Improved mechanical properties of V-microalloyed dual phase steel by enhancing martensite deformability

A good combination of ultimate tensile strength(UTS)up to 1365 MPa and total strain to failure(StF)to 15.5％has been achieved due to deformable martensite in the invented vanadium-microalloyed dual-phase(DP)steel,which was manufactured by two-stage annealing of cold rolled steel strip.The employed extensive characterizations revealed that the ductile martensitic phase in this DP steel differ-entiated from ordinarily low-carbon martensitic lath in both morphology and lattice structure.Complex coherent orientation relationships between ferrite,reverse austenite,martensitic phase and vanadium carbide(VC)do exist,leading to a new martensitic transformation mechanism and resultant dual-phase microstructure.Besides,a detailed characterization including essential phase transformation analysis in combination with in situ TEM observation,shows that,all the essential processing including recrystal-lization,reverse austenitic and martensitic transformation,in debt to the particular effects of VC,can be recognized as phase transformations with higher thermodynamic driving force and higher kinetic energy barrier as compared to previously common processing,which actually changes the microstructure and,indirectly leads to higher strength and higher ductility.This synergy of thermodynamics and kinetics can be generalized to improve mechanical properties of present steels.     

Relationship between the martensite phase transition and pitting susceptibility of AISI-321 stainless steel in acidic solutions of NaCl

Specimens of AISI-321 (1Cr18Ni9Ti) stainless steel with various contents of austenite are stretched to different degrees at a temperature of -70°C. The X-ray diffraction method and the method of transmission electron microophotography are applied to study the phase and structural transformations. The potentiodynamic tests and electrochemical impedance spectroscopy are used to determine the susceptibility of steel to pitting formation in acidic solutions of NaCl. The experimental investigations demonstrate that the martensite phase consists mainly of -martensite and its content increases with the level of strains. The interphase electrochemical parameters demonstrate that if the content of the martensite phase is lower than 6% or higher than 22%, then the susceptibility of the material to pitting formation increases with the content of the martensite phase. However, within the range 6–22%, the indicated characteristic is a decreasing function of the content of martensite.Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 2, pp. 83–89, March–April, 2004.     

Effect of corrosive medium on fatigue crack growth behaviour and fracture in high martensite dual phase steel

Fatigue crack growth (FCG) behaviour in both near-threshold and higher stress intensity range (ΔK) in intercritically annealed dual-phase (DP) steel containing martensite between 32% and 76% in ferrite has been studied in 3·5% NaCl solution. It is shown that the amount of martensite content in dual phase steel has a significant effect on threshold (ΔK _th) values and FCG rates. Higher content of martensite in ferrite leads to higher threshold values and lower FCG rates. Further, ΔK _th is much higher in 3·5% NaCl solution as compared to that in laboratory air. Fractography studies reveal that in the near-threshold region, fracture surfaces are characterized mainly by intergranular cracking in corrosive (3·5% NaCl solution) environment. Higher threshold values in 3·5% NaCl solution is attributed to the higher crack closure induced by rougher fracture surface and by the strong wedge effects of corrosion products.     

Quantitative phase analysis by X-ray diffraction of martensite and austenite in strongly oriented orthodontic stainless steel wires

A method is described for measuring the volume fractions and textures of martensite and austenite in strongly textured stainless steel orthodontic wires using a conventional X-ray diffractometer. These wires display a classic fibre texture with the 111 of the FCC austenite phase and the 110 of the BCC martensite phase aligned parallel to the wire axis. The samples analysed consisted of wire cross-sections bundled together and chemically polished in an epoxy disc. In this form the dominant lines in the XRD patterns are the austenite (111) and the martensite (110). On the basis of X-ray diffraction results from these two lines only, procedures are described for, (a) correcting the X-ray intensity data for both the finite size and irregular cross-sectional shape of the specimens in relation to the X-ray beam footprint, (b) separately measuring the texture of the austenite and martensite phases and, (c) correcting the 111 and 110 integrated intensities for texture. These procedures are illustrated using X-ray data from four different orthodontic wires. The factors limiting the accuracy of the phase analysis are discussed.     

Strain-induced martensite formation in austenitic stainless steel

In situ measurements of the strain-induced martensitic transformation (SMTs) of SUS304 stainless steel that takes place during tensile loading at room temperature were performed around the notch of a dumbbell-shaped specimen where high stress concentration occurs. Even in the low plastic strain regime, with loading to 0.2 % proof stress (σ _0.2), some SMTs occurred. However, the area fraction of the Fe-α′-martensite phase did not increase significantly even when the sample was loaded to the ultimate tensile strength (σ _UTS). After the σ _UTS point, the total fraction of the Fe-α′ phase increased dramatically to the fracture point (σ _f). The phase textures of Fe-α′ and Fe-γ were almost equal at (σ _UTS ? σ _f)/2, and the Fe-α′ phase was observed over almost the entire measurement area around the notch at the σ _f point. However, the area fraction of the Fe-α′ phase at the σ _f point decreased far away from the fracture surface, to an extent that the total fraction of the Fe-α′ phase was almost the same as that of the Fe-γ phase in an area about 1.7 mm from the fracture face. Different martensite characteristics were detected in the stainless steel, depending on the applied load level. This was attributed to the severity of deformation. In particular, deformation twinning, created around σ _UTS, and severe plastic deformation before fracture make a strong Fe-α′ phase. Details of this phenomenon are interpreted using various approaches, including electron backscatter diffraction analysis and finite element analysis.     

