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The role of connectivity of martensite on the tensile properties of a low alloy steel
Materials & Design Materials and Design 28 (2007) 1928–1933 www.elsevier.com/locate/matdes
Short communication
The role of connectivity of martensite on the tensile properties of a low alloy steel M. Sarwar *, T. Manzoor, E. Ahmad, N. Hussain Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan Received 5 December 2005; accepted 8 May 2006 Available online 11 July 2006
Abstract The effects of martensitic morphology and its distribution in a ferrite matrix on tensile properties of a low alloy steel have been studied. Two distinct microstructures were developed solely by heat treatment, i.e., no thermo-mechanical processing was involved; one consisted of continuous ferrite matrix with embedded islands of martensite (MD: martensite dispersed). The other was continuous martensite phase with embedded islands of ferrite (MC: martensite continuous). The MC structure results in a tensile strength of 977 MPa as compared to 938 MPa for the MD structure, despite the fact that the latter had an 8% higher volume fraction of martensite. The increase is attributed to the continuous distribution of the martensite. 2006 Elsevier Ltd. All rights reserved.
1. Introduction Fuel economy together with safety consideration, is the driving force for the steady increasing usage of higher strength in automobile and composites. The increased strength to weight ratio is the additional advantage that processing is similar to that of conventional steel. Therefore, the overall fabrication cost will be reduced in addition to benefit of weight saving, which is exactly the opposite for all other competing materials. The microstructure of dual-phase steel (DPS) is comprised of a soft ductile ferrite matrix and 20–30% volume fraction of hard phase (martensite). This type and proportion of microstructure exhibits unique combination of strength and ductility [1–4] compared with high strength low alloy steels and thus has been considered for application which requires good formability. The other higher strength materials such as micro-alloyed steels, suffers from the disadvantages that they show lower formability com*
Corresponding author. Tel.: +92 51 2207222. E-mail address: [email protected] (M. Sarwar).
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.05.010
pared with conventional low carbon steel. To rectify this problem, DPSs have been developed in order to obtain the best balance among the desired mechanical properties. In this connection, thermo-mechanical processing was employed previously [5,6] to get reasonable enhancement in the tensile properties. This was done by introducing the fibrousity in microstructure for better stress transfer from soft ferrite matrix to hard martensite particles by increasing their area of contact. In these findings the fibrous microstructures were oriented along the tensile axis. The connectivity of martensite particles attained by changing the heat treatment cycles [7,8] can offer an increase in the interfacial area for better stress transfer in tensile deformation. The purpose of the present communication is to investigate the role of degree of connectivity of martensite on tensile properties of dual-phase steel and to ascertain whether the material with connectivity of martensite has a superior effect compared with other martensite distribution and morphology. This information should aid in understanding the role of martensite morphology on unique combination of strength and ductility of dual-phase steel.
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
1929
2. Experimental procedure
2.4. Tensile testing
2.1. Material
Round tensile specimen, shown in Fig. 2, was machined from the heat treated blanks. The tensile tests were performed on the Instron tensile testing machine model 8562 at room temperature at strain rate of 10 3/s. Three tensile specimens were tested for each microstructure and average tensile data is taken.
Material selected for this study was 10 mm thick hot rolled plate of low alloy steel. The chemical composition of steel (wt%) is given in Table 1.
2.2. Heat treatment 2.5. Scanning electron microscopy The microstructural studies revealed that the as-received material contained banded ferrite and pearlite microstructure. The samples were heat treated at 1200 C for 1 h to homogenize and remove the banded structure of as-received material. These heat treated samples were used in the following heat treatment experiments. The samples of 10 · 10 mm size were heat treated at series of intercritical annealing temperatures (ICAT) from 730 to 875 C for 30 min and quenched in iced brine solution ( 6 C) in order to determine the volume fraction of austenite at intercritical annealing temperatures. On the basis of this experimental data a heat treatment temperature of 750 C was selected where 50% austenite formed on intercritical annealing. Two heat treatment cycles were employed on sample A and B of 10 · 10 · 75 mm size, to develop different morphologies of martensite, used for mechanical testing. The details of the heat treatment cycles are presented schematically in Fig. 1a and b.
2.3. Metallography After grinding and polishing, all the samples were etched in 2% nital solution. Quantitative metallographic techniques were employed to determine volume fraction and degree of connectivity of martensite. The connectivity of martensite is defined as ratio of the number of ferrite/martensite boundaries per unit length to the total number of boundaries (i.e., ferrite/martensite and ferrite/ferrite) per unit length, expressed as percentage [8].
Microvoid formation and fracture surfaces of the specimens were studied on scanning electron microscope (SEM). The broken tensile specimens were cut through the centerline along the tensile axes to study the microvoid formation at the centre of the specimens.
3. Results and discussion 3.1. Effect of intercritical annealing temperature on volume fraction of martensite Fig. 3 shows the influence of intercritical annealing temperature on volume fraction of martensite. The volume fraction of martensite increased with the increase of intercritical annealing temperatures; similar trend was reported by other investigators [5,6,9,10]. All the specimens were quenched in the iced brine solution and, therefore, it is assumed that austenite has fully transformed to martensite, if no retained austenite is left. The amount of martensite which is in fact a measure of austenite present at the ICAT depends upon the carbon content
Table 1 Chemical composition of steel used C
Mn
Si
S
P
Fe
0.20
0.90
0.22
0.04
0.035
Bal
1h
1h
1200°C
1200°C
5h
950°C
1h
950°C
Temperature
5h
Temperature
Fig. 2. Geometry of tensile sample used.
750°C
F.C 1h
750
F.C
F.C
1h
A.C
750
1h
A.C
750
1h
A.C
1h
750°C
1h
200°C F.C
F.C
(a)
Time
Ice Brine Quench
Heat Treatment Cycle A
200°C
(b)
Time
Ice Brine Quench
Heat Treatment Cycle B
Fig. 1. (a, b) Schematic representation of heat treatment cycles A and B for development of different microstructural morphologies.
Temperature, °C
1930
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
860 850 840 830 820 810 800 790 780 770 760 750 740 730 720 20
30
40
50
60
70
80
90
100
110
% of Martensite
Fig. 3. Dependence of austenite contents on intercritical annealing temperatures for holding times of 30 min. Fig. 5. Microstructure of MC.
and other alloying elements of the steel. In present study, 50% of martensite was obtained at ICAT of 750 C which is lower than previous results of Sarwar and Priestner [5] and Ahmad and Priestner [6] having the carbon content of 0.16% and 0.09%, respectively, with similar manganese contents of the steels. 3.2. Microstructural characterization SEM photomicrographs of samples MD and MC resulted from heat treatments cycles are shown in Figs. 4 and 5. In sample MD, step quenching (SQ) treatment yields a microstructure consisting of a continuous ferrite network encapsulating islands of martensite (Fig. 4) from residual austenite on brine quenching. The step quenching from austenite into the (a + c) region causes ferrite to nucleate at prior austenite grain boundaries and grow into the austenite, resulting in a coarse and roughly equiaxed martensite surrounded by a continuous ferrite matrix. Repeated intercritical annealing (RIA) yields a MC microstructure (Fig. 5) in which martensite have a form and are located exclusively along the ferrite grain boundaries and/or at the triple points. The martensite also shows the marked tendency to encircle the ferrite grains.
Speich et al. [11] have studied the formations of austenite at temperatures between 740 and 900 C in a series of 1.5 wt% Mn steels containing 0.06–0.20 wt%C and having a ferrite and pearlite starting microstructure. They found that the growth of austenite into ferrite, at low temperatures (750 C), was controlled by manganese diffusion in ferrite, which is much easier along the grain boundaries, so that the principal direction of growth of austenite must be parallel to the grain boundaries. This model of formation of austenite can easily explain the distribution of martensite along the ferritic grain boundaries. Similar microstructure has been observed in the present study. About 8% more volume of martensite was determined in sample MD microstructure at the same intercritical temperature and cooling rate compare to specimen MC microstructure. As shown in Fig. 4, the MD microstructure have continuous ferrite network which encapsulated volumes of martensite formed from the austenite on brine quenching and resulted microstructure consist of equiaxed and bigger martensite grains, whereas MC microstructure consists of a microstructure wherein reverted austenite has transformed to martensite and forms a continuous network encapsulating ferrite. The resulted microstructure is of connected, randomly oriented and elongated martensite grains (Fig. 5). 3.3. Tensile properties
Fig. 4. Microstructure of MD.
