Vacuum 80 (2005) 332–342 Structural characterization of fluidized bed nitrided steels Ahmet Tu¨rk, Orcun Ok, Cuma Bindal à Department of Metallurgical and Materials Engineering, Engineering Faculty, Sakarya University, Esentepe Campus, 54187 Sakarya, Turkey Received 8 June 2005 Abstract The nitriding behaviour of 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels was investigated nitrided in the fluidized bed processes. The nitriding processes were carried out at a temperature of 575 1C for treatment times of 6, 12 and 18 h. The nitrided samples were fully characterized using metallographic, microhardness and XRD techniques. Test results indicated that thickness of the compound layer on the steel surface changed in the range from 10 to 18 mm depending on steel type and treatment time, and g 0 -Fe 4 N and e-Fe 23 N formed in the compound layer. The hardness of the diffusion layer was over 1000 HV. Depending on the chemical composition of steels, the case depth ranged from 155 to 525 mm. Kinetics studies showed that the effective diffusion coefficients are 298  10 14 , 525  10 14 and 68.8  10 14 m 2 s 1 , for 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels, respectively. The fluidized bed process realizes the highest hardness of the case layer, 1095 HV, with fairly high growth rates, 27 mm/h. r 2005 Elsevier Ltd. All rights reserved. Keywords: Fluidized bed nitriding; Hardness; Diffusion layer; Layer thickness; Steels 1. Introduction Nitriding is a low-temperature method used to diffuse nitrogen into the surface of steel without changing the phase structure of steel. The ferrite or cementite phase (depending on the carbon content of the steel) remains the same as original, and it does not transform into austenite. Thus, there is minimal distortion because no phase transforma- tion occurs in the steel. The nitriding process has been successfully applied to carbon steels, alloy steels, tool steels and stainless steels [1–10]. Traditional methods such as gas nitriding and salt-bath nitriding have been used by engineers to improve the surface hardness and wear resistance of parts and components in a number of different industries for many years [11]. In recent years, new techniques of nitriding have been developed, known as ion (or plasma) nitrid- ing and fluidized bed nitriding. Ion nitriding has many advantages (short treatment time, low process temperature, minimal distortion, clean ARTICLE IN PRESS www.elsevier.com/locate/vacuum 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.06.001 à Corresponding author. Tel.: +90 264 3460353; fax: +90 264 3460351. E-mail address: bindal@sakarya.edu.tr (C. Bindal).