Gas carburizing with quenching is one of the most useful heat treatment processes for steel parts. However, after quenching distortion is still occurs. The nitriding and nitrocarburizing are the surface hardening heat treatment methods with low distortion, but these methods require the long treating time to obtain a thick hardened layer. Austenitic nitriding and quenching (ANQ) solves these problems. In ANQ process, nitrogen is infiltrated into the steel parts in austenite phase, and they are quenched to harden. The ANQ process can also be applied to cheap low carbon steel such as the Cold Rolled Carbon Steel Sheet. In this study, the effect of ANQ on mechanical properties was examined. For infiltrating the nitrogen into the steel parts, the steel parts were heating to 750°C or higher in an ammonia atmosphere and heating to 750°C or higher in a nitrogen glow discharge. After the ANQ process, hardness profiles, structure, nitrogen and carbon concentration profiles were observed. Also, distortion, tribological properties, impact value and fatigue strength were examined. The effective case depth, which is treated by ANQ, is larger than the effective case depth of gas nitrocarburizing for same period of time. Distortion of ANQ is much smaller than that of gas carbonitriding, and it is almost equal with that of gas nitrocarburizing. The seizure load is same as with other surface hardening heat treatment processes. The wear loss of ANQ is a lower, in the amount of about 1/2 that of the carbonitrided specimen and 1/3 that of the gas nitrocarburized specimen. The ANQ is an effective heat treