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Effects of heat treatment cycle on equilibrium between ferrite and austenite during intercritical annealing
Materials Science and Engineering, A161 (1993) 267-271
267
Effects of heat treatment cycle on equilibrium between ferrite and austenite during intercritical annealing H . K. K h a i r a a, A . K. J e n a b a n d M . C. C h a t u r v e d F
aDepartment of Mechanical Engineering, MA CT, Bhopal (India) bDepartment of MetallurgicalEngineering, HT, Kanpur (India) CDepartmentof Mechanical Engineering, Universityof Manitoba, WinnipegR3T 2N2 (Canada) (Received July 10, 1992; in revised form October 5, 1992)
Abstract A steel containing 0.2% C, 0.98% Mn, 0.46% Si and 0.47% Cr was intercritically annealed at 750, 770, 790 and 810 °C for 15 minutes and quenched to room temperature to produce dual-phase structures containing ferrite and martensite. Three heat-treatment cycles--IN, IQ and IA--were applied to the steel such that the structure at the beginning of intercritical annealing consisted of pearlite, martensite and austenite respectively. During intercritical annealing the steel first approaches partial equilibrium, which involves no partitioning of substitutional solutes and equilibrium with respect to carbon. In the IQ cycle the partial equilibrium state is attained. The steel remains in that state up to 770 °C, but above 770 °C departs from that state early during annealing because of partitioning of substitutional solutes. In the IN and IA cycles, the transformation rates are too slow for the partial equilibrium state to be attained in 15 minutes. In the case of the IA cycle, the departure from the partial equilibrium state increases with an increase in annealing temperature because of a decrease in the transformation rate with an increase in temperature.
I. Introduction
Intercritical annealing of steels between Ae~ and A e 3 temperatures results in the formation of ferrite and austenite, and involves partitioning of the alloying elements between these two phases. The amount of austenite that forms in the steel is determined by the extent of partitioning of the alloying elements. The kinetics of partitioning depends upon the diffusivities of the solutes. The diffusivity of carbon in steel is nearly 105 to 106 times larger than that of the substitutional solutes [1]. Therefore, establishment of complete equilibrium may require high intercritical annealing temperatures and a longer annealing time. However, at low annealing temperatures and a short annealing time partial equilibrium [2, 3] involving no partitioning of substitutional solutes and equilibrium with respect to carbon may be established. Suitable cooling after intercritical annealing produces dual-phase steels containing ferrite and martensite. The martensite content of the steel is critical in determining its properties [4]. The percentage of martensite in a dual-phase steel is essentially equal to that of austenite created during annealing at the intercritical temperature. It has been shown that the percentage of martensite in a number of dual phase steels can be 0921-5093/93/$6.00
accurately predicted on the basis of the partial equilibrium model [5]. The investigation of a dual phase steel has shown that partial equilibrium was established in the steel up to the annealing temperature 770 °C and complete equilibrium was approached at higher annealing temperatures [6]. However, the extent of partitioning and the establishment of equilibrium should depend upon the initial structure of the steel which is established by the heat treatments given to the steel prior to intercritical annealing. A number of heat treatment cycles are employed for producing the dualphase structure. In this study the effects of three widely different heat treatment cycles on the establishment of equilibrium during intercritical annealing have been investigated.
2. Experimental details
The steel used in this investigation was swaged, cleaned and homogenized in vacuum-sealed quartz tubes at about 1250°C for 24 h in order to have a segregation-free alloy. The homogenized alloy was aircooled to room temperature. Optical and scanning metallography did not give any indication of inhomogeneity. Chemical analysis of samples of the homogen© 1993 - Elsevier Sequoia. All rights reserved
268
H. K. Khaira et al.
/
Effect of heat treatment of ferrite-austenite equilibrium
ized steel yielded 0.20% C, 0.98% Mn, 0.46% Si and 0.47% Cr by weight. Specimens cut from this steel were subjected to three different heat treatment cycles. In one of the cycles, the homogenized specimens were austenitized at 900 oC for 90 min and air-cooled to produce a normalized structure. Such specimens were upquenched to the intercritical temperature, held there for 15 min and quenched to room temperature in iced brine. This cycle is designated intermediate normalizing, IN, as the steel prior to intercritical annealing contains a fine pearlitic structure characteristic of normalizing. In another cycle homogenized specimens were austenized at 900 °C for 90 min and water-quenched to room temperature to produce a martensitic structure. These specimens were upquenched to the intercritical temperature and after holding for 15 min, quenched in iced brine to room temperature. This is called the intermediate quenching cycle, IQ, as the steel contained a martensitic structure prior to intercritical annealing. The third cycle is designated intermediate austenitizing, IA. It involved heating the homogenized specimens to 900 °C for 90 min for austenitization, and quenching from the austenitizing temperature directly to the intercritical annealing temperature. After holding at the intercritical temperature for 15 min the specimens were quenched to room temperature in iced brine. All the specimens to be heat treated were vacuumsealed in quartz tubes to avoid oxidation and decarburization. For quenching the specimens, the quartz tubes were broken under iced brine. Intercritical annealing was carried out in salt baths and lead baths. Four intercritical annealing temperatures, 750, 770, 790 and 810°C were used. The temperatures of the baths were controlled to within _+ 1 °C. The intercritical annealing time was kept constant at 15 min. The intercritically annealed specimens contained ferrite and martensite. Microhardness measurements were made to identify the phases. Each reported hardness value was the average of at least 10 measurements. The microhardness of martensite was always higher ~than that of ferrite. The Volume fraction of martensite was measured by the point counting technique. About 50 measurements were made on the same specimen. Several repeat experiments showed the results to be reproducible.
