International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6480(Print), ISSN 0976 – 6499...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6480(Print), ISSN 0976 – 6499...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 64...
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Microstructural characterization and elastoplastic behaviour of high strengt

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  1. 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 7 MICROSTRUCTURAL CHARACTERIZATION AND ELASTOPLASTIC BEHAVIOUR OF HIGH STRENGTH LOW ALLOY STEEL Shatrughan Soren1 , R.N. Gupta2 , N. Prasad3 and M. K. Banerjee4 1 Assistant Professor, Dept. of Fuel & Mineral Engineering, ISM Dhanbad, India 2 Associate Professor & Head, Dept. of Metallurgical Engineering, BIT Sindri, India 3 Ex-Professor, Dept. of Metallurgical Engineering, BIT Sindri, India 4 Steel Chair Professor, Dept. of Metallurgical and Materials Engineering, MNIT Jaipur, India ABSTRACT The ferrite grain refinement is a powerful mechanism to improve the strength and toughness in steels. High strength low alloy steel is controlled rolled at a temperature just above its A3 temperature and then water cooled. In the present investigation an attempt has been made to produced ultrafine ferrite grained (1–3 µm ) steels through relatively simple Thermomechanical Controlled Processing (TMCP). The microstructure of the steel was characterized by Electron Back Scattered Diffraction (EBSD) technique and nanoindentation method was used to characterize the elastoplastic behaviour of the steel. It is found that about 20 percent prior austenite undergoes dynamic strain induced transformation with grain size 3µ or less. The ferrite formed after direct cooling having varying elastoplastic characteristics and that the observed variation owes its origin to difference in carbon content of ferritic formed at different temperatures. Keywords: Thermomechanical controlled processing, ultra fined ferrite, Electron back scattered diffraction, Nanoindentation. 1. INTRODUCTION Due to excellent formability advanced high strength steels are widely employed in automotive industries [1, 2]. Quite often high strength low alloy steels are subjected to conventional thermo-mechanical treatment. Controlled thermo-mechanical processing has often more use direct cooling after controlled rolling. Under such situation of continuous cooling multiphase microstructure are reported to result [3, 4]. However the micro constituents formed are dependent upon cooling rate. In recent times dynamic strain induced transformation (DSIT) of austenite to ferrite is reported by a number of workers; in this case deformation and transformation takes place INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 4, Issue 6, September – October 2013, pp. 07-16 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com IJARET © I A E M E
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 8 simultaneously [5-7]. In order to have a deeper understanding about microstructural change during DSIT, electron back scatter diffraction has been used by previous workers [8]. It appears that the ferrite formed in case of continuous cooling after hot working is of transient character in respect of mechanism of formation. It is anticipated that the ferrite formed through different mechanism will have different elastoplastic behaviour. Nanoindentation technique has been used by some researchers to study the elastoplastic behaviour of microconstituent in multiphase microstructure [9, 10]. However there is no report of relating the elastoplastic behaviour of mechanistically transient ferrite crystals with chemical composition and deformation parameters. The present investigation aims at characterizing elastoplastic behaviour of ferrite crystals formed through different mechanism during continuous cooling of high strength low alloy steels following control rolling just above upper critical temperature. 2. EXPERIMENTAL The chemical composition of the steel used for the present investigation is furnished below in Table 1 Table 1 Chemical composition of the experimental steel (weight %) C Mn Si Cr Mo Ni Cu Al Ti Nb Fe 0.13 2.24 1.25 0.34 0.027 0.048 0.111 0.017 0.007 0.061 bal As supplied steel plate of thickness 6mm was cut into small pieces (20mmx20mm); the sample pieces were soaked at different temperatures viz. 800o C, 845o C, and 900o C in the electrical resistance furnace for fixed holding time of 20 minutes. After soaking, the samples were rolled upto 50 % thickness reduction and directly quenched in water. The mechanical behaviour of the rolled specimen were studied by nanoindentation test. The microstructural characterization were carried out using optical, scanning electron microscope. Electron back scattered diffraction technique was used to understand the character of transformation of austenite. DSC studies was made to get the idea of As and Af temperatures. 