The Lehrer diagram often serves as a guide for selecting gas mixtures for nitriding alloy steels, but it is only accurate for ammonia gas nitriding processes when hydrogen is used as the diluting gas. This paper presents the results of a study showing that the use of pure nitrogen as a diluent has a marked effect on the phase boundary lines of the standard Lehrer diagram, essentially shifting them to the left. The paper also includes examples showing where the use of nitrogen is advantageous and where it is not.

Gas nitriding is proving to be a viable low temperature case hardening process for stainless steels used in numerous applications. In this study, a comparison between austenitic (grade 304) and martensitic (grade 401) stainless steels shows how pre-oxidation temperature affects the thickness and porosity of the compound layer produced as well as hardness and nitriding diffusion depth. The results indicate that austenitic stainless steel would be the best choice for a part requiring wear resistance and strength, and that a standard rolled martensitic stainless steel would suffice if only a wear resistant surface is needed.

Because of its qualities such as surface uniformity and high load density, controlled gas nitriding is recognized as one of the best nitriding techniques available. It guarantees greater dimensional and surface morphology stability and, therefore, is applied to a variety of stainless steels components. Classical depassivation methods often contribute to a decreased corrosion resistance. In certain applications, alternative methods may be used to achieve the same extent of surface uniformity and similar results, without the use of classical halogen activation methods.

Gas nitriding has been performed in many ways with many variations over the years (using NH 3 only, NH 3 + dissociated NH 3 , and NH 3 + N 2 ). Practical heat treating today demands lower costs and gas and energy consumption. With the challenges of ammonia storage, its costs and emissions, the ZeroFlow process reduces ammonia gas consumption and still offers the same capability to affect the nitrided layer, resulting in lower emissions of ammonia and greenhouse gases. Prior analysis has shown that ZeroFlow control allows control of not only the nitriding potential, but also phase composition of the nitrided layer, its thickness and the constituents of the atmosphere used for nitriding. Tremendous experience via actual commercial production has proven that this method of gas nitriding, and the atmosphere used, still allows for precision in controlling growth kinetics of the nitrided layer. The impact on the ammonia consumption and emissions is significant.

To achieve optimal component properties through controlled gas nitriding, a precise approach is required. This method utilizes nitride layers with a defined structure, tailored to the specific stresses each component will encounter. Success relies on three key factors: firstly, understanding the ideal layer structure for a given stress profile, which informs the desired properties of the surface layer. Secondly, in-depth knowledge of material behavior during nitriding, considering both the material itself and the nitriding conditions, is crucial for material selection and parameter optimization. Finally, achieving consistent and predictable results hinges on the precise measurement and control of nitriding conditions, particularly the nitriding potential.

Plasma nitriding is a thermochemical surface heat treatment of steel components to produce nitride layers which increase wear-, corrosion- and fatigue resistance. Research into plasma nitriding lately showed that there is a significant and characteristic amount of ammonia formed off the process gases nitrogen and hydrogen. This research paper is aimed to analyze the influence of plasma treatment parameters, such as pressure, voltage, temperature and nitrogen to hydrogen ratio on the atmosphere and the formation of ammonia during plasma nitriding. The ammonia content is measured in the exhaust gas. By correlating the measured ammonia with the treatment parameters and modeling the nitriding process, the ammonia content can then be predicted. Further a plasma nitriding potential, comparable to the gas nitriding potential, based on ammonia content is calculated and its practicability as process control parameter is shown by correlating the potential with the nitriding results, e.g. the formation of ε and γ’ nitride phases.

Gas Nitriding is a common industry process which can improve the hardness, wear resistance, and fatigue strength of steel components. Proper surface activation is reported to be critical in achieving a uniform and repeatable nitriding response, but little data is available to compare various activation techniques for common nitriding alloys. This paper reports the early hour compound layer formation for six activation techniques on both a low alloy steel and a specialized nitriding steel. Both grades of steel showed the best performance when a multi-stage nitride washer was used to prepare the surfaces. Two other processes, namely nitric acid etching and a neutral wash and rinse cycle, were also shown to provide acceptable early hour performance for both alloys under the test conditions in this study.

Surface morphology of 1018 steel samples was studied after early stages of gaseous nitriding using AFM technique. Oscillation in measured roughness and gas/solid interface area values at different stages of process has been reported. The amplitude and period of such oscillation depends on initial surface finishing.