Room temperature unidirectional tensile curves together with the mechanical properties are shown in Fig. 6. The metallographic and tensile data are presented in Table 2, respectively. MD and MC showed continuous yielding in tensile test while the as-received specimen invariably showed discontinuous yielding. This difference in yielding behaviour has been investigated by several workers [12,13] and it has been suggested that the expansion associated with austenite to martensite transformation produces stresses which are relieved by the generation of mobile dislocations in ferrite [14,15] adjacent to the martensite. These mobile disloca-
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
Fig. 6. Stress–strain curves of MD, MC and as-received material.
tions together with the residual stresses remaining after relaxation, promoted continuous yielding. Present study of MD and MC microstructure gives rise to continuous yielding, which is one of the typical characteristics of dual-phase steels. MC sample exhibited a bit higher tensile and yield strength than specimen MD. It can be seen from Fig. 5 that martensite grains in the microstructure are interconnected with each other and connectivity of the martensite was 0.92 indicating that 92% of the grain boundary area of each ferrite grain was encapsulated by the martensite phase. Approximately 5% more tensile and yield strength of MC may be attributed due to fibrousity of martensite particles, which would be expected to improve stress transfer from the ferrite matrix to the martensite particle
1931
during plastic deformation results in an increase tensile strength [5,6]. The tensile and yield strengths of MD are lower than MC even having 8% less volume fraction of martensite. From this comparison it is clear that almost, for a constant volume of martensite, connectivity of the martensite plays an important role to increase the yield and tensile strengths. The geometrical distribution of the hard phase (martensite) can influence strongly the mode of deformation of dual-phase steels. When hard particles are distributed along the grain boundaries of a soft matrix (ferrite), it is almost impossible to deform the specimen without cracking the some of the hard particles. The hard particles having geometry distribution as shown (Fig. 5) restrict plastic flow of the soft (matrix) due to which strength of the martensite is improved. Total and uniform elongations increased with the connectivity of martensite as shown in Table 2. It is clear from Fig. 6 and Table 2 that strength and elongations of MC are higher than MD. This comparison suggests that connectivity of martensite played an important role to improve the strength and ductility of material. 3.4. Tensile fracture Fractured tensile specimens of MD and MC were studied on scanning electron microscope to reveal the fracture mechanism. The microstructure examination near the fracture surface shows that ‘‘Microcracks’’ were developed in the necked region before the ultimate failure to occur, Figs. 7 and 8.
Table 2 Metallographic and tensile data S. No.
Sample description
Volume fraction of martensite (%)
Connectivity of martensite (%)
0.2% proof stress (MPa)
Ultimate tensile strength (MPa)
Uniform elongation (%)
Total elongation (%)
1 2 3
MD MC ARa
60 52 –
25 92 –
680 717 344
938 977 581
12 14 24
12.6 17 35
a
AR, as-received sample.
Fig. 7. (a, b) Formation of microcracks near the fracture surface of MD Sample.
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M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
Fig. 8. (a, b) Formation of microcracks near the fracture surface of MC sample.
