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Grain refinement in FeC alloys by thermal cycling
185
Materials Science and Engineering, 11 (1973) 185-193 @ American Society for Metals, Metals Park, Ohio, and Elsevier Sequoia S.A., Lausanne
Printed in the Netherlands
Grain Refinement in Fe-C Alloys by Thermal Cycling BIRGER KARLSSON Department of En,qineerin,q Metals, Chalmers University of Technology, Fack, S-402 20 G6tebor,q 5 (Sweden) (Received July 17, 1972)
Summao'* Grain refinement of plain, hypoeutectoid steels by means of repetitive short time austenitization is' studied. For carbon levels of 0.38, 0.I8 and O.05 wt. '% this' technique allows a reduction of grain size of up to 1: I0 in Jerritic-pearlitic condition for original grain sizes of 50-100 t~m, The rapid nucleation of ferrite during controlled high speed cooling from the austenite temperature is important for the mechanism of yrain refinement. 1. I N T R O D U C T I O N
The influence of grain size on the mechanical properties of polycrystalline materials is well known 1. In order to improve these properties, ways of reducing the grain size are of considerable engineering interest. In the case of plain, hypoeutectoid steels used in the normalized condition, this reduction can be achieved by either thermomechanical or thermal techniques. The latter technique involves repetitive austenitizing at relatively low temperatures in the austenite range. The effect of such treatments has been studied on differen t, slightly hypoeutectoid steels z- '~.A noticeable decrease in austenite grain size is reported for ordinary quench-hardened steels 3'4. A much more pronounced effect of grain refinement in the ferriticpearlitic state is attained if the short time austenitizing is instead interrupted by relatively rapid air cooling 2. In the latter study optimum grain refinement was achieved after several cycles with austenitizing periods of around 20 seconds at temperatures approximately 25 deg C above the A3 temperature. However, this technique was applied to steels of nearly eutectoid compositions which transform * R6sume en fran~ais fi la fin de Farticle. Deutsche Z u s a m m e n f a s s u n g am Schlul3 des Artikels.
rapidly to homogeneous austenite. For many reasons, some of which are discussed below, grain refinement is much easier at this relatively high carbon content level. The earlier studies do not make it clear whether the grain refinement originates from a reduction in austenite grain size or from the austenite transforming to a finer aggregate of ferrite and pearlite upon cooling. There seems to be some indication that austenitization at low temperature results in finer austenite grains, owing to a combination of low growth rate and moderately high nucleation rate 4'5. Utilization of this thermal cycling technique on steels of other than eutectoid composition requires a deeper understanding of the phenomenon. The purpose of this work is to extend the thermal cycling technique of grain refinement to pure carbon steels of low carbon content, and in particular to elucidate some of the different contributory effects causing grain refinement. 2. E X P E R I M E N T A L P R O C E D U R E
The material used in the experiments originates from a vacuum sintered iron* with a typical analysis given in Table 1. Desired carbon levels were achieved by carburizing strips of 2 mm thickness in a Hz/CTH 8 atmosphere followed by homogenization in purified argon at 935°C. Eight hours at this temperature TABLE 1 Typical chemical analysis of the material employed (according to supplier)
Composition, p.p.m. C 40
O N ~ 1 0 .45
P ~1
Ni <100
Mo <100
Mn <50
Si <~50
-- . . . . . . . . . . . * Vacuumschmelze G.m.b.H. "Magnetreineisen $2".
Fe Balance
186
o
Anneo temp.
o~ Q.
E
B. KARLSSON
, ~J
r
upquench
R oom temp.
900
quen
oir cooling
I
o o.
E
o)
Time
Fig. 1. Thermal process for developing fine ferrite-pearlite
I-8(%
structures. (One or more austenitization cycles are used.) A3
were sufficient to produce almost homogeneous carbon distribution. From the homogenization temperature the specimens were furnace cooled down to the A1 temperature followed by air cooling. This cooling sequence results in a comparatively coarse, well-defined ferrite-pearlite structure, where the pearlite has the equilibrium composition of 0.80 wt. % C. The thermal treatments are shown schematically in Fig. 1. The specimens were austenitized by immersion in a lead bath, the temperature of which was controlled within ___5 deg C. A 10 % NaC1 brine solution was used as quenching medium. Heating and cooling the strip specimens took less than one second, allowing the austenitization time to be defined within 2 seconds. The slower cooling rate was achieved by air cooling at a measured rate of 15 dog C/sec in the temperature interval 900-600 ° C. These heating and cooling rates were established on
Fig. 3. Microstructure of original material A, Fe 0.05 C. ( x 200)
700
A, [I,e Carbon
conc.,
~Vo
Fig. 2. Annealing temperatures for the alloys A, B and C. {Phase
diagram according to Hanson6.)
