Xiaodan Zhang

Study on dislocation slips in ferrite and deformation of cementite in cold drawn pearlitic steel wires from medium to high strain

Abstract The deformation of cementite was studied via optical microscopy and SEM in the longitudinal sections of pearlitic steel wires from medium to high strain. The cementite shows good deformability, the angle between the deformation direction of cementite and drawing direction decreases with increasing strain, and finally the deformation directions of cementite turn to the drawing axis at high strains. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, S-bands and cracks across cementite observed parallel to either {110}α-Fe or {112}α-Fe plane traces determined by the largest Schmid factors.

EVOLUTIONS OF MICROSTRUCTURE AND FERRITIC MICRO–ORIENTATION AND TEXTURE IN A PEARLITIC STEEL WIRE DURING COLD DRAWING

Cold drawn high carbon pearlitic steel wires have the highest strength of all massproduced steel materials and are widely used in industry for a variety of applications,including cables for suspension bridges,steel cords for automobile tires and springs.At present the maximum tensile strength of high-carbon steel wires has already reached a value of 5.7 GPa.The properties of steel wires,including strength,fatigue properties and torsinal properties,are deeply affected by microstructure and ferritic micro-orientation and texture in the deformed pearlite.In this study,the evolutions of microstructure and ferritic micro-orientation and texture were investigated in a pearlitic steel wire during cold drawing using electron channel contrast(ECC) and electron backscatter diffraction(EBSD) techniques.The results show that there exist shear-bands(S-bands) in the deformed pearlite microstructure. Their appearance is related to the angle between cementite plates and the drawing axis: the larger the angle is,the more S bands appear in the structure.The pearlite structure turns to the drawing direction and the angle between S band and drawing axis decreases with the increase of strain. The S-bands in the deformed pearlite colony induce the rapid change of local orientation of ferrite, and make the pearlite colony subdivide into several areas by high angle boundaries of ferrite.The strong〈110〉fibre texture of ferrite parallel to the drawing direction forms with the increase of strain,but the intensity of〈110〉fibre texture of ferrite is inhomogeneous from the centre to surface in longitudinal section with the strongest in the centre and the weakest near the surface.

Composition change during creep in colonies oriented for easy-slip of Ti–46.5Al–2Cr–3Nb–0.2W

Abstract Alloy Ti–46.5Al–2Cr–3Nb–0.2W with fully lamellar microstructure was crept under 1073 K and 270 MPa, and element distribution before and after creep deformation has been compared. In the undeformed microstructure, Cr concentrates in the α 2 phase, W concentrates in α 2 phase and segregates at the γ/α 2 interface and Nb concentrates in the γ phase. An explanation based on site occupying and electronic bonding has been given. During creep deformation, α 2 →β phase transformation occurs. The segregation of W atoms at the γ/α 2 interface promotes nucleation of β phase precipitates, then β phase precipitates grow with the help of the immigration of Cr and W atoms from α 2 phase to β phase, and Cr and W do not concentrate in α 2 phase any more. During migration, Cr atoms diffuse in the α 2 phase directly and W atoms mainly diffuse along the γ/α 2 interface. The composition of sub-lamellae in deformed microstructure has also been studied; the composition of α 2 laths in sub-lamellae is influenced significantly by the formation of β phase precipitates.

Structure and strength of sub-100 nm lamellar structures in cold-drawn pearlitic steel wire

ABSTRACT Pearlitic steel wire, with a representative sub-100 nm lamellar structure, is the strongest mass-produced steel with an excellent combination of formability and strength. This overview summarises investigations of cold-drawn pearlitic steel wire in the last decades, covering the microstructural evolution and strengthening mechanisms. Based on quantitative structural parameters, this overview covers a quantitative and extensive analysis of structure–strength relationships. By focusing on the structure, challenges and future strategy are outlined to further improve the mechanical behaviour and performance of pearlitic steel wire to widen its use in society. This is part of a thematic issue on Pearlitic Steel Wires.

Effect of shot peening on the residual stress and mechanical behaviour of low-temperature and high-temperature annealed martensitic gear steel 18CrNiMo7-6

A martensitic gear steel (18CrNiMo7-6) was annealed at 180 °C for 2h and at ∼ 750 °C for 1h to design two different starting microstructures for shot peening. One maintains the original as-transformed martensite while the other contains irregular-shaped sorbite together with ferrite. These two materials were shot peened using two different peening conditions. The softer sorbite + ferrite microstructure was shot peened using 0.6 mm conditioned cut steel shots at an average speed of 25 m/s in a conventional shot peening machine, while the harder tempered martensite steel was shot peened using 1.5 mm steel shots at a speed of 50 m/s in an in-house developed shot peening machine. The shot speeds in the conventional shot peening machine were measured using an in-house lidar set-up. The microstructure of each sample was characterized by optical and scanning electron microscopy, and the mechanical properties examined by microhardness and tensile testing. The residual stresses were measured using an Xstress 3000 G2R diffractometer equipped with a Cr Kα x-ray source. The correspondence between the residual stress profile and the gradient structure produced by shot peening, and the relationship between the microstructure and strength, are analyzed and discussed.