3. Results and discussion 3.1. Mole fraction o f austenite
The intercritically annealed specimens contained ferrite and martensite. The phases were easily distinguishable in the microstructure. In order to establish the identities of the phases, microhardness measure-
ments were made. The hardness values of the two phases were quite different. The microhardness of ferrite was 310 VPN which is comparable to values reported in the literature [7]. The hardness of martensite varied from 435 to 660 VPN when the annealing temperature was changed from 750 to 810°C. These results are consistent with the reported variation in hardness of martensite with carbon content [8]. The presence of retained austenite or other phases was not detected. Thus, the mole fraction of martensite, N~,, present in a specimen was the same as that of austenite present at the intercritical temperature, N r = N ~,
(1)
The volume fraction of martensite, V.,, is the experimentally measured quantity. Va, may be expressed as: Va, = 1 - {ma/[m~ + (1 - Na)(v~,/v~)]}
(2)
where Na is the mole fraction of the ferrite, and v~, and v~ are the molar volumes of martensite and ferrite respectively. It follows from eqns. ( 1 ) and (2) that
where V~ is the volume fraction of a and Av is equal to (v~,- v~). In the experimental steel, V~ varies between 0 and 0.65. Taking the values of v,, and v~ from literature [9], the maximum difference between N~ and V~, is less than 2%. The calculated mole fraction of austenite is plotted in Fig. 1 as a function of intercritical annealing temperature. The A e 1 and A e 3 temperatures of the experimental steel are also shown in the figure. The A e l temperature was calculated using Andrews' equation [10]. The A e 3 temperature was calculated after a model developed on the basis of data available on 173 steels [11]. The mole fraction of ~ should be one at A e 3 temperature, should decrease with a decrease of intercritical annealing temperature, and should have the lowest value at the A e l temperature. The three heattreatment cycles show a decrease of austenite content with a decrease of annealing temperature. However, the differences between the austenite contents produced in the three heat-treatment cycles are large. During the IQ cycle, martensite present in the steel is tempered during upquenching to the intercritical annealing temperature, and produces a structure consisting of fine particles of carbide distributed in ferrite with considerable substructure. In such a material nucleation of austenite can occur easily at the carbide-ferrite interfaces during intercritical annealing. The nucleation rate would be high. The growth rate would also be high because of reduced diffusion dis-
H. K. Khaira et al.
1.0
Treotment Cycle oIN
Heat
AIQ
Effect of heat treatment of ferrite-austenite equilibrium
o
//
olA
~- 0.8 Z l"m
0
Z 0.6
Q
I0
o
/
0.4
Ae I
o.2
7o0
I
Ae 3
I
750
I
800
,1
850
INTERCRITICAL TEMPERATURE,°C Fig. t. Variation of the mole fraction of austenite with inter-
critical annealingtemperature.
269
Therefore, the applicability of the partial equilibrium model to the dual-phase steel heat treated in the abovementioned three cycles is considered. Steel containing homogeneous austenite is intercritically annealed during the IA cycle. Hence, formation of ferrite is expected to be governed by partial equilibrium. The steel treated in the IQ cycle contains tempered martensite prior to intercritical annealing. Tempered martensite consists of fine carbides distributed in a matrix of heavily substructured ferrite. Therefore, the carbides are expected to dissolve in a short time and partial equilibrium is likely to be applicable. During the IN cycle the specimens are normalized before intercritical annealing. Normalizing involves formation of pearlite at low temperatures owing to air cooling. Substitutional solute partitioning between ferrite and cementite of pearlite is usually limited at low transformation temperatures as the free energy available for the transformation is large at high supercooling. For example, the partition coefficient of manganese between cementite and ferrite is one at temperatures below about 660 °C. Therefore, the partitioning of substitutional solutes between ferrite and cementite is expected to be small and the austenite that forms at the intercritical temperature from the normalized pearlite is expected to have almost the same substitutional solute content as steel. If the time required for the dissolution of carbides is small compared with the intercritical annealing time, the partial equilibrium model is likely to be applicable to the steel treated in the IN cycle. Partial equilibrium between ferrite and austenite at an intercritical temperature exists when the substitutional solutes do not partition between ferrite and austenite, but carbon equilibrium is maintained between the two phases, If the substitutional solutes, Mn, Cr and Si do not partition, the ratio of the atom fraction of a substitutional solute and that of iron is the same in all phases. Hence,
tance. Thus, the transformation rate in the IQ cycle would be high. The structure of the steel just before intercritical annealing during the other two cycles is comparatively coarse. The steel has lamellar pearlitic structure in the IN cycle and only austenite grains in the IA cycle. For these cycles the rates of transformation at the intercritical range would be much lower than that for the IQ cycle. Therefore, the structure produced in the IQ cycle should be closest to the equilibrium structure, and the amount of austenite that forms during the other two cycles should differ appreciably from that formed during the IQ cycle (see Fig. 1). For the IA cycle, the initial mole fraction of anstenite is one and it decreases and approaches the equilibrium value with an increase in annealing time. Therefore, the austenite content in specimens treated in the LA cycle is higher than that in specimens treated in the IQ cycle. The initial mole fraction of austenite in the IN cycle is zero and it increases with an increase in annealing time. Therefore, the mole fraction of austenite that forms in the IN cycle is lower than that in the IQ cycle (Fig. 1 ). Figure 1, however, does not clarify the nature of the equilibrium.