a typical specimen with a thermocouple joint fixed in the centre of its mid-section. All heat-treated specimens were examined by light microscopy. In these metallographic investigations the standard etchant 2% nital was used; to reveal the former austenite structure in the quenched samples an etch solution of 1 g picric acid, 5 cm 3 hydrochloric acid and 95 cm 3 ethanol served satisfactorily. 3. E X P E R I M E N T A L RESULTS
Three materials with different carbon contents
GRAIN REFINEMENT IN Fe C ALLOYS were investigated: 0.05, 0.18 and 0.38 wt.°o~C, henceforth denoted A, B and C respectively. To ensure meaningful comparisons, all specimens were austenitized at 20 deg C over their respective A3 temperature (Fig. 2). The sensitivity to the degree of superheating was checked for each carbon level by separate experiments with austenitization temperatures 30 deg C higher and lower than normal.
187
3.1. Ferrite-pearlite structures after different austenitization treatments Isothermal experiments were made at A 3 + 2 0 d e g C to determine suitable austenitization periods. Insufficient holding time results in undissolved ferrite; excessive grain growth sets the upper limit. For materials A and B, Figs. 3 and 5, a suitable austenitizing time was found to be
Fig. 4. Microstructure of material A after annealing 20 rain al 905°C and air cooling. ( x 200)
Fig. 5, Microstructure of original material B, Fe 0.18 C. (x 200)
188 20 minutes, producing (after air cooling) the structures in Figs. 4 and 6. Similarly, material C, Fig. 8, was given a 10 minutes austenitization to bring about the structure shown in Fig. 9. Additional austenitization cycling, with successively shorter times, resulted in further grain refinement for
B. KARLSSON materials A, B and C but had only a small effect on material A. The final grain size for materials B and C was achieved after three austenitization cycles, as shown in Figs. 7 and 10. The grain size for the ferrite-pearlite structures was determined by the linear-intercept method,
Fig. 6. Microstructure of material B after annealing 20 min at 855°C and air cooling. ( x 200)
Fig. 7. Microstructure of material B after three annealing treatments at 855°C (20 min, 15 sec and 5 sec),with intermediate air cooling. ( x 200)
189
GRAIN P,E F I N E M E N T IN Fe C ALLOYS
Table 2. The total length traversed was in each case 2 mm except for the specimens with the original structures, where 30 mm was used. These lengths correspond, for each specimen type, to approximately 200 pearlite regions. The original pearlite regions are effectively broken up already during the first austenitization.
Apart from the pearlite colonies the initial structure also contains rather coarse cementite particles situated mainly at the ferrite grain boundaries; these too are seen to be acting as nucleation centres for austenite, producing finer ferrite-carbide aggregates upon cooling. For materials B and C also the largest grain size
Fig. 8. Microstructure of original material C, Fe 0.38 C. ( × 200)
Fig. 9. Microstructure of material C after annealing 10 rain at 805°C and air cooling. ( x 200)
190
B. KARLSSON
Fig. 10. Microstructure of material C after three annealing treatments at 805°C (10 rain, 15 sec and 5 sec), with intermediate air cooling. ( x 200) TABLE 2 The grain size of the materials in air cooled condition, initially and after one and three austenitization cycles.
Material
Wt.% C
Heat treatment
Pearlite colony mean size, #m
Mean free path between pearlite colonies, I~m
Mean free path between pearlite colonies and/or ferrite grain boundaries, #m
A A B B B
0.05 0.05 0.18 0.18 0.18
33 3.1 27.8 3.9 3.5
(2.600) 126 116 16.1 15.8
96.4 17.8 54.5 9.0 8.8
C C C
0.38 0.38 0.38
Original A. 20 min Original A. 20 min A. 20 m i n + A . 15 s e c + A . 5 sec Original A. 10 min A. 10 m i n + A . 15 sec + A . 5 sec
48.3 8.3 4.2
51.9 8.0 6.5
35.8 5.4 4.9
(A. = austenitization).
reduction occurs during the first austenitization cycle. However, this heat treatment results in a fairly uneven grain size distribution. The largest of the pearlite regions are frequently observed to be broken up by Widmannstatten ferrite, as seen in Figs. 6 and 9. After further cycling a still finer structure with a good degree of homogeneity is obtained. Moreover, repeated austenitization makes the ferrite regions rounder, as shown in Figs. 7 and 10.
