Pearlite

About: Pearlite is a research topic. Over the lifetime, 6028 publications have been published within this topic receiving 65695 citations.

Influence of morphology and structural size on the fracture behavior of a nanostructured pearlitic steel

Abstract: Subjecting pearlitic steels to severe plastic deformation is known to transform the original colony structure to a nanostructured pearlite with the microstructural constituents aligned parallel to the deformation direction. Besides a huge increase in strength due to an enormous reduction of the interlamellar distance, other mechanical properties such as fracture toughness are deteriorated especially when the crack propagation is parallel to the shear deformed structure. Post-deformation heat-treatments up to 600 °C were applied to modify the oriented nanostructured pearlite after applying high pressure torsion to a micro-duplex structure of ultrafine grained ferrite with spherical cementite dispersoids. Already moderate annealing led to a pronounced increase of the fracture toughness accompanied only by a slight drop in strength. Nevertheless, at those low annealing temperatures the deformation structure was partially present which crucially influenced the crack propagation behavior and thereby the mechanical anisotropy. By raising the annealing temperature it was possible to produce a fully spheroidized microstructure and in further consequence mechanical isotropy was achieved. The decrease in strength due to microstructural coarsening is balanced by a remarkable gain in fracture toughness.

The Tensile Properties of Pearlite, Bainite, and Spheroidite

Abstract: The tensile properties of four steels have been determined as a quantitative function of the measured dimensions of the aggregate structures pearlite and spheroidite, and of the austenite decomposition temperature for the structures pearlite and bainite. Studies of the recalescence effect have been performed in connection with the measurement of the reaction temperature. The strength indices (stress at corresponding strains, tensile strength, hardness) vary linearly with the reaction temperature and the logarithm of the dimensions of the aggregate. Mixtures of pearlite and bainite are intermediate in strength. The ductility indices are low for mixed structures, coarse pearlite and low temperature bainite; higher for bainite and pearlite in the middle of the reaction temperature range for each. It has been observed that spheroidized eutectoid specimens have a typical mild steel yield point; pearlitic specimens of the same tensile strength do not. The spacing of pearlite is shown to be proportional to the carbon diffusion coefficient in austenite, the logarithm of the spacing plotting as a straight line against the reciprocal of the absolute reaction temperature, with the same slope as a similar plot for the diffusion coefficient. Because of this it is concluded that a measurement of the spacing at one temperature permits its calculation at another, using the measured energy of activation for the diffusion of carbon in the steel. A rule of strength for aggregates is proposed, based on these studies, as follows: The resistance to deformation of a metallic aggregate consisting of a hard phase dispersed in a softer one is proportional to the logarithm of the mean straight path through the continuous phase. The rule works for a comparison of the properties of pearlite and spheroidite, as well as for pearlite alone over a wide range of spacings, and extrapolates to reasonable particle sizes for the finest spheroidites (tempered martensite). A simple explanation of the semilogarithmic character of the relationship is advanced.

Effect of hardening friction treatment with hard-alloy indenter on microstructure, mechanical properties, and deformation and fracture features of constructional steel under static and cyclic tension

Abstract: It has been studied how a hardening friction treatment with a sliding hard-alloy indenter influences the chemical composition and roughness of the surface, the structure, the distribution of the microhardness and the density of dislocations in depth of the surface layer, the mechanical properties upon static tension and the features of deformation and fracture upon cyclic tension of the annealed low-carbon (0.17 wt.% C) steel grade 20. It has been found that friction hardening of the steel (an increase in the microhardness up to 4.25 GPa) is due to a considerable dispersion of the ferritic base (with the formation of alpha-phase fragments no less than 100 nm in size) and pearlite colonies (crushing and partial dissolution of cementite plates) under the action of a severe friction deformation; hardening is not connected with the carryover of separate particles of the hard alloy to the steel surface. A finite-element model describing the process of the friction treatment with a sliding cylindrical indenter has been constructed. This model was used to determine how the number of the indenter strokes, the friction coefficient, and the shear component of the deformation affect the value of the accumulated deformation. It has been shown that the friction treatment improves the strength characteristics of the steel upon static tension. The hardened surface layer is susceptible to further considerable hardening (an increase in the microhardness up to 5.6–5.9 GPa) at the initial stages of the cyclic deformation and, therefore, has some margin of plasticity. The plastic flow in the surface-hardened steel under cyclic loading is due to the formation of numerous bands of localized deformation. It has been found that the friction-hardened surface layer is more prone to cracking under cyclic loading. The methods by which cracking of the hardened layer can be diminished have been discussed.

Structure-properties relationship of ultra-fine grained V-microalloyed dual phase steels

Abstract: The effect of vanadium microalloying on ultra-high strength dual phase (DP) ferrite-martensite steel microstructure and properties was studied. It was found that the addition of 0.14 wt% V to a Fe-0.18C-1.5Mn-0.3Si-0.008N reference alloy introduced very significant ferrite grain size refinement in the cold rolled and annealed state. During continuous annealing the initial ferrite to austenite transformation kinetics were strongly retarded, however under slow cooling both pearlite and bainite transformations were suppressed indicating increased hardenability. After cold rolling and intercritical annealing at 750 ⁰C intense V(C,N) precipitates (mean radius 3.7 nm) were observed in the ferrite phase whereas precipitates were scarce in martensite (austenite) and much larger (mean radius 6.7 nm). Significant gains in YS, UTS and work hardening rate were observed at low martensite fractions due to a combination of selective precipitation strengthening and grain refinement of ferrite. However, at higher martensite fractions (> 45%) the YS, UTS and work hardening rate became lower than the reference, primarily due to softening of the martensite. The latter was attributed to the fixing of solute carbon by V(C,N). The net increase in tensile strength with martensite content of the vanadium alloy was ~ 4 MPa/%α’ compared to ~ 16 MPa/%α’ for the reference alloy. A recently developed size-sensitive mean field structure-properties model was extended to capture these microalloying effects. At iso-tensile strength both the fracture strain and hole expansion behaviour of the new microalloyed steel showed improved performance over the reference.