Xi, a = KiXFe, a
(5a)
Xi, y = K i X F e , ~ ,
(50)
3.2. Partial equilibrium
The sum of the mole fractions of components in a phase is one. Therefore, it follows from eqn. (5) that:
Diffusivity of carbon in steel is many orders of magnitude higher than the diffusivities of the substitutional solutes in steel, and partial equilibrium a t intercritical annealing temperature involving no partitioning of substitutional solutes and partitioning of carbon is known to exist in a number of dual-phase steels [5, 6].
( X i / X F e ) ~ = ( X i / X F e ) a -~ ( X i / X F e ) s = K i
(4)
where X is the atom fraction, i = Mn, Cr, Si and the subscripts a, y and s refer to ferrite, austenite and the steel respectively. As K i for a given substitutional solute is a constant,
Xi. a =[K,/(1 + ZK,)](I -Xc.a)
(6a)
XFe.a = [1/( 1 + X Ki)]( 1 -Xc.
(6b)
I + ZKi)](1 -Xc, y)
(6c)
XFe. = [l/(1 + XKi)](1 - Xc, )
(6d)
H. K. Khaira et al.
270
/
Effect of heat treatment of ferrite-austenite equilibrium
Thus, a single composition variable X c determines the properties of a phase. For carbon equilibrium between ferrite and austenite,
co, =co,
z~og ,.
L0-
(8)
A third relation between the three unknowns, Xc, ~, Xc, and N~ is obtained by assuming that the transformation minimizes the free energy of the system. For the free energy of a multicomponent system in partial equilibrium to be a minimum, the following relation is valid
[5]:
o
o IN
ix IQ
o
VZx
~
/ ~n
o.8
(7)
where Gc,~ and (~, ~ are partial Gibbs free energies of carbon in phases a and 7 respectively. Also G~.a and G~,y are functions of only the carbon contents of the phases, Xc. ~ and X~,y respectively. The mass balance of carbon shows:
Xc,~,Na+Xc,7(1-Na)=X~,s
Heat Treatment Cycle
oy
u..J (~ 0.6-
13
I-./ o.,-
wO~ne ~ W 0.2 Q > 0
0
0.2
0.4
0.6
0.8
1.0
MEASUREDVOLUMEFRACTION OF MARTENSITE
Fig. 2. Comparison of the measured volume fractions of martensite with that predicted by the partial equilibrium model.
[XFe, a GFe, o "1-ZXi, a a/, a]/[ 1 - X c , a ]
= [XFe,, CFe,~+ ZX,. yG,, r]/[1 - X~, r]
(9)
Equations (7), (8) and (9) were solved using available data on free energy interaction coefficients. The procedure is outlined elsewhere in detail [5]. Knowing N~, the values of the volume fractions of martensite were computed using eqns. (1) and (2). The calculated volume fraction of martensite is plotted in Fig. 2 with that determined experimentally. The figure also contains values obtained with another dual-phase steel which had been treated in the IQ cycle with 20 min intercritical annealing time. The data points in Fig. 2 should lie on the straight line shown in that figure when the experimentally determined values and the values computed on the basis of partial equilibrium are identical. It is clear that at low volume fractions of martensite, the IQ cycle yields results which are in excellent agreement with the predictions of the partial equilibrium model. The specimens treated in the IN and IA cycles also approach partial equilibrium at low volume fractions of martensite. However, in these two cycles the partial equilibrium is not attained in 15 min because of their low transformation rates. The specimens treated in all three cycles deviate appreciably from partial equilibrium at high volume fractions of martensite. High volume fraction of martensite imply high intercritical annealing temperature. Thus, at high temperatures the system appears to deviate appreciably from partial equilibrium. 3.3.