The small amount of ferrite which remains during austenitization at 10 deg C below A3 does not markedly affect the appearance of the resulting ferrite-pearlite structure upon cooling. On the other hand, increasing the austenitization temperature to 50 deg C above A 3 gives no observable change in grain size after one heat treatment cycle compared with the results above. Thus, the microstructure is not especially sensitive to slight changes in the degree of overheating o v e r A 3.
191
G R A I N R E F I N E M E N T IN F e - C A L L O Y S
3.2. Austenite 9rain size durin 9 the austenitization anneal The growth of austenite grain size during austenitization of material B (0.18 wt. ~o C) is recorded in Table 3. Because of poor definition of the austenite grain size, a comparison method was found to give the best results in grain size measurements. The TABLE 3 Austenite grain growth during annealing at 885°C for material B
(0.18 wt. °,oC). Annealinq time, sec
Grain size A S T M No.
4
8 15 30 60 240 600 1200
_
9 8 7.5 6.5 6 6 5.5
Correspondin9 intercept length, i, pm m
14 19 23 33 39 39 46
ASTM standard grain size chart was used. The ASTM number G can be translated to a corresponding mean intercept length i [in cm], assuming the structure to consist of equal sized tetrakaidecahedral grains v : 1 G = -- 10.00+6.64 1°log ~.
Grain sizes obtained by ASTM and intercept methods on real structures will differ slightly owing to varying size distribution and shape of the grains, but this variation is insignificant for reasonably uniform, equiaxed structures. The reproducibility of the grain size measurements was found to be _+½grain size number, corresponding to an error in the estimated linear grain size of approximately + 20 ~. Attention should be paid to the variation in mean carbon content during the growth of austenite regions, a problem that has been discussed elsewhere s. Not until all ferrite has disappeared and the austenite is homogeneous all through the specimen can the austenite grain growth be compared with the normal grain growth kinetics of single-phase materials. The austenite grain size for material A after 20 minutes of annealing during the first cycle was
60 #m. The corresponding value for material C after 10 minutes of annealing was 40/zm. 3.3. Formation of ferrite and pearlite in supercooled austenite When cooled below the A 3 temperature the austenite (in the hypoeutectoid range) becomes supersaturated in carbon. Ferrite normally nucleates at the austenite grain boundaries. On further cooling below A 1 the carbon-enriched regions of remaining austenite provide nucleation sites for pearlite. The ferrite, growing from the grain boundaries, often appears in a plate-like shape (Widmannst~itten plates). In large grains, ferrite can nucleate also within the austenite grains and can grow as Widmannst/itten plates or blocky ferrite. Increased undercooling will enhance homogeneous nucleation of ferrite relative to grain boundary nucleation, producing further refinement of the ferrite-pearlite structure. In order to study the nucleation of ferrite in our actual case, material A was annealed for 10 minutes at 875°C, i.e. slightly below A 3. It was then air cooled at 15 deg C/sec to 750°C and quenched from this temperature. Annealing in the two-phase field leaves some undissolved ferrite and restricts austenite grain growth. Figure l l a indicates that the proeutectoid ferrite nucleated predominantly at the austenite grain boundaries at small undercooling. With increased undercooling, some blocky ferrite forms within the austenite grains (Fig. llb). Although there are some indications of Widmannstfitten ferrite growing from austenite grain boundaries, this does not occur frequently with this mode of cooling. Transformation by the normal cooling cycle (Fig. 4) produces only equiaxed ferrite grains. For more carbon-rich austenite a Widmannst~itten morphology of ferrite growth is more likely, Figs. 6 and 9.
