Mechanical Properties of Cementite

Abstract

This review focuses on the mechanical properties of single-phase cementite. The mechanical properties of interest are 1) sound velocity, 2) elastic constants, 3) hardness, 4) plastic deformation mechanism, 5) wear, 6) fracture toughness, and 7) crystal orientation anisotropy. The effects of temperature, magnetic transition, and alloying elements on the sound velocity, elastic constants, and hardness were reviewed. Furthermore, experimental values of the above mechanical properties as well as other parameters, such as the specimen shape, the quantity of alloying elements, and the measurement method, were collected. A large variation was observed in the reported experimental values. The main reason for this is that cementite is metastable, and it is difficult to prepare large single-phase samples. Other factors, such as sample shape, measurement method, alloying element, magnetic transformation, and crystal orientation anisotropy, also influenced the measured values. Studies using first-principles calculations of cementite were also reviewed. The crystal orientation anisotropy of each elastic constant of single-crystal cementite based on the first-principles calculations are summarized, and its comparison with the experimental results is discussed. By comparing the elastic constants obtained by the first-principles calculations with the measured values, the former values of Young's modulus and shear modulus are several % and bulk modulus and Poisson's ratio are several tens of % larger than those of the latter. This is thought to be because of the difference in temperature between 0 K (first-principles calculations) and room temperature (measured value), and theoretical and experimental studies in which the temperature is changed are expected.

Determining the Role of Acicular Ferrite Carbides in Cleavage Fracture Crack Initiation for Two Medium Carbon Microalloyed Steels

“…Taking into account that all sites of cleavage fracture initiation were placed in the area of high plastic strains, in the vicinity of the notch roots of the 4PB specimens, and bearing in mind the previous analysis, it follows that initial microcracks were formed by damage accumulation by plastic deformation in pearlite, a mechanism referred to as Miller-Smith's, characteristic of high pearlitic steels [48]. When the lamellae in pearlite are aligned at angles close to 45 • relative to the direction of applied tension, dislocations move freely in the ferrite matrix without encountering ferrite/cementite interfaces, allowing significant plastic deformation without straining the cementite phase.…”

“…Taking the values of effective surface energy, γ, of 9 Jm −2 [47], modulus of elasticity, E, of 189 GPa and Poisson's coefficient, ν, of 0.301 for cementite [48], and taking the average maximal thickness of cementite plates measured in this work of 0.798 µm and 0.966 µm, the calculation renders stress values of 1970 MPa and 1791 MPa, for V and TiV steel respectively. When plotted in the stress distribution graphs in Figure 10a,b, it is clear that a fracture of the cementite plates could be possible in the narrow zone around the peak stress.…”

“…Furthermore, the point that peak values of the normal tensile stress, σ 11 , located away from the notch root were not sufficient for fracture of coarse TiN particles, which is a common mechanism of the cleavage fracture initiation in medium carbon microalloyed steels [29]. Taking the values of the peak stresses in the range from approximately 1722 to 1876 MPa for V steel and 1617 to 1779 MPa for TiV steel, and the effective surface energy for fracture of TiN particles of 7 Jm −2 [34,47], the minimum size of spherical TiN particles that could fracture can be calculated using Griffith's equation for spherical particles [47][48][49][50]:…”