Abstract
A low-alloy medium-carbon bainitic steel was isothermally tempered at 300 °C for up to 24 hours which led to a significant hardness decrease. In order to explain the decreasing hardness, extensive microstructural characterization using scanning and transmission electron microscopy, X-ray diffraction, and atom probe tomography was conducted. The experimental work was further supplemented by thermodynamic and kinetic simulations. It is found that the main underlying reason for the hardness reduction during tempering is related to dislocation annihilation, possibly also with corresponding changes in Cottrell atmospheres. On the other hand, cementite precipitate size, effective grain size of the bainite, and retained austenite fraction appear unchanged over the whole tempering cycle.
Highlights
BAINITIC steels are used in applications such as crankshafts, seat belt buckles, rails or bearings, all of which require high performance in terms of hardness and wear resistance combined with a good toughness
The isothermal transformation time to obtain a fully bainitic structure was determined to about 8 minutes using dilatometry
The dilatometry curve can be seen in Figure 3(a); it takes about 350 seconds to reach 95 pct transformation and 499 seconds to reach 99 pct transformation, evaluated as a ratio with respect to the maximum length change
Summary
BAINITIC steels are used in applications such as crankshafts, seat belt buckles, rails or bearings, all of which require high performance in terms of hardness and wear resistance combined with a good toughness. When choosing the optimal material for a certain component, geometrical constraints together with desired properties are determining factors. Alloying must be used to obtain the desired hardenability in order to avoid the formation of unwanted phases leading to poor mechanical properties or thermal cracking, but at the same time the cost should be minimized. The alloying content is kept to a minimum and the processing time is kept short to reduce cost. The bulk mechanical properties of a steel product can often be achieved by a single heat treatment; additional processing steps such as coating in order to Manuscript submitted May 5, 2020; accepted September 9, 2020.
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