al., 2012). In a recent study Kimura et al. reported that crack arrester type delamination .... manganese sulfides, silicates, alumina and oxides which can be either elongated along the ...... measurement of more than 100 carbide films, Table 4.1.
Effect of Microstructure and Crystallographic Texture on Impact Toughness in Low Carbon Ferritic Steel
Thesis submitted to the Indian Institute of Technology, Kharagpur For award of the degree of
Doctor of Philosophy by
Abhijit Ghosh
Under the supervision of
Dr. Debalay Chakrabarti
Department of Metallurgical and Materials Engineering Indian Institute of Technology Kharagpur
February, 2016 © 2016 Abhijit Ghosh. All rights reserved.
Dedicated to Baba
APPROVAL OF THE VIVA-VOCE BOARD 05/02/2016
Certified that the thesis entitled EFFECT OF MICROSTRUCTURE AND CRYSTALLOGRAPHIC TEXTURE ON IMPACT TOUGHNESS IN LOW CARBON FERRITIC STEEL submitted by ABHIJIT GHOSH to the Indian Institute of Technology, Kharagpur, for the award of the degree Doctor of Philosophy has been accepted by the external examiners and that the student has successfully defended the thesis in the viva-voce examination held today.
(Member of the DSC)
(Member of the DSC)
(Member of the DSC)
(Supervisor)
(External Examiner)
(Chairman)
CERTIFICATE This is to certify that the thesis entitled Effect of Microstructure and Crystallographic Texture on Impact Toughness in Low Carbon Ferritic Steel, submitted by Abhijit Ghosh to Indian Institute of Technology, Kharagpur, is a record of bona fide research work under my supervision and we consider it worthy of consideration for the award of the degree of Doctor of Philosophy of the Institute.
__________________________ (Dr. Debalay Chakrabarti) Associate Professor Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur-721 302, INDIA
Date:
DECLARATION
I certify that a. The work contained in the thesis is original and has been done by myself under the general supervision of my supervisors. b. The work has not been submitted to any other Institute for any degree or diploma. c. I have followed the guidelines provided by the Institute in writing the thesis. d. I have conformed to the norms and guidelines given in the Ethical Code of Conduct of the Institute. e. Whenever I have used materials (data, theoretical analysis, and text) from other sources, I have given due credit to them by citing them in the text of the thesis and giving their details in the references. f. Whenever I have quoted written materials from other sources, I have put them under quotation marks and given due credit to the sources by citing them and giving required details in the references.
(Abhijit Ghosh)
ACKNOWLEDGEMENTS I would like to express my special thanks to my advisor Dr. D. Chakrabarti for his invaluable guidance and for constant inspiration and continuous motivation in each and every stage of this research program. In addition, I really feel that this work could not be completed without your friendly treatment and exceptional patience during my tough time. I am thankful to the Department of Science and Technology, DST, New Delhi and IIT Kharagpur for financial support behind the project. I am indebted to RDCIS, Ranchi & DMRL, Hyderabad for providing study material. I would like to acknowledge Prof. G.G. Roy, Prof. Karabi Das, Prof. Tapas Laha and Prof. Arghya Deb, the faculty members of my Doctoral Scrutiny Committee for constantly guiding and encouraging me during my Ph.D. tenure. I am grateful to the Dept. of Metallurgy and Materials Engg. (MME), Central Research Facility (CRF) and Steel Technology Centre (STC), IIT Kharagpur, for the provision of research facilities. I would also like to express my sincere gratitude to Prof. I. Samajdar from IIT Bombay, Dr. S. Kundu from Tata Steel R&D, Jamshedpur, Dr. Vinod Kumar from RDCIS, SAIL, Ranchi and Dr. R. Balamuralikhishnan from DMRL Hyderabad for their kind help in providing material and experimental support and for sharing their knowledge on various aspects related to this work. I thank Prof. Claire Davis and Dr. Martin Strangwood for their critical review of some of my work when they visited IIT Kharagpur. I take this opportunity to thank Prof. S. K. Pabi, Prof. K. K. Ray, Prof. N. Chakrabarti, Prof. R. Mitra, Prof. S. B. Singh and all the faculty members of the Department of Metallurgical and Materials Engineering for their valuable suggestions during the course of this investigation. The author also wishes to thank Mr. Pradip Sarkar, Mr. Tinku Thomas, Mr. Sudipta Das, Mr. Biswarup Kar, Mr. Prasanta Das, Mr. T.K. Tiary, Mr. Niloy Bhoumick (CRF) and all the staff members of CRF and Dept. of MME for their various types of assistance during experimental work. Above all, I acknowledge my friends, my parent and my wife for their unconditional support. Abhijit Ghosh
vi
List of Symbols Symbol TNR γs γp γ γpm γmm σf σ0 ˆ PGY Pmax Pi Pa Kpcf D t Y µ w B Cc Cs G Fp 1, , 2 Davg Deff σyd σp σd σsg l vf x b θc θi θm
