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.Keywords Microstructure Á Tensile properties Á Mechanical properties Á Pearlite Á Bainite This paper is the second of a series dealing with the quantitative correlation of the microstructures observed in metals and alloys with the mechanical properties of these structures. It deals with the mechanical properties of structures consisting of a hard phase dispersed in a soft one, and particularly with those aggregate structures in steel known as pearlite, spheroidite, and bainite. The structure of pearlite is lamellar; the particles in spheroidite are approximately spheroidal, while the fine structure of bainite is still uncertain. Work on the specific effect of various alloying elements in solution in ferrite is in progress, and work on the effect of grain size on the properties of ferrite will be reported in the near future. It is hoped that these studies will help in the development of a theory of the strength of metals and alloys, and perhaps be of use in their practical utilization.Two y...

Size Effects of High Strength Steel Wires

“…Since the introduction of TEM in the materials research, earlier microscopic studies have often been overlooked. Of interest for the present discussion are two papers by Gensamer et al [72,73]. They used optical microscopy (presumably with photographic enlargements) and determined pearlite spacings varied by using different isothermal phase transformation temperatures.…”

“…This spacing data set was corrected by a factor of 0.5 because of their use of the random intercept method [74]. Correlations with the mechanical parameters were obtained in terms of the logarithm of pearlite spacing [73]. Their tensile strength data (in red points) is plotted against the pearlite spacing, the inverse pearlite spacing, and the inverse square-root of pearlite spacing in Figure 7a-c.…”

“…Comparable measurements using TEM or atom probe are practically impossible considering time and cost. Tensile strength versus length parameters using data from [3,29,30,32,34,40,49,50,73,75]. (a) Pearlite spacings, in log-log scales.…”

Some Aspects of Estimation Accuracy of Mean True Interllamelar Spacing

“…Gensamer [15] and next the others (DeHoff [4], Underwood [5], Sa³tykow [16]) determine the mean random spacing r l as follows:…”

Effects of Vanadium Addition on Microstructure and Tribological Performance of Bainite Hardfacing Coatings

“…Generally, the hardfacing consumables are divided into three main groups : pearlite consumable, austenite consumable and martensite consumable. When the pearlite hardfacing consumable is used to remanufacture failed rail, both the strength and toughness of the hardfacing coating couldn't fully meet the requirement on mechanical properties of high speed rail [7]. When the failed rail was remanufactured using the austenite hardfacing consumables, work hardening will occur on the surface of the hardfacing coating after being used for a period of time.…”