There is no indication of microvoid formation in both the specimens. It appears from Fig. 7(a) and (b) that MD has less density of microcrack compared with the MC (Fig. 8(a) and (b)). In early studies of micorovid formation Stenbruner et al. [16] found that in dual-phase steels, with 0.08%C, during localized necking the microvoids were concentrated near the fracture surface and their density decreased rapidly with increasing distance from the fracture surface. In diffuse necking they found a more uniform distribution of microvoids in the necked region. These microvoids were formed either by decohesion at the ferrite–martensite interface by elongation of martensite particles along the tensile axes or by fracture of individual martensite particle. Ahmad et al. [17] with 0.32%C of steel observed that at higher martensite (>30%), failure occurred by microcrack initiation in stead of void formation. Sarwar and Priestner [5] observed that the plastic deformation of ferrite and martensite in the necked region at >55% martensite while in the present study the specimens having almost same fraction of martensite showed minimal of plastic deformation. This may be due to the lower carbon content of Sarwar and Priestner [5] steel (0.17%C) than the steel used in the present studies, resulting in the lower carbon content of martensite. Higher carbon content in the martensite as in the present case means that there is more carbon atom at the interstitial position in the martensite creating more dislocations in the ferrite, resulting in brittle failure. The (MC) structure consisting of interconnected martensite showed partially brittle fracture; martensite particles break and forms cracks perpendicular to the tensile direction (Fig. 8(a) and (b)). Both broken halves of specimens generally showed microcrack formation in the necked region near the fracture surface and no indication of structural elongation along the tensile axes before the failure. It is observed that cleavage cracks are initiated within the ferrite grains and stopped at the martensite particles. It is also noted that the MD specimen (Fig. 9) exhibited higher density of cleavage facets compared with MC specimen (Fig. 10).
Fig. 9. Fracture surface of the MD sample, indicating cleavage facets.
Fig. 10. Fracture surface of the MC sample, indicating mixed mode of failure.
4. Conclusions Considering the strength of the dual-phase steels martensite particles with better area of contact with ferrite provide better stress transfer from ferrite (soft matrix) to
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
martensite and improve the tensile properties. Such improvements were obtained in the past by applying warm rolling in the two phase region but due to the development of anisotropy in mechanical properties such steels have not improved their worth for practical applications. The continuous martensitic network produced by heat treatment cycles certainly improved the area of contact between ferrite and martensite resulted in enhancement in the tensile properties. Acknowledgements The authors thank Mr. Ghulam Jalini, Mr. Amir Alam, Mr. Shahbaz Ahmad Chatta and Mr. Masood Ahmad for their help in experimental work and arrangement of technical equipment. References [1] Rashid MS. Relationship between steel microstructure and formability. In: Formable HSLA and dual-phase steels. Davenport AT: AIME; 1977. p. 1–24. [2] Hansen SS, Pradhan RR. Structure/property relationship and continuous yielding behavior in dual-phase steels. In: Kot RA, Bramfitt BL, editors. Proceedings fundamentals of dual-phase steels, 1981. p. 113–44. [3] Speich GR, Miller RL. Mechanical properties of ferrite–martensite steels. In: Davenport AT, editor. Structure and properties of dualphase steels. New York (NY): TMS-AIME; 1979. p. 145–82. [4] Balliger NK, Gladman T. Work hardening of dual-phase steels. Met Sci 1981;15(3):95–108.
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[5] Sarwar M, Priestner R. Influence of ferrite–martensite microstructural morphology on tensile properties of dual-phase steel. J Mater Sci 1996;31:2091–5. [6] Ahmad E, Priestner R. Effect of rolling in the intercritical region on the tensile properties of dual-phase steel. J Mater Eng Perform 1998;7(6):772–6. [7] Suzuki H, McEvily AJ. Microstructural effects on fatigue crack growth in a low carbon steel. Metall Trans A 1979;10:475–81. [8] Minakawa K, Matsuo Y, McEvily AJ. The influence of a duplex microstructure in steel on fatigue crack growth in near-threshold region. Metal Trans A 1982;13:439–45. [9] Sarwar M, Priestner R. Hardenability of austenite in a dual-phase steel. J Mater Eng Perform 1999;8(3):380–4. [10] Sun Shoujin, Pugh Martin. Properties of thermomechanically processed dual-phase steels containing fibrous martensite. Mater Sci Eng 2002;335(1–2):298–308. [11] Speich GR, Demarest VA, Miller RL. Formation of austenite during intercritical annealing of dual-phase steels. Metall Trans A 1981;12:1419–28. [12] Rigsbee JM, Vanderarend PJ. Laboratory of microstructures and structure–property relationship in dual-phase HSLA steels. In Proceedings of the AIME symposium on modern developments in HSLA formable steels, Chicago; 1977. [13] Davies RG. Early stages of yielding and strain aging of a vanadium containing dual-phase steel. Metall Trans A 1979;10:1549–55. [14] Magee CL, Davies RG. On the volume expansion accompanying the fcc and bcc transformation in ferrous alloys. Acta Metall 1972;20:1031–43. [15] Moyer JM, Ansell GS. The volume expansion accompanying the martensite transformation in iron–carbon alloys. Metall Trans A 1975;6:1785–91. [16] Steinbrunner DL, Matlock DK, Krauss G. Void formation during tensile testing of dual-phase steels. Metall Trans A 1988;19:579–89. [17] Ahmad E, Manzoor T, Ali KL, Akhter JI. Effect of microvoid formation on the tensile properties of dual-phase steel. J Mater Eng Perform 2000;9(3):306–10.
Short communication
The role of connectivity of martensite on the tensile properties of a low alloy steel M. Sarwar *, T. Manzoor, E. Ahmad, N. Hussain Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan Received 5 December 2005; accepted 8 May 2006 Available online 11 July 2006
Abstract The effects of martensitic morphology and its distribution in a ferrite matrix on tensile properties of a low alloy steel have been studied. Two distinct microstructures were developed solely by heat treatment, i.e., no thermo-mechanical processing was involved; one consisted of continuous ferrite matrix with embedded islands of martensite (MD: martensite dispersed). The other was continuous martensite phase with embedded islands of ferrite (MC: martensite continuous). The MC structure results in a tensile strength of 977 MPa as compared to 938 MPa for the MD structure, despite the fact that the latter had an 8% higher volume fraction of martensite. The increase is attributed to the continuous distribution of the martensite. 2006 Elsevier Ltd. All rights reserved.
1. Introduction Fuel economy together with safety consideration, is the driving force for the steady increasing usage of higher strength in automobile and composites. The increased strength to weight ratio is the additional advantage that processing is similar to that of conventional steel. Therefore, the overall fabrication cost will be reduced in addition to benefit of weight saving, which is exactly the opposite for all other competing materials. The microstructure of dual-phase steel (DPS) is comprised of a soft ductile ferrite matrix and 20–30% volume fraction of hard phase (martensite). This type and proportion of microstructure exhibits unique combination of strength and ductility [1–4] compared with high strength low alloy steels and thus has been considered for application which requires good formability. The other higher strength materials such as micro-alloyed steels, suffers from the disadvantages that they show lower formability com*
Corresponding author. Tel.: +92 51 2207222. E-mail address: [email protected] (M. Sarwar).
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.05.010
pared with conventional low carbon steel. To rectify this problem, DPSs have been developed in order to obtain the best balance among the desired mechanical properties. In this connection, thermo-mechanical processing was employed previously [5,6] to get reasonable enhancement in the tensile properties. This was done by introducing the fibrousity in microstructure for better stress transfer from soft ferrite matrix to hard martensite particles by increasing their area of contact. In these findings the fibrous microstructures were oriented along the tensile axis. The connectivity of martensite particles attained by changing the heat treatment cycles [7,8] can offer an increase in the interfacial area for better stress transfer in tensile deformation. The purpose of the present communication is to investigate the role of degree of connectivity of martensite on tensile properties of dual-phase steel and to ascertain whether the material with connectivity of martensite has a superior effect compared with other martensite distribution and morphology. This information should aid in understanding the role of martensite morphology on unique combination of strength and ductility of dual-phase steel.
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
1929
2. Experimental procedure
2.4. Tensile testing
2.1. Material
Round tensile specimen, shown in Fig. 2, was machined from the heat treated blanks. The tensile tests were performed on the Instron tensile testing machine model 8562 at room temperature at strain rate of 10 3/s. Three tensile specimens were tested for each microstructure and average tensile data is taken.