Description recrystallization stop temperature energy required to create a unit area of new fracture surface energy required for plastic work prior to the creation of new fracture surface effective plastic work effective surface energies for particle-matrix interface effective surface energies for matrix-matrix interface critical stress required for cleavage fracture temperature dependency of the yield strength peak stress in the plastic zone general yield load maximum load brittle crack initiation load crack arrest load plastic constraint factor grain size carbide size precipitation hardening effects of alloying elements shear modulus sample width sample thickness plane-normal in crystal reference frame plane-normal in sample reference frame orientation of any ferrite grain in the sample reference frame fracture plane of the sample Eulers angle in Bunge notation representing the orientation of any crystal in sample reference frame. average grain size effective grain size dynamic yield strength precipitation strengthening dislocation strengthening sub grain strengthening the average sub-grain size measured in terms of the intercept length dislocation density precipitate volume fraction average precipitate size burger’s vector cleavage angle cleavage angle at EBSD scan plane measured angle vii
List of Abbreviations Abbreviation Ar1 Ar3 Ae1 Ae3 BCC DBTT FRT EBSD ECD EDS EGS FATT HSLA ITT LSE m-m p-m SEM TEM UTS USE YS PCF LAB HAB T-L RD TD ND TMCR
Description lower critical temperature during cooling upper critical temperature during cooling equilibrium lower critical temperature equilibrium upper critical temperature body centered cubic ductile to brittle transition temperature finish rolling temperature electron back scattered diffraction equal circle diameter energy dispersive spectroscopy effective grain size fracture appearance transition temperature high strength low alloy impact transition temperature lower shelf energy matrix-matrix particle-matrix scanning electron microscope transmission electron microscope ultimate tensile strength uppershelf energy yield strength plastic constraint factor low-angle boundaries high-angle boundaries transverse-longitudinal rolling direction transverse direction normal direction thermo-mechanical controlled rolling
viii
List of Tables Table 1.1:
Mechanical properties required for the existing grades of high strength steel plates for structural, line-pipe and naval applications.
Table 1.2:
Influence of compositional and processing parameters on microstructural features favoring high impact toughness and low impact transition temperature.
Table 2.1:
Proportion (in percentage) of HSLA steel used for different end products and applications relative to carbon and alloy steels (Tither, 1990).
Table 2.2:
Different Plastic Constraint Factor (PCF) values reported in the literature.
Table 2.3:
Threshold criterion used for the determination of effective grain size for different microstructures in steel.
Table 2.4:
Various reasons mentioned in the literature behind the formation of fissure during Charpy impact testing.
Table 3.1:
Chemical composition of the investigated samples, (wt.%).
Table 3.2:
Sample codes with processing details.
Table 4.1:
Microstructural parameters and transition temperatures of the investigated samples of steel S1.
Table 5.1:
Microstructural parameters, low-angle boundary fractions and the effective grain size of the investigated samples of steel S2.
Table 5.2:
Characterization of MnS inclusions in the as-rolled samples of steel S2.
Table 5.3:
Hardness and tensile properties of the investigated samples from steel S2.
Table 5.4:
Charpy impact toughness parameters of the investigated samples of steel S2 measured from instrumented impact test.
Table 6.1:
Microstructural parameters and hardness of the tested samples of steel S3.
Table 6.2:
Tensile properties of the investigated samples of steel S3.
Table 6.3:
Charpy impact properties of the investigated samples of steel S3.
ix
Table 6.4:
Boundary density of the investigated samples of steel S3
Table 7.1:
Microstructure, texture, impact toughness and fractrographic parameters of the investigated samples from S2 steel.
Table 7.2:
Calculated grain boundary misorientation angle and the cleavage crack deviation angle on the transverse section as obtained from EBSD scan in Fig. 7.5(a, b).
Table 7.3:
Strengthening contributions and the prediction of the general yield temperature (TGY) for S2 steel samples.
Table 8.1:
Fissure, texture and banding quantification of the investigated samples.
Table 8.2:
Average ferrite grain size and ‘effective grain size’ on RD-ND and RDTD planes of the investigated samples.
Table 9.1:
Tensile and Charpy impact properties of the investigated samples of steel S2FRT800 and S4FRT800.
Table 9.2:
Effective gain size and grain boundary density of the investigated samples obtained from EBSD analysis.
x
List of Figures
Fig. 2.1:
Flow chart showing the typical processing route for TMCR microalloyed steels. The dark shaded stages are important from the point of view of grain refinement (Roy, 2013).
Fig. 2.2:
Schematic showing the typical rolling schedule and the associated microstructural changes for thermomechanical controlled rolling (TMCR) (Tanaka, 1981) of steels.
Fig. 2.3:
Ductile fracture surface at (a) lower magnification and (b) higher magnification, showing micro-voids (arrowed) nucleated from spheroid carbide particles. Brittle fracture surface at (c) lower magnification and (d) higher magnification, showing a cleavage facet (nucleated from TiN particle) and river lines (both arrowed).
Fig. 2.4:
Schematic diagram showing variation in yield stress and fracture stress with temperature. TGY denotes the general yield temperature.