Low temperature annealing of cold-drawn pearlitic steel wire

Cold-drawn pearlitic steel wires are nanostructured and the flow stress at room temperature can reach values above 6 GPa. A typical characteristic of the nanostructured metals, is the low ductility and thermal stability. In order to optimize both the processing and application of the wires, the thermal behaviour is of interest. This has been studied by annealing the wires for 1h at temperatures from ambient temperature to 300 °C (573 K). It is expected that a raising temperature may lead to structural changes and a reduction in strength. The change in strength is however not expected to be large. For this reason we have applied a very precise technique to measure the tensile properties of the wires from a strain of 10-4 to the maximum strain of about 1-2%. The structural changes have also been followed to estimate and relate strength changes to changes in structural parameters and morphology.

Local microstructure and flow stress in deformed metals

The microstructure and flow stress of metals are related through many well-known strength-structure relationships based on structural parameters, where grain size and dislocation density are examples. In heterogeneous structures, the local stress and strain are important as they will affect the bulk properties. A microstructural method is presented which allows the local stress in a deformed metal to be estimated based on microstructural parameters determined by an EBSD analysis. These parameters are the average spacing of deformation introduced boundaries and the fraction of high angle boundaries. The method is demonstrated for two heterogeneous structures: (i) a gradient (sub)surface structure in steel deformed by shot peening; (ii) a heterogeneous structure introduced by friction between a tool and a workpiece of aluminum. Flow stress data are calculated based on the microstructural analysis, and validated by hardness measurement and 2D numerical simulations. A good agreement is found over a plastic strain range from ~1 to 5.

EBSD characterization of deformed lath martensite in IF steel

Rolling deformation results in the transformation of a lath martensite structure to a lamellar structure characteristic to that of IF steel cold-rolled to medium and high strains. The structural transition takes place from low to medium strain, and electron backscatter diffraction analysis shows that the frequency of medium angle boundaries with misorientation angles of 5-10° decreases with increasing strain, while the frequencies of boundaries with angles in the ranges of 1-5° and 10-25° increase, resulting in the evolution of a bimodal misorientation angle distribution. The microstructural evolution and the strength are characterized for lath martensite rolled to a thickness reduction of 30%, showing that large changes in the misorientation take place, while the strain hardening rate is low.

Gradient microstructure and microhardness in a nitrided 18CrNiMo7-6 gear steel: Paper

A commercial gear steel (18CrNiMo7-6) containing a tempered martensite structure was nitrided using a pressurized gas nitriding process under a pressure of 5 atm at 530 °C for 5 hours. The mechanical properties and microstructure of the nitrided sample were characterized by Vickers hardness measurements, X-ray diffraction, and backscatter electron imaging in a scanning electron microscope. A micro-hardness gradient was identified over a distance of 500 μm with hardness values of 900 HV at the top surface and 300 HV in the core. This micro-hardness gradient corresponds to a gradient in the microstructure that changes from a nitride compound layer at the top surface (~ 20 μm thick) to a diffusion zone with a decreasing nitrogen concentration and precipitate density with distance from the surface, finally reaching the core matrix layer with a recovered martensite structure.

Microstructure and hardness development in a copper-nickel diffusion gradient model system

Cu has been electrolytically coated with Ni and subsequently deformed by rotary swaging up to a strain of e=2 to create a chemical gradient at the interface of the two elements. The extend of this chemical intermixing has been investigated through Energy Dispersive X- ray (EDX) spectroscopy in the Scanning and Transmission Electron Microscope (SEM and TEM). The depth, in which intermixing takes place, is about 1pm from the interface. Because of the uniform deformation, the structure does not get elongated but rather uniformly reduced in size. Microindentation hardness measurement shows a hardness increase from 120 to 135kp/mm2 in the Cu phase with increasing strain. After annealing at 200°C for up to 4h the hardness first decreases, but raises above the value for the highly strained sample. The experimental findings are discussed with emphasis on surface mechanical alloying as a process of both scientific and technological interest.