Effects of intercriticalannealing temperature
The volume fractions of martensite determined in the experimental steel are plotted in Fig. 3 against the intercritical annealing temperature. The data reported on another dual-phase steel [6] treated in the IQ cycle
Heat Treatment Cycle us I.-
OlA
o IN
~ 0.8
~
/-portiol Equilibrium 6
-
0.6 -
~ 0.4 uJ _
o~0.2
0
700
Ael
I
Ae 5
I
,
750 800 INTERCRITICAL TEMPERATURE ,°C
,I
850
Fig. 3. Change of volume fraction of martensite with intercritical annealing temperature. Values due to partial equilibrium and complete equilibrium are also shown.
are also shown. The results obtained with both steels are identical. The composition of the second steel [6] (1.0% Mn, 0.5% Cr, 0.5% Si and 0.2% C) is also very close to that used in this investigation. Therefore, the reported volume fractions of martensite obtained in the second steel at equilibrium at the intercritical annealing temperatures are reproduced in Fig. 3 as a smooth straight line. These values are expected to be close to those expected in the experimental steel at equilibrium. Volume fractions of martensite calculated assuming partial equilibrium are also shown as a smooth curve in Fig. 3.
H. K. Khaira et al.
/
Effect of heat treatment of ferrite-austenite equilibrium
The figure shows that in the specimens treated in the IQ cycle partial equilibrium is attained up to about 770°C. Above this temperature the departure from partial equilibrium is appreciable. Consider the transformation at low annealing temperatures. When the annealing time is zero, the amount of austenite is zero. With an increase in anneafing time the amount of austenite increases beyond the value expected at equilibrium (Fig. 3) and approaches a higher value characteristic of partial equilibrium. When the annealing time is increased further the amount of austenite tends to decrease due to substitutional solute partitioning, and approaches a lower value characteristic of equilibrium. The same process occurs at high annealing temperatures. However, in the IQ cycle the amount of 7 at the start of transformation is zero, and the rate of transformation should increase with an increase in temperature. Because of high transformation rates at high intercritical annealing temperatures the system departs from the partial equilibrium state early during annealing. The specimens heat treated in the IN and IA cycles behave in the same manner as specimens treated in the IQ cycle. In the IN cycle, the amount of austenite is zero at the beginning of intercritical annealing and increases with an increase in time. Figure 3 suggests that because of low transformation rate the specimens treated in the IN cycle do not even attain the partial equilibrium state. For similar reasons the specimens treated in the IA cycle also do not attain the partial equilibrium state. During intercritical annealing austenite forms from ferrite and carbide in the IQ and IN cycles. Therefore, the driving force and the transformation rate increase with an increase in temperature. In the IA cycle the specimens contain 100% austenite at the start of intercritical annealing and ferrite precipitates from austenite. The driving force and the transformation rate are low at high temperatures, and increase with a decrease in temperature. Therefore, the departure from the partial equilibrium state is much higher at high temperatures than at low temperatures.
4. Summary and conclusion (1) A steel containing 0.2% C, 0.98% Mn, 0.46% Si and 0.47% Cr was intercritically annealed at 750, 770,
271
790 and 810 °C for 15 min and quenched in iced brine to produce dual-phase steels containing ferrite and martensite. Three heat-treatment cycles, IN, IQ and IA, were employed so as to have pearlite, martensite and austenite respectively prior to intercritical annealing. (2) The austenite content in the three cycles decreased with a decrease in annealing temperature. However, the amount of austenite was widely different in different cycles. (3) Calculations were made for partial equilibrium, which implies no partitioning of substitutional solutes and equilibrium with respect to carbon. The results showed that a partial equilibrium state in the steel was attained in the IQ cycle and was approached in the IN and IA cycles. (4) In the IQ cycle the steel departed from the partial equilibrium state early during annealing at high temperatures as the transformation rate in this cycle increased with an increase in temperature. The partial equilibrium state was not attained in the IN and IA cycles because of low rates of transformation. However, the steel in the IA cycle remained away from the partial equilibrium state even at high temperatures due to an increase in the rate of transformation with an increase in temperature.
References 1 C. Smithels and E. A. Brandes, Metals Reference Book, Butterworths, London, 1967. 2 M. Hillert, JernkontoretsAnn., 136 (1952) 25. 3 J. B. Gilrnour, G. R. Purdy and J. S. Kirkaldy, Metall. Trans., 3 (1972) 1455. 4 A.R. Marder, Metall. Trans., 13A (1982) 85. 5 H. K. Khaira and A. K. Jena, in S. E Mehrotra and T. R. Ramaehandran (eds.), Prog. in Metallurgical Research, TataMcGraw Hill, New Delhi, 1986, p. 559. 6 M.C. Chaturvedi and A. K. Jena, Mater. Sci. Eng., 94 (1987) L1. 7 A.R. Marder, in A. T. Davenport (ed.), Formable HSLA and Dual Phase Steels, TMS-AIME, New York, 1979, p. 89. 8 A. K. Jena and M. C. Chaturvedi, Mater. Sci. Eng., 100 (1988) 1. 9 W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London, 1964. 10 K.W. Andrews, J. Iron Steel Inst., London, 203 (1965) 721. 11 H.K. Khaira, Ph.D. Thesis, IIT, Kanpur, India, 1986.