4. D I S C U S S I O N
Grange 2'4 observed that the austenite grain size can be considerably refined by successive short time austenitization. These observations indicated that the grain refinement in the final structure at room temperature might be due to the reduced austenite grain size. This conclusion was shared by Gladman 5. In the present work the comparatively coarse pearlite distribution required long initial annealing times to obtain homogeneous austenite s. In the
192
g. KARLSSON
Fig. 1la. Microstructureof material A, Fe 0.05 C, after annealing 10 min at 875°C, air cooling to 750°C and quenching. (× 100)
Fig. 11b. Enlargedviewof the area marked in Fig. 11a. B indicates a region with blocky ferrite. ( × 500) process, considerably larger austenite grains (Table 3) developed than in the samples studied by Grange. Comparison of the data in Tables 2 and 3 shows that the austenite grain size developed under these conditions (20 rain anneal) was about 15 per cent smaller than the ferrite grain size in the original structure. Air cooling from this state already produces a ferrite grain size roughly six times smaller
than that of the original ferrite-pearlite structure. This reduction is almost independent of carbon content, being only slightly smaller in the low carbon material. This striking reduction indicates that a decisive role in grain refinement is played by the nucleation of ferrite from austenite. According to the preceding section, homogeneous nucleation of ferrite contributes to the initial grain refinement.
193
GRAIN R E F I N E M E N T IN Fe-C ALLOYS
The data in Table 2 indicate that the refinement effect diminishes on further cycling. In recycling after the initial refinement stage, austenite will be nucleated at the cementite which is now finely dispersed at the ferrite grain boundaries. The short austenitization periods employed will produce small austenite grains 4. In these small grains, ferrite nucleation will occur only at the grain boundaries. The final ferrite grain size is then determined by the number of nuclei per unit volume and the amount of growth allowed to take place under the given cooling conditions. As the austenite grain size is further reduced in successive cycles the growth becomes the dominating factor and the density of nucleation loses its importance. This explains the diminishing effect of repeated cycling.
steels of original grain sizes of about 50 to 100 gin. The size reduction is more pronounced for steels with higher carbon contents. 2. The marked refinement in the first cycle is attributed to multiple nucleation of ferrite in the large austenite grains. In the following cycles, where the refinement effect is less, the growth of transformed regions both in the ferrite and in the austenite state seems to determine the final grain size. ACKNOWLEDGEMENTS
Helpful suggestions by Prof. H. F. Fischmeister and Dr. G. Linden are gratefully acknowledged. This work was partly sponsored by the Swedish Board for Technical Development.
5. C O N C L U S I O N S REFERENCES
A technique has been worked out whereby a very marked grain refinement in low carbon steels can be obtained by comparatively simple heat treatments : 1, Repetitive short time austenitization slightly above A> interrupted by relatively fast air cooling, results in a grain refinement of up to 1 : 10 in intercept grain size for steels with carbon contents of from 0.38 down to at least 0.05 wt. ~o. This is valid for
1 2 3 4 5
R. W. Armstrong, Met. Trans., 1 (1970) 1169. R. A. Grange, Trans. Am. Soc. Metals, 59 (1966) 27. W. Peter and H. Finkler, Hgirterei-Techn. Mitt.. 24 (1969) 210. R. A. Grange, Met. Trans., 2 (1971) 65. T. Gladman, Grain Control, Special Rept., Iron Steel Inst., London, 1969. 6 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 7 J. E. Hilliard, Metal Progr., 85 (1964) 99. 8 B. Karlsson, Z. Metallk., 63 (1972) 160.
Ra/'~'nement des 9rains, par traitement therrnique alternd, dens un alliaqe Fe-C
Kornt'erfeinerun9 in Fe-C-Le.qierun,qen durch thermische Behandlun.qszyklen
On a etudie le raffinement des grains d'aciers hypoeutectoides ordinaires par austbnitisations breves et repetdes. Dens le cas de proportions de charbon de 0,38, 0,18 et 0,05~ en poids, cette methode permet une reduction des dimensions du grain allant jusqu'fi 1:10 lorsqu'on a des conditions de ferrite-perlite, et des dimensions initiales des grains de 50 fi 100 #m. La nucleation rapide du ferrite pendant le refroidissement fi vitesse elevee et regl6 fi partir de la temperature d'austdnite, est importante pour le mecanisme de raffinement des grains.
Die Kornverfeinerung in ebenen, hypoeutektoiden Stfihlen mit Hilfe wiederholter KurzzeitAustenitisierung wurde untersucht. Bei Kohlenstoffgehalten von 0.38, 0.18 und 0.05 Gew.'~,~{,erlaubt diese Methode eine Reduktion der Korngrege im Verh/iltnis 1:10 unter ferritisch-perlitischen Bedingungen und bei Ausgangskorngragen von 50-100 l~m. Die schnelle Keimbildung von Ferrit w/ihrend der kontrollierten schnellen Abkfihlung yon der Austenittemperatur ist wichtig for den Mechanismus der Kornverfeinerung.
Materials Science and Engineering, 11 (1973) 185-193 @ American Society for Metals, Metals Park, Ohio, and Elsevier Sequoia S.A., Lausanne
Printed in the Netherlands
Grain Refinement in Fe-C Alloys by Thermal Cycling BIRGER KARLSSON Department of En,qineerin,q Metals, Chalmers University of Technology, Fack, S-402 20 G6tebor,q 5 (Sweden) (Received July 17, 1972)
Summao'* Grain refinement of plain, hypoeutectoid steels by means of repetitive short time austenitization is' studied. For carbon levels of 0.38, 0.I8 and O.05 wt. '% this' technique allows a reduction of grain size of up to 1: I0 in Jerritic-pearlitic condition for original grain sizes of 50-100 t~m, The rapid nucleation of ferrite during controlled high speed cooling from the austenite temperature is important for the mechanism of yrain refinement. 1. I N T R O D U C T I O N
The influence of grain size on the mechanical properties of polycrystalline materials is well known 1. In order to improve these properties, ways of reducing the grain size are of considerable engineering interest. In the case of plain, hypoeutectoid steels used in the normalized condition, this reduction can be achieved by either thermomechanical or thermal techniques. The latter technique involves repetitive austenitizing at relatively low temperatures in the austenite range. The effect of such treatments has been studied on differen t, slightly hypoeutectoid steels z- '~.A noticeable decrease in austenite grain size is reported for ordinary quench-hardened steels 3'4. A much more pronounced effect of grain refinement in the ferriticpearlitic state is attained if the short time austenitizing is instead interrupted by relatively rapid air cooling 2. In the latter study optimum grain refinement was achieved after several cycles with austenitizing periods of around 20 seconds at temperatures approximately 25 deg C above the A3 temperature. However, this technique was applied to steels of nearly eutectoid compositions which transform * R6sume en fran~ais fi la fin de Farticle. Deutsche Z u s a m m e n f a s s u n g am Schlul3 des Artikels.
rapidly to homogeneous austenite. For many reasons, some of which are discussed below, grain refinement is much easier at this relatively high carbon content level. The earlier studies do not make it clear whether the grain refinement originates from a reduction in austenite grain size or from the austenite transforming to a finer aggregate of ferrite and pearlite upon cooling. There seems to be some indication that austenitization at low temperature results in finer austenite grains, owing to a combination of low growth rate and moderately high nucleation rate 4'5. Utilization of this thermal cycling technique on steels of other than eutectoid composition requires a deeper understanding of the phenomenon. The purpose of this work is to extend the thermal cycling technique of grain refinement to pure carbon steels of low carbon content, and in particular to elucidate some of the different contributory effects causing grain refinement. 2. E X P E R I M E N T A L P R O C E D U R E
The material used in the experiments originates from a vacuum sintered iron* with a typical analysis given in Table 1. Desired carbon levels were achieved by carburizing strips of 2 mm thickness in a Hz/CTH 8 atmosphere followed by homogenization in purified argon at 935°C. Eight hours at this temperature TABLE 1 Typical chemical analysis of the material employed (according to supplier)
Composition, p.p.m. C 40
O N ~ 1 0 .45
P ~1
Ni <100
Mo <100
Mn <50
Si <~50
-- . . . . . . . . . . . * Vacuumschmelze G.m.b.H. "Magnetreineisen $2".
Fe Balance
186
o
Anneo temp.
o~ Q.
E
B. KARLSSON
, ~J
r
upquench
R oom temp.
900
quen
oir cooling
I
o o.
E
o)
Time
Fig. 1. Thermal process for developing fine ferrite-pearlite
I-8(%
structures. (One or more austenitization cycles are used.) A3
were sufficient to produce almost homogeneous carbon distribution. From the homogenization temperature the specimens were furnace cooled down to the A1 temperature followed by air cooling. This cooling sequence results in a comparatively coarse, well-defined ferrite-pearlite structure, where the pearlite has the equilibrium composition of 0.80 wt. % C. The thermal treatments are shown schematically in Fig. 1. The specimens were austenitized by immersion in a lead bath, the temperature of which was controlled within ___5 deg C. A 10 % NaC1 brine solution was used as quenching medium. Heating and cooling the strip specimens took less than one second, allowing the austenitization time to be defined within 2 seconds. The slower cooling rate was achieved by air cooling at a measured rate of 15 dog C/sec in the temperature interval 900-600 ° C. These heating and cooling rates were established on
Fig. 3. Microstructure of original material A, Fe 0.05 C. ( x 200)
700
A, [I,e Carbon
conc.,
~Vo
Fig. 2. Annealing temperatures for the alloys A, B and C. {Phase
diagram according to Hanson6.)