Tungsten carbides and W-C phase diagram

The crystal structures of the tungsten monocarbide δ-WC and the disordered lower carbide β-W₂C are studied. Using magnetic susceptibility measurements, the hexagonal carbide δ-WC is shown to be stable from 300 to 1200 K. The sequence of phase transformations associated with β-W₂C ordering is analyzed. The temperature and composition stability limits of the cubic carbide γ-WC_1?x are evaluated, and the first data are presented on the variation of its lattice parameter with composition. An optimized W-C phase diagram is proposed which takes into account detailed structural and phase-equilibrium data for tungsten carbides.     

Pressure induced martensite transformation in plain carbon steel

Abstract

In the present study, plain low carbon steel with 0·033 wt-% carbon content was subjected to severe pressure during continuous cooling from austenite region. The pressure increased gradually and then suddenly released by the breakdown of ram under pressure. As a result, a microstructure composed of 80% lath martensite and 20% ferrite was produced. Results showed that the martensite formation is not due to the effect of cooling rate but the effect of hydrostatic pressure on the austenite to ferrite transformation start temperature Ar₃.     

Influence of martensite morphology on the work-hardening behavior of high strength ferrite–martensite dual-phase steel

This study concerns influence of martensite morphology on the work-hardening behavior of high-strength ferrite–martensite dual-phase (DP) steel. A low-carbon microalloyed steel was subjected to intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA) to develop different martensite morphologies, i.e., fine and fibrous, blocky and banded, and island types, respectively. Analyses of work-hardening behavior of the DP microstructures by differential Crussard–Jaoul technique have demonstrated three stages of work-hardening for IQ and IA samples, whereas the SQ sample revealed only two stages. Similar analyses by modified Crussard–Jaoul technique showed only two stages of work-hardening for all the samples. Among different treatments, IQ route has yielded the best combination of strength and ductility due to its superior work-hardening behavior. The influence of martensite morphology on nucleation and growth of microvoids/microcracks has been correlated with the observed tensile ductility.     

Effect of thermomechanical treatment parameters on mechanical properties of duplex ferrite–martensite structure in dual phase steel

Abstract

The quantitative effects of the variables used in the thermomechanical treatment (TMT) of a dual phase steel, in the temperature region of intercritical annealing, have been studied by statistical design of experiments. The initial microstructure has tremendous influence on the final microstructure and properties of the steel. The kinetics of transformation is enhanced by the deformation process as has been evidenced by optical and TEM microstructures. The mechanical properties such as tensile strength, yield stress, and relative elongation have been correlated with the TMT parameters and are brought out in the form of regression equations. Percentage phase of ferrite or martensite formed owing to thermomechanical treatment by two different routes has also been quantified in the form of regression equations. The adequacies of the equations were assessed by a Fisher F test and the accuracies of the equations have been further verified by performing random experiments in the range of variation of the variables. Isoproperty lines have been constructed using the regression equations developed. The equations can predict the properties within the range of variation of the variables.     

Anomalous kinetics of lath martensite formation in stainless steel

The kinetics of lath martensite formation in Fe–17·3 wt-%Cr–7·1 wt-%Ni–1·1 wt-%Al–0·08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.     

EBSD investigation of the crystallographic features of deformation-induced martensite in stainless steel

Variant selection during the martensitic transformation in steels may play an important role in determining the transformation kinetics and the resulting mechanical properties.In this study,the variant selection and crystallographic features of deformation-induced martensite were investigated by quasi in situ electron backscatter diffraction(EBSD) in grade SUS321 during tensile deformation.Significant differences in variant selection between austenite(γ)→hcp-martensite(ε)→bcc-martensite(α') and γ→α'transformation routes were observed and reported in detail,which demonstrated that s-martensite plays an important role in the variant selection of α'.Variant selection at diffe rent deformation stages was also analysed and revealed that α' variants with the highest priority and variant pairs were preferred at the initial and last deformation stages in the γ→ε→α'sequence,respectively.Meanwhile,the single α' variant nucleated at the thin slip band keeps its crystallography feature upon further deformation in the γ→α'sequence.In addition,the strain work of the martensitic transformation for applied loads was quantitatively estimated to explain the variant selection and associated mechanism.When these calculations are compared to the experimental results it is found that they are not able to predict which α' variant is forming pre ferentially during either during the γ→α' or the ε→α' sequences,while only accurate predictions are obtained for the γ→ε-transformation which indicates that the γ→α' variant selection is more complex.     

Effect of martensite content,its dispersion,and epitaxial ferrite content on Bauschinger behaviour of dual phase steel

Abstract

Dual phase microstructures were produced in a low carbon steel, in which the martensite volume fraction was kept constant at two levels, of 18 and 25%, while the epitaxial ferrite content was varied independently. The microstructures were produced with two dispersions of martensite, a relatively coarse dispersion by intercritical annealing of a ferrite/pearlite starting microstructure and a finer dispersion from an initial martensitic microstructure. Bauschinger tests were conducted using prestrains in both tension and compression of 0.4, 1, and 2.2%. Prestrain direction had no measurable effect on plastic flow behaviour after strain reversal. Mean back stresses increased with increasing strain and martensite content, and were higher for the finer martensite dispersion. They decreased significantly with increasing epitaxial ferrite content in the case of the finer dispersion, but less significantly in the coarser dispersion. These effects of microstructure are discussed in terms of stress transfer to martensite, work hardening, and tensile properties. It is concluded that about half of the mean back stress developed during early plastic deformation was generated by stress transfer to the martensite, the remainder arising from strain hardening of the matrix. A small permanent softening in the Bauschinger test resulted from a reduction of effective stress in the ferrite matrix when the strain path was reversed.