Factors influencing ferrite/pearlite banding and origin of large pearlite nodules in a hypoeutectoid plate steel

Abstract

The microstructure and distribution of alloying elements in a hot rolled, low alloy plate steel containing (wt-%) 0·15%C, 0·26%Si, l·49%Mn, and 0·03%Al were examined using light microscopy and electron probe microanalysis. Microstructural banding was caused by microchemical banding of manganese, where alternate bands of proeutectoid ferrite and pearlite were located in solute lean and solute rich regions, respectively. Bands were well defined for a cooling rate of 0·1 K s^?1, but banding was much less intense after cooling at 1 K s^?1. At a cooling rate of 0·1 K s^?1 and for austenite grains smaller than the microchemical band spacing, austenite decomposition occurred via the formation of ‘slabs’ of proeutectoid ferrite in manganese lean regions resulting in the growth of ferrite grains across austenite grain boundaries. Abnormally large austenite grains result in the formation of large, irregularly etching pearlite nodules which traversed several bands. In specimens cooled at 1 K s^?1, ferrite/pearlite banding did not exist in regions where austenite grains were two or more times larger than the microchemical band spacing.

MST/1397     

Texture evolution in hot band and annealed hot bands of low alloyed ferritic stainless steel

Abstract

The texture evolution in hot band and annealed hot bands of low alloyed ferritic stainless steel with about 11 wt-%Cr was experimentally investigated using quantitative texture analysis. While the hot band texture was composed of components of α fibre and in particular δ fibre, its microstructure was a banded structure of mostly relaxed martensite and retained ferrite. Both the texture and microstructure of the hot band was derived from partially recrystallised austenite. During single phase hot band annealing, there was a strong sharpening in the strength of the texture components of δ fibre by strain induced boundary migration of the retained ferrite and formation of fine carbide sheets leading to the persistence of ferrite banding. In contrast, two phase hot band annealing resulted in the formation of a nearly equiaxed duplex ferrite grain structure with an aggregate of precipitated carbides within the transformed ferrite grains and complete elimination of microstructural banding of the hot band, and also led to the occurrence of a texture memory phenomenon.     

Microstructural control by secondary processing of strip cast low carbon steel

Abstract

The secondary processing of low carbon steel strip produced by twin roll casting was investigated to examine its effect on microstructural development and mechanical properties. The as cast microstructure is predominantly acicular ferrite with regions of bainitepearlite and polygonal ferrite. Deformation at temperatures below Ar1 produces a heterogeneous microstructure with regions of moderately deformed acicular ferrite adjacent to highly deformed regions containing shear bands. Cold rolled and warm rolled steels show similar behaviour to conventional hot band in that dynamic recovery during warm rolling results in sluggish recrystallisation and produces a coarse final grain size. However, the initial as cast microstructure recrystallises at a slower rate than conventional hot band and produces a weaker recrystallisation texture. This can be attributed to the heterogeneous microstructure of the as cast strip such that, after rolling, nucleation occurs within shear bands and more ill defined sites, which results in nucleation of randomly oriented grains thereby producing a weak final texture. It was found that austenitising the as cast strip followed by rolling in the vicinity of Ar3 produces a uniform distribution of equiaxed, ultrafine ferrite UFF grains throughout the thickness of the strip. The production of UFF by twin roll casting and subsequent rolling represents a simple processing route for the production of fine grained low carbon sheet steel products.     

Microstructure in adiabatic shear bands in a pearlitic ultrahigh carbon steel

Abstract

Adiabatic shear bands, obtained in compression deformation at a strain rate of 4000 s^?1, in a pearlitic 1·3%C steel, were investigated. Shear bands initiated at 55% compression deformation with the width of the band equal to 14 μm. Nano-indentor hardness of the shear band was 11·5 GPa in contrast to the initial matrix hardness of 3·5 GPa. The high strength of the shear band is attributed to its creation from two sequential events. First, large strain deformation, at a high strain rate, accompanied by adiabatic heating, led to phase transformation to austenite. Second, retransformation upon rapid cooling occurred by a divorced eutectoid transformation (DET). The result is a predicted microstructure consisting of nano size carbide particles within a matrix of fine ferrite grains. It is proposed that the DET occurs in iron–carbon steels during high rate deformation in ball milling, ball drop tests and in commercial wire drawing.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Deformation of sintered copper and 50Cu–50Fe mixture to large strains by cyclic extrusion and compression

Abstract

Sintered compacts of copper and a 50Cu–50Fe mixture have been plastically deformed to large strains (total strain ?_t=13·8) by cyclic extrusion and compression. The hardness changes after deformation indicate that no further work hardening occurs with either material when ?_t>4·6. With copper, strain accommodation at large strains would appear to occur solely by dynamic recovery and recrystallisation. With the Cu–Fe mixture, shear banding is still found at the highest strains used.

