Modification of Surface Structure by Diffusion Processes

In the paper an effect of a diffusion technology such as gaseous ferritic nitrocarburizing on the surface properties of selected alloyed case-hardening steel was tested. The steel 18CrNiMo7-6, primarily predetermined for carburizing and frequently utilized in manufacturing of highly strained components, where high core tensile strength as well as hard surface is demanded, was exposed to gaseous ferritic nitrocarburizing. Such treated surface was subjected to experimental methods. The microstructure observation and the determination of the white layer thickness was performed on the Opto-digital microscope Olympus DSX500i. The nitriding hardness depth of the surface layer from the microhardness profiles obtained by the microhardness tester LM247 AT LECO was deduced. The wear resistance was assessed by utilizing the Scratch test method performed on the tribometer Bruker UMT-3 TriboLab. Results of the measurements present an effect of gaseous ferritic nitrocarburizing on the surface properties of the steel 18CrNiMo7-6 and provide a perception of possibility to substitute the frequently utilized carburizing by the gaseous ferritic nitrocarburizing.

Nitriding and Ferritic Nitrocarburizing of Quenched and Tempered Steels

A physics-based software model is being developed to predict the nitriding and ferritic nitrocarburizing (FNC) performance of quenched and tempered steels with tempered martensitic microstructure. The microstructure of the nitrided and FNC steels is comprised of a white compound layer of nitrides (ε and γ’) and carbides below the surface with a hardened diffusion zone (i.e., case) that is rich in nitrogen and carbon. The composition of the compound layer is predicted using computational thermodynamics to develop alloy specific nitriding potential KN and carburizing potential KC phase diagrams. The thickness of the compound layer is predicted using parabolic kinetics. The diffusion in the tempered martensite case is modeled using diffusion with a reaction. Diffusion paths are also developed on these potential diagrams. These model predictions are compared with experimental results.

How Does Precise Control Affect the Outcome of Nitriding and Ferritic Nitrocarburizing Cycles?

Controlled nitriding and ferritic nitrocarburizing cycles have become commonly used to improve wear resistance, mechanical properties, and corrosion resistance. The framework for process control is based on standards such as AMS 2759/10 and 2759/12A. In all cases, these standards allow for a tolerance in control parameters. This work presents the influence of standard allowed deviations in control parameters, for example, nitriding potential and temperature for a given furnace class, on actual results when comparing iron, carbon steel and low alloy steel samples. The results point to interesting variations in white layer thickness. We also discuss other potential factors such as the shift of actual results due to carbon and alloying elements, and the subsequent alteration of results between expected and actual as demonstrated by shifting boundaries on the Lehrer diagram. As a conclusion, the study underlines the fact that while tolerances are allowed, precise control in specific furnace classes is necessary to obtain an ideal repetition of results. Additionally, improved control lowers gas usage and gas flows to a strict minimum, with a direct impact on the cost of quality, cost of treatment, and emissions affecting the environment.