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Phase Diagrams
Published in Yip-Wah Chung, Monica Kapoor, Introduction to Materials Science and Engineering, 2022
What will happen if we quench the sample from slightly above 727°C to between 300°C and 540°C? The transformation of the fcc (austenite) to bcc (ferrite) phase will occur, requiring only small atomic displacements on the order of 0.1 nm. This is known as displacive or diffusionless transformation. The result is an assembly of irregular parallel plates of ferrite, with typical dimensions of about 10 μm in length and 0.2 μm in thickness. The amount of carbon trapped in ferrite is significantly higher than the solubility limit (0.02 w/o). Temperatures between 300°C and 540°C are high enough that carbon diffusion out of the ferrite plates subsequently occurs, followed by the formation of cementite adjacent to the ferrite plates, as shown in Figure 5.14. This microstructure is known as upper bainite. If the quenching is from above 727°C to between 200°C and 300°C, the resulting microstructure is known as lower bainite, which still consists of irregular ferrite plates along with the precipitation of cementite adjacent to the ferrite plates (Figure 5.14). However, there are two major differences from upper bainite. One is that the microstructural features will be finer. Another is the precipitation of cementite within the ferrite plates, due to slow carbon diffusion at these lower temperatures. These finer microstructures of lower bainite make it a stronger and tougher alloy.
Phase Transformation in Steels
Published in Bankim Chandra Ray, Rajesh Kumar Prusty, Deepak Nayak, Phase Transformations and Heat Treatments of Steels, 2020
Bankim Chandra Ray, Rajesh Kumar Prusty, Deepak Nayak
Martensitic transformation is a diffusionless transformation, as martensite has the same composition as the parent austenite. Carbon remains in dissolved solid solution state in martensite, even if the other alloying elements are present.
Phase Transformations and Kinetics
Published in Zainul Huda, Metallurgy for Physicists and Engineers, 2020
In general, phase transformations are divided into three classifications: (a) simple diffusion-dependent transformation (e.g. melting, solidification of a pure metal, allotropic transformation, recrystallizations, and grain growth) (see Chapter 14); (b) diffusion-dependent transformation involving change in phase composition and/or number of phases present (e.g. eutectoid reaction); and (c) diffusionless transformation (e.g. martensitic transformation).
A novel analytical approach to ascertain hardenability of plain carbon steel
Published in Philosophical Magazine, 2023
Putting the value of F(X) from Equation (7) and with further simplification we finally get: Equation (16) provides a direct relationship that predicts quantitative microstructure evolution (in terms of X) with time-temperature paradigm of phase transformation. As tN(CCT) for different diameter (D) is known from Equation (15), ignoring retained austenite, the volume fraction of diffusional transformation product (X) or the volume fraction of diffusionless transformation product (1−X) can readily be correlated to D. Conceptually, as Equation (16) suggests, more cooling time (tN(CCT)) required to reach CCT nose temperature is eventually responsible for greater extent of diffusional transformation (represented by X) at the centre of the rod.
Effect of stress on the thermal hysteresis of martensitic transformations - A continuum based particle dynamics model
Published in Mechanics of Advanced Materials and Structures, 2022
Since the shape memory alloys undergo a diffusionless transformation between austenite to martensite, there is no rearrangement of particles. We take the square austenite phase as a reference configuration for which the corresponding Lagrangian strain is zero (). The martensite phase is obtained by stretching the reference square configuration along the two axis by α and β. The corresponding Lagrangian strains are where are the Bain stretch tensors. and represent the strains corresponding to the martensite variants and are explicitly given as
Numerical investigation of mechanical properties in spur gears during quenching process
Published in International Journal of Modelling and Simulation, 2023
Ali Koohi Esfahani, Mahdi Babaei, Saeid Sarrami, Machel Morrison
A large number of studies have been conducted to model the cooling curves and predict the microstructural formation of the present phases and the hardness profile, while no attempts have been made to address the stress effect on the austenite to martensite diffusionless transformation. According to Magee’s rule [13], the austenite–martensite transformation depends on both temperature and stress state, compared to the transformation kinetics being only temperature-dependent as described in the Koistinen–Marburge equation [14]. Several studies been conducted to develop a fully simulated algorithm where heat equation with associated latent heat generating from phase transformation was numerically solved to estimate hardness value [14–16].