obtained after annealing at 300-350°C for 0.3-3 h. With increasing isothermal holding temperature and quantity of bainite the fracture toughness decreases.

We (with the help of A. V. Telekhova and N. N. Kalabanovoi) have obtained cast and heat treated BCISG alloyed with Ni, Cu, Mo, or V possessing, depending upon composition and modification technique, o u = 600-1400 N/rR~ 2 , ~0.2 = 550-1370 N/l~m 2 , 6 = 1.7%, a = 5-56 J/cm 2 ,

and 240-500 HB.

The deformation of BCISG is not connected with the ferrite or austenite constituents of the structure, but occurs via a multitude of voids, often connected to each other. This deformation mechanism results from the enhanced ductility of BCISG. Only small zones of quasi-cleavage are found in the fracture, due to the difficulty in crack extension by the mechanism of quasi-cleavage. In the case of isothermal quenching, zones of quasi-cleavage are more numerous because of the presence of martensite in the structure. Regions of quasi- cleavage do not appear in BCISG with additions of Cu and Ni. In BCISG with additions of Cu and Mo regions of quasi-cleavage occur in locations where Mo has segregated, and, therefore, its concentration should not exceed 0.3%. It is desirable that the Mn and Si concentrations should not exceed 0.2% and 3%, respectively [61].

The abrasive wear resistance of isothermally quenched CISG is appreciably higher than that of ordinary CISG of the same composition, and increases with decrease in the isothermal quenching temperature. The wear resistance decreases with increase of the isothermal holding time between 2 and 30-60 min, and does not change at longer holding times [62].

The abrasive wear resistances of BCISG with hardnesses of 260-400 HB are approximately equal. This is explained by the transformation of residual austenite with low hardness in the cast iron during testing producing equalization of the hardnesses. The stability of the residual austenite, which increases with increasing isothermal holding time, affects the abrasive wear resistence. The abrasive wear resistance of BCiSG decreases with increasing austenite stability. Cast CISG with a bainitic structure has lower abrasive wear resistance then isothermally quenched CISG. The abrasive wear resistance of BCISG is less dependent on hardness than that of steel. The abrasive wear resistance of BCISG with a hardness of 350 HB is twice that of rail steel with a hardness of 255 HB, and three times that of low alloyed Cr--Mo steel with a hardness of 304 HB [63].

FIELDS OF APPLICATION OF BCISG

The principal sphere of application of BCISG is in machine construction, where it is used for the fabrication of parts in which high strength, ductility, impact toughness, wear resistance and fatigue strength are required. This combination of properties is attained as a result of surface hardening by the decomposition of retained austenite. The wear rate of BCISG in dry friction is less than that of other types of CISG, as well as of carbon and low steels, although it is inferior to the Ni Hard type of white cast iron.

769

Most of all, BCISG is used for the production of cast pinion and toothed gears. The institute of gear transmissions (USA) has undertaken development of the technology and cre- ation of a data bank on the properties of BCISG, directed toward its production and wide- spread use in the manufacture of toothed gears, the potential requirements for which in the USA and Canada alone amount to about i0 million tons per year. The possibility of utilizing BCISG instead of steel for the production of transmission and main drive gears, and of parts for the universal joints of automobiles is under investigation by PO "ZIL" together with NAMI [2].

BCISG is a prospective material for transport, agricultural, and heavy-machine construction; for the production of automobile parts (steering-ear joints, parts of the fluid drive, crankshafts, and engines, particularly diesel), wheelhubs, caterpillar drive sprockets, coup- lings, rolling-mill rolls, railway wheels, etc. Examples of the use of BCISG for various parts weighing 2.5 to 600 kg as a substitute for Cr--Ni--Mo steels are presented in [15], and data concerning the practical application of GGG-140, GGG-120, GGG-90, and GGG-80B gades of BCISG are generalized in [64]. The properties of castings used instead of forged steel are given for the following parts: toothed and worm gears of various types, disc cams, refrigerator compressor crankshafts, truck spring supports, rear-axle differential spiders, wear resistant inserts for sludge pumps, hooks and hinges for BCISG containers. This substitution allows a 10% decrease in weight, and cost savings of 10-30%. Automobile crankshafts produced from BCISG have higher o u and 0o. 2 than forged. Furthermore, their

properties are isotropic in all directions.

LITERATURE CITED

i. K. E. Vashchenko, A. M. Petrichenko, Yu. A. Shul'te, et al., Progress of Foundry Produc- tion in the Ukrainian SSR [in Russian], Naukova Dumka, Kiev (1988).

2. Ya. G. Kletskin and M. M. Levitan, Liteinoe Proizvodstvo, No. 9, 9-13 (1987). 3. R. B. Gundlach and J. P. Janowak, in: First Int. Conf. Austempered Ductile Iron, Metals

Park, Ohio (1984), pp. 1-12. 4. K. C. Voigt and C. R. Loper, in: Met. Cast Iron: Proc. 3rd Int. Symp., Stockholm

(1984), New York, et al. (1985), pp. 377-386. 5. J. M. Schissler and J. Saverna, in: First Int. Conf. Austempered Ductile Iron, Metals

Park, Ohio (1984), pp. 71-82. 6. T. Podrabsky, J. Svej'car, E. Dorazil and B. Barta, Slevarenstvi, 35, No. i, 27-37

(1987). 7. J . M . Sch iss ler and J. P. Chobaut, Harter-Techn. M i t t . , 42, No. 4, 205-210 (1987). 8. R. P. Gundlach, J. P. Janowak, S. Bechet, and K. Rohrig, in: Phys. Met. Cast Iron:

Proc. 3rd Int. Symp., Stockholm (1984), New York,-et al., (1985), pp. 399-409. 9. J. D. Verhoeven, A. E1 Nagar, B. E1 Sarnagawa, et al., in: Phys. Met. Cast Iron:

Proc. 3rd Int. Symp., Stockholm (1984), New York et al. (1985), pp. 387-398. I0. M. Gagne, Giesserei-Prax., No. I0, 155-164 (1987). ii. D. J. Moore, T. N. Rouns, and K. B. Rundman, Trans. Amer. Foundrymens. Soc., 94, 255-

264 (1986). 12. Y. J. Park, P. A. Morton, M. Gagne, and R. Goller, Giesserei-Prax., No. i-2, 1-7 (1980). 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

R. A. Harding, Metals and Materials, 2, No. 2, 65-72 (1986). R. Viau, M. Gagne, and R. Thibau, Giesserei-Prax., No. 9-10, 117-125 (1988). G. Barbezat and H. Mayer, Mater'. et Tech., 75, No. 9, 364-370 (1987). S. K. Yu, G. R. Loper, and H. H. Cornell, Giesserei-Prax., No. 15-16, 228-237 (1987). M. Gagne and P. A. Fallon, Can. Met. Quart., 25, No. i, 79-90 (1986). S. Peitrowski, Prz. odlew, 3~-, No. 5, 157-161 (1987). Disclosure 61 - 288013, Japan. Disclosure 61 - 60855, Japan. Disclosure 61 - 291919, Japan. Disclosure 63 - 166928, Japan. Disclosure 63 - 126604, Japan. Disclosure 61 - 243121, Japan. Disclosure 63 - 140037, Japan. A. s. 1157113 SSSR, MKI $21S 37/00. C. Yoshida and S. Shibutani, Kobe Steel Eng. Rpts., 37, No. 3, 59-62 (1987). M. P. Shebatinov, A. A. Zhokov, and V. M. Kovalenko, Vestn. Mashin., No. 3, 61-63 (1986).

770

29. S. E. Stenfors, in: Phys. Met. Cast Iron: Proc. 3rd Int. Symp., Stockholm (1984), New York, et al. (i985), pp. 423-432.

30. O. Yanagisawa, T. Yano, and H. Fukuhara, J. Jpn. Soc. Heat Treat., 28, No. 5, 314-319 (1988).

31. D. J. Moore, T. N. Round, and K. B. Rundman, Giesserei-Prax., No. 23-24, 309-318 (1988). 32~ J. P. Janowak and R. P. Gundlach, Trans. Am. Foundrymens Soc., 93, 377-388 (1983). 33. G. J. Cox, Brit. Foundryman, 7_99, No. 5, 215-219 (1986). 34. J. Fargues and S. Parent-Simon±n, Mem. et etude Sci. Rev. Met., 85, No. 9, 510 (1988). 35. F. I. Yakovlev, Liteinoe Proizvodstvo, No. 2, 18 (1986). 36. D. J. Moore, T. N. Rouns, and K. B. Rundman, Heat Treat., i, No. i, 7-24 (1985). 37. T. Kobayasi, Soke'zai, 27, No. i, 15 (1986). 38. H. Yamamoto, J. Iron Steel Inst. Japan, 72, No. 13, 1489 (1986). a~. ~±~u±OSUL~ 61 -- ±4~42z, Japan. L~ ~u. Disclosure o~ ± ~Ju~1~, ...... Japan. Jl ~±. Disclosure 61 - 149934, Japan. 42. Disclosure 61 - 174333, Japan. 43. Disclosure 60 - 243216, Japan. 44. Disclosure 62 - 44522, Japan. 45. Disclosure 61 - 143514, Japan. 46. Disclosure 63 - 432, Japan 47. Patent 4619713, USA. 48. Disclosure 61 - 79717, Japan. 49. Disclosure 61 - 79718, Japan. 50. Disclosure 61 - 288011, Japan. 51. Disclosure 61 - 288012, Japan. 52. Disclosure 62 - 83419, Japan. 53. Disclosure 63 - 140030, Japan. 54. Disclosure 61 - 60854, Japan. 55. M. Brunner, Met. y elec., 49, No. 564, 36-37 (1985). 56. A. Alagarsamy, in: First Int. Conf. Austempered Ductile Iron, Metals Park, Ohio,

pp. 105-106. 57. W. Hauke, VDI-Ber. No. 489, 105-111 (1983). 58. M. M. Shea and E. F. Ryntz, Trans. Am. Foundrymens Soc., 94, 683-688 (1986). 59. M. Hui, C. Li, J. Liang, et al., Chin. J. Mech. Eng., 24, No. 2, 6-11 (1988). 60. J. L. Doong and C. S. Chen, Fatigue Fract. Eng. Mater. and Struct., 12, No. 2,

155-165 (1989). 61. M. Gagn~, M&m. e t ~tud. Sci . Rev. Met . , 84, No. 1, 35-45 (1987). 62. S. Shepperson and C. Allen, Wear, 121, No. 3, 271-287 (1988). 63. K. Rohrig, Konstr. + Giessen, No. 2, 7-12 (1988). 64. K. Rohrig, Konstr. + Giessen, No. 4, 43-51 (1986).

771

bainitic high-strength cast iron with spheroidal graphite (review)

Documents