MST/1609     

Thermoplastic shear localisation in titanium alloys during dynamic deformation

Abstract

Cylindrical specimens (4 mm diameter and 4 mm height) of titanium alloy bar were given various heat treatments to provide a wide range of microstructures and mechanical parameters. These specimens were then subjected to high plastic strain at a large strain rate (10³ s^-1 ) during dynamic compression by a split Hopkinson bar at ambient temperature. The microstructures of the localised shear bands were examined by optical and transmission electron microscopy. The results show that there are two types of localised shear bands: deformed and white shear bands. A detailed observation reveals that there is no difference in the nature of the deformed and white shear bands, but they occur at different stages of localised deformation. It is found that there is a burst of strain, corresponding to a critical strain rate at which the white shear band occurs and no phase transformation occurs in the shear bands.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

Ductile aluminium and brittle trialuminides

Abstract

The ductility of aluminium is attributed to the low elastic shear modulus of this fcc metal. Its trivalency endows a strong electrostatic resistance to shear, but this is almost entirely offset by a negative band structure contribution, due to a special Brillouin zone feature, with the result that the C₄₄ shear component is very small. Several effects are associated with the addition of transition metals, for example in trialuminides such as Al₃Ti. Outstandingly, the band structure contribution is severely reduced, because the large scattering cross-sections of the transition metal atoms greatly reduce the Brillouin zone effect. As a result, C₄₄ becomes large and the alloyed crystal is thereby embrittled. Because similar scattering occurs in dilute random solutions of transition metals in aluminium, there is a direct link between electrical resistivity and brittleness in these materials.

MST/1527     

Effects of vanadium addition on nucleation and growth of pearlite in high carbon steel

Abstract

The influence of vanadium addition on the microstructure of high carbon steels has been investigated. A careful examination of the initial stages of austenite decomposi~ion has been made, using a range of high resolution metallographic techniques. It has been confirmed that vanadium addition results in the formation of grain boundary ferrite films, even in the eutectoid composition range. It is argued that this ferrite is the product of eutectoid transformation, and is not proeutectoid ferrite. This is because the first event is the nucleation of carbide particles along the grain boundaries. These carbides have been identified mainly as cementite. The presence of vanadium appears to change the morphology and distribution of the grain boundary cementite, so that rather than forming a grain boundary network, the cementite occurs in the form of a high density of small discrete particles along the boundaries. It is proposed that this occurs because vanadium increases the driving force for cementite nucleation. The formation of the grain boundary cementite depletes the surrounding region of carbon and encourages the formation of ferrite, but because of their discrete and fine dispersion, the cementite particles are engulfed by the more voluminous ferrite phase. In such regions, the onset of afully cooperative growth regime is delayed. Pearliteforms later at the ferrite/austenite interfaces.

MST/1923     

Effects of nano-sized α<Superscript>2</Superscript> (Ti<Superscript>3</Superscript>Al) particles on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy with bimodal microstructure

A bimodal microstructure containing very fine α²(Ti³Al) particles was produced by over-aging a Ti-6Al-4V alloy. The effects of α² precipitation on quasi-static and dynamic deformation behavior were investigated in comparison with an unaged bimodal microstructure. Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar. The quasi-static torsional test data indicated that the over-aged bimodal microstructure showed higher fracture shear strain than the unaged bimodal microstructure, while their maximum shear stresses were similar. Under dynamic torsional loading, both maximum shear stress and maximum shear strain of the over-aged microstructure were higher than those of the unaged microstructure. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed by introducing concepts of critical shear strain, absorbed deformation energy, and void initiation. In the over-aged microstructure, the energy required for forming adiabatic shear bands was higher than that in the unaged microstructure, thereby lowering the possibility of the adiabatic shear band formation. The α² precipitation in the over-aged microstructure was effective in both the improvement of quasi-static and dynamic torsional properties and the reduction in the adiabatic shear banding, which suggested a new approach to improve ballistic performance of Ti alloy armor plates.     

Characterisation and quantification of microstructural banding in dual phase steels Part 1 – General 2D study

Abstract

In order to quantify the degree of banding in dual phase steel, optical micrographs of various dual phased steel microstructures have been analysed. From the binary images of the micrographs, greyscale profiles are created and used to define the bands within the image. Two parameters are calculated from the defined bands: band continuity index C_b and perpendicular continuity index C_p. The first parameter quantitatively describes the continuity of each band along the direction of banding, while the second parameter quantitatively describes the continuity of the bands perpendicular to the banding direction. Both parameters are bounded on [0,1], with 0 indicating no banding and 1 indicating strong banding.     