Fig. 2.5:
Schematic diagram showing different steps involved in cleavage fracture: Step 1: nucleation of sharp microcrack nucleates at some microstructural feature, Step 2: propagation of microcrack across particle-matrix interface, Step 3: propagation of microcrack across matrix-matrix boundaries, (Ray, 2011).
Fig. 2.6:
Cottrell’s mechanism for cleavage crack initiation by dislocation interaction, (Curry and Knott, 1978; Knott, 1976).
Fig. 2.7:
Formation of cleavage crack at the intersection of twins, (Knott and Cottrell, 1963).
Fig. 2.8:
Cleavage initiation from MnS inclusion (Zhang et al., 1986).
Fig. 2.9:
Cleavage initiation from TiN inclusion (Fairchild et al., 2000a, 2000b).
Fig. 2.10:
Cleavage crack propagation (a) through particle-matrix interface and (b) matrix-matrix interface cheverr and odrig e -Ibabe, 2003).
Fig. 2.11:
Stress distribution ahead of a notch and the cleavage crack initiation (Wallin et al., 1984).
xi
Fig. 2.12:
Schematic diagram showing the different fracture mechanisms operating at different temperatures determined considering the variation in yield strength σ0), the peak stress in the plastic zone ( ˆ ) and the cleavage fracture stresses considering particles (σpm) and grains (σmm) (Lin et al., 1987).
Fig. 2.13:
Characteristic points on the Charpy impact transition curves. Solid line represents impact energy transition curve and dotted line represents fracture appearance transition curve.
Fig. 2.14:
Characteristic points on schematic load-time profile in the transition region.
Fig. 2.15:
Schematic diagram showing the variation in maximum load (Pmax) and General yield load (PGY) with temperature.
Fig. 2.16:
Variation in cleavage fracture stress (F, MPa) with D-1/2 showing the increase in F with the decrease in grain size (D, mm) (Ritchie et al., 1973).
Fig. 2.17:
Influence of grain size (mm) and carbide thickness (m) on 27 J-ITT (Mintz et al., 1982).
Fig. 2.18:
Deformation and transformation textures develop in TMCR HSLA steel (Ray and Jonas, 1990).
Fig. 2.19:
Different text re components of HSLA steel at φ2=45° cross section of ler’s space are their origin.
Fig. 2.20:
(a) A cleavage facet from TMCR steel on which orientation imaging was carried out; (b) Corresponding OIM image of one cleavage facet from TMCR steel (four different grains are marked); (c) Inverse pole figure of the orientation of crystals detected on fracture surface (Bhattacharjee et al., 2004).
Fig. 2.21:
Schematic representation of (a) Twist (b) Tilt angle.
Fig. 2.22:
Fissures in HSLA steel at different testing temperatures: (a) at room temperature, (b) at -18 °C and (c) at -73 °C (Bramfitt and Marder, 1977).
Fig. 2.23:
Variation in Charpy impact transition curve at different finish rolling temperatures (Bourell, 1983)
xii
Fig. 2.24:
Schematic diagram showing how fissures form during impact testing over certain test temperature range (Mintz and Morrison, 2007).
Fig. 2.25:
Variation in Charpy impact transition curve with variation in the orientation of Charpy impact samples (Joo, 2012)
Fig. 3.1:
Typical rolling and normalizing schedules used in the present study.
Fig. 3.2:
Typical dimension and orientation of samples for (1) microstructural study, (2) microtexture study, (3) macrotexture study, (4) tensile testing and (5) Charpy impact testing with respect to the original rolled plate.
Fig. 3.3:
Schematic diagram showing the dimensions of sub-size tensile Specimen.
Fig. 3.4:
Schematic diagram showing the dimensions of full size Charpy impact sample.
Fig. 3.5:
Schematic diagram showing the orientation of Charpy impact samples with respect to the rolling direction for anisotropy study.
Fig. 3.6:
Schematic diagram showing the deviation in estimation of DBTT ±ΔDBTT) and US ±ΔUS ) val es from tanh c rve fitting.
Fig. 4.1:
Optical micrographs of the investigated samples of steel S1: (a) S1FRT765, (b) S1FRT935, (c) S1HT940, and (d) S1HT1150.
Fig. 4.2:
SEM micrographs of the investigated samples of steel S1: (a) S1FRT765, (b) S1FRT935, (c) S1HT940, and (d) S1HT1150.
Fig. 4.3:
Ferrite grain size distributions in terms of area-frequency of the investigated samples of steel S1.
Fig. 4.4:
SEM micrographs of TiN particles in steel S1. TiN either present (a) individually or (b to d) along with Al2O3 and MnS inclusions.
Fig. 4.5:
Number density (per mm2) of TiN particles for different particle sizes (maximum cord length) in steel S1.
Fig. 4.6:
Thermo-Calc software prediction of precipitation sequence in steel S1.
Fig. 4.7:
Orientation maps of (a) S1FRT765, (b) S1FRT935, (c) S1HT940 and (d) S1HT1150 sample. Color code is based on the ler’s angle. Low angle boundaries are indicated by arrows.
xiii
Fig. 4.8:
Grain boundary misorientation histograms of S1FRT765, S1FRT935, and S1HT940 samples.