3. RESULTS AND DISCUSSIONS When the steel is rolled at 800o C, within the two phase field, both austenite and proeutectoid ferrite are deformed. The austenite so deformed is converted to bainite during subsequent cooling (Fig. 1a). The proeutectoid ferrite is also deformed and undergoes partial recovery. Some austenite undergoes dynamic strain induced transformation (DSIT) and forms very small grained ferrite. The SEM picture in Fig. 1(b) substantiates the above observation by way of showing acicular bainitic ferrite, ultrafine DSIT ferrite and recovered ferrite. When rolling of the steel is carried out at a temperature, 845o C, which is near to A3 temperature of the steel, DSIT ferrite formation is enhanced (Fig. 2a); the deformed austenite remaining in the microstructure finally transform to bainite. As corroborated by the scanning electron microscopic observation some proeutectoid ferrite of small size is also found in the microstructure (Fig. 2b). When the rolling is raised to 900o C, the austenite is recrystallized; this crystallized austenite undergoes bainite transformation following Kardjumov- Sach relationship. Some austenite also undergoes dynamic strain induced transformation to ferrite. This bainite structure is clearly seen in Fig. 3(a & b).
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 9 Fig. 1 Microstructure of steel deformed 50% after soaking at 800o C (a) Optical (b) SEM Fig. 2 Microstructure of steel deformed 50% after soaking at 845o C (a) Optical (b) SEM In view of the fact that microstructures indentified by microorientation can provide extra information than the mere microstructural study, EBSD analysis was performed on samples deformed at 900o C by 50% reduction. The orientation data (Table. 2) and corresponding histogram (Fig. 4) shows that 20% of the boundaries ferrite crystals are low angle boundaries having misorientation 3.5o or less. However 73% of the boundaries are high angle boundaries having misorientation greater than 15o . The corresponding microstructure shows the predominance of acicular ferrite/bainite as microconstituent. This means that rolling just above the A3 line with a moderate deformation of 50% thickness reduction has undergone recrystallization of austenite. The recrystallized austenite during fast cooling has given rise to the formation of acicular bainite αo B. It is pertinent to state that the existence of appreciable amount of boundaries of low misorientation, is indicative of occurrence Fig. 3 Microstructure of steel deformed 50% after soaking at 900o C (a) Optical (b) SEM 20 µ a b a 20 µ b 20 µ a b
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue of dynamic strain induced transformation of austenite, where deformation and transformation take place simultaneously; however the ferrite so formed could not be recrystallized even statically owing to pining effect of microalloy carbides microstructure does not show any evidence of dynamic recrystallization of ferrite and most of ferrite with or without low angle boundaries appears acicular in shape. Table.2 Fig. 4 The final microstructure of the said steel is therefore acicular ferrite with large angle boundaries; these ferrite has originated from recrystallized austenite; the ferrite of small misorientation can result only if transformation of austenite to ferrite and deformation of ferrite to acicular morphology, take place simultaneously. found that only 8.5 percent grains have sizes one micron and less whereas 23 percent of grains are found to be of size less than 3 microns (Table. low misorientation, one finds a good correspondence between percentage of grains of size less than 3 micron and the percentage of ferrite of low misorientation of grains. This tends to lead one to conclude that dynamic strain induced transformation (DSIT) ca conventional thermomechanical treatment. As evident from the microstructure, DSIT ferrite is deformed and has not undergone recrystallization (Fig. 5). Therefore it is expected that they should exhibit a deformation t However recrystallized austenite of 77 percent has undergone bainitic transformation and it is known that this bainite transformation leads to stronger texture in comparison to polygonal ferrite formed urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 10 of dynamic strain induced transformation of austenite, where deformation and transformation take place simultaneously; however the ferrite so formed could not be recrystallized even statically owing to pining effect of microalloy carbides formed, strain induced, at relatively low temperature. This microstructure does not show any evidence of dynamic recrystallization of ferrite and most of ferrite with or without low angle boundaries appears acicular in shape. Table.2 Showing Misorientation Angle Fig. 4 Misorientation histogram The final microstructure of the said steel is therefore acicular ferrite with large angle ferrite has originated from recrystallized austenite; the ferrite of small misorientation can result only if transformation of austenite to ferrite and deformation of ferrite to acicular morphology, take place simultaneously. From the analysis of grain size distribution, it is found that only 8.5 percent grains have sizes one micron and less whereas 23 percent of grains are found to be of size less than 3 microns (Table. 3). Comparison of this observation with the ferrite of w misorientation, one finds a good correspondence between percentage of grains of size less than 3 micron and the percentage of ferrite of low misorientation of grains. This tends to lead one to conclude that dynamic strain induced transformation (DSIT) can lead to grain refinement better than conventional thermomechanical treatment. As evident from the microstructure, DSIT ferrite is deformed and has not undergone recrystallization (Fig. 5). Therefore it is expected that they should exhibit a deformation t However recrystallized austenite of 77 percent has undergone bainitic transformation and it is known that this bainite transformation leads to stronger texture in comparison to polygonal ferrite formed urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – October (2013), © IAEME of dynamic strain induced transformation of austenite, where deformation and transformation take place simultaneously; however the ferrite so formed could not be recrystallized even statically owing formed, strain induced, at relatively low temperature. This microstructure does not show any evidence of dynamic recrystallization of ferrite and most of ferrite The final microstructure of the said steel is therefore acicular ferrite with large angle ferrite has originated from recrystallized austenite; the ferrite of small misorientation can result only if transformation of austenite to ferrite and deformation of ferrite to From the analysis of grain size distribution, it is found that only 8.5 percent grains have sizes one micron and less whereas 23 percent of grains are ). Comparison of this observation with the ferrite of w misorientation, one finds a good correspondence between percentage of grains of size less than 3 micron and the percentage of ferrite of low misorientation of grains. This tends to lead one to n lead to grain refinement better than As evident from the microstructure, DSIT ferrite is deformed and has not undergone recrystallization (Fig. 5). Therefore it is expected that they should exhibit a deformation texture. However recrystallized austenite of 77 percent has undergone bainitic transformation and it is known that this bainite transformation leads to stronger texture in comparison to polygonal ferrite formed
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue intragranularly under slower cooling rate duri expected to be weak, due to the fact that a less volume fraction of this phase has been formed. When recrystallized austenite produces polygonal ferrite in the microstructure a weak and random texture would result. However in this particular case a further cooling has produced acicular morphology of ferrite. This transformation takes place with definite habit and orientation between parent and product phases. Also its percentage is quite high (~77%); ther characteristic grain orientation may be reflected in the form of microscopic texture. Admittedly the microstructure consists of ferrite of similar morphology but with different genesis. Major fraction is bainitic ferri The other type is dynamic strain induced transformation of austenite to ferrite with limited dynamic recrystallization in ferritic phase. However the pole figure (Fig. 6) generated in EBSD anal not show isolines but shows dots, thereby being less conclusive in information. Again the pole figure does not show symmetry along RD or TD; therefore the deformation process could not be orthorhombic. Again only a limited number of grain are inv information being less statistical it is difficult to derive the representative texture if there be any. This is for why no specific comments on the evolution of distinctly different textures for ferrites of different origin can be made. Table 3. Grain size distribution and Fig.5 SEM microstructure showing deformed DSIT urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 11 intragranularly under slower cooling rate during TMCP (ref bavis). In case of DSIT, the texture is expected to be weak, due to the fact that a less volume fraction of this phase has been formed. When recrystallized austenite produces polygonal ferrite in the microstructure a weak and random texture uld result. However in this particular case a further cooling has produced acicular morphology of ferrite. This transformation takes place with definite habit and orientation between parent and product phases. Also its percentage is quite high (~77%); therefore there is reason to believe that characteristic grain orientation may be reflected in the form of microscopic texture. Admittedly the microstructure consists of ferrite of similar morphology