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The heat treatment of plain-carbon steels
Published in William Bolton, R.A. Higgins, Materials for Engineers and Technicians, 2020
Clearly, things do not happen in this way when we quench a steel. Austenite, which is the phase present in a steel above its upper critical temperature, is a soft malleable material – which is why steel is generally shaped by hot-working processes. Yet when we quench austenite, instead of trapping the soft malleable structure, a very hard, brittle structure is produced, which is most unlike austenite. Under the microscope, this structure appears as a mass of uniform needle-shaped crystals and is known as martensite (Figure 12.1 A). Even at very high magnifications, no pearlite can be seen, so we must conclude that all of the cementite (which is one of the components of pearlite) is still dissolved in this martensitic structure. So far, this is what we would expect. However, investigations using X-ray methods tell us that, although the rapid cooling has prevented the formation of pearlite, it has not arrested the polymorphic change from face-centred cubic to body-centred cubic.
Machining of DTC Materials (Stainless Steels and Super Alloys) by Traditional and Non-Traditional Methods
Published in Helmi Youssef, Hassan El-Hofy, Non-Traditional and Advanced Machining Technologies, 2020
Austenitic alloys are costly; thus, they should not be specified where less expensive ferritic or martensitic alloys would be adequate. Austenitic alloys of standard series 300 may cost twice as much as the ferrite variety due to their expensive alloying elements (Ni and Cr). Mn group (series 200) is of lower cost, but is a somewhat lower quality alloy. Austenitic alloys are used in a wide variety of applications, such as kitchenware, fittings, welded constructions, lightweight transportation equipment, furnace and heat exchanger parts, and equipment for severe chemical environments. The austenitic 304 is the most commonly used SS in the world. The excellent forming and welding characteristics make it the standard steel for many applications, architecture, and transportation. The grade 316 is the second most commonly used austenitic SS. Like 304, it has excellent forming characteristics, but the added Mo gives 316 an improved corrosion resistance, so it is usually regarded as “marine steel grade” (Youssef, 2016). Table 5.2 summarizes typical compositions of the basic alloys of SSs. The microstructure that a stainless steel attains depends primarily on its composition, in which the main alloy components Cr and Ni are most important.
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
The carbon steel contains a mixture of ferrite and pearlite, only pearlite, or pearlite and cementite at the room temperature depending on the carbon content of the steel. When the steel is heated into the austenite range, the formation of austenite occurs. For example, for hypoeutectoid steels, the formation of austenite starts when the temperature of the steel crosses the lower critical temperature, AC1. Similarly, austenite starts to form just at the eutectoid temperature, i.e., 723°C for eutectoid steels. The transformation begins with the nucleation of austenite at the interfaces between the ferrite and cementite of pearlite, as shown in site 1 of Figure 8.1.
An experimental study on a self-centering damper based on shape-memory alloy wires
Published in Mechanics Based Design of Structures and Machines, 2023
Afsaneh Falahian, Payam Asadi, Hossein Tajmir Riahi, Mahmoud Kadkhodaei
The effect of amplitude and strain rate of cyclic loading was investigated on the SMA wires' temperature. Austenite is a stable phase at high temperatures and low stress; whereas, martensite is stable at lower temperatures and higher stress. The transformation of the austenite to martensite phase begins at a point with higher stress (Morais et al. 2017). Initially, the strain rate was 0.015 s−1, while the amplitude of the cyclic loadings rose incrementally. Figure 15 exhibits the location of nucleation, forward movement of temperature, and temperature distribution by using a thermal camera (Testo camera). The increase in the temperature of a point of the wire can indicate the nucleation of the new martensite phase since initially the temperature of all areas is uniform. Nucleation is the beginning of the transformation in the potential points and starts from the points that were pressed by the nuts, due to the stress concentration in them. As the phase transformation develops, the high-temperature front moves along the wires, until all parts of the wires transform from austenite to martensite phase. During the cyclic loadings, the SMA wires’ temperature increases from 21.6 to 33.1 °C.
Crack initiation and early propagation in case hardened sintered PM steels under cyclic load
Published in Powder Metallurgy, 2023
Anders Holmberg, Urban Wiklund, Per Isaksson, Åsa Kassman Rudolphi
Machine components of PM steels, such as gears, are almost always hardened in some way. The most common is case hardening, which provides the surface with a hard, wear-resistant martensitic layer while keeping the ductility and toughness of the interior. The case hardening procedure involves surface heating, resulting in phase transformations from ferrite and pearlite to austenite. As the material is then rapidly cooled, the austenite is transformed into martensite. The relevant structure to relate to after case hardening is thus prior austenite grains rather than prior particle grains. For sintered steel, the pores, if they are large enough, will pin down grain boundaries and retard austenitic grain growth. Therefore the initial particle size practically restricts the maximum size of the austenite grains [10,11], meaning that the size of a prior austenite grain depends on the size of the prior particle grain size.
Free vibration of pseudoelastic NiTi wire: finite element modeling and numerical design
Published in Mechanics of Advanced Materials and Structures, 2022
Liangdi Wang, Jun Wang, Yingjie Xu, Jihong Zhu, Weihong Zhang
Pseudoelasticity, sometimes referred to as superelasticity, is manifested by the large inelastic but recoverable deformation in NiTi alloys [9]. It is derived from the stress-induced phase transformation between martensite and austenite. Figure 1 shows a typical pseudoelastic stress-strain response of NiTi alloy. When the material is loaded at the temperature above Af, austenite transforms to martensite along the applied stress in three stages: elasticity of austenite (a b), forward phase transformation (b c), and elasticity of martensite (c d). Upon unloading, martensite transforms back to austenite in a similar way, elastic unloading of martensite (d e), reverse phase transformation (e f), and elastic unloading of austenite (f g). The two most remarkable features in pseudoelasticity are the large nonlinear recoverable strain of more than 8% and the significant hysteresis loop between the forward and reverse phase transformation.