Fig. 4.9:
Impact energy transition curves of the samples from steel S1.
Fig. 4.10:
(a) River lines indicating the cleavage initiation site in S1FRT765 sample broken at – 65 °C and (b) TiN particle at the initiation site for the same sample. (c and d) TiN particles at the cleavage origin of the S1FRT935 sample and S1HT1150 sample, respectively, showing completely cleavage fracture.
Fig. 4.11:
Study of crack initiation on the transverse section perpendicular to the fracture surface: (a) TiN particle cracking, (b) interface separation, (c) TiN particle-twin interaction, (d) cracking to TiN particle (identified by EDS analysis) and the crack propagation through the ferrite grains till the grain boundaries, where the cracks are deviated or stopped completely, (e) cracking of grain boundary carbides at multiple points.
Fig. 4.12:
Variation of local fracture stress with grain size and particle size (Ray, 2011).
Fig. 4.13:
Vickers macro-hardness and yield strength (obtained from tensile test) of the investigated samples.
Fig. 4.14:
TEM images showing (a and b) high dislocation density and dislocation cell formation in S1FRT765 sample; (c) Interaction between VC precipitates and dislocations in S1FRT765 sample; (d) low dislocation density in S1FRT935 sample. A diffraction spot from (111) ring of VC is circled in (c).
Fig. 4.15:
Comparison between experimentally measured and predicted DBTT values for the investigated samples.
Fig. 5.1:
SEM micrographs of the investigated samples of steel S2: (a) S2FRT820, (b) S2FRT730, (c) S2FRT650, (d) S2HT940 and (e) S2HT1150. Rolling direction (RD) is indicated in all the images.
Fig. 5.2:
Band contrast images of the investigated samples of steel S2 as obtained from the EBSD analysis: (a) S2FRT820, (b) S2FRT730, (c) S2FRT650, and (d) S2HT940.
Fig. 5.3:
Cumulative misorientation angle distribution of the investigated samples of steel S2 as obtained from the EBSD analysis. xiv
Fig. 5.4:
Transmission electron micrographs of (a) S2FRT820, (b) S2FRT730, (c) S2FRT650 and (d) S2HT940 samples; (e) Bright field and dark field TEM images of (Nb,V)(C, N) precipitate with the corresponding EDS analysis and (f) distribution of fine V(C,N) precipitates (arrowed) in the ferrite matrix with the corresponding SADP analysis.
Fig. 5.5:
(a, b) MnS inclusions elongated along the rolling direction (RD) in the ferrite matrix and (c) an example of a fragmented MnS inclusion in S2FRT650 sample. Arrows indicate decohesion of MnS inclusions from the ferrite matrix.
Fig. 5.6:
Engineering stress-strain curves obtained from the tension tests on the investigated samples from steel S2.
Fig. 5.7:
Impact transition curves obtained from the Charpy impact test on the investigated samples of steel S2.
Fig. 5.8:
(a) Presence of MnS inclusions (arrowed) inside the voids on the fracture surface of the Charpy impact sample tested at room temperature; SEM images on the transverse section just below the fracture surface of impact tested specimens showing the formation of cracks and voids from (b) the MnS inclusions and (c) MnS-Al2O3 complex inclusion.
Fig. 5.9:
(a) Variation in strain hardening rate with the true strain as obtained from the tensile test of the investigated samples; (b) Variation of crack initiation energy with the low-angle boundary fraction and (c) the variation of crack propagation energy with the effective grain size of the investigated samples as obtained from the instrumented impact testing.
Fig. 5.10:
SEM fractographs of the (a) S2FRT820, (b) S2FRT730, (c) S2FRT650, (d) S2HT940 samples Charpy impact tested at -100C showing the cleavage facets; (e) Presence of the secondary cleavage cracks just below the fracture surface in an impact tested S2FRT820 sample.
Fig. 6.1:
Optical micrographs of the investigated samples of steel S3: (a) S3FRT850, (b) S3HT940FC, (c) S3HT940AC, (d) S3HT940WQ, (e) S3HT1150FC and (f) S3HT1250FC.
Fig. 6.2:
SEM micrographs of the investigated samples of steel S3: (a) S3FRT850, (b) S3HT940FC, (c) S3HT940AC, (d) S3HT940WQ (e) S3HT1150FC and (f) S3HT1250FC. B: Upper bainite, GB: Granular bainite, P: Lameller pearlite, DP: Degenerated pearlite, C: Carbide, M: Martensite. xv
Fig. 6.3:
Ferrite grain size distributions of the tested samples of steel S3.
Fig. 6.4:
Optical micrographs showing the prior austenite grain structures developed after reheating to (a) 950C, (b) 1150C, (c) 1250C and (d) the corresponding austenite grain size distributions. Coarse- and fine-austenite grains in (b) are indicated by arrows.
Fig. 6.5:
Molar fraction of precipitates as the function of temperature in steel S3 predicted using Thermo-Calc® thermodynamic software.
