Pearlite

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

Influence of the Initial Microstructure on the Spheroidization of SAE 52100 Bearing Steel

Abstract: High carbon Cr-bearing SAE 52100 is widely used for bearing production. The in-service microstructure of bearing steel is tempered high C martensite, which has superior mechanical properties in severe rolling fatigue conditions. The bearings are produced from steel wire with a pearlitic microstructure. Spheroidization annealing is used to obtain a soft microstructure prior to cold deformation. Spheroidization results in a ferritic matrix with coarse globular cementite. The spheroidization annealing process is lengthy, involving considerable economic cost and an environmental impact related to CO2 emissions. Avoiding spheroidization annealing altogether or substantially reducing the annealing time is, therefore, of industrial relevance. Spheroidization annealing may be omitted if a very soft, i.e., coarse pearlitic microstructure, can be obtained after hot rolling. This is generally considered to be the case for a coarse pearlitic microstructure with a hardness of approximately 270 HV. In practice, the requirement for a guaranteed homogeneous low hardness results in the spheroidization annealing being often still necessary in many circumstances. The present work reports on a study of the potential for the reduction of the spheroidization annealing time by spheroidization of different initial microstructures. The two main findings are (i) that the martensitic and bainitic microstructures spheroidize more rapidly than pearlite and (ii) that an initial martensitic microstructure can achieve a hardness lower than 240 HV within 10 h of spheroidization at 710 °C.

Effect of prior-austenite grain size and transformation temperature on nodule size of microalloyed hypereutectoid steels

Abstract: The effect of prior-austenite grain size and transformation temperature on nodule size and colony size of hypereutectoid steels containing 1 pct carbon with different levels of vanadium and silicon was investigated. Specimens of the various steels were thermally processed at various temperatures ranging from 900 °C to 1200 °C and transferred to salt bath conditions at 550 °C, 580 °C, and 620 °C to examine the structural evolution of pearlite. The heat-treatment work showed that for only the hypereutectoid steel without vanadium there was a continuous grain boundary cementite network, the thickness of which increased with increasing reheat temperature. Analysis of the thermally processed hypereutectoid steels also indicated that the prior-austenite grain size and transformation temperature controlled the nodule size, while the colony size was dependent on the latter only.

Effects of Rare Earth Element on Isothermal and Martensitic Transformations in Low Carbon Steels

Abstract: Rare earth elements (RE) may segregate at the grain boundaries of austenite, lead to form carbide and refine the austenite grain. In case of no change of grain size and carbon content of austenite, an addition of RE is beneficial to the hardenability of steels. In case of a marked refinement of austenite grain, addition of RE will deteriorate the hardenability. The incubation period of the proeutectoid ferrite can be expressed as a function of grain boundary energy, grain size, activation energy for growth and the driving force for transformation and the calculated results are in good agreement with the experimental data. RE may retard the isothermal pearlitic transformation, because RE diminishes the diffusion coefficient of carbon as well as tends to segregate at Fe3C/α interface, showing a pinning effect on the transformation. RE reduces the lamella spacing of the pearlite owning to lowering the interfacial energy, e.g., from 0.7 to 0.53 J/m2 in 0.27C–1Cr–RE steel. RE tends to segregate at ferrite/island interface in the granular bainite. In grain refined steel, at the earlier stage of bainite formation, the transformation rate is high while at later stage it becomes sluggish. The activation energies of pearlitic and bainitic transformations increase by the addition of RE. The segregation of RE at ferrite/island interface may act as a drag effect. A drag factor α is expressed as a function of transformation fraction and calculated in a 0.27–1Cr–RE steel. RE segregates at the grain boundary of austenite and this kind of distribution will not be changed during the martensitic transformation. It is reasonable to predict that RE will lower the martensite/austenite interface energy, resulting in the formation of a finer lath structure. RE lowers Ms, decreases the amount of the retained austenite and retards the autotempering process. It is emphasized that the amount of the retained austenite, γ, in quenched low carbon steel depends on not only the Ms and the temperature of quenching medium, Tq, but also the influence of alloying elements on the carbon diffusion during quenching. A general equation modified from the Magee's equation is derived as γ=exp{α(C1–C1)-β(Ms–Tq)} where C0 and C1 are carbon concentrations in austenite before and after quenching respectively, α and β are constants. RE decreases Ms but also lowers C1 so as to reduce the amount of the retained austenite.

High-stress abrasive wear behaviour of a zinc-based alloy and its composite compared with a cast iron under varying track radius and load conditions

Abstract: An attempt has been made in this investigation to study the high-stress abrasive wear behaviour of a zinc-based alloy composite containing SiC particle reinforcement. The influence of the reinforcement phase on wear behaviour was investigated by characterizing the zinc-based matrix alloy under identical test conditions. A cast iron was also investigated on similar lines for comparison purposes. The effects of track radius, test duration and applied load on the wear response of the samples were studied. The wear properties studied were wear rate, friction coefficient and frictional heating. The zinc-based matrix alloy comprised of primary α dendrites, eutectoid α + η and e phase. The composite revealed presence of SiC particles in addition to the features of the matrix alloy. The cast iron showed graphite in a matrix of pearlite and ferrite. The zinc-based matrix alloy attained maximum wear rate and frictional heating, while those for the composite were the least; the response of the cast iron was intermediate between the two in general. The trend observed by friction coefficient was somewhat different in the sense that it was the least for the cast iron while the maximum was still noted for the zinc-based matrix alloy, the composite revealing intermediate response. The wear rate and frictional heating increased with test duration while the friction coefficient was affected in an opposite manner. Increasing load and track radius caused the wear rate, frictional heating and friction coefficient to increase. The wear behaviour of the samples has been substantiated through the characteristics of wear surfaces, debris particles and the abrasive medium.