Material selected for this study was 10 mm thick hot rolled plate of low alloy steel. The chemical composition of steel (wt%) is given in Table 1.
2.2. Heat treatment 2.5. Scanning electron microscopy The microstructural studies revealed that the as-received material contained banded ferrite and pearlite microstructure. The samples were heat treated at 1200 C for 1 h to homogenize and remove the banded structure of as-received material. These heat treated samples were used in the following heat treatment experiments. The samples of 10 · 10 mm size were heat treated at series of intercritical annealing temperatures (ICAT) from 730 to 875 C for 30 min and quenched in iced brine solution ( 6 C) in order to determine the volume fraction of austenite at intercritical annealing temperatures. On the basis of this experimental data a heat treatment temperature of 750 C was selected where 50% austenite formed on intercritical annealing. Two heat treatment cycles were employed on sample A and B of 10 · 10 · 75 mm size, to develop different morphologies of martensite, used for mechanical testing. The details of the heat treatment cycles are presented schematically in Fig. 1a and b.
2.3. Metallography After grinding and polishing, all the samples were etched in 2% nital solution. Quantitative metallographic techniques were employed to determine volume fraction and degree of connectivity of martensite. The connectivity of martensite is defined as ratio of the number of ferrite/martensite boundaries per unit length to the total number of boundaries (i.e., ferrite/martensite and ferrite/ferrite) per unit length, expressed as percentage [8].
Microvoid formation and fracture surfaces of the specimens were studied on scanning electron microscope (SEM). The broken tensile specimens were cut through the centerline along the tensile axes to study the microvoid formation at the centre of the specimens.
3. Results and discussion 3.1. Effect of intercritical annealing temperature on volume fraction of martensite Fig. 3 shows the influence of intercritical annealing temperature on volume fraction of martensite. The volume fraction of martensite increased with the increase of intercritical annealing temperatures; similar trend was reported by other investigators [5,6,9,10]. All the specimens were quenched in the iced brine solution and, therefore, it is assumed that austenite has fully transformed to martensite, if no retained austenite is left. The amount of martensite which is in fact a measure of austenite present at the ICAT depends upon the carbon content
Table 1 Chemical composition of steel used C
Mn
Si
S
P
Fe
0.20
0.90
0.22
0.04
0.035
Bal
1h
1h
1200°C
1200°C
5h
950°C
1h
950°C
Temperature
5h
Temperature
Fig. 2. Geometry of tensile sample used.
750°C
F.C 1h
750
F.C
F.C
1h
A.C
750
1h
A.C
750
1h
A.C
1h
750°C
1h
200°C F.C
F.C
(a)
Time
Ice Brine Quench
Heat Treatment Cycle A
200°C
(b)
Time
Ice Brine Quench
Heat Treatment Cycle B
Fig. 1. (a, b) Schematic representation of heat treatment cycles A and B for development of different microstructural morphologies.
Temperature, °C
1930
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
860 850 840 830 820 810 800 790 780 770 760 750 740 730 720 20
30
40
50
60
70
80
90
100
110
% of Martensite
Fig. 3. Dependence of austenite contents on intercritical annealing temperatures for holding times of 30 min. Fig. 5. Microstructure of MC.
and other alloying elements of the steel. In present study, 50% of martensite was obtained at ICAT of 750 C which is lower than previous results of Sarwar and Priestner [5] and Ahmad and Priestner [6] having the carbon content of 0.16% and 0.09%, respectively, with similar manganese contents of the steels. 3.2. Microstructural characterization SEM photomicrographs of samples MD and MC resulted from heat treatments cycles are shown in Figs. 4 and 5. In sample MD, step quenching (SQ) treatment yields a microstructure consisting of a continuous ferrite network encapsulating islands of martensite (Fig. 4) from residual austenite on brine quenching. The step quenching from austenite into the (a + c) region causes ferrite to nucleate at prior austenite grain boundaries and grow into the austenite, resulting in a coarse and roughly equiaxed martensite surrounded by a continuous ferrite matrix. Repeated intercritical annealing (RIA) yields a MC microstructure (Fig. 5) in which martensite have a form and are located exclusively along the ferrite grain boundaries and/or at the triple points. The martensite also shows the marked tendency to encircle the ferrite grains.
Speich et al. [11] have studied the formations of austenite at temperatures between 740 and 900 C in a series of 1.5 wt% Mn steels containing 0.06–0.20 wt%C and having a ferrite and pearlite starting microstructure. They found that the growth of austenite into ferrite, at low temperatures (750 C), was controlled by manganese diffusion in ferrite, which is much easier along the grain boundaries, so that the principal direction of growth of austenite must be parallel to the grain boundaries. This model of formation of austenite can easily explain the distribution of martensite along the ferritic grain boundaries. Similar microstructure has been observed in the present study. About 8% more volume of martensite was determined in sample MD microstructure at the same intercritical temperature and cooling rate compare to specimen MC microstructure. As shown in Fig. 4, the MD microstructure have continuous ferrite network which encapsulated volumes of martensite formed from the austenite on brine quenching and resulted microstructure consist of equiaxed and bigger martensite grains, whereas MC microstructure consists of a microstructure wherein reverted austenite has transformed to martensite and forms a continuous network encapsulating ferrite. The resulted microstructure is of connected, randomly oriented and elongated martensite grains (Fig. 5). 3.3. Tensile properties
Fig. 4. Microstructure of MD.
Room temperature unidirectional tensile curves together with the mechanical properties are shown in Fig. 6. The metallographic and tensile data are presented in Table 2, respectively. MD and MC showed continuous yielding in tensile test while the as-received specimen invariably showed discontinuous yielding. This difference in yielding behaviour has been investigated by several workers [12,13] and it has been suggested that the expansion associated with austenite to martensite transformation produces stresses which are relieved by the generation of mobile dislocations in ferrite [14,15] adjacent to the martensite. These mobile disloca-
M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
Fig. 6. Stress–strain curves of MD, MC and as-received material.
tions together with the residual stresses remaining after relaxation, promoted continuous yielding. Present study of MD and MC microstructure gives rise to continuous yielding, which is one of the typical characteristics of dual-phase steels. MC sample exhibited a bit higher tensile and yield strength than specimen MD. It can be seen from Fig. 5 that martensite grains in the microstructure are interconnected with each other and connectivity of the martensite was 0.92 indicating that 92% of the grain boundary area of each ferrite grain was encapsulated by the martensite phase. Approximately 5% more tensile and yield strength of MC may be attributed due to fibrousity of martensite particles, which would be expected to improve stress transfer from the ferrite matrix to the martensite particle
1931
during plastic deformation results in an increase tensile strength [5,6]. The tensile and yield strengths of MD are lower than MC even having 8% less volume fraction of martensite. From this comparison it is clear that almost, for a constant volume of martensite, connectivity of the martensite plays an important role to increase the yield and tensile strengths. The geometrical distribution of the hard phase (martensite) can influence strongly the mode of deformation of dual-phase steels. When hard particles are distributed along the grain boundaries of a soft matrix (ferrite), it is almost impossible to deform the specimen without cracking the some of the hard particles. The hard particles having geometry distribution as shown (Fig. 5) restrict plastic flow of the soft (matrix) due to which strength of the martensite is improved. Total and uniform elongations increased with the connectivity of martensite as shown in Table 2. It is clear from Fig. 6 and Table 2 that strength and elongations of MC are higher than MD. This comparison suggests that connectivity of martensite played an important role to improve the strength and ductility of material. 3.4. Tensile fracture Fractured tensile specimens of MD and MC were studied on scanning electron microscope to reveal the fracture mechanism. The microstructure examination near the fracture surface shows that ‘‘Microcracks’’ were developed in the necked region before the ultimate failure to occur, Figs. 7 and 8.
Table 2 Metallographic and tensile data S. No.
Sample description
Volume fraction of martensite (%)
Connectivity of martensite (%)
0.2% proof stress (MPa)
Ultimate tensile strength (MPa)
Uniform elongation (%)
Total elongation (%)
1 2 3
MD MC ARa
60 52 –
25 92 –
680 717 344
938 977 581
12 14 24
12.6 17 35
a
AR, as-received sample.
Fig. 7. (a, b) Formation of microcracks near the fracture surface of MD Sample.
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M. Sarwar et al. / Materials and Design 28 (2007) 1928–1933
Fig. 8. (a, b) Formation of microcracks near the fracture surface of MC sample.
There is no indication of microvoid formation in both the specimens. It appears from Fig. 7(a) and (b) that MD has less density of microcrack compared with the MC (Fig. 8(a) and (b)). In early studies of micorovid formation Stenbruner et al. [16] found that in dual-phase steels, with 0.08%C, during localized necking the microvoids were concentrated near the fracture surface and their density decreased rapidly with increasing distance from the fracture surface. In diffuse necking they found a more uniform distribution of microvoids in the necked region. These microvoids were formed either by decohesion at the ferrite–martensite interface by elongation of martensite particles along the tensile axes or by fracture of individual martensite particle. Ahmad et al. [17] with 0.32%C of steel observed that at higher martensite (>30%), failure occurred by microcrack initiation in stead of void formation. Sarwar and Priestner [5] observed that the plastic deformation of ferrite and martensite in the necked region at >55% martensite while in the present study the specimens having almost same fraction of martensite showed minimal of plastic deformation. This may be due to the lower carbon content of Sarwar and Priestner [5] steel (0.17%C) than the steel used in the present studies, resulting in the lower carbon content of martensite. Higher carbon content in the martensite as in the present case means that there is more carbon atom at the interstitial position in the martensite creating more dislocations in the ferrite, resulting in brittle failure. The (MC) structure consisting of interconnected martensite showed partially brittle fracture; martensite particles break and forms cracks perpendicular to the tensile direction (Fig. 8(a) and (b)). Both broken halves of specimens generally showed microcrack formation in the necked region near the fracture surface and no indication of structural elongation along the tensile axes before the failure. It is observed that cleavage cracks are initiated within the ferrite grains and stopped at the martensite particles. It is also noted that the MD specimen (Fig. 9) exhibited higher density of cleavage facets compared with MC specimen (Fig. 10).
Fig. 9. Fracture surface of the MD sample, indicating cleavage facets.
Fig. 10. Fracture surface of the MC sample, indicating mixed mode of failure.
4. Conclusions Considering the strength of the dual-phase steels martensite particles with better area of contact with ferrite provide better stress transfer from ferrite (soft matrix) to
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martensite and improve the tensile properties. Such improvements were obtained in the past by applying warm rolling in the two phase region but due to the development of anisotropy in mechanical properties such steels have not improved their worth for practical applications. The continuous martensitic network produced by heat treatment cycles certainly improved the area of contact between ferrite and martensite resulted in enhancement in the tensile properties. Acknowledgements The authors thank Mr. Ghulam Jalini, Mr. Amir Alam, Mr. Shahbaz Ahmad Chatta and Mr. Masood Ahmad for their help in experimental work and arrangement of technical equipment. References [1] Rashid MS. Relationship between steel microstructure and formability. In: Formable HSLA and dual-phase steels. Davenport AT: AIME; 1977. p. 1–24. [2] Hansen SS, Pradhan RR. Structure/property relationship and continuous yielding behavior in dual-phase steels. In: Kot RA, Bramfitt BL, editors. Proceedings fundamentals of dual-phase steels, 1981. p. 113–44. [3] Speich GR, Miller RL. Mechanical properties of ferrite–martensite steels. In: Davenport AT, editor. Structure and properties of dualphase steels. New York (NY): TMS-AIME; 1979. p. 145–82. [4] Balliger NK, Gladman T. Work hardening of dual-phase steels. Met Sci 1981;15(3):95–108.
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