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Effect of arrangement of acicular ferrite in a Widmanstatten microstructure on the fracture of mild steel
Materials Science and Engineering, A l l 9 (1989) 211-217
211
Effect of Arrangement of Acicular Ferrite in a Widmanstatten Microstructure on the Fracture of Mild Steel Z. H U A N G
Institute of Physics, Academia Sinica, Beijing (China) M. YAO
Harbin Institute of Technology, Harbin (China) (Received February 7, 1989; in revised form June 7, 1989)
Abstract
The effect of the morphology and distribution of ferrite, especially acicular ferrite in a Widmanstatten structure, on the fracture behaviour of mild steel at low temperatures is discussed in detail. The microstructures of polygonal ferrite, intersecting acicular ferrite and parallel acicular ferrite were obtained using an interrupted cooling procedure, and the different effects on the fracture toughness at low temperature were found. If an intersecting acicular ferrite microstructure is present, then the steel has superior toughness at low temperature in comparison with that of polygonal ferrite. If the microstructure is mainly composed of parallel acicular ferrite, however, the steel toughness is comparable to that of polygonal ferrite. 1. Introduction The effect of microstructures containing acicular ferrite on the mechanical properties of mild steel, especially the fracture toughness at low temperature, has been widely studied [1-5], yet even now there is intense debate about the nature of this effect. In a previous investigation into the influence of an interrupted cooling procedure on the microstructure and mechanical properties of steel [6], it was found that the microstructures obtained could be respectively characterized as polygonal ferrite, intersecting acicular ferrite, parallel acicular ferrite and bainite. The microstructure characterized by intersecting acicular ferrite has the best toughness at low temperature. These phenomena imply that the morphology and distribution of ferrite might be a significant factor in the fracture behaviour of mild steel. It is therefore the objective of the present investiga0921-5093/89/$3.50
tion to determine the main factors governing the fracture process at low temperature and to gain physical understanding of the fracture mechanism in microstructures containing ferrite, especially in the Widmanstatten structure characterized by acicular ferrite.
2. Experimental materials and procedures Two plates of 16Mn steel 20 mm thick were taken as specimens. The chemical compositions are given in Table 1. The steel plates were austenitized at 1150 °C and hot rolled with a reduction of 25% at 1080 or 980 °C, followed by accelerated cooling in water or oil to a temperature in the range from 750 to 500 °C and then cooling in air. The procedure is shown schematically in Fig. 1. To control the accelerated cooling end temperature (ACET), the cooling curves of the plate of chosen geometry in water or oil were measured by means of an optical oscillograph (SC60). The signals were taken from the centre of the plate by Le Chatelier thermocouples. The relationships of temperature and time in the different cooling media were established and hence the ACET could be controlled by controlling the time in the quench medium. The cooling time was controlled within 0.2 s and hence the error in the cooling temperature was less than 2 °C in oil or 4 °C in water because the maximum rate of cooling is 9 °C s-I in oil or TABLE 1
Chemicalcomposition of the steels (wt.%)
C
Mn
Si
S
P
0.16 0.16
1.32 1.27
0.66 0.33
0.016 0.020
0.013 0.022
© Elsevier Sequoia/Printed in The Netherlands
212
1000 °
4O
Oo ,..~8
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0 °C
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~,. ~'-,~.~
~v..._'"-,~ ~-..~ ~ \ ~
20
IN AIR
75ooc
600
720°C
69o °c 65o °c s2o oc s8o °c - 5 0 0 °C
u'/
3. Experimental results and analyses 3.1. Effect of A C E T on microstructure and properties Figure 2 illustrates the tensile properties and the fracture appearance transition temperature (FATT) as a function of the ACET. Here, only two groups of test results are presented because similar results were found in all of the groups. It can be seen from Fig. 2 that the yield strength (a0.2) and tensile strength (oh) increase and the reduction of area (~) and elongation (~) decrease with decreasing ACET, but it must be emphasized that these trends become obvious only if the ACET is below 650 °C. As far as the low temperature fracture toughness is concerned, the minimum occurs at about 690 °C on the FATT vs. ACET curve; that is to say, there is a range of ACET which optimizes the ductility of steel at low temperature. The same conclusion can be
~ 0 ffh
40o tO
TIME Fig. 1. Schematic illustration of the procedure used in this investigation. 16 °C s- 1 in water in the temperature range from 750 to 550 °C. The temperature of the medium was monitored to ensure consistent heat transfer. Charpy impact tests were made over a temperature range from - 78 to + 20 °C to determine the fracture properties of the steel at low temperature. Specimens of 10 mm gauge diameter and 50 mm gauge length were used to determine the tensile properties. The specimens were taken from the centre of the treated plates in longitudinal and transverse directions, and several groups of specimens were obtained. Observation of the microstructure was carried out with a transmission electron microscope, and analysis of the fracture appearance of the specimens impacted at low temperature was done by means of scanning electronic microscopy (SEM).