a typical specimen with a thermocouple joint fixed in the centre of its mid-section. All heat-treated specimens were examined by light microscopy. In these metallographic investigations the standard etchant 2% nital was used; to reveal the former austenite structure in the quenched samples an etch solution of 1 g picric acid, 5 cm 3 hydrochloric acid and 95 cm 3 ethanol served satisfactorily. 3. E X P E R I M E N T A L RESULTS
Three materials with different carbon contents
GRAIN REFINEMENT IN Fe C ALLOYS were investigated: 0.05, 0.18 and 0.38 wt.°o~C, henceforth denoted A, B and C respectively. To ensure meaningful comparisons, all specimens were austenitized at 20 deg C over their respective A3 temperature (Fig. 2). The sensitivity to the degree of superheating was checked for each carbon level by separate experiments with austenitization temperatures 30 deg C higher and lower than normal.
187
3.1. Ferrite-pearlite structures after different austenitization treatments Isothermal experiments were made at A 3 + 2 0 d e g C to determine suitable austenitization periods. Insufficient holding time results in undissolved ferrite; excessive grain growth sets the upper limit. For materials A and B, Figs. 3 and 5, a suitable austenitizing time was found to be
Fig. 4. Microstructure of material A after annealing 20 rain al 905°C and air cooling. ( x 200)
Fig. 5, Microstructure of original material B, Fe 0.18 C. (x 200)
188 20 minutes, producing (after air cooling) the structures in Figs. 4 and 6. Similarly, material C, Fig. 8, was given a 10 minutes austenitization to bring about the structure shown in Fig. 9. Additional austenitization cycling, with successively shorter times, resulted in further grain refinement for
B. KARLSSON materials A, B and C but had only a small effect on material A. The final grain size for materials B and C was achieved after three austenitization cycles, as shown in Figs. 7 and 10. The grain size for the ferrite-pearlite structures was determined by the linear-intercept method,
Fig. 6. Microstructure of material B after annealing 20 min at 855°C and air cooling. ( x 200)
Fig. 7. Microstructure of material B after three annealing treatments at 855°C (20 min, 15 sec and 5 sec),with intermediate air cooling. ( x 200)
189
GRAIN P,E F I N E M E N T IN Fe C ALLOYS
Table 2. The total length traversed was in each case 2 mm except for the specimens with the original structures, where 30 mm was used. These lengths correspond, for each specimen type, to approximately 200 pearlite regions. The original pearlite regions are effectively broken up already during the first austenitization.
Apart from the pearlite colonies the initial structure also contains rather coarse cementite particles situated mainly at the ferrite grain boundaries; these too are seen to be acting as nucleation centres for austenite, producing finer ferrite-carbide aggregates upon cooling. For materials B and C also the largest grain size
Fig. 8. Microstructure of original material C, Fe 0.38 C. ( × 200)
Fig. 9. Microstructure of material C after annealing 10 rain at 805°C and air cooling. ( x 200)
190
B. KARLSSON
Fig. 10. Microstructure of material C after three annealing treatments at 805°C (10 rain, 15 sec and 5 sec), with intermediate air cooling. ( x 200) TABLE 2 The grain size of the materials in air cooled condition, initially and after one and three austenitization cycles.
Material
Wt.% C
Heat treatment
Pearlite colony mean size, #m
Mean free path between pearlite colonies, I~m
Mean free path between pearlite colonies and/or ferrite grain boundaries, #m
A A B B B
0.05 0.05 0.18 0.18 0.18
33 3.1 27.8 3.9 3.5
(2.600) 126 116 16.1 15.8
96.4 17.8 54.5 9.0 8.8
C C C
0.38 0.38 0.38
Original A. 20 min Original A. 20 min A. 20 m i n + A . 15 s e c + A . 5 sec Original A. 10 min A. 10 m i n + A . 15 sec + A . 5 sec
48.3 8.3 4.2
51.9 8.0 6.5
35.8 5.4 4.9
(A. = austenitization).
reduction occurs during the first austenitization cycle. However, this heat treatment results in a fairly uneven grain size distribution. The largest of the pearlite regions are frequently observed to be broken up by Widmannstatten ferrite, as seen in Figs. 6 and 9. After further cycling a still finer structure with a good degree of homogeneity is obtained. Moreover, repeated austenitization makes the ferrite regions rounder, as shown in Figs. 7 and 10.
