Martensite and Bainite in Steels : Transformation Mechanism & Mechanical Properties

Abstract

Abstract:Many essential properties of iron alloys depend on what actually happens when one allotropic form gives way to another, i.e. on the mechanism of phase change. The dependence 05 the$ mechanical properties on the atomic mechanism by which bainite and martensite grow is the focus of this paper. The discussion is illustrated in the context of some common engineering design parameters, and with a brief example of the inverse problem in which the mechanism may be a function of the mechanical properties.

Loading-rate dependence of fracture absorption energy of low-carbon carbide-free bainitic steel

“…3). Now, it may be expected that more martensitic transformation occurring at lower loading rates favors improving fracture absorption energy due to the well-known TRIP effect [17,18]. However, the fact is that the samples failed at lower loading rates do not show increased fracture absorption energy.…”

Microstructures and Mechanical Properties of Austempering SUS440 Steel Thin Plates

“…The tensile properties of the 300˝C specimen are significantly better than those of the other specimens. The main reason for this is that the austempering temperature of 200˝C is too low for complete austempering and thus the matrix also contains martensite, which increases the brittleness of the material [5,17,18]. The 400˝C specimen has more retained austenite, which results in lower tensile mechanical properties.…”

“…In addition, the salt-bath specimen at 300 °C, became coarser was the austempering duration increased (Figure 4). A fully bainite matrix with finer primary carbides can be achieved with a longer austempering time [5,[12][13][14]. However, if the tempering time was insufficient, then no feathery structure was seen in the matrix after etching.…”

“…However, if this process is carried out improperly, then this will adversely affect the mechanical properties of the resulting materials [3,4]. This is because the quenching rate and temper-brittleness, and especially quench-tempering, can lead to reliability problems with regard to thin plates [5,6]. In the current study, SUS440 plate is made into a double-loop type thin plate specimen by a punch-shear process, in order to highlight the stress concentration and study the effects on brittleness.…”

Correlation of the Effects of Alloying Elements on the Hardenability of Steels to the Diffusion Coefficients of Elements in Fe

“…Martensite is then usually understood to form by a diffusionless process [24,25]. The orders of magnitude of the diffusion coefficients of the elements discussed are speculated to be valid in this metastable γ-phase.…”