0 o
0
-20 200
¶ ~ ¢ \ at 980"C
_
-40
0
<# 60 8 0 _ ~ ~ ~ ~
- 30 -
0
o
:::
40-
20
I
E-i
\ I
]
20 0
_10
o
]
800 700 600 500 COOLING-END TEMPERATURE IN WATER,"C Fig. 2. Variation of mechanical properties of 16Mn steel with cooling end temperature (broken line shows FATT of specimen rolled at 980 °C).
reached from Fig. 3, in which the variation of impact energy with temperature is shown. The corresponding variation of microstructure is shown in Fig. 4. If the ACET is higher than 720 °C, the microstructure obtained is composed of polygonal ferrite-pearlite (Figs. 4(a) and 4(b)). When the ACET varies over the temperature range from 720 to 620 °C, the microstructure consists mainly of acicular ferrite-pearlite, but the arrangement of acicular ferrite changes from intersecting (Fig. 4(c)) to parallel (Fig. 4(e)) and the fraction of parallel acicular ferrite increases with a decrease in the ACET. In this range the proportion of acicular ferrite (about 4%) changes little and hence the yield stress does not change greatly [4]. The corresponding FATT, as mentioned above, first shifts to a lower temperature and then returns. The minimum value of the FATT corresponds to the intersecting acicular ferrite microstructure. When the ACET is lower than 580 °C, bainite is formed (Fig. 4(f)). These phenomena were found in other research work, where similar heat treatments were followed and the formation of ferrite in the microstructure was
213 7o I
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•
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o •
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TEMPERATURE, C
Fig. 3. Impact energy of specimens with different microstructures (marked temperatures are ACETs).
Fig. 4. Variation of microstructure with cooling end temperature: (a) cooling in air; (b) cooling in water to 720 °C; (c) to 690 °C; (d) to 650 °C; (e) to 620 °C; (f) to 500 °C, then cooling in air.
214
studied by using a dilatometer [7]. The ACET was controlled directly by controlling the temperature. All control was carried out by computer. The results showed that the ACET can be characterized by several temperature ranges: above 720 °C, the morphology of the ferrite is mainly polygonal; between 710 and 670 °C, intersecting acicular; and between 650 and 550 °C, parallel acicular. Evidently the above variation is the same as that mentioned earlier.
3.2. Morphological features and crystallographic properties In order to clarify the effect of the arrangement of acicular ferrite, the distinctions between the different acicular ferrite morphologies were studied by means Of transmission electron microscopy. Figure 5 shows a replica of the morphology of acicular ferrite in detail. In the microstructure of intersecting ferrite (Fig. 5(a)), pearlite blocks can frequently be found to exist among the ferrite needles and thus each needle could be regarded as a unit; but in the parallel ferrite microstructure, the parallel ferrite needles form a package (Fig. 5(b)) in which the ferrite needles are arranged in parallel even though the ferrite boundary is curved (Fig. 6). Thus it is proposed that each of these packages of parallel needles be regarded as one microstructural unit. This supposition is strongly supported by analyses of electron diffraction patterns and microfractographs. Figure 7 shows a transmission electron micrograph of parallel acicular ferrite and corresponding diffraction patterns. It can be seen that the patterns of the needles in a package are the same. In fact, it was found that the same phenomena could be observed in other crystallographic directions and in other packages. Therefore the suggestion is that the package also be taken as a unit for cleavage crack propagation in respect of the crystallography. Details of the crystallography of acicular ferrite have been reported in ref. 8. As far as the intersecting acicular ferrite is concerned, however, the patterns which belong to different ferrite needles are not the same. As shown by transmission electron microscopy (TEM) (Fig. 8), the indexed diffraction patterns belong to three lattice arrays whose axes are respectively
the crystallographic relationship between acicular ferrite and prior austenite was inferred. On the supposition that a K-S relationship exists between a-Fe (b.c.c. lattice) and 7-Fe (f.c.c. lattice), it was found that a crystallographic direction [145]r is parallel to [315]~,, [133]~ and [lil]~ 3, and is related to them by the following K-S relationship:
[3 lS]:,//[1 ~3]:J/[1 i 1]~ 3
[315]:1//[ 145]r corresponding to
By means of a programme of matrix analysis [9],
Fig. 5. Replica of two types of arrangement of acicular ferrite: (a) intersecting; (b) parallel.
