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Heat Treatment Defects and Their Determination
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
Quench cracks generally appear in zigzag form at the grain boundaries, and they may be external or internal as well as small or large. They appear as straight lines that run from the surface toward the center of the quenched specimen as shown in Figure 15.2. This may be formed due to the presence of stresses produced during the transformation of austenite to martensite. They often appear after a steel sample undergoes quenching. The differential cooling in the surface and core during martensitic transformation is accompanied by the volume mismatch, which results in the generation of compressive stresses and thus, causes cracking. Generation of quench crack is detrimental, as the steel with quench cracks cannot be used further and, therefore, has only scrap value.
Strengthening mechanisms
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
From the above list, the dominant mechanism is dislocation glide. Twinning is activated in metals, which do not exhibit multiple slip systems or if for some reason dislocation glide is hindered (e.g., at low temperatures). Martensitic transformation takes place in specific alloy systems and is accompanied by significant shear-like deformations. Diffusional flow is the deformation caused by the diffusional transfer of mass and contributes to the creep deformation of a metal, when loaded at high temperatures. As discussed above, dislocation glide is the dominant deformation mechanism in metals. Accordingly, impeding dislocation glide, constitutes the basis for the strengthening of metals and alloys. The most important obstacles to dislocation glide and the respective strengthening mechanisms are depicted in Table 8.5. The information in Table 8.5 is important for two reasons:
Shape Memory Materials
Published in D I Arun, P Chakravarthy, R Arockiakumar, B Santhosh, Shape Memory Materials, 2018
D I Arun, P Chakravarthy, R Arockiakumar, B Santhosh
The martensitic transformation is a shear-dominant, diffusionless transformation that occurs via the nucleation and growth of the martensitic phase from the austenitic phase. Referring to Figure 2.2, the austenite begins to change to martensite on cooling to the martensite start (Ms) temperature of the alloy. The change strictly finishes when it cools to beneath the martensite finish (Mf) temperature. On heating, a martensitic–austenitic phase change is obtained at the austenite start (As) temperature. When the alloy is heated to above the austenite finish (Af) temperature, the change will end, and beyond the Af temperature, it will be completely austenitic. Austenite and martensite exist together in the middle of the Af and Mf temperatures. There is often hysteresis between the As–Af and Ms–Mf transformation regions, as can be seen on the temperature axis in Figure 2.2.
Stress-induced martensitic transformation in metastable austenite grains during nanoindentation investigation
Published in Philosophical Magazine Letters, 2021
Martensitic transformation is a first-order solid-state phase transformation in steels [1–4], resulting in the formation of hard martensite to strengthen the advanced high-strength steel (AHSS) [5]. The martensitic transformation could take place during the deformation process [6,7], enabling the transformation-induced plasticity (TRIP) effect to improve the work hardening behaviour of AHSS [8,9]. In general, the deformation-induced martensitic transformation can be classified into stress- and strain-induced martensitic transformation [6,7,10]. The occurrence of martensitic transformation prior to the initiation of dislocations in austenite during deformation is termed as the stress-induced martensitic transformation [7,11]. In contrast, the formation of martensite after the plastic deformation of austenite is defined as the strain-induced martensitic transformation [7]. It is clarified by Olson and Cohen that the main difference between the stress and strain-induced martensitic transformations is on the nucleation site of martensite [12]. The stress-induced martensitic transformation takes place on the nucleation site that is the same as the one for quenching-induced martensite while the strain-induced martensitic transformation occurs on the nucleation site created during the plastic deformation, such as the slip band intersection and slip bands [6,12].
Effects of ultrasonic vibration on SUS304 stainless steel subjected to uniaxial plastic deformation
Published in Journal of the Chinese Institute of Engineers, 2018
Chun-yuan Chen, Valentino Anok Melo Cristino, Chinghua Hung
SUS304 stainless steel is a metastable austenitic stainless steel that presents excellent mechanical properties accompanied by corrosion resistance, and has been widely used in various products in the automotive, chemical and food industries, just to name a few. The material has face-centered cubic (FCC) austenite as its primary phase, and it undergoes a deformation-induced martensitic transformation during its plastic deformation behavior (Mcguire 2008). The phenomenon of deformation-induced martensitic transformation depends on the chemical composition (Andrade et al. 2004), temperature (Müller-Bollenhagen, Zimmermann, and Christ 2010), strain rate, and microstructure of the material. Previous researches indicated that cold plastic deformations of austenitic stainless steel considerably increase the density of dislocations (Cigada et al. 1982), (Odnobokova, Belyakov, and Kaibyshev 2015). Iyer (1988) observed the effect of ultrasonic vibration on the transformation of austenite into martensite in 304L stainless steel, attributing the observed effects to the increase of dislocation density and point defects. However, the mechanism of deformation-induced martensitic transformation on the ultrasonic vibration-assisted forming of SUS304 stainless steel is still not fully understood.