The small amount of ferrite which remains during austenitization at 10 deg C below A3 does not markedly affect the appearance of the resulting ferrite-pearlite structure upon cooling. On the other hand, increasing the austenitization temperature to 50 deg C above A 3 gives no observable change in grain size after one heat treatment cycle compared with the results above. Thus, the microstructure is not especially sensitive to slight changes in the degree of overheating o v e r A 3.
191
G R A I N R E F I N E M E N T IN F e - C A L L O Y S
3.2. Austenite 9rain size durin 9 the austenitization anneal The growth of austenite grain size during austenitization of material B (0.18 wt. ~o C) is recorded in Table 3. Because of poor definition of the austenite grain size, a comparison method was found to give the best results in grain size measurements. The TABLE 3 Austenite grain growth during annealing at 885°C for material B
(0.18 wt. °,oC). Annealinq time, sec
Grain size A S T M No.
4
8 15 30 60 240 600 1200
_
9 8 7.5 6.5 6 6 5.5
Correspondin9 intercept length, i, pm m
14 19 23 33 39 39 46
ASTM standard grain size chart was used. The ASTM number G can be translated to a corresponding mean intercept length i [in cm], assuming the structure to consist of equal sized tetrakaidecahedral grains v : 1 G = -- 10.00+6.64 1°log ~.
Grain sizes obtained by ASTM and intercept methods on real structures will differ slightly owing to varying size distribution and shape of the grains, but this variation is insignificant for reasonably uniform, equiaxed structures. The reproducibility of the grain size measurements was found to be _+½grain size number, corresponding to an error in the estimated linear grain size of approximately + 20 ~. Attention should be paid to the variation in mean carbon content during the growth of austenite regions, a problem that has been discussed elsewhere s. Not until all ferrite has disappeared and the austenite is homogeneous all through the specimen can the austenite grain growth be compared with the normal grain growth kinetics of single-phase materials. The austenite grain size for material A after 20 minutes of annealing during the first cycle was
60 #m. The corresponding value for material C after 10 minutes of annealing was 40/zm. 3.3. Formation of ferrite and pearlite in supercooled austenite When cooled below the A 3 temperature the austenite (in the hypoeutectoid range) becomes supersaturated in carbon. Ferrite normally nucleates at the austenite grain boundaries. On further cooling below A 1 the carbon-enriched regions of remaining austenite provide nucleation sites for pearlite. The ferrite, growing from the grain boundaries, often appears in a plate-like shape (Widmannst~itten plates). In large grains, ferrite can nucleate also within the austenite grains and can grow as Widmannst/itten plates or blocky ferrite. Increased undercooling will enhance homogeneous nucleation of ferrite relative to grain boundary nucleation, producing further refinement of the ferrite-pearlite structure. In order to study the nucleation of ferrite in our actual case, material A was annealed for 10 minutes at 875°C, i.e. slightly below A 3. It was then air cooled at 15 deg C/sec to 750°C and quenched from this temperature. Annealing in the two-phase field leaves some undissolved ferrite and restricts austenite grain growth. Figure l l a indicates that the proeutectoid ferrite nucleated predominantly at the austenite grain boundaries at small undercooling. With increased undercooling, some blocky ferrite forms within the austenite grains (Fig. llb). Although there are some indications of Widmannstfitten ferrite growing from austenite grain boundaries, this does not occur frequently with this mode of cooling. Transformation by the normal cooling cycle (Fig. 4) produces only equiaxed ferrite grains. For more carbon-rich austenite a Widmannst~itten morphology of ferrite growth is more likely, Figs. 6 and 9.
4. D I S C U S S I O N
Grange 2'4 observed that the austenite grain size can be considerably refined by successive short time austenitization. These observations indicated that the grain refinement in the final structure at room temperature might be due to the reduced austenite grain size. This conclusion was shared by Gladman 5. In the present work the comparatively coarse pearlite distribution required long initial annealing times to obtain homogeneous austenite s. In the
192
g. KARLSSON
Fig. 1la. Microstructureof material A, Fe 0.05 C, after annealing 10 min at 875°C, air cooling to 750°C and quenching. (× 100)
Fig. 11b. Enlargedviewof the area marked in Fig. 11a. B indicates a region with blocky ferrite. ( × 500) process, considerably larger austenite grains (Table 3) developed than in the samples studied by Grange. Comparison of the data in Tables 2 and 3 shows that the austenite grain size developed under these conditions (20 rain anneal) was about 15 per cent smaller than the ferrite grain size in the original structure. Air cooling from this state already produces a ferrite grain size roughly six times smaller
than that of the original ferrite-pearlite structure. This reduction is almost independent of carbon content, being only slightly smaller in the low carbon material. This striking reduction indicates that a decisive role in grain refinement is played by the nucleation of ferrite from austenite. According to the preceding section, homogeneous nucleation of ferrite contributes to the initial grain refinement.
193
GRAIN R E F I N E M E N T IN Fe-C ALLOYS
The data in Table 2 indicate that the refinement effect diminishes on further cycling. In recycling after the initial refinement stage, austenite will be nucleated at the cementite which is now finely dispersed at the ferrite grain boundaries. The short austenitization periods employed will produce small austenite grains 4. In these small grains, ferrite nucleation will occur only at the grain boundaries. The final ferrite grain size is then determined by the number of nuclei per unit volume and the amount of growth allowed to take place under the given cooling conditions. As the austenite grain size is further reduced in successive cycles the growth becomes the dominating factor and the density of nucleation loses its importance. This explains the diminishing effect of repeated cycling.
steels of original grain sizes of about 50 to 100 gin. The size reduction is more pronounced for steels with higher carbon contents. 2. The marked refinement in the first cycle is attributed to multiple nucleation of ferrite in the large austenite grains. In the following cycles, where the refinement effect is less, the growth of transformed regions both in the ferrite and in the austenite state seems to determine the final grain size. ACKNOWLEDGEMENTS
Helpful suggestions by Prof. H. F. Fischmeister and Dr. G. Linden are gratefully acknowledged. This work was partly sponsored by the Swedish Board for Technical Development.
5. C O N C L U S I O N S REFERENCES
A technique has been worked out whereby a very marked grain refinement in low carbon steels can be obtained by comparatively simple heat treatments : 1, Repetitive short time austenitization slightly above A> interrupted by relatively fast air cooling, results in a grain refinement of up to 1 : 10 in intercept grain size for steels with carbon contents of from 0.38 down to at least 0.05 wt. ~o. This is valid for
1 2 3 4 5
R. W. Armstrong, Met. Trans., 1 (1970) 1169. R. A. Grange, Trans. Am. Soc. Metals, 59 (1966) 27. W. Peter and H. Finkler, Hgirterei-Techn. Mitt.. 24 (1969) 210. R. A. Grange, Met. Trans., 2 (1971) 65. T. Gladman, Grain Control, Special Rept., Iron Steel Inst., London, 1969. 6 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 7 J. E. Hilliard, Metal Progr., 85 (1964) 99. 8 B. Karlsson, Z. Metallk., 63 (1972) 160.
Ra/'~'nement des 9rains, par traitement therrnique alternd, dens un alliaqe Fe-C
Kornt'erfeinerun9 in Fe-C-Le.qierun,qen durch thermische Behandlun.qszyklen
On a etudie le raffinement des grains d'aciers hypoeutectoides ordinaires par austbnitisations breves et repetdes. Dens le cas de proportions de charbon de 0,38, 0,18 et 0,05~ en poids, cette methode permet une reduction des dimensions du grain allant jusqu'fi 1:10 lorsqu'on a des conditions de ferrite-perlite, et des dimensions initiales des grains de 50 fi 100 #m. La nucleation rapide du ferrite pendant le refroidissement fi vitesse elevee et regl6 fi partir de la temperature d'austdnite, est importante pour le mecanisme de raffinement des grains.
Die Kornverfeinerung in ebenen, hypoeutektoiden Stfihlen mit Hilfe wiederholter KurzzeitAustenitisierung wurde untersucht. Bei Kohlenstoffgehalten von 0.38, 0.18 und 0.05 Gew.'~,~{,erlaubt diese Methode eine Reduktion der Korngrege im Verh/iltnis 1:10 unter ferritisch-perlitischen Bedingungen und bei Ausgangskorngragen von 50-100 l~m. Die schnelle Keimbildung von Ferrit w/ihrend der kontrollierten schnellen Abkfihlung yon der Austenittemperatur ist wichtig for den Mechanismus der Kornverfeinerung.