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Properties and applications of engineering materials
Published in Alan Darbyshire, Charles Gibson, Mechanical Engineering, 2023
Alan Darbyshire, Charles Gibson
Stainless steels can be classified as ‘austenitic’ (which accounts for 70% of the production), ‘ferritic’ and ‘martensitic’ depending on their crystal structure. The term ‘ferrite’ has already been mentioned. It refers to grains of BCC iron, containing a small amount of interstitial carbon. The term ‘austenite’ refers to FCC iron containing larger amounts of interstitial carbon. The term ‘martensite’ refers to a needle-like crystal structure that occurs when the material is quenched.
Material
Published in Subhash Reddy Gaddam, Design of Pressure Vessels, 2020
Martensite: It has a very hard needle-like structure of iron and carbon and is formed by rapid cooling of the austenitic structure (above the upper critical temperature), which needs to be modified by tempering before acceptable properties are reached.
Phase Transformations and Kinetics
Published in Zainul Huda, Metallurgy for Physicists and Engineers, 2020
Transformation Temperatures. The kinetics of the eutectoid reaction controls the arrangement of ferrite and cementite in the reaction product. It is evident in the TTT diagram that two types of micro-constituents are generated as a result of the austenite transformation. Above 550oC, pearlite is formed, whereas below the temperature, bainite is formed. It means that rapid cooling from austenitic temperature to a lower temperature (215–540oC), holding for a long enough time to complete the transformation followed by quenching to room temperature, results in bainite. If austenite is cooled at very high cooling rate (e.g. quenched in cold water) to a temperature in the vicinity of the ambient, the resulting microstructure is martensite (see Figure 7.10).
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.
Use of additive manufacturing for the fabrication of cellular and lattice materials: a review
Published in Materials and Manufacturing Processes, 2021
Esmeralda Uribe-Lam, Cecilia D. Treviño-Quintanilla, Enrique Cuan-Urquizo, Oscar Olvera-Silva
Martensite phase is one of the most common phases found in the microstructure of electron-beam melting parts due to the difference in cooling rates of the process. Hardness and strength are properties affected by the martensite formation. In their study, Murr and coworkers [118] fabricated titanium–aluminum–vanadium alloy (Ti–6Al–4 V) electron-beam melting open-cellular materials Fig. 5g with controlled relative density and porosities. The 3D complex structures were exposed to different cooling rates that promote the formation of martensite phase with refined and acicular Widmanstätten structure. Martensite microstructure improves the hardness of the cellular structure by 40% compared with bulk solid parts also made by electron-beam melting process.