267
Effects of heat treatment cycle on equilibrium between ferrite and austenite during intercritical annealing H . K. K h a i r a a, A . K. J e n a b a n d M . C. C h a t u r v e d F
aDepartment of Mechanical Engineering, MA CT, Bhopal (India) bDepartment of MetallurgicalEngineering, HT, Kanpur (India) CDepartmentof Mechanical Engineering, Universityof Manitoba, WinnipegR3T 2N2 (Canada) (Received July 10, 1992; in revised form October 5, 1992)
Abstract A steel containing 0.2% C, 0.98% Mn, 0.46% Si and 0.47% Cr was intercritically annealed at 750, 770, 790 and 810 °C for 15 minutes and quenched to room temperature to produce dual-phase structures containing ferrite and martensite. Three heat-treatment cycles--IN, IQ and IA--were applied to the steel such that the structure at the beginning of intercritical annealing consisted of pearlite, martensite and austenite respectively. During intercritical annealing the steel first approaches partial equilibrium, which involves no partitioning of substitutional solutes and equilibrium with respect to carbon. In the IQ cycle the partial equilibrium state is attained. The steel remains in that state up to 770 °C, but above 770 °C departs from that state early during annealing because of partitioning of substitutional solutes. In the IN and IA cycles, the transformation rates are too slow for the partial equilibrium state to be attained in 15 minutes. In the case of the IA cycle, the departure from the partial equilibrium state increases with an increase in annealing temperature because of a decrease in the transformation rate with an increase in temperature.
I. Introduction
Intercritical annealing of steels between Ae~ and A e 3 temperatures results in the formation of ferrite and austenite, and involves partitioning of the alloying elements between these two phases. The amount of austenite that forms in the steel is determined by the extent of partitioning of the alloying elements. The kinetics of partitioning depends upon the diffusivities of the solutes. The diffusivity of carbon in steel is nearly 105 to 106 times larger than that of the substitutional solutes [1]. Therefore, establishment of complete equilibrium may require high intercritical annealing temperatures and a longer annealing time. However, at low annealing temperatures and a short annealing time partial equilibrium [2, 3] involving no partitioning of substitutional solutes and equilibrium with respect to carbon may be established. Suitable cooling after intercritical annealing produces dual-phase steels containing ferrite and martensite. The martensite content of the steel is critical in determining its properties [4]. The percentage of martensite in a dual-phase steel is essentially equal to that of austenite created during annealing at the intercritical temperature. It has been shown that the percentage of martensite in a number of dual phase steels can be 0921-5093/93/$6.00
accurately predicted on the basis of the partial equilibrium model [5]. The investigation of a dual phase steel has shown that partial equilibrium was established in the steel up to the annealing temperature 770 °C and complete equilibrium was approached at higher annealing temperatures [6]. However, the extent of partitioning and the establishment of equilibrium should depend upon the initial structure of the steel which is established by the heat treatments given to the steel prior to intercritical annealing. A number of heat treatment cycles are employed for producing the dualphase structure. In this study the effects of three widely different heat treatment cycles on the establishment of equilibrium during intercritical annealing have been investigated.
2. Experimental details
The steel used in this investigation was swaged, cleaned and homogenized in vacuum-sealed quartz tubes at about 1250°C for 24 h in order to have a segregation-free alloy. The homogenized alloy was aircooled to room temperature. Optical and scanning metallography did not give any indication of inhomogeneity. Chemical analysis of samples of the homogen© 1993 - Elsevier Sequoia. All rights reserved
268
H. K. Khaira et al.
/
Effect of heat treatment of ferrite-austenite equilibrium
ized steel yielded 0.20% C, 0.98% Mn, 0.46% Si and 0.47% Cr by weight. Specimens cut from this steel were subjected to three different heat treatment cycles. In one of the cycles, the homogenized specimens were austenitized at 900 oC for 90 min and air-cooled to produce a normalized structure. Such specimens were upquenched to the intercritical temperature, held there for 15 min and quenched to room temperature in iced brine. This cycle is designated intermediate normalizing, IN, as the steel prior to intercritical annealing contains a fine pearlitic structure characteristic of normalizing. In another cycle homogenized specimens were austenized at 900 °C for 90 min and water-quenched to room temperature to produce a martensitic structure. These specimens were upquenched to the intercritical temperature and after holding for 15 min, quenched in iced brine to room temperature. This is called the intermediate quenching cycle, IQ, as the steel contained a martensitic structure prior to intercritical annealing. The third cycle is designated intermediate austenitizing, IA. It involved heating the homogenized specimens to 900 °C for 90 min for austenitization, and quenching from the austenitizing temperature directly to the intercritical annealing temperature. After holding at the intercritical temperature for 15 min the specimens were quenched to room temperature in iced brine. All the specimens to be heat treated were vacuumsealed in quartz tubes to avoid oxidation and decarburization. For quenching the specimens, the quartz tubes were broken under iced brine. Intercritical annealing was carried out in salt baths and lead baths. Four intercritical annealing temperatures, 750, 770, 790 and 810°C were used. The temperatures of the baths were controlled to within _+ 1 °C. The intercritical annealing time was kept constant at 15 min. The intercritically annealed specimens contained ferrite and martensite. Microhardness measurements were made to identify the phases. Each reported hardness value was the average of at least 10 measurements. The microhardness of martensite was always higher ~than that of ferrite. The Volume fraction of martensite was measured by the point counting technique. About 50 measurements were made on the same specimen. Several repeat experiments showed the results to be reproducible.
3. Results and discussion 3.1. Mole fraction o f austenite
The intercritically annealed specimens contained ferrite and martensite. The phases were easily distinguishable in the microstructure. In order to establish the identities of the phases, microhardness measure-
ments were made. The hardness values of the two phases were quite different. The microhardness of ferrite was 310 VPN which is comparable to values reported in the literature [7]. The hardness of martensite varied from 435 to 660 VPN when the annealing temperature was changed from 750 to 810°C. These results are consistent with the reported variation in hardness of martensite with carbon content [8]. The presence of retained austenite or other phases was not detected. Thus, the mole fraction of martensite, N~,, present in a specimen was the same as that of austenite present at the intercritical temperature, N r = N ~,
(1)
The volume fraction of martensite, V.,, is the experimentally measured quantity. Va, may be expressed as: Va, = 1 - {ma/[m~ + (1 - Na)(v~,/v~)]}
(2)
where Na is the mole fraction of the ferrite, and v~, and v~ are the molar volumes of martensite and ferrite respectively. It follows from eqns. ( 1 ) and (2) that
where V~ is the volume fraction of a and Av is equal to (v~,- v~). In the experimental steel, V~ varies between 0 and 0.65. Taking the values of v,, and v~ from literature [9], the maximum difference between N~ and V~, is less than 2%. The calculated mole fraction of austenite is plotted in Fig. 1 as a function of intercritical annealing temperature. The A e 1 and A e 3 temperatures of the experimental steel are also shown in the figure. The A e l temperature was calculated using Andrews' equation [10]. The A e 3 temperature was calculated after a model developed on the basis of data available on 173 steels [11]. The mole fraction of ~ should be one at A e 3 temperature, should decrease with a decrease of intercritical annealing temperature, and should have the lowest value at the A e l temperature. The three heattreatment cycles show a decrease of austenite content with a decrease of annealing temperature. However, the differences between the austenite contents produced in the three heat-treatment cycles are large. During the IQ cycle, martensite present in the steel is tempered during upquenching to the intercritical annealing temperature, and produces a structure consisting of fine particles of carbide distributed in ferrite with considerable substructure. In such a material nucleation of austenite can occur easily at the carbide-ferrite interfaces during intercritical annealing. The nucleation rate would be high. The growth rate would also be high because of reduced diffusion dis-
H. K. Khaira et al.
1.0
Treotment Cycle oIN
Heat
AIQ
Effect of heat treatment of ferrite-austenite equilibrium
o
//
olA
~- 0.8 Z l"m
0
Z 0.6
Q
I0
o
/
0.4
Ae I
o.2
7o0
I
Ae 3
I
750
I
800
,1
850
INTERCRITICAL TEMPERATURE,°C Fig. t. Variation of the mole fraction of austenite with inter-
critical annealingtemperature.
269
Therefore, the applicability of the partial equilibrium model to the dual-phase steel heat treated in the abovementioned three cycles is considered. Steel containing homogeneous austenite is intercritically annealed during the IA cycle. Hence, formation of ferrite is expected to be governed by partial equilibrium. The steel treated in the IQ cycle contains tempered martensite prior to intercritical annealing. Tempered martensite consists of fine carbides distributed in a matrix of heavily substructured ferrite. Therefore, the carbides are expected to dissolve in a short time and partial equilibrium is likely to be applicable. During the IN cycle the specimens are normalized before intercritical annealing. Normalizing involves formation of pearlite at low temperatures owing to air cooling. Substitutional solute partitioning between ferrite and cementite of pearlite is usually limited at low transformation temperatures as the free energy available for the transformation is large at high supercooling. For example, the partition coefficient of manganese between cementite and ferrite is one at temperatures below about 660 °C. Therefore, the partitioning of substitutional solutes between ferrite and cementite is expected to be small and the austenite that forms at the intercritical temperature from the normalized pearlite is expected to have almost the same substitutional solute content as steel. If the time required for the dissolution of carbides is small compared with the intercritical annealing time, the partial equilibrium model is likely to be applicable to the steel treated in the IN cycle. Partial equilibrium between ferrite and austenite at an intercritical temperature exists when the substitutional solutes do not partition between ferrite and austenite, but carbon equilibrium is maintained between the two phases, If the substitutional solutes, Mn, Cr and Si do not partition, the ratio of the atom fraction of a substitutional solute and that of iron is the same in all phases. Hence,
tance. Thus, the transformation rate in the IQ cycle would be high. The structure of the steel just before intercritical annealing during the other two cycles is comparatively coarse. The steel has lamellar pearlitic structure in the IN cycle and only austenite grains in the IA cycle. For these cycles the rates of transformation at the intercritical range would be much lower than that for the IQ cycle. Therefore, the structure produced in the IQ cycle should be closest to the equilibrium structure, and the amount of austenite that forms during the other two cycles should differ appreciably from that formed during the IQ cycle (see Fig. 1). For the IA cycle, the initial mole fraction of anstenite is one and it decreases and approaches the equilibrium value with an increase in annealing time. Therefore, the austenite content in specimens treated in the LA cycle is higher than that in specimens treated in the IQ cycle. The initial mole fraction of austenite in the IN cycle is zero and it increases with an increase in annealing time. Therefore, the mole fraction of austenite that forms in the IN cycle is lower than that in the IQ cycle (Fig. 1 ). Figure 1, however, does not clarify the nature of the equilibrium.
