Martensitic phases, carbides

Carbon plays an important role in steels, both in the form of solid solution and as component of carbide phases. The cubic face centered modification of iron (y-Fe) dissolves as much as 8at-% (2weight-%) carbon, which randomly occupies the octahedral voids of the cubic close packed iron atoms. This cubic phase is called austenite. On quenching, this phase distorts by a displacive ( martensitic ) phase transition to the corresponding tetragonal structure (martensite). The degree of the distortion is proportional to the carbon content. For a carbon content of zero, the distortion extrapolates to zero, that is, the lattice constants of the (tetragonal) martensite extrapolate to those of pure y-Fe [76,77). 

Mur] Light microscopy, magnetic analysis cooling from 1200, 1100, and 1000°C up to 28 mass% Cr and up to 1.6 mass% C, a and y phases, martensite, double carbides... 

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. 

Hardenable stainless steels usually contain up to 0.6% carbon. This is added in order to change the Fe-Cr phase diagram. As Fig. 12.7 shows, carbon expands the y field so that an alloy of Fe-15% Cr, 0.6% C lies inside the y field at 1000°C. This steel can be quenched to give martensite and the martensite can be tempered to give a fine dispersion of alloy carbides. 

F, Ferrite, C, Carbides, A, Austenite M, Martensite TS, as tempered IP, Intermetallic phases PH, Precipation hardened H T, Heat treated and tempered, H, Heat-treated. 

The crystallography of the f.c.c.— b.c.t. martensitic transformation in the Fe-Ni-C system (with 22 wt. %Ni and 0.8 wt. %C) has been described in Section 24.2. In this system, the high-temperature f.c.c. solid-solution parent phase transforms upon cooling to a b.c.t. martensite rather than a b.c.c. martensite as in the Fe-Ni system. Furthermore, this transformation is achieved only if the f.c.c. parent phase is rapidly quenched. The difference in behavior is due to the presence of the carbon in the Fe-Ni-C alloy. In the Fe-Ni alloy, the b.c.c. martensite that forms as the temperature is lowered is the equilibrium state of the system. However, in the Fe-Ni-C alloy, the equilibrium state of the system in the low-temperature range is a two-phase mixture of a b.c.c. Fe-Ni-C solid solution and a C-rich carbide phase.5 This difference in behavior is due to a much lower solubility of C in the low-temperature b.c.c. Fe-Ni-C phase than in the high-temperature f.c.c. Fe-Ni-C phase. If the high-temperature... 

In most systems the martensitic reaction is geometrically reversible. On heating, the martensite will start to form the higher temperature phase at the As temperature and the reaction will be complete at an Af temperature, as illustrated in Figure 11.19. Martensite in the iron-carbon system is an exception. On heating, the iron-carbon martensite decomposes into iron carbide and ferrite before the As temperature is reached. Martensite can be induced to form at temperatures somewhat above the Ms by deformation. The highest temperature at which this can occur is called the Md temperature. Likewise, the reverse transformation can be induced by deformation at the Ad temperature somewhat below the As. The temperature at which the two phases are thermodynamically in equilibrium must lie between the Ad and Md temperatures. 

The cause for the martensitic-type transition that occurs when RXH phases are hydrogenated to RXH2 is easily understood along the lines of arguments presented for the layered carbide halides. In the structure of RXH (x < 1) only tetrahedral voids between the metal atom layers are occupied by H atoms. Therefore, positions near the octahedral voids are electrostatically favorable for the X atoms. In RXH2 both the tetrahedral voids and the octahedral voids are occupied by H atoms. As the number of... 

In contrast to the pearlite structure, which is lamellar (Figure 2.15), tempered martensite contains the carbide particles as a spheroidal dispersed phase. While the tempered martensite is soft and tough, the parent martensite is hard and abrasion resistant. 

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