but with different genesis. Major fraction is bainitic ferrite formed by shear transformation of recrystallized austenite. The other type is dynamic strain induced transformation of austenite to ferrite with limited dynamic recrystallization in ferritic phase. However the pole figure (Fig. 6) generated in EBSD anal not show isolines but shows dots, thereby being less conclusive in information. Again the pole figure does not show symmetry along RD or TD; therefore the deformation process could not be orthorhombic. Again only a limited number of grain are involved in mapping; thus textural information being less statistical it is difficult to derive the representative texture if there be any. This is for why no specific comments on the evolution of distinctly different textures for ferrites of Grain size distribution and corresponding area fraction SEM microstructure showing deformed DSIT urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – October (2013), © IAEME ng TMCP (ref bavis). In case of DSIT, the texture is expected to be weak, due to the fact that a less volume fraction of this phase has been formed. When recrystallized austenite produces polygonal ferrite in the microstructure a weak and random texture uld result. However in this particular case a further cooling has produced acicular morphology of ferrite. This transformation takes place with definite habit and orientation between parent and efore there is reason to believe that Admittedly the microstructure consists of ferrite of similar morphology but with different te formed by shear transformation of recrystallized austenite. The other type is dynamic strain induced transformation of austenite to ferrite with limited dynamic recrystallization in ferritic phase. However the pole figure (Fig. 6) generated in EBSD analysis does not show isolines but shows dots, thereby being less conclusive in information. Again the pole figure does not show symmetry along RD or TD; therefore the deformation process could not be olved in mapping; thus textural information being less statistical it is difficult to derive the representative texture if there be any. This is for why no specific comments on the evolution of distinctly different textures for ferrites of
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 12 Fig. 6 Pole figure generated by Fig. 7 Map of phases generated by EBSD technique EBSD technique The colour coded EBSD map of phases have recoded the presence of about 2% retained austenite (Fig. 7). This follows from transformation mechanism of γ→α which in this case has been martensitic for about 77% recrystallized austenite; this produces bainite ferrite of high angle misorientation. Thus our conjecture that bainitic transformation occurs in this steel to a large extent is supported by the observation that some retained austenite is present in the material. In the present experiments, the steel was rolled at 900o C after soaking for 20 minutes and a total of 50% deformation was given. From the DSC heating curve it is also clear that the above temperature of deformation is just above the A3 temperature of the experimental steel. From the microstructure of the steel it is further observed that the structure is constituted by bainitic ferrite, DSIT and some proeutectoid ferrite (Fig.8). The microstructure of the sample shows acicular ferrite, some ferrite of irregular morphology occasional presence of proeutectoid ferrite of massive appearance is occasionally observed. The deformation temperature being above Ar3, the austenite is stable and transformation during straining of stable austenite is characteristically different from that of the metastable austenite. The deformation load within austenite leads to creation of intragranular nucleation sites for ferrite. The ferrite nucleated intra-granularly undergoes continuous deformation and recrystallization and become finer. Such equiaxed ferrite formed after recrystallization appears extremely fine in the microstructure. During the course of rolling, deformed austenite cooled fast subsequently transforms at a lower temperature and produces bainite or granular bainite depending upon transformation temperature. Elastoplastic behaviour of ferrite present in its microstructure is attempted to be studied by nanoindentation methods. The result of nanoindentation in respect of hardness of the measured phase, its yield strength and elastic modulus are furnished in the Table.4. It is known that nanoindentation involves stresses along all directions and compressive, tensile and shear stresses are produced in nanoindentation against where pure tensile or compressive stresses is involved [9]. It is reported that construction of indentation stress-strain curve from the observed load-depth of indentation curve enables one to relate them [8]. With the concept used elsewhere [11] the present data have been generated to describe the elastoplastic behaviour of microconstituents in this rolled steel.
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 13 Fig. 8 Microstructure of steel showing different phases Table. 4 Mechanical properties of steel characterized by Nanoindentation methods Fig: 9. Nanoindentation Hardness Vs Yield Fig. 10 Load versus penetration dept showing Strength curve elastic recovery for indentation From the result of nanoindentation in Table 3 it is observed that hardness, yield strength and also the elastic constants are different for different ferritic phase. Elastic constant varies with yield strength of ferrite; it is known that yield strength is highly structure sensitive properties and owes it origin to the interaction of interstitial atom with Cottrell-Lomer barriers. Table 3 however does not SL. No. Hardness (VHN) Yield Strength (GPa) Elastic Constant (GPa) 1 535 5.78 246 2 622 6.70 263 3 778 8.40 316 4 649 7.00 279 5 691 7.40 275 6 703 7.50 271 7 766 8.20 315 8 973 10.50 360 9 439 4.70 439 400 500 600 700 800 900 1000 4 5 6 7 8 9 10 11 YieldStrength,GPa Hardness, VHN 0 100 200 300 400 0 4 8 12 16 20 Indent 1 Indent 2 Indent 3 Indent 4 Indent 5 Indent 6 Indent 7 Indent 8 Indent 9 Indent10 Load,mN Penetration Dept, nm
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 14 show any specifically defined pattern of variation; nevertheless it is found that increasing yield strength has increased elastic constant of the materials. It is known that elastic constant is a material property and it is structure insensitive. Therefore the observed variation of this structure insensitive property along with a structure sensitive property must stem from the difference in chemistries of ferrite appearing in the microstructure. Fig 9 shows that hardness of the ferrite crystals vary linearly with yield strength of the phase. Such one to one correspondence is indicative of predominance of shear deformation in determining the hardness of the phase. Yield strength is the manifestation of onset of plastic deformation through shear. Indentation test similarly measures the resistance to permanent deformation; so it is natural that they will have co-relation. From all these relations it appears that dynamic transformation of austenite under rolling strain has envisaged mechanistically transient phase transformation. In fact such transient transformation of austenite has been reported earlier. The microstructural evidence is also suggestive of the same (Fig 8). Due to difference in mechanism of transformation, there is kinetic constraints in solute partitioning. The ferrite formed later at lower temperatures are rich in solute content due to transformation being diffusionless, not permitting solute partitioning. However carbon rich bainitic ferrite is unlikely to exhibit such a large difference in Young Modulus (228 GPa in test 1 to 360 GPa in test 9); rather it may be conjectured that other substitutional elements are also responsible for creating difference in elastic constants in the steel. Fig. 11 (a) Nanoindentation Load-penetration curves and (b) Corresponding stress- strain curves for spot ten Ferrite of different carbon content forming at different transformation temperature will envisage different degrees of breaking away of Cottrell atmosphere and hence will exhibit different yield strength; higher is the ferritic carbon, more will be the stress required to break the barrier and to initiate plastic deformation and hence yield strength will be higher. Elastically strained lattice of high solute ferrite will require high stress to produce equivalent elastic strain; hence modulus of elasticity will increase. From the degree of elastic recovery (Fig. 10), it seems that about four distinct compositionally different ferrite has formed. The ferrite at spot ten has a massive look and appears to be proeutectoid ferrite formed at early stage of transformation (Fig. 8). The ferrite at spot nine showing maximum hardness, yield strength and Young modulus is clearly the bainitic ferrite rich in solute content and being highly dislocated (Fig. 8). The microstructure is seen to corroborate the nanoindentation results. Dynamically transformed austenite produces ferrite of different solute contents and dislocation densities and accordingly they show different elastoplastic behaviour. In view of low carbon of this steel, the elastic recovery is usually very low. There are few ferritic areas for which elastic recovery is very small (Fig 10). Such a high level plasticity entices to conjectured 0 1 00 20 0 300 400 500 0 4 8 12 16 20 Load,mN P enetration depth, nm 0.00 0.05 0.10 0.15 0.20 0 1x10 8 2x10 8 3x10 8 4x10 8 Stress,Pa Strain
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 15 that this ferrite is formed through low range diffusional means and is of extremely low carbon, leaving behind a carbon richer austenite. This is of globular morphology and shows low hardness. The load penetration curves for this phase exhibits discontinuities (Fig. 11a). As observed elsewhere such discontinuities are normally associated with the formation of cracks. For such large plastic deformation, initiate that a very high shear stress had been generated. This high shear stress is seemingly responsible for the formation of cracks in this material. In the corresponding indentation stress strain curve (Fig. 11b), a sudden decrease in indentation stress is observed at high strain. This leads to lower hardness. A similar observation is made for spot 1 where the elastic recovery is 17% (Fig 12a & 12b). The general value of elastic recovery in the ferrite found through dynamic transformation of austenite is found to lie within 20-23% . The corresponding Fig. 12 (a) Nanoindentation Load-penetration curves and (b) Corresponding stress- strain curves for spot one stress-strain curve also reveals plastic deformation with little elastic strain. Hence plastic deformation has also led to discontinuities in concerned load-penetration curves which are of crack formation indicative. high shear stress had been generated. This high shear stress is seemingly responsible for the formation of cracks in this material. In the corresponding indentation stress strain curve (Fig. 11b), a sudden decrease in indentation stress is observed at high strain. This leads to lower hardness. A similar observation is made for spot 1 where the elastic recovery is 17% (Fig 12a & 12b). The general value of elastic recovery in the ferrite found through dynamic transformation of austenite is found to lie within 20-23%. The corresponding stress-strain curve also reveals plastic deformation with little elastic strain. Hence plastic deformation has also led to discontinuities in concerned load-penetration curves which are of crack formation indicative. 4. CONCLUSIONS The authors wish to conclude that rolling of the experimental HSLA steel just above upper critical temperature insures dynamic strain induced transformation of austenite to ferrite. About 75% of recrystallized austenite transforms to bainite. It is further concluded that the grain size of the DSIT ferrite is quite small, ~ 3µm or less. The authors also conclude that ferrite of different compositions are present in the microstructure and these ferrite crystals exhibit different degrees of elastic recovery during nanoindentation; again due to variation in composition these ferrites have different elastic modulii. The highly plastic ferrite present in the microstructure undergoes cracking due to shear involved in indentation test. 0 100 200 300 400 0 4 8 1 2 1 6 2 0 Load,mN P en etratio n d epth (n m ) 0.00 0.05 0.10 0.15 0.20 0 1x10 8 2x10 8 3x10 8 4x10 8 5x10 8 Stres,Pa strain
  10. 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME 16 REFERENCES [1] Ph. Harlet, F. Beco, P. Cantinieaus, D. Bouquegneau, P. Messien, J. C. Herman, in: Ra. Asfahani and G. Tither (eds.) International Symposium on Low Carbon Steel for the 90s, 1993, p. 389 [2] M. R. Barnett and J.J. Jonas, ISIJ Int. 39(1999) 856 [3] M.K. Banerjee, P.S. Banerjee and S. Datta, ISIJ Int. Vol. 41 (2001) No. 3, pp. 257-261 [4] M. K. Banerjee, D. Ghosh and S. Datta: Scand J. Metall., 29 (2000). [5] A. J. DeArodo: ISIJ Int., 35 (1995), 946. [6] G. Krauss and S. W. Thompson: ISIJ Int., 35 (1995), 937. [7] K. Shibata and K. Asakura: ISIJ Int., 35 (1995), 982. [8] A. Oudin, P.D. Hodgson and M.R. Barnett, Mater. Sci. Eng. A 486(2008) 72-79 [9] E. Martinnez, J. Romero, A. Lousa and J. Esteve, Appl. Phys. A 77, 419-426 (2003) [10] P. Hones, M. Diserens, R. Sanjines and F. Levy, J. Vac. Sci. Technol. B 18, 2851 (2000) [11] W.C. Oliver and G.M. Pharr.J. Mater. Res. 7, 1564 (1992) [12] Bevis Hutchinson, Lena Ryde, Eva Lindh, et al J. Mat. Sc. Engg. (A) 257, 9-17(1998)

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