Laws of alloyed cementite particles nucleation during heat-resistant steels carburizing

• Main Author: M. Yu. Semenov
• Format: Article
• Language: Russian
• Published: MGTU im. N.È. Baumana 2014-01-01
• Series: Nauka i Obrazovanie
• Subjects: carburizing; heat-resistant steels; cementite type carbides; kinetic theory of nucleation; defects of the crystal structure; gear wheels for turbofan engines
• Online Access: http://technomag.edu.ru/jour/article/view/614

<p>The article considers a problem analyzing a nucleation of cementite type carbides in carburized heat-resistant steels for the turbofan engines gear wheels.</p><p>The verification of previously hypothesized mechanism of dislocation nucleation particles chromium-alloyed cementite during process of carburizing was accepted as an objective of the work.</p><p>As a methodological basis of this paper were accepted the numerical experiments based on the kinetic theory of nucleation, as well as on the known results of experimental studies.</p><p>According to the kinetic theory of nucleation, a new phase in the solid solutions take place in the defects of the crystal structure of the metal such as inter-grain boundaries and dislocations clusters. A principle feature of the inter-grain boundary mechanism of nucleation is formation of carbide lattice. It is of great practical interest because the cementite lattice drops mechanical properties of hardened parts.</p><p>According to the experimental studies, the average chromium concentration in the alloyed cementite twice exceeds its Cr content in the heat-resistant steels. Furthermore, the areas of abnormally high (more than ten times in comparison with the average content) chromium concentration in cementite have been experimentally revealed.</p><p>Numerical experiments have revealed that the nucleation of cementite particles alloyed with chromium (chromium concentration of 3% or more) occurs, mainly, by the dislocation mechanism on the concentration fluctuations of the alloying element. According to calculations, an obligatory prerequisite to start an active nucleation process of new phase in the solid solution is a local increase of the chromium concentration up to 40%.</p><p>Despite the lack of physical prerequisites for the formation of chromium precipitates, this phenomenon is explained by a strong chemical affinity of chromium and carbon, causing diffusion of chromium atoms in the region of the carbon atoms clusters. The formation of carbon fluctuations in steel structural defects caused by high binding energy of carbon atoms with dislocations, which is a well known fact.</p><p>A determined primary mechanism of cementite particles nucleation on chromium fluctuations satisfactorily explains the other known facts known from experimental studies.</p><p>So the introduction of significant amounts of nickel (about 3-4 %) in the 3% chromium- alloyed steel leads to the formation of the cementite lattice. This fact is explained by the known fact that is a reducing chromium diffusion rate in the steel with available nickel, thereby preventing the formation of chromium atom segregations diffusing in the areas of defects saturated with carbon.</p><p>Also a phenomenon of the cementite lattice formation with carburizing temperature increasing over 980 °C could be explained by suggested theory. The rise of process temperature causes a sharp increase of carbon and chromium diffusibility that results in destruction of carbon atoms bonds and defects in the crystalline structure and a more uniform distribution of chromium and carbon in steel.</p><p>So it is advisable to form diffusion layers in heat-resistant steels of new generation with high nickel content without cementite phase. This layers must by hardened by refractory carbides of tungsten, molybdenum, vanadium, and other strong carbide forming elements.</p>