Fig. 6. Transmission electron micrograph of parallel acicular ferrite and its boundary.
(i 11)J/(O 11)~, [01 i]r//[111]~
215
( 200 )
~
020 )
7
Fig. 7. Transmission electron micrograph of parallel acicular ferrite and corresponding diffraction patterns of regions A, B and C. A/B=4~I2/2 ~/z= 1.414; B =[001].
(7>ol)
_
Fig. 8. Transmission electron micrograph of intersecting acicular ferrite and corresponding diffraction pattern of boundary A.
[133]aJ/[145]~, corresponding to (i 11 )~//(011 )a, []0 ]]J/[111]a [1 ] 1]~3//[145]r corresponding to (11
[1 iol,///[11115
That is to say, there is a certain crystallographic relationship between acicular ferrite and prior austenite but the relationships for different needles may not be exactly the same. In addition, because the ferrite needles as proeutectoid phase are able to separate prior austenite grain into a number of smaller parts so that the growing of pearlite in each part is restricted by
surrounding acicular ferrite, the intersecting acicular ferrite is able to refine pearlite block. As seen from Fig. 5, the pearlite blocks in the different parts have different arrangement orientations. Obviously, this separation is beneficial for improving the toughness of steel [10]. However, the effect of parallel acicular ferrite on the separation of prior austenite grain and the restriction on the growing of pearlite is almost the same as that of polygonal ferrite on account of the package being considered as a unit. All of these analyses demonstrate that, although both kinds of microstructure can be classified as a Widmanstatten structure, they exhibit significant
216
morphological differences which obviously affect the steel toughness differently at low temperature.
3.3. Microfractographic features SEM was used to distinguish the microfractographic features of steel with different ferrite morphologies to obtain more direct evidence of the previous hypothesis. Figure 9 shows SEM fractographs of the specimens impacted at - 5 0 °C. The fractograph of the specimen with intersecting acicular ferrite obtained at 690 °C ACET (Fig. 9(b)) indicates that the cleavage facets are obviously smaller than those of the polygonal ferrite obtained at 720 °C ACET (Fig. 9(a)) and the parallel acicular ferrite obtained at 650 °C ACET (Fig. 9(c)), and shows that the river
pattern is more confused and a large number of tear ridges and cleavage steps exist. In the fractograph of the parallel acicular ferrite specimen the facets, in sharp contrast to the above features, are large and the river pattern is regular, these features evidently being related to the crystallographic properties of the parallel acicular ferrite microstructure. This point is proved by another group of SEM microfractographs (Fig. 10) in which the correlation of cleavage facet with microstructure is exhibited directly by using the method of etching the fracture surface with Nital. It can be seen that in a package of parallel acicular ferrite the cleavage stream, whether it is across or along the parallel acicular ferrite, hardly twists (Figs. 10(a) and 10(b)), but in the intersecting
Fig. 9. SEM fractographs of specimens impacted at - 5 0 *C: (a) polygonal ferrite (ACET 720 °C); (b) intersecting acicular ferrite (ACET 690 °C); (c) parallel acicular ferrite (ACET 650 °C).
Fig. 10. SEM micrographs of the etched fractures: (a), (b) parallel acicular ferrite; (c) intersecting acicular ferrite.
217
acicular ferrite microstructure it twists so sharply that the plane or regular facet is barely visible (Fig. lO(c)). All of these observations demonstrate further the significant role of the arrangement of acicular ferrite in cleavage fracture.
4. Discussion
With respect to the discussion of the effect of the Widmanstatten structure, the approach can be divided into two aspects: affirmation of the improvement in steel toughness and negation thereof. In the affirmation, the properties of acicular ferrite, such as high density of dislocations, small effective grain size and absence of carbide precipitation in the grain and on the grain boundary [2], were emphasized. In the negation, the integrity of the microstructure was considered and the acicular ferrite was regarded as a phase which would play the role of separating the microstructure and hence diminish the steel toughness [1]. On the basis of the above results and analyses, we believe that the arrangement of acicular ferrite in the Widmanstatten structure is a dominant factor in determining steel toughness at low temperature, and the influence of the Widmanstatten structure depends upon whether the acicular ferrite maintains certain characteristics such as small effective grain size (particularly important), high density of dislocations and absence of carbide precipitation in the grain and on the grain boundary. Since intersecting acicular ferrite maintains the characteristic of small effective grain size and has the ability to refine pearlite block, it is beneficial for the improvement of steel toughness at low temperature. Parallel acicular ferrite, however, does not retain the characteristic of fine grain, because the needles in a package exhibit almost identical crystallographic properties and hence the package can be regarded as an effective grain for cleavage crack propagation [11, 12], so that parallel acicular ferrite should diminish steel toughness at low temperature. This is evidently a valid model for the illustration of not only the positive but also the negative influence of the Widmanstatten structure.
5. Conclusions
When an interrupted accelerated cooling procedure is used, the morphology of ferrite in the microstructure of 16Mn steel will change from polygonal to intersecting acicular and finally to parallel acicular as the accelerated cooling end temperature decreases. Once a microstructure composed mainly of intersecting acicular ferrite has been obtained, the steel should exhibit the lowest FATT (FATT50%) and the smallest cleavage facets in the fracture; that is, it has the greatest resistance to brittle fracture at low temperature. This is because the intersecting acicular ferrite maintains the characteristic of fine effective grain and, moreover, refines the pearlite block. If a microstructure consists mainly of parallel acicular ferrite, the steel toughness at low temperature will deteriorate, which results in a higher FATT50% and larger cleavage facets. The essential cause of this is that the ferrite needles in a package display identical arrangement orientation and have the same crystallographic characteristics. References 1 R. E. Greaves and H. Wrighton, Practical Microscopical Metallography, Chapman, London, 4th edn., 1957. 2 V. I. Izotov and B. A. Leontov, Fiz. Met. Metalloved., 32 (1971)96. 3 L. G. Taylor and R. A. Farrar, Welding Met. Fabr., 43 (5) (1975)305. 4 D.J. Widgery, Welding Res. Suppl., 41 (1976) 57-s. 5 R. Otterberg, R. Sandstrom and A. Sandberg, Met. Technol., 10 (1980) 397. 6 Z. Huang and M. Yao, Iron SteelSin., 12 (1989)in the press. 7 Z. Huang and M. Yao, WUL1CESH1 (Physical Testing), 3 (1986) 10 (in Chinese). 8 A.D. King and T. Bell, Met. Sci. J., 8 (1974) 253. 9 C.Z. Li, Acta Phys. Sin., 28 (1979) 314. 10 E B. Pickering, Physical Metallurgy and The Design of Steels, Elsevier, Amsterdam, 1978, p. 89. 11 J. Dvorak, in G. C. Sih (ed.), Proc. Int. Conf on Dynamic Crack Propagation, Sijthoff and Noordhoff, Alphen aan den Rijn, 1973, p. 49. 12 M. Yao, Y. K. He, D. M. Li and C. C. Zhou, in S. R. Vallari, D. M. Taplin, P. Roma Rao, J. F. Knott and R. Dukey (eds.), Advances in Fracture Research, Proc. 6th Int. Conf. on Fracture, New Delhi, Pergamon, Oxford, 1985, p. 1423.
211
Effect of Arrangement of Acicular Ferrite in a Widmanstatten Microstructure on the Fracture of Mild Steel Z. H U A N G
Institute of Physics, Academia Sinica, Beijing (China) M. YAO
Harbin Institute of Technology, Harbin (China) (Received February 7, 1989; in revised form June 7, 1989)
Abstract
The effect of the morphology and distribution of ferrite, especially acicular ferrite in a Widmanstatten structure, on the fracture behaviour of mild steel at low temperatures is discussed in detail. The microstructures of polygonal ferrite, intersecting acicular ferrite and parallel acicular ferrite were obtained using an interrupted cooling procedure, and the different effects on the fracture toughness at low temperature were found. If an intersecting acicular ferrite microstructure is present, then the steel has superior toughness at low temperature in comparison with that of polygonal ferrite. If the microstructure is mainly composed of parallel acicular ferrite, however, the steel toughness is comparable to that of polygonal ferrite. 1. Introduction The effect of microstructures containing acicular ferrite on the mechanical properties of mild steel, especially the fracture toughness at low temperature, has been widely studied [1-5], yet even now there is intense debate about the nature of this effect. In a previous investigation into the influence of an interrupted cooling procedure on the microstructure and mechanical properties of steel [6], it was found that the microstructures obtained could be respectively characterized as polygonal ferrite, intersecting acicular ferrite, parallel acicular ferrite and bainite. The microstructure characterized by intersecting acicular ferrite has the best toughness at low temperature. These phenomena imply that the morphology and distribution of ferrite might be a significant factor in the fracture behaviour of mild steel. It is therefore the objective of the present investiga0921-5093/89/$3.50
tion to determine the main factors governing the fracture process at low temperature and to gain physical understanding of the fracture mechanism in microstructures containing ferrite, especially in the Widmanstatten structure characterized by acicular ferrite.
2. Experimental materials and procedures Two plates of 16Mn steel 20 mm thick were taken as specimens. The chemical compositions are given in Table 1. The steel plates were austenitized at 1150 °C and hot rolled with a reduction of 25% at 1080 or 980 °C, followed by accelerated cooling in water or oil to a temperature in the range from 750 to 500 °C and then cooling in air. The procedure is shown schematically in Fig. 1. To control the accelerated cooling end temperature (ACET), the cooling curves of the plate of chosen geometry in water or oil were measured by means of an optical oscillograph (SC60). The signals were taken from the centre of the plate by Le Chatelier thermocouples. The relationships of temperature and time in the different cooling media were established and hence the ACET could be controlled by controlling the time in the quench medium. The cooling time was controlled within 0.2 s and hence the error in the cooling temperature was less than 2 °C in oil or 4 °C in water because the maximum rate of cooling is 9 °C s-I in oil or TABLE 1
Chemicalcomposition of the steels (wt.%)
C
Mn
Si
S
P
0.16 0.16
1.32 1.27
0.66 0.33
0.016 0.020
0.013 0.022
© Elsevier Sequoia/Printed in The Netherlands
212
1000 °
4O
Oo ,..~8
© - - 0"o.~ A 800 -- O FATT.~3 % / /
0 °C
~~,....~.....~ ..,~,., -
~,. ~'-,~.~
~v..._'"-,~ ~-..~ ~ \ ~
20
IN AIR
75ooc
600
720°C
69o °c 65o °c s2o oc s8o °c - 5 0 0 °C
u'/
3. Experimental results and analyses 3.1. Effect of A C E T on microstructure and properties Figure 2 illustrates the tensile properties and the fracture appearance transition temperature (FATT) as a function of the ACET. Here, only two groups of test results are presented because similar results were found in all of the groups. It can be seen from Fig. 2 that the yield strength (a0.2) and tensile strength (oh) increase and the reduction of area (~) and elongation (~) decrease with decreasing ACET, but it must be emphasized that these trends become obvious only if the ACET is below 650 °C. As far as the low temperature fracture toughness is concerned, the minimum occurs at about 690 °C on the FATT vs. ACET curve; that is to say, there is a range of ACET which optimizes the ductility of steel at low temperature. The same conclusion can be
~ 0 ffh
40o tO
TIME Fig. 1. Schematic illustration of the procedure used in this investigation. 16 °C s- 1 in water in the temperature range from 750 to 550 °C. The temperature of the medium was monitored to ensure consistent heat transfer. Charpy impact tests were made over a temperature range from - 78 to + 20 °C to determine the fracture properties of the steel at low temperature. Specimens of 10 mm gauge diameter and 50 mm gauge length were used to determine the tensile properties. The specimens were taken from the centre of the treated plates in longitudinal and transverse directions, and several groups of specimens were obtained. Observation of the microstructure was carried out with a transmission electron microscope, and analysis of the fracture appearance of the specimens impacted at low temperature was done by means of scanning electronic microscopy (SEM).
0 o
0
-20 200
¶ ~ ¢ \ at 980"C
_
-40
0
<# 60 8 0 _ ~ ~ ~ ~
- 30 -
0
o
:::
40-
20
I
E-i
\ I
]
20 0
_10
o
]
800 700 600 500 COOLING-END TEMPERATURE IN WATER,"C Fig. 2. Variation of mechanical properties of 16Mn steel with cooling end temperature (broken line shows FATT of specimen rolled at 980 °C).
reached from Fig. 3, in which the variation of impact energy with temperature is shown. The corresponding variation of microstructure is shown in Fig. 4. If the ACET is higher than 720 °C, the microstructure obtained is composed of polygonal ferrite-pearlite (Figs. 4(a) and 4(b)). When the ACET varies over the temperature range from 720 to 620 °C, the microstructure consists mainly of acicular ferrite-pearlite, but the arrangement of acicular ferrite changes from intersecting (Fig. 4(c)) to parallel (Fig. 4(e)) and the fraction of parallel acicular ferrite increases with a decrease in the ACET. In this range the proportion of acicular ferrite (about 4%) changes little and hence the yield stress does not change greatly [4]. The corresponding FATT, as mentioned above, first shifts to a lower temperature and then returns. The minimum value of the FATT corresponds to the intersecting acicular ferrite microstructure. When the ACET is lower than 580 °C, bainite is formed (Fig. 4(f)). These phenomena were found in other research work, where similar heat treatments were followed and the formation of ferrite in the microstructure was
213 7o I
70
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bO 5O
m
60
~o-
/,
,,=, z W
2OF ~i~/i~/ tO 6~°C
20
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I -60
-40
I
I
~
-20
0
+20
TEHPERATURE. C
+40
10
0 -8O
-60
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Z~ ~
720"C
e
~
6 5 0 "C
•
I
620"C
o •
~ ~
580 °C 500"C
I
I
0
+20
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TEMPERATURE, C
Fig. 3. Impact energy of specimens with different microstructures (marked temperatures are ACETs).
Fig. 4. Variation of microstructure with cooling end temperature: (a) cooling in air; (b) cooling in water to 720 °C; (c) to 690 °C; (d) to 650 °C; (e) to 620 °C; (f) to 500 °C, then cooling in air.
214
studied by using a dilatometer [7]. The ACET was controlled directly by controlling the temperature. All control was carried out by computer. The results showed that the ACET can be characterized by several temperature ranges: above 720 °C, the morphology of the ferrite is mainly polygonal; between 710 and 670 °C, intersecting acicular; and between 650 and 550 °C, parallel acicular. Evidently the above variation is the same as that mentioned earlier.