Fig. 6.6:
Continuous cooling transformation (CCT) diagram of S3 steel as prepared by dilatometric study on S3FRT850 sample.
Fig. 6.7:
(a) SEM micrograph showing the TiN particle in S3FRT850 sample and (b) the corresponding TiN particle size distribution. (c) Bright field TEM image showing the presence of (Nb,V)(C,N) precipitates (arrowed) located inside recrystallized ferrite grain.
Fig. 6.8:
Engineering stress-strain curve of the investigated samples of steel S3.
Fig. 6.9:
Charpy impact transition curve of the tested samples of steel S3.
Fig. 6.10:
EBSD map of the tested samples of steel S3 (a) S3FRT860, (b) S3HT940FC, (c) S3HT940AC, (d) S3HT940WQ (e) S3HT1150FC and (f) S3HT1250FC.
Fig. 6.11:
Orientation distrib tion f nction ODF) at Φ2=45° cross section of a) Asrolled S3FRT850 plate, (b) S3HT940FC, (c) S3HT940AC (d) S3HT940WQ (e) S3HT1150FC, (f) S3HT1250FC and (g) typical orientations fo nd on Ф2= 45o section of Eular space in ferritic steel.
Fig. 6.12:
Distribution of (a) Gamma fibre (ND||) and (b) Alpha fibre (RD||) in the S3 steel samples.
Fig. 6.13:
Volume fraction of cube component ({001}) (at 15° threshold) in the S3 steel samples.
Fig. 6.14:
(a) Variations in cleavage fracture stress with effective grain size; (b) Comparison between measured and predicted cleavage fracture stress of the investigated samples.
Fig. 7.1:
(a) The schematic diagram showing a crystal, having an arbitrary orientation with respect to the fracture plane of the sample. RD, TD and xvi
ND stands for rolling direction, transverse direction and normal direction, respectively. (b) Variation in plastic constraint factor with the crystal orientation represented by colo r code on φ2=45° section of ler space. Fig. 7.2:
The schematic diagram showing the increase in general yield temperature from TGY1 to TGY2 with the increase in plastic constraint factor from PCF1 to PCF2.
Fig 7.3:
Orientation distribution f nction at φ2=45° section of the investigated samples: (a) S2FRT820, (b) S2FRT730, (c) S2FRT650, and (d) S2HT940.
Fig. 7.4:
Schematic diagram showing the two neighbouring crystals having (001) and (001) orientations. (b) Schematic diagram showing the new concept of effective grain size.
Fig 7.5:
EBSD analysis around a secondary cleavage crack on the transverse plane perpendicular to fracture surface.
Fig 7.6:
Schematic diagram showing the angle between the cleavage facets (θc), the angle between the traces of cleavage plane-normal on the plane of EBSD scan (θi) and the cleavage crack deviation angle on the plane of EBSD scan (θm).
Fig 7.7:
Prediction of general yield temperature for the investigated samples considering the average grain size (Davg) and effective grain size (Deff).
Fig. 8.1:
Optical micrographs of the investigated samples on RD-ND plane: (a) S4FRT820, (b) S4FRT730 and (c) S4FRT650.
Fig. 8.2:
Optical micrographs of the investigated samples on RD-TD plane: (a) S4FRT820, (b) S4FRT730 and (c) S4FRT650.
Fig. 8.3:
Charpy transition curves of the investigated samples of steel of steel S4.
Fig. 8.4:
Macro-view on the fracture surfaces of the Charpy impact samples (tested at –40 °C) of (a) S4FRT820 (b) S4FRT730 and (c) S4FRT650.
Fig. 8.5:
Schematic diagram showing the T-L (Transverse –Longitudinal) orientation of the Charpy impact sample in a rolled plate. Main fracture plane and fissure plane in the sample are indicated by Grey and Red colour, repectively.
xvii
Fig. 8.6:
SEM images showing the fracture surfaces of S4FRT650 specimen tested at -40°C; (a, b) on the main fracture plane and (c, d) inside the split, i.e. on the fissure plane.
Fig. 8.7:
Inverse pole figure map (IPF) on main fracture plane (RD-ND) of (a) S4FRT820, (b) S4FRT730 and (c) S4FRT650 samples; the color legend is given (d).
Fig. 8.8:
Inverse pole figure (IPF) map on fissure plane (RD-TD) of (a) S4FRT820, (b) S4FRT730 and (c) S4FRT650 sample; (d) IPF map of S4FRT650 sample at higher magnification showing a prominent clustering of cube oriented grains indicated by dotted circle, (e) Colour legend of IPF, (f) a schematic showing the clustering of cube oriented grain on fissure plane.
Fig. 8.9:
(a, b) IPF map of the region surrounding a hairline fissure crack on RDND plane (parallel to main fracture plane). The scan in (b) was performed over the dotted region of (a); (c) colour legend of IPF; (d) Taylor factor map for plane strain compression deformation for different crystallographic orientation represented within the Φ2=45° section of ler’s space colo r legend is given); e) S M image showing the propagation of fissure crack through the deformed and undeformed grains.
Fig. 8.10:
(a) Schematic diagram showing two adjacent cube textured grains (G1 and G2) having high misoritation angle, but 0° angle between the corresponding (001) cleavage planes; (b) Schematic diagram showing the ‘effective grain si e’ concept, c,d) Φ2=45° section of the lers space showing (c) the grain boundary misorientation angle and (d) the angle between the {001} cleavage planes of the adjacent crystals. The first crystal is considered to be orientated at (001)[110] , whilst, the second crystal is having any arbitrary orientation represented by any point inside the lers’ space.
Fig. 8.11:
Schematic diagram showing elongated grain boundary at (a) Fissure plane (b) Main fracture plane. The boundaries that provide maximum obstacle to cleavage crack propagation are indicatted by arrows.
Fig. 8.12:
φ2=45° section of the lers’ space showing a) minim m twist angle on main fracture plane, (b) minimum twist angle on fissure plane and (c) tilt angle on fissure plane corresponding to minimum twist angle, (d) minimum tilt angle on fissure plane, (e) twist angle on fissure plane corresponding to minimum tilt angle. The first crystal is considered to be orientated at (001), whilst, the second crystal is having any xviii
arbitrary orientation represented by any point inside the axis: φ1, Y axis: φ).
lers’ space X
Fig. 8.13:
(a, b) SEM micrograph showing ridge at the interface of two facets (c) Schematic diagram showing the angle between the cleavage facets θc) and the twist angle θt).
Fig. 8.14:
The ‘effective grain si e’ estimated considering the 15° misorientation threshold between {001} cleavage planes of the neighboring crystals on the fissure plane (RD-TD) of the investigated samples: (a) S4FRT820, (b) S4FRT730 and (c)S4FRT650
Fig. 8.15:
Schematic diagram showing the alternate cube and gamma bands on the main fracture plane (RD-ND) and cube textured grain cluster on the fissured plane (RD-TD) in Chapy impact sample.
Fig. 9.1:
Schematic diagram showing the orientation of Charpy impact samples with respect to the rolling direction, RD. Grey colored planes are the macroscopic fracture plane for Charpy impact samples at different orientations.
Fig. 9.2:
Optical micrographs of the investigated samples of steel S2FRT800: (a) S2R0, (b) S2R30, (c) S2R60 and (d) S2R90.
Fig. 9.3:
Optical micrographs of the investigated samples of steel S4FRT800: (a) S4R0, (b) S4R30, (c) S4R60 and (d) S4R90.
Fig. 9.4:
Grain size distributions of the investigated samples of steel (a) S2FRT800 and (b) S4FRT800.
Fig. 9.5:
Morphology of the MnS inclusions on the plane parallel to the main fracture plane of S2R0 and S2R90 samples.
Fig. 9.6:
Tensile stress-strain curves of the investigated samples having different orientations with respect to the rolling direction of steel (a) S2FRT800 and (b) S4FRT800.
Fig. 9.7:
Charpy impact transition curves of the investigated samples of steel (a) S2FRT800 and (b) S4FRT800.
Fig. 9.8:
Inverse pole figure (RD-IPF) maps of the investigated samples of steel S2FRT800: (a) S2R0, (b) S2R30, (c) S2R60 and (d) S2R90. (e) Color legend to the corresponding IPF maps. xix
Fig. 9.9:
Inverse pole figure (RD-IPF) maps of the investigated samples of steel S4FRT800: (a) S4R0, (b) S4R30, (c) S4R60 and (d) S4R90. (e) Color legend to the corresponding IPF maps.
Fig. 9.10:
Orientation distribution function (ODF) of (a) S2FRT800 sample and (b) S4F T800 sample presented at φ2=45° section of lar’s space. Inverse pole figures with respect to RD, TD and ND directions for (c) S2FRT800 sample and (d) S4FRT800 sample.
Fig. 9.11:
(a) Gamma fibre (ND||), (b) Alpha fibre (RD||) and (c) Cube fibre (ND||) distributions in S2FRT800 and S4FRT800 samples.
Fig. 9.12:
Variation in DBTT with the area fraction of grains having {001} plane parallel to the macroscopic fracture plane of (a) S2FRT800 and (b) S4FRT800 samples.
Fig. 9.13:
Variation in impact energy absorption with the area fraction of grains having {011} planes parallel to the shear plane of (a) S2FRT800 and (b) S4FRT800 samples. Schematic diagram showing the shear planes in a Charpy impact sample.