This article addresses an investigation of the influence of plastic deformation on low-temperature surface hardening by gaseous nitriding of three commercial austenitic stainless steels: AISI 304, EN 1.4369 and Sandvik Nanoflex with various degrees of austenite stability. The materials were plastically deformed to different equivalent strains by uniaxial tension. Gaseous nitriding of the strained material was performed in ammonia at atmospheric pressure in the temperature range 693-703 K (420-430°C). Microstructural characterization of the as-deformed states and the nitrided case included X-ray diffraction analysis, reflected light microscopy and microhardness indentation. The results demonstrate that a case of expanded austenite develops and that, in particular, the presence of strain-induced martensite in the initial (deformed) microstructure has a large influence on the nitrided zone.

Controlled gaseous nitriding of ternary Fe-Cr-Mo alloys leads to the development of ternary, mixed nitrides in the ferrite matrix, which show complex chemical, structural, and morphological transformations as a function of nitriding time: initially continuous precipitation of fine, coherent, cubic, NaCl-type (Cr,Mo)N x nitride platelets develop, which later transform to a novel, hexagonal CrMoN 2 nitride by a discontinuous precipitation reaction. Some relatively coarse cubic nitrides also occur in the ferrite lamellae. The Fe-Cr-Mo alloys with varying Cr/Mo ratio, but all containing a total alloying element (Cr+Mo) content of 2 at.%, showed similar kinetics of continuous precipitation of cubic (Cr,Mo)N x nitride. The kinetics of the discontinuous precipitation reaction is faster for the alloys with lower Cr/Mo ratio.

This article examines the critical role of process control in nitriding and ferritic nitrocarburizing (FNC) treatments, focusing on the measurement and regulation of nitriding potential (K N ) and carbon activity (K C ). The author demonstrates how modern sensor technology combined with automated programmable control systems can effectively eliminate process variability and ensure consistent, high-quality outcomes. The discussion covers fundamental principles of K N and K C control through proper utilization of sensors, analysis of key input variables, and the relationship between gas ratios, temperature, and time on treatment results. Various monitoring technologies are reviewed, including hydrogen sensors, CO/CO 2 analyzers, oxygen probes, associated technical challenges, and maintenance requirements. By highlighting the importance of precise process parameter control, the article provides practical insights for achieving repeatable, high-quality surface treatments in industrial applications.

This study examines the critical but often overlooked role of surface roughness in gaseous nitriding processes. While nitriding technology fundamentally relies on gas-solid interactions through multiple stages—including ammonia diffusion to the metal surface, dissociation reactions, nitrogen transfer, and bulk diffusion—the authors highlight how surface conditions significantly impact treatment outcomes, particularly at the relatively low processing temperatures (380-590°C) where surface reactions become rate-limiting. The research investigates how surface roughness affects the gas-metal contact area and consequently influences nitrogen uptake kinetics, challenging the traditional assumption that nitriding produces negligible changes in surface morphology. Working with commercial furnaces that use nitriding potential as the primary process control parameter, the researchers correlate various surface finishes with nitrogen absorption rates, ammonia dissociation, and atmosphere activity. The ultimate goal is to incorporate surface roughness—a specification widely used in metalworking industries—as a formal parameter in control systems, thereby enhancing process predictability and meeting increasingly stringent industrial standards for surface quality in nitrided components.

This paper describes the development of a simulation program to model gas nitriding processes for steels and replace traditional trial-and-error methods that are expensive, time-consuming, and often inaccurate. The authors introduce a compound layer growth model that simulates key nitriding outcomes, including phase composition, compound layer thickness, and nitrogen concentration profiles based on process parameters (temperature, time, and nitriding potential or dissociation rate). The model employs computational thermodynamics to construct alloy-specific Lehrer diagrams that describe phase stabilities, uses parabolic law to simulate compound layer growth kinetics, and applies constant effective nitrogen diffusivity in the diffusion zone for specific alloys at given temperatures. Validation testing on AISI 4140 steel under various nitriding conditions demonstrated excellent agreement between simulated and experimental results for both as-washed and pre-oxidized specimens, confirming that the software provides an effective tool for determining optimal nitriding parameters to meet specifications and ensure reliable performance through accurate process control.

A vacuum-purge gas nitriding furnace was modified to develop a process and a furnace enhancement to produce a controlled in situ oxide layer on the surfaces of steel parts using various oxidation techniques. The process is an effective alternative to conventional grit blasting of materials as a means of surface preparation for uniform and consistent nitriding results. Pre-oxidation is known to enhance receptivity of steel part surfaces to the effects of nitriding, and in situ oxidation is inherently efficient and economical. Topics discussed include the type of oxidizing carrier used in the furnace, practical methods used to control the oxidation, and a gas delivery system developed to inject gases with an elevated dew point for the purpose of providing a controlled oxidizing atmosphere. Comparative tests with other activation techniques, and results with no activation, will be discussed along with approaches to technical process difficulties encountered.