Carbonitriding: kinetic modeling of ammonia and acetylene decomposition at high temperature and low pressure

Abstract

The process of carbonitriding is similar to the process of carburization yet with additional ammonia to increase the hardness of the metal surface. Carbonitriding is performed at approximately 850°C-880°C, which is lower compare to carburizing and low pressure (10-50 mbar). The process consists of two stages: decomposition of ammonia and diffusion of "activated nitrogen", and decomposition of acetylene and diffusion of carbon. The decomposition of acetylene is a very complex phenomenon, depending on the temperature, pressure and residence time (the time of presence of acetylene in a furnace). Different reaction products form: small molecules (H 2 , CH 4 , C 2 H 4 , C 6 H 6 , etc.), fine-crystalline graphite (the one that diffuses into the metal surface), polycyclic aromatic hydrocarbons (PAHs), soot etc. The current investigation is based on detailed kinetic modeling (using Chemkin 17.1) of the acetylene decomposition in the atmosphere of a reactor. For this modeling three different comprehensive mechanisms from the literature are considered: the mechanism of K. Norinaga (including 227 species, 827 reactions), the mechanism of T. Bensabath (including 364 species, 1245 reactions) and the mechanism of C. Saggese (including 350 species, more than 10,000 reactions). Comparison of simulation results with experimental data from the literature showed good agreement, demonstrating their applicability for modeling of industrial process. A parametric study suggests the best parameters for acetylene decomposition in a furnace.

Low‐Pressure Nitrocarburizing in Standard Vacuum Furnaces—Preliminary Studies

“…Thermodynamic analysis indicates that the homogeneous pyrolysis of ammonia occurs in a gaseous atmosphere during conventional nitrocarburizing at temperatures and pressures typical for this process, although the iron surface also acts as a catalyst. Pyrolysis starts with decomposition of ammonia according to reaction (1): [21] 2NH 3ðgÞ ⇌ N 2ðgÞ þ 3H 2ðgÞ (1) However, studies have shown that heterogeneous surface reactions play a significant role in ammonia pyrolysis and account for most of the ammonia decomposition reactions. Unfortunately, monitoring reactions at the gas/solid interface is quite difficult, but there are some rules for surface reactions involving ammonia.…”

“…[30] In addition, PAHs with higher molecular weights are precursors of soot, [31] which impedes diffusion, deposits on the surface of the furnace and causes damage to the pump. [21] However, it should be emphasized that the amount of PAHs produced in lowpressure process is definitely lower than in the process carried out at atmospheric pressure, and therefore the advantages of the vacuum process include greater purity and safety. [21,32] As far as we know, there are no papers devoted to the investigations of chemical reactions occurring during low-pressure FNC.…”

“…[21] However, it should be emphasized that the amount of PAHs produced in lowpressure process is definitely lower than in the process carried out at atmospheric pressure, and therefore the advantages of the vacuum process include greater purity and safety. [21,32] As far as we know, there are no papers devoted to the investigations of chemical reactions occurring during low-pressure FNC. Vyazmina et al [21] studied the pyrolysis products of ammonia and acetylene at low pressure (p = 10-50 mbar).…”

“…[21,32] As far as we know, there are no papers devoted to the investigations of chemical reactions occurring during low-pressure FNC. Vyazmina et al [21] studied the pyrolysis products of ammonia and acetylene at low pressure (p = 10-50 mbar). Simulations of ammonia and acetylene pyrolysis were carried out independently.…”

“…Thermodynamic analysis indicates that the homogeneous pyrolysis of ammonia occurs in a gaseous atmosphere during conventional nitrocarburizing at temperatures and pressures typical for this process, although the iron surface also acts as a catalyst. Pyrolysis starts with decomposition of ammonia according to reaction (): [ 21 ] …”