Xi, a = KiXFe, a
(5a)
Xi, y = K i X F e , ~ ,
(50)
3.2. Partial equilibrium
The sum of the mole fractions of components in a phase is one. Therefore, it follows from eqn. (5) that:
Diffusivity of carbon in steel is many orders of magnitude higher than the diffusivities of the substitutional solutes in steel, and partial equilibrium a t intercritical annealing temperature involving no partitioning of substitutional solutes and partitioning of carbon is known to exist in a number of dual-phase steels [5, 6].
( X i / X F e ) ~ = ( X i / X F e ) a -~ ( X i / X F e ) s = K i
(4)
where X is the atom fraction, i = Mn, Cr, Si and the subscripts a, y and s refer to ferrite, austenite and the steel respectively. As K i for a given substitutional solute is a constant,
Xi. a =[K,/(1 + ZK,)](I -Xc.a)
(6a)
XFe.a = [1/( 1 + X Ki)]( 1 -Xc.
(6b)
I + ZKi)](1 -Xc, y)
(6c)
XFe. = [l/(1 + XKi)](1 - Xc, )
(6d)
H. K. Khaira et al.
270
/
Effect of heat treatment of ferrite-austenite equilibrium
Thus, a single composition variable X c determines the properties of a phase. For carbon equilibrium between ferrite and austenite,
co, =co,
z~og ,.
L0-
(8)
A third relation between the three unknowns, Xc, ~, Xc, and N~ is obtained by assuming that the transformation minimizes the free energy of the system. For the free energy of a multicomponent system in partial equilibrium to be a minimum, the following relation is valid
[5]:
o
o IN
ix IQ
o
VZx
~
/ ~n
o.8
(7)
where Gc,~ and (~, ~ are partial Gibbs free energies of carbon in phases a and 7 respectively. Also G~.a and G~,y are functions of only the carbon contents of the phases, Xc. ~ and X~,y respectively. The mass balance of carbon shows:
Xc,~,Na+Xc,7(1-Na)=X~,s
Heat Treatment Cycle
oy
u..J (~ 0.6-
13
I-./ o.,-
wO~ne ~ W 0.2 Q > 0
0
0.2
0.4
0.6
0.8
1.0
MEASUREDVOLUMEFRACTION OF MARTENSITE
Fig. 2. Comparison of the measured volume fractions of martensite with that predicted by the partial equilibrium model.
[XFe, a GFe, o "1-ZXi, a a/, a]/[ 1 - X c , a ]
= [XFe,, CFe,~+ ZX,. yG,, r]/[1 - X~, r]
(9)
Equations (7), (8) and (9) were solved using available data on free energy interaction coefficients. The procedure is outlined elsewhere in detail [5]. Knowing N~, the values of the volume fractions of martensite were computed using eqns. (1) and (2). The calculated volume fraction of martensite is plotted in Fig. 2 with that determined experimentally. The figure also contains values obtained with another dual-phase steel which had been treated in the IQ cycle with 20 min intercritical annealing time. The data points in Fig. 2 should lie on the straight line shown in that figure when the experimentally determined values and the values computed on the basis of partial equilibrium are identical. It is clear that at low volume fractions of martensite, the IQ cycle yields results which are in excellent agreement with the predictions of the partial equilibrium model. The specimens treated in the IN and IA cycles also approach partial equilibrium at low volume fractions of martensite. However, in these two cycles the partial equilibrium is not attained in 15 min because of their low transformation rates. The specimens treated in all three cycles deviate appreciably from partial equilibrium at high volume fractions of martensite. High volume fraction of martensite imply high intercritical annealing temperature. Thus, at high temperatures the system appears to deviate appreciably from partial equilibrium. 3.3.
Effects of intercriticalannealing temperature
The volume fractions of martensite determined in the experimental steel are plotted in Fig. 3 against the intercritical annealing temperature. The data reported on another dual-phase steel [6] treated in the IQ cycle
Heat Treatment Cycle us I.-
OlA
o IN
~ 0.8
~
/-portiol Equilibrium 6
-
0.6 -
~ 0.4 uJ _
o~0.2
0
700
Ael
I
Ae 5
I
,
750 800 INTERCRITICAL TEMPERATURE ,°C
,I
850
Fig. 3. Change of volume fraction of martensite with intercritical annealing temperature. Values due to partial equilibrium and complete equilibrium are also shown.
are also shown. The results obtained with both steels are identical. The composition of the second steel [6] (1.0% Mn, 0.5% Cr, 0.5% Si and 0.2% C) is also very close to that used in this investigation. Therefore, the reported volume fractions of martensite obtained in the second steel at equilibrium at the intercritical annealing temperatures are reproduced in Fig. 3 as a smooth straight line. These values are expected to be close to those expected in the experimental steel at equilibrium. Volume fractions of martensite calculated assuming partial equilibrium are also shown as a smooth curve in Fig. 3.
H. K. Khaira et al.
/
Effect of heat treatment of ferrite-austenite equilibrium
The figure shows that in the specimens treated in the IQ cycle partial equilibrium is attained up to about 770°C. Above this temperature the departure from partial equilibrium is appreciable. Consider the transformation at low annealing temperatures. When the annealing time is zero, the amount of austenite is zero. With an increase in anneafing time the amount of austenite increases beyond the value expected at equilibrium (Fig. 3) and approaches a higher value characteristic of partial equilibrium. When the annealing time is increased further the amount of austenite tends to decrease due to substitutional solute partitioning, and approaches a lower value characteristic of equilibrium. The same process occurs at high annealing temperatures. However, in the IQ cycle the amount of 7 at the start of transformation is zero, and the rate of transformation should increase with an increase in temperature. Because of high transformation rates at high intercritical annealing temperatures the system departs from the partial equilibrium state early during annealing. The specimens heat treated in the IN and IA cycles behave in the same manner as specimens treated in the IQ cycle. In the IN cycle, the amount of austenite is zero at the beginning of intercritical annealing and increases with an increase in time. Figure 3 suggests that because of low transformation rate the specimens treated in the IN cycle do not even attain the partial equilibrium state. For similar reasons the specimens treated in the IA cycle also do not attain the partial equilibrium state. During intercritical annealing austenite forms from ferrite and carbide in the IQ and IN cycles. Therefore, the driving force and the transformation rate increase with an increase in temperature. In the IA cycle the specimens contain 100% austenite at the start of intercritical annealing and ferrite precipitates from austenite. The driving force and the transformation rate are low at high temperatures, and increase with a decrease in temperature. Therefore, the departure from the partial equilibrium state is much higher at high temperatures than at low temperatures.
4. Summary and conclusion (1) A steel containing 0.2% C, 0.98% Mn, 0.46% Si and 0.47% Cr was intercritically annealed at 750, 770,
271
790 and 810 °C for 15 min and quenched in iced brine to produce dual-phase steels containing ferrite and martensite. Three heat-treatment cycles, IN, IQ and IA, were employed so as to have pearlite, martensite and austenite respectively prior to intercritical annealing. (2) The austenite content in the three cycles decreased with a decrease in annealing temperature. However, the amount of austenite was widely different in different cycles. (3) Calculations were made for partial equilibrium, which implies no partitioning of substitutional solutes and equilibrium with respect to carbon. The results showed that a partial equilibrium state in the steel was attained in the IQ cycle and was approached in the IN and IA cycles. (4) In the IQ cycle the steel departed from the partial equilibrium state early during annealing at high temperatures as the transformation rate in this cycle increased with an increase in temperature. The partial equilibrium state was not attained in the IN and IA cycles because of low rates of transformation. However, the steel in the IA cycle remained away from the partial equilibrium state even at high temperatures due to an increase in the rate of transformation with an increase in temperature.
References 1 C. Smithels and E. A. Brandes, Metals Reference Book, Butterworths, London, 1967. 2 M. Hillert, JernkontoretsAnn., 136 (1952) 25. 3 J. B. Gilrnour, G. R. Purdy and J. S. Kirkaldy, Metall. Trans., 3 (1972) 1455. 4 A.R. Marder, Metall. Trans., 13A (1982) 85. 5 H. K. Khaira and A. K. Jena, in S. E Mehrotra and T. R. Ramaehandran (eds.), Prog. in Metallurgical Research, TataMcGraw Hill, New Delhi, 1986, p. 559. 6 M.C. Chaturvedi and A. K. Jena, Mater. Sci. Eng., 94 (1987) L1. 7 A.R. Marder, in A. T. Davenport (ed.), Formable HSLA and Dual Phase Steels, TMS-AIME, New York, 1979, p. 89. 8 A. K. Jena and M. C. Chaturvedi, Mater. Sci. Eng., 100 (1988) 1. 9 W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London, 1964. 10 K.W. Andrews, J. Iron Steel Inst., London, 203 (1965) 721. 11 H.K. Khaira, Ph.D. Thesis, IIT, Kanpur, India, 1986.