3.2. Morphological features and crystallographic properties In order to clarify the effect of the arrangement of acicular ferrite, the distinctions between the different acicular ferrite morphologies were studied by means Of transmission electron microscopy. Figure 5 shows a replica of the morphology of acicular ferrite in detail. In the microstructure of intersecting ferrite (Fig. 5(a)), pearlite blocks can frequently be found to exist among the ferrite needles and thus each needle could be regarded as a unit; but in the parallel ferrite microstructure, the parallel ferrite needles form a package (Fig. 5(b)) in which the ferrite needles are arranged in parallel even though the ferrite boundary is curved (Fig. 6). Thus it is proposed that each of these packages of parallel needles be regarded as one microstructural unit. This supposition is strongly supported by analyses of electron diffraction patterns and microfractographs. Figure 7 shows a transmission electron micrograph of parallel acicular ferrite and corresponding diffraction patterns. It can be seen that the patterns of the needles in a package are the same. In fact, it was found that the same phenomena could be observed in other crystallographic directions and in other packages. Therefore the suggestion is that the package also be taken as a unit for cleavage crack propagation in respect of the crystallography. Details of the crystallography of acicular ferrite have been reported in ref. 8. As far as the intersecting acicular ferrite is concerned, however, the patterns which belong to different ferrite needles are not the same. As shown by transmission electron microscopy (TEM) (Fig. 8), the indexed diffraction patterns belong to three lattice arrays whose axes are respectively
the crystallographic relationship between acicular ferrite and prior austenite was inferred. On the supposition that a K-S relationship exists between a-Fe (b.c.c. lattice) and 7-Fe (f.c.c. lattice), it was found that a crystallographic direction [145]r is parallel to [315]~,, [133]~ and [lil]~ 3, and is related to them by the following K-S relationship:
[3 lS]:,//[1 ~3]:J/[1 i 1]~ 3
[315]:1//[ 145]r corresponding to
By means of a programme of matrix analysis [9],
Fig. 5. Replica of two types of arrangement of acicular ferrite: (a) intersecting; (b) parallel.
Fig. 6. Transmission electron micrograph of parallel acicular ferrite and its boundary.
(i 11)J/(O 11)~, [01 i]r//[111]~
215
( 200 )
~
020 )
7
Fig. 7. Transmission electron micrograph of parallel acicular ferrite and corresponding diffraction patterns of regions A, B and C. A/B=4~I2/2 ~/z= 1.414; B =[001].
(7>ol)
_
Fig. 8. Transmission electron micrograph of intersecting acicular ferrite and corresponding diffraction pattern of boundary A.
[133]aJ/[145]~, corresponding to (i 11 )~//(011 )a, []0 ]]J/[111]a [1 ] 1]~3//[145]r corresponding to (11
[1 iol,///[11115
That is to say, there is a certain crystallographic relationship between acicular ferrite and prior austenite but the relationships for different needles may not be exactly the same. In addition, because the ferrite needles as proeutectoid phase are able to separate prior austenite grain into a number of smaller parts so that the growing of pearlite in each part is restricted by
surrounding acicular ferrite, the intersecting acicular ferrite is able to refine pearlite block. As seen from Fig. 5, the pearlite blocks in the different parts have different arrangement orientations. Obviously, this separation is beneficial for improving the toughness of steel [10]. However, the effect of parallel acicular ferrite on the separation of prior austenite grain and the restriction on the growing of pearlite is almost the same as that of polygonal ferrite on account of the package being considered as a unit. All of these analyses demonstrate that, although both kinds of microstructure can be classified as a Widmanstatten structure, they exhibit significant
216
morphological differences which obviously affect the steel toughness differently at low temperature.
3.3. Microfractographic features SEM was used to distinguish the microfractographic features of steel with different ferrite morphologies to obtain more direct evidence of the previous hypothesis. Figure 9 shows SEM fractographs of the specimens impacted at - 5 0 °C. The fractograph of the specimen with intersecting acicular ferrite obtained at 690 °C ACET (Fig. 9(b)) indicates that the cleavage facets are obviously smaller than those of the polygonal ferrite obtained at 720 °C ACET (Fig. 9(a)) and the parallel acicular ferrite obtained at 650 °C ACET (Fig. 9(c)), and shows that the river
pattern is more confused and a large number of tear ridges and cleavage steps exist. In the fractograph of the parallel acicular ferrite specimen the facets, in sharp contrast to the above features, are large and the river pattern is regular, these features evidently being related to the crystallographic properties of the parallel acicular ferrite microstructure. This point is proved by another group of SEM microfractographs (Fig. 10) in which the correlation of cleavage facet with microstructure is exhibited directly by using the method of etching the fracture surface with Nital. It can be seen that in a package of parallel acicular ferrite the cleavage stream, whether it is across or along the parallel acicular ferrite, hardly twists (Figs. 10(a) and 10(b)), but in the intersecting
Fig. 9. SEM fractographs of specimens impacted at - 5 0 *C: (a) polygonal ferrite (ACET 720 °C); (b) intersecting acicular ferrite (ACET 690 °C); (c) parallel acicular ferrite (ACET 650 °C).
Fig. 10. SEM micrographs of the etched fractures: (a), (b) parallel acicular ferrite; (c) intersecting acicular ferrite.
217
acicular ferrite microstructure it twists so sharply that the plane or regular facet is barely visible (Fig. lO(c)). All of these observations demonstrate further the significant role of the arrangement of acicular ferrite in cleavage fracture.
4. Discussion
With respect to the discussion of the effect of the Widmanstatten structure, the approach can be divided into two aspects: affirmation of the improvement in steel toughness and negation thereof. In the affirmation, the properties of acicular ferrite, such as high density of dislocations, small effective grain size and absence of carbide precipitation in the grain and on the grain boundary [2], were emphasized. In the negation, the integrity of the microstructure was considered and the acicular ferrite was regarded as a phase which would play the role of separating the microstructure and hence diminish the steel toughness [1]. On the basis of the above results and analyses, we believe that the arrangement of acicular ferrite in the Widmanstatten structure is a dominant factor in determining steel toughness at low temperature, and the influence of the Widmanstatten structure depends upon whether the acicular ferrite maintains certain characteristics such as small effective grain size (particularly important), high density of dislocations and absence of carbide precipitation in the grain and on the grain boundary. Since intersecting acicular ferrite maintains the characteristic of small effective grain size and has the ability to refine pearlite block, it is beneficial for the improvement of steel toughness at low temperature. Parallel acicular ferrite, however, does not retain the characteristic of fine grain, because the needles in a package exhibit almost identical crystallographic properties and hence the package can be regarded as an effective grain for cleavage crack propagation [11, 12], so that parallel acicular ferrite should diminish steel toughness at low temperature. This is evidently a valid model for the illustration of not only the positive but also the negative influence of the Widmanstatten structure.
5. Conclusions
When an interrupted accelerated cooling procedure is used, the morphology of ferrite in the microstructure of 16Mn steel will change from polygonal to intersecting acicular and finally to parallel acicular as the accelerated cooling end temperature decreases. Once a microstructure composed mainly of intersecting acicular ferrite has been obtained, the steel should exhibit the lowest FATT (FATT50%) and the smallest cleavage facets in the fracture; that is, it has the greatest resistance to brittle fracture at low temperature. This is because the intersecting acicular ferrite maintains the characteristic of fine effective grain and, moreover, refines the pearlite block. If a microstructure consists mainly of parallel acicular ferrite, the steel toughness at low temperature will deteriorate, which results in a higher FATT50% and larger cleavage facets. The essential cause of this is that the ferrite needles in a package display identical arrangement orientation and have the same crystallographic characteristics. References 1 R. E. Greaves and H. Wrighton, Practical Microscopical Metallography, Chapman, London, 4th edn., 1957. 2 V. I. Izotov and B. A. Leontov, Fiz. Met. Metalloved., 32 (1971)96. 3 L. G. Taylor and R. A. Farrar, Welding Met. Fabr., 43 (5) (1975)305. 4 D.J. Widgery, Welding Res. Suppl., 41 (1976) 57-s. 5 R. Otterberg, R. Sandstrom and A. Sandberg, Met. Technol., 10 (1980) 397. 6 Z. Huang and M. Yao, Iron SteelSin., 12 (1989)in the press. 7 Z. Huang and M. Yao, WUL1CESH1 (Physical Testing), 3 (1986) 10 (in Chinese). 8 A.D. King and T. Bell, Met. Sci. J., 8 (1974) 253. 9 C.Z. Li, Acta Phys. Sin., 28 (1979) 314. 10 E B. Pickering, Physical Metallurgy and The Design of Steels, Elsevier, Amsterdam, 1978, p. 89. 11 J. Dvorak, in G. C. Sih (ed.), Proc. Int. Conf on Dynamic Crack Propagation, Sijthoff and Noordhoff, Alphen aan den Rijn, 1973, p. 49. 12 M. Yao, Y. K. He, D. M. Li and C. C. Zhou, in S. R. Vallari, D. M. Taplin, P. Roma Rao, J. F. Knott and R. Dukey (eds.), Advances in Fracture Research, Proc. 6th Int. Conf. on Fracture, New Delhi, Pergamon, Oxford, 1985, p. 1423.