xx
Abstract: Combined effect of inclusion, microstructure and crystallographic texture on Charpy impact properties of low-carbon ferritic steels has been studied after different finish rolling (935 °C - 650 °C) and normalizing (1250 °C - 940 °C) treatments. Finish rolling within the austenite-ferrite two phase region leads to the formation of low-angle boundaries, which strengthen ferrite matrix but are ineffective in restricting the cleavage crack propagation; thereby deteriorate the upper shelf energy (USE) and increase the ductile to brittle transition temperature (DBTT). The presence of coarse cuboidal TiN particles (>1µm) also increase the DBTT, whereas, stringer shaped MnS inclusions deteriorate the USE. In spite of the presence of large TiN particles, refinement in ‘effective grain size’ of ferrite can improve the impact toughness. Finish rolling just above the austenite to ferrite transformation start temperature (820 C) or normalizing of as-rolled plates at low austenitization temperature (940 C) develop fine strain-free ferrite grains with small ‘effective grain size’, and therefore, can be recommended for achieving high USE and low DBTT. Crystallographic texture can influence the general yield temperature through its effect on the plastic constraint factor. Local texture also determines the ‘effective grain size’, which is found to be dependent on the angle between {001} cleavage planes of the neighbouring crystals, rather than the grain boundary misorientation angle considering angle-axis pair. The severity of delamination on the fracture surface of Charpy impact tested samples of low-carbon steel has been found to be dependent on finish rolling temperature and texture. Strain incompatibility between the through thickness texture bands of cube (ND ║ ) and gamma (ND ║ ) orientations developed during the inter-critical rolling treatment causes fissure cracking on the main fracture plane in an intergranular fashion. The crack subsequently propagates through the transverse ‘fissure plane’ in transgranular fashion, due to the lower cleavage fracture stress of that plane. Presence of strong crystallographic texture results in non-uniform distribution of crystallographic planes along the different directions of the rolled plates, which causes anisotropy in Charpy impact properties. The volume fraction of grains having {001} plane parallel to fracture plane control the DBTT whereas the volume fraction grains having {011} plane parallel to shear plane dictates the USE. Keywords: Low-carbon ferritic Steel, Crystallographic texture, Inclusions, Charpy impact testing, Effective grain size, Fissure, Anisotropy.
xxi
Contents Subject Title Page Certificate of Approval Certificate Declaration Acknowledgements List of Symbols List of Abbreviations List of Tables List of Figures Abstract Contents Chapter 1 Introduction 1.1 General background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Factor affecting Charpy impact toughness. . . . . . . . . . . . . . . . . . . . . . 1.3 Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Thesis organization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 2 Literature Review 2.1 Principles of high strength low alloy (HSLA) steel. . . . . . . . . . . . . . . . 2.2
1
2.3 2.4 2.5 2.6 2.7
2.1.1 Processing of HSLA steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fracture Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Ductile fracture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Brittle fracture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Basic mechanisms for cleavage fracture. . . . . . . . . . . . . . . . . 2.2.4 Crack nucleation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4.1 Dislocation piles up. . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4.2 Twin-Twin interaction. . . . . . . . . . . . . . . . . . . . . . . 2.2.4.3 Carbide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4.4 Inclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Crack propagation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Definition of the energetic parameters (γ) . . . . . . . . . . . . . . . . 2.2.7 Effect of stress distribution on cleavage crack initiation. . . . . 2.2.8 Dependency of controlling mechanism with temperature. . . . Instrumented Charpy Impact Testing. . . . . . . . . . . . . . . . . . . . . . . . . . Micro-cleavage fracture Stress. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of ferrite grain size and carbide particle size on fracture. . . . . . Prediction of impact transition temperature. . . . . . . . . . . . . . . . . . . . . Effect of crystallographic texture on Charpy impact toughness. . . . . . 2.7.1 Development of crystallographic texture during processing .of HSLA steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.2 Effect of crystallographic texture on upper shelf energy. . . . . 2.7.3 Effect of crystallographic texture on transition temperature. . xxii
Page no. i iii iv v vi vii viii ix xi xxi xxii 1-8 1 2 5 5 9-51 9 10 14 14 15 16 17 17 18 19 20 20 22 23 24 26 29 30 32 34 35 37 37
2.7.4
Effect of Crystallographic texture on Cleavage fracture mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.4.1 Role of Cube texture. . . . . . . . . . . . . . . . . . . . . . . . 2.7.4.2 Effective grain size. . . . . . . . . . . . . . . . . . . . . . . . . 2.7.4.3 Role of tilt/ twist angle. . . . . . . . . . . . . . . . . . . . . . . 2.8 Fissure / Splitting / Delamination in impact testing . . . . . . . . . . . . . . . 2.8.1 Splitting and its effect on Charpy impact toughness. . . . . . . 2.8.2 Mechanism of fissure formation. . . . . . . . . . . . . . . . . . . . . . 2.8.3 Different reasons behind fissure formation. . . . . . . . . . . . . . . 2.8.3.1 Elongated ferrite grain structure. . . . . . . . . . . . . . . 2.8.3.2 High dislocation density within ferrite grains. . . . . 2.8.3.3 Inclusions like sulphur, phosphorus and silicate . . 2.8.3.4 Ferrite-pearlite banded microstructure. . . . . . . . . . . 2.8.3.5 Crystallographic Texture. . . . . . . . . . . . . . . . . . . . . 2.9 Anisotropy in Charpy impact properties. . . . . . . . . . . . . . . . . . . . . . . . 2.9.1 Different reasons behind anisotropy in Charpy impact properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.1.1 Elongated inclusions. . . . . . . . . . . . . . . . . . . . . . . . 2.9.1.2 Shape and size of ferrite grains. . . . . . . . . . . . . . . . 2.9.1.3 Pearlite banding. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.1.4 Crystallographic texture. . . . . . . . . . . . . . . . . . . . . . . . 2.10 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental details. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 3 3.1 Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Processing and heat treatment schedules . . . . . . . . . . . . . . . . . . . . . . . 3.3 Microstructural characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Macro-texture study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Macro-, Micro- and Nano-hardness testing. . . . . . . . . . . . . . . . . . . . . 3.6 Tensile testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Instrumented Charpy impact testing. . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Fractography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 4 Effect of ferrite grain structure and grain boundary misorientation on impact toughness in presence of TiN particles 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Microstructural and precipitate characterization . . . . . . . . . . . . . . . . . 4.3 EBSD analysis of grain boundary character distribution. . . . . . . . . . . . 4.4 Charpy impact testing to determine the impact transition curves. . . . . 4.5 Concept of critical grain size. . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . 4.6 Prediction of DBTT using regression equations . . . . . . . . . . . . . . . . . . 4.7 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 5 Effect of ferrite grain structure and grain boundary misorientation on impact toughness in presence of MnS inclusions 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Characterization of microstructure and inclusions. . . . . . . . . . . . . . . . xxiii
38 38 39 42 43 43 45 46 47 47 47 47 48 49 49 50 50 50 50 51 53-61 53 53 55 57 57 58 59 60 63-84
63 64 69 72 76 79 83 85-103
85 86
5.3 Hardness and tensile testing of the investigated samples of S2 steel . . 5.4 Charpy impact testing to determine the impact transition curves . . . .. 5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 6 Effect of reheating temperature on the microstructure, texture and impact transition behaviour of naval grade HSLA steel 6.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Microstructural characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Charpy impact properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Grain boundary misorientation and Crystallographic texture of the investigated samples of S3 steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Effect of microstructure on cleavage fracture stress. . . . . . . . . . . . . . . 6.7 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 7 Effect of crystallographic texture on cleavage fracture micro-mechanism and effective grain size of ferritic steel 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Theoretical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Prediction of transition temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 8 Effect of local crystallographic texture on the fissure formation during Charpy impact testing of low carbon steel 8.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Result and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 9 Effect of MnS inclusion and crystallographic texture on anisotropy in Charpy impact toughness of low carbon ferritic steel 9.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Microstructural Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Tensile properties of S2 and S4 steel samples. . . . . . . . . . . . . . . . . . . . 9.4 Study of grain boundary misorientation and texture. . . . . . . . . . . . . . . 9.5 Effect of microstructure and texture on the anisotropy in impact toughness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 10 Conclusions and further scope of work 10.1 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Further scope of work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A: List of publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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92 94 102 105-128
105 106 113 116 118 125 127 129-144 129 130 140 142 144 145-167
145 146 167 169-182
169 170 173 175 179 182 183-189 183 189 191 193-205
CHAPTER 1: Introduction 1.1
General background: Thermo-mechanically controlled processed (TMCP) high strength low alloy
(HSLA) steels are widely used for the fabrication or construction of buildings, bridges, pressure vessels, ship hulls, line-pipes and various other strategic or defence applications. Excellent combination of strength and toughness are essential requirements for the above mentioned applications. The advantage of using low carbon steel over the other steel grades is its superior weldability. High impact energy absorption and low ductile to brittle transition temperature (DBTT) are essential in line-pipe steels as several oil rigs are located in the freezing climate of Siberia, Alaska and in several other places (Shin et al., 2007). Low ductile to brittle transition temperature (DBTT) ensures sufficient toughness at sub-zero working temperature. Due to the simple nature of testing, Charpy impact testing is widely used in the steel industries as a measure of impact toughness and the impact transition behavior (Broek, 1986; Dieter and Bacon, 1986; Hertzberg et al., 1996). The requirement of mechanical properties in various structural, linepipe and naval grades are summarized in Table 1.1. Table 1.1: Mechanical properties required for the existing grades of high strength steel plates for structural, line-pipe and naval applications. Grade
Minimum yield strength (MPa)
S420 S460 S500 S550 S600 S700
420 460 500 550 600 680
X52
358
Tensile Minimum strength elongation (MPa) (%) Structural grades 500-580 20 520-670 17 550-700 14 600-760 14 650-820 13 750-900 12 Line-pipe grades 455
30
Typical yield ratio
Required Charpy impact toughness
0.84 0.77 0.82 0.82 0.83 0.86
40 J at –20°C 40 J at –20°C 40 J at –20°C 40 J at 0°C 40 J at 0°C DBTT0.03 wt%) is present in steel large stringer shaped MnS inclusions can form (Biswas et al., 1992; Dehghan187
Chapter 10
Conclusion and future scope of work
Manshadi and Dippenaar, 2010; Tomita, 1988) which drastically reduce the USE at transverse to rolling direction. The alternatives are to either reduce the sulfur below S(