Nucleation of Widmanstätten ferrite

Abstract

Recently published experimental data on the variation of the highest temperature at which Widmanstiittenferrite can be seen to form at a detectable rate, as a function of steel chemistry, are analysed theoretically. It is found that the data can be predicted to a fair accuracy if it is assumed that the nucleation of Widmanstiitten ferrite occurs by a mechanism similar to that of martensitic nucleation, but with the diffusion of carbon during nucleation, and if it is additionally assumed that the growth of Widmanstiitten ferrite can only be sustained when the chemical driving force exceeds a specific stored energy term.

MST/1210     

Initiation and growth of microfractures along adiabatic shear bands in Ti–6AI–4V

Abstract

An experimental study has been made of the manner in which microfractures initiate and grow along adiabatic shear bands formed in the titanium alloy Ti–6AI–4V by the normal impact of hard steel spheres at velocities up to 340 m s^?1. It is suggested that a critical shear strain must be exceeded along the shear bands for microvoids to nucleate, or to cause significant local thermal softening in the bands, leading to the formation of single voids or arrays of voids and smooth-sided cracks when the stress state became predominantly tensile. The final shape of the micro fractures within the shear bands and the morphology of the resultant fracture surfaces are explained in terms of the density of void nucleation sites and the tensile-stress state across the shear bands.

MST/179     

High-strain deformation of dual-phase steel

Abstract

A commercial type dual-phase steel has been heat treated to develop a conventional dual-phase structure and, by a double-quench heat treatment, a dual-phase structure with a small martensite island size. These specially heat treated materials together with the normalized material have been plastically deformed by rolling to a reduction of 98% (ε_t = 4·0). The tensile properties have been determined after deformation and correlated with the microstructure. It has been found that within the strain range ε_t = 0·5–1·5 the work hardening modulus is similar to that of pure iron. Over a narrow strain range little work hardening occurs but within the range ε_t = 2·5–4·0 the work hardening modulus is greater than that of ferrite. The increase in modulus seems to be associated with the plastic deformation of the martensite islands which, at the highest strains, give a fibre reinforcing effect. The results are discussed in relation to the work hardening mechanisms involved. It is concluded that changes in the ferrite grain size, established during the development of deformation bands at lower strains and subsequently deformed at higher strains, greatly influence the flow stress through a Hall-Petch relationship.

MST/241     

Comparative study of antimicrobial and photocatalytic activity in titania encapsulated composite nanoparticles with different dopants

Abstract

The present study describes a relative comparison of antimicrobial and photocatalytic activity in titania encapsulated nickel ferrite nanoparticles with different dopants. The photocatalytic and antimicrobial activity in doped titania composite nanoparticles follows the sequence: W⁴⁺ doped > Nd³⁺ doped > Zn²⁺ doped > undoped titania. The maximum enhancement in tungsten doped titania is attributed to the greater inhibition of electron hole recombination process and decrease in band gap in titania. The ferrite magnetic nanoparticles encapsulated with the photocatalytic shell retain superparamagnetic characteristics and magnetic strength encouraging their potential application as removable antimicrobial photocatalytic composite nanoparticles. The combination of reverse micelle and hydrolysis method is recognised as a promising method for the synthesis of these composite nanoparticles.     

Behaviour of low and medium carbon free cutting steels during deformation to large strains

Abstract

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.     

Diffusion-controlled growth of ferrite plates in plain-carbon steels

Abstract

The implementation of the theory of diffusion-controlled growth of ferrite plates in plain-carbon steels is critically assessed. It is found that the use of empirically extrapolated diffusion coefficients, phase boundaries, and thermodynamic functions leads to errors in calculations of growth rate. The errors become most important for low transformation temperatures, leading to exaggerated growth rates. Ways of avoiding these difficulties are suggested, and a new analysis of experimental data indicates that the lengthening of Widmanstatten ferrite plates in Fe–C alloys occurs at a rate which is influenced by the diffusion of carbon in the austenite ahead of the interface, assuming that the plates adopt a tip radius consistent with the maximum growth velocity. However, there is a systematic discrepancy between theory and experiment: plate-growth theory seems to underestimate the lengthening rate by some 5 μm s^?1. This may have something to do with the lath shape of Widmanstatten ferrite, but an analysis using needle–growth theory does not resolve the problem for data obtained at low lengthening rates. In general, plate–growth theory gives a better explanation of experimental data. The growth of bainite sheaves occurs at a rate much faster than expected from carbon diffusion-controlled growth. If the maximum-velocity hypothesis is incorrect (as it is for dendritic solidification), the above-mentioned discrepancies would be larger.

MST/192     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Width of adiabatic shear bands formed under combined stresses

Abstract

An equation for the half width of an adiabatic shear band formed under combined stresses is derived. The importance of strain rate, stress, temperature rise, and thermal conductivity is described. Confirmation is found for the proposition that shear band width is independent of stress state and this is confirmed by comparing the theory with the experimental results of other workers.

MST/933