Papers by Keyword: Low Temperature Bainite

Abstract: In the current study, a novel approach was employed to produce a unique combination of ultrafine ferrite grains and low temperature bainite in a low carbon steel with a high hardenability. The thermomechanical route included warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to bainitic transformation regime (i.e. 400-250°C). The resultant microstructure was ultrafine ferrite grains (i.e. <4μm) and very fine bainite consisting of bainitic ferrite laths with high dislocation density and retained austenite films. This microstructure offers a unique combination of ultimate tensile strength and elongation due to the presence of ductile fine ferrite grains and hard low temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy techniques.

Abstract: Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechanical characteristics together with near zero distortion during heat-treating. Such heat-treatments are often carried out at temperatures close to but above the martensite start temperature, where transformation kinetics are unfortunately very sluggish. The formation of a small amount of martensite prior to isothermal holding is known to accelerate transformation kinetics. However, the influence of the amount transformed remains to clarify, and the consequence on final properties have not been explored. In the following, the influence of partial martensite formation on the kinetics of bainite formation is investigated in 100Cr6 (AISI 52100) and similar grades. It is shown that, although the formation of martensite allows for shorter heat-treatment durations, it does not seem to affect the bainite kinetics, inasmuch as the bainite formation kinetics after formation of X% martensite are identical to that which would be expected if the first X% transformation had been obtained through isothermal bainitic transformation.

Abstract: Rolling contact fatigue (RCF) properties of low-temperature bainitic microstructure in the surface layer of a low-carbon steel, obtained by means of carburization and successive lowtemperature austempering were studied. Comparisons were made with those properties of a martensite steel obtained by quenching and tempering. The results showed that the low-temperature bainitic steel exhibits an excellent RCF resistance and reaches a RCF life more than twice that of the martensite steel, presumably as a consequence of the very thin bainite plates (~80 nm in thickness) and the fine-scale dispersion of austenite between the plates. The apparently increased hardness in the surface layer may partly be the cause of the enhanced RCF resistance.

Abstract: The mechanical properties of a bainitic microstructure with slender ferrite plates (20-65 nm in thickness) in a matrix of carbon-enriched retained austenite were characterized. The microstructure is generated by isothermal transformation at temperatures in the range 200-300°C. A yield strength as high as 1.5 GPa and an ultimate tensile strength between 1.77 to 2.2 GPa was achieved, depending on the transformation temperature. Furthermore, the high strength is frequently accompanied by ductility (£ 30%) and respectable levels of fracture toughness (< 45 MPa m0.5). This unusual combination of properties is attributed to the exceptionally fine scale of the carbidefree bainitic microstructure and the associated retained austenite.