Austenite start temperature

The formation of a heavily twinned material on cooling can be reversed by an increase in temperature, which causes the material to transform to the untwinned pre-martensite state. The transformation starts at a temperature, usually called As, the austenite start temperature, and is complete at a temperature Af, the austenite finish temperature (Figure 8.16). (These terms are related to the fact that the best-known martensitic transformation is that of austenite to martensite, in steels.) For the alloy NiTi, As is 71 °C, and Af is 77 °C. It is seen that Ms and Mf differ from As and Af. This is a hysteresis phenomenon, commonly found in solid-state transformations. 

A NiTiNOL shape memory metal alloy can exist in two different temperature-dependent crystal structures or phases called martensite (i.e., lower-temperature phase) and austenite (i.e., higher-temperature or parent phase). Several properties of the austenite and martensite phases are notably different. When martensite is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called the austenite start temperature A). The temperature at which the phenomenon is complete is called the austenite finish temperature (A). When austenite is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called the martensite start temperature (M ). The temperature at which martensite is again completely reverted is called the martensite finish temperature (Mj). Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTiNOL can have three different forms ... 

Forward and reverse transformation occur at different temperatures, resulting in a hysteresis as can be seen in Fig. 6.50. The start and end of the transformation from martensite to austenite are given by As (austenite start temperature) and At (austenite finish temperature). The reverse transformation takes place in the temperature interval from Ms to Mt (martensite start and finish temperatures). The shape of the hysteresis curve in Fig. 6.50 strongly depends on the thermomechanical treatment of the shape memory alloy (see also Sect. 6.4.1). 

To stress relieve or postweld heat treat carbon and low-alloy steels, they are typically heated to 1100°F to 1350°F (595°C to 730°C) for extended time, followed by air coohng. The minimum time is specified by the relevant engineering code, and the temperature must be less than the lower transformation temperature of the steel, which is the lowest temperature at which austenite starts to form, for example, 1333°F (720°C) for plain carbon steels. In order to avoid degrading the required mechanical properties of a heat treated alloy, subsequent fabrication heat treatment temperatures, such as those for stress relief and PWHT, must not exceed the tempering temperature (discussed in the next section). 

The temperature at which the transformation to martensite takes place is found to be composition-dependent. Martensite starts to form when the temperature reaches about 700 °C for the lowest-carbon-content steels but not until a temperature of about 200 °C for austenite with a carbon content of 1.2 wt%. The temperature at which the martensite starts to form is usually labelled Mg, the martensite start temperature, and the temperature at which the transformation is complete is labelled M(. the martensite hnish temperature. 

Notes R, As and Ms are starting temperatures for transformations to R-phase, austenite and martensite, respectively. AH is for the overall transformation between martensite and austenite. 

Martensite is formed when steel with a carbon content above 0.2 % is rapidly cooled from the austenite temperature range to a temperature below the martensite starting temperature. Due to the prompt cooling, the carbon dissolved in austenite is forced to remain dissolved in the mixed crystal. Martensite has a fine-acicular, very hard, and brittle microstmcture which causes increased abrasive wear and high mechanical and thermal stresses during machining. 

Fig. 3.1-103 Martensitic transformation temperatures of Fe-rich Fe—Ni alloys. The reverse transformation is characterized by the Ag (austenite start) and Af (austenite finish) temperatures [1.82]...

A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures below the pearlite, but above the martensite start temperature. 

When a steel is cooled sufficiendy rapidly from the austenite region to a low (eg, 25°C) temperature, the austenite decomposes into a nonequilihrium phase not shown on the phase diagram. This phase, called martensite, is body-centered tetragonal. It is the hardest form of steel, and its formation is critical in hardening. To form martensite, the austenite must be cooled sufficiently rapidly to prevent the austenite from first decomposing to the softer stmeture of a mixture of ferrite and carbide. Martensite begins to form upon reaching a temperature called the martensite start, Af, and is completed at a lower temperature, the martensite finish, Mj, These temperatures depend on the carbon and alloy content of the particular steel. 

Fig. 1. Schematic of the hysteresis loop associated with a shape-memory alloy transformation, where M. and Afp correspond to the martensite start and finish temperatures, respectively, and and correspond to the start and finish of the reverse transformation of martensite, respectively. The physical property can be volume, length, electrical resistance, etc. On cooling the body-centered cubic (bcc) austenite (parent) transforms to an ordered B2 or E)02...

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

The Ms temperature, at which the diffusion-less martensitic transformation starts, depends on the alloy considered (its composition, etc.) it can be above or below room temperature. For the so-called austenitic steels Ms < < ambient temperature, whereas Ms > ambient temperature for the martensitic steels. 

When specified, a removable steam heating element external to the oil reservoir or a thermostatically controlled electric immersion heater with a sheath of austenitic stainless steel shall be provided for heating the charge capacity of oil before start-up in cold weather. The heating device shall have sufficient capacity to heat the oil in the reservoir from the specified minimum site ambient temperature to the manufacturer s required start-up temperature within 12 hours. If an electric immersion heater is used, the watt density shall not exceed 2.33 watts per sq. cm (15 watts per sq. in.). 

Isothermal Transformation Diagram. To separate the effects of transformation temperature from those of heat flow, it is essential to understand the nature of the transformation of austenite at a given, preselected temperature below the A. Information needed includes the starting time, the amount transformed as a function of time, and the time for complete transformation. A convenient way to accomplish this is to form austenite in specimens so thin (usually about 1-mm thick) that heat flow is not an issue, rapidly transfer the specimens to a Hquid bath at the desired temperature, and foUow the transformation with time. The experiment is repeated at several other transformation temperatures. On the same specimens, the microstmcture and properties of the transformation products can be assessed. These data can be summarized on a single graph of transformation temperature versus time known as an isothermal transformation (IT) diagram or, more usually, a time—temperature—transformation (ITT) diagram. A log scale is used for... 

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