Martensites formation

Ferritic Nitrocarburizing. This process is similar to carbonitriding, except that it is carried out in the temperature range of the stabiHty of ferrite and carbide (<723° C). Therefore hardening is not by martensite formation, but because of the formation of very hard carbonitrides. 

Characteristics and implementation of the treatments depend on the expected results and on the properties of the material considered a variety of processes are employed. In ferrous alloys, in steels, a eutectoid transformation plays a prominent role, and aspects described by time-temperature-transformation diagrams and martensite formation are of relevant interest. See a short presentation of these points in 5.10.4.5. Titanium alloys are an example of the formation of structures in which two phases may be present in comparable quantities. A few remarks about a and (3 Ti alloys and the relevant heat treatments have been made in 5.6.4.1.1. More generally, for the various metals, the existence of different crystal forms, their transformation temperatures, and the extension of solid-solution ranges with other metals are preliminary points in the definition of convenient heat treatments and of their effects. In the evaluation and planning of the treatments, due consideration must be given to the heating and/or cooling rate and to the diffusion processes (in pure metals and in alloys). 

The remainder of the book treats discontinuous transformations. Nucleation, which is necessary for the production of a new phase, is treated in Chapter 19. The growth of new phases under diffusion- and interface-limited conditions is treated in Chapter 20. Concurrent nucleation and growth is treated in Chapter 21. Specific examples of discontinuous transformations are discussed in detail these include solidification (Chapter 22), precipitation from solid solution (Chapter 23), and martensite formation (Chapter 24). 

A martensitic transformation occurs over a temperature range. The temperature at which the martensite first starts to form on cooling is called the Ms temperature. More martensite will form only if the temperature is lowered. The temperature at which the reaction is complete is called the Mf temperature. However, the concept of an Mf temperature may be more of a convenience than a reality because often there is no sharp completion of martensite formation. 

Equation 9 quantifies the effects of dopant concentrations on the temperature required for the onset of martensite formation, Mg. The greatest effects are seen for the austenite-forming elements of C, Mn, and Ni where even small concentrations result in a sharp decrease in Mg. Whereas pure y-iron may be converted to martensite at temperatures in excess of 500°C, hypereutectoid steel is not transformed to martensite until a temperature of ca. 160°C is reached during quenching. At carbon concentrations above 0.7%, martensite is still being formed at temperatures well below 0°C. Hence, high-C steels must be quenched in low-temperature media e.g., dry ice/acetone, liquid nitrogen) to ensure full conversion of austenite to martensite. 

The shape-memory effect is observed when the temperature of a piece of alloy is cooled to below that required to form the martensite phase Ms (initial martensite formation) until Mf (martensite formation complete), as seen in Figure 3.25. Upon heating the martensitic material, a reformation of austenite begins to occur at Hg... 

Coating of soft substrates, cooling after coating sufficient for martensite formation. 

Hull F. C., Delta ferrite and martensite formation in stainless steels. Weld. J. Res. Suppl., 52 (1973) 193s-203s... 

Although low values of CE are specified to prevent martensite formation in welds [31], these regions are often still harder than the surrounding pipeline base metal. As documented in Section 3.4.1, higher hardness (i.e. higher yield strength) makes steel welds more susceptible to hydrogen embrittlement. 

The thermodynamic analysis of martensite formation was performed in [2000Kun]. 

Met] Metauer, G., Schissle, J.-M., Study of Mechanism of Martensite Formation in Certain Iron-Carbon-Silicon and Iron-Carbon-Nickel Alloys (in French), Mem. Scient Rev. Metall, 71(5), 295-306 (1974) (Experimental, Crys. Stmcture, Morphology, 13)... 

Since martensite formation is used as a main hardening mechanism in steels, the hardenabUity is a main concern of alloy design and consequence of alloy composition. The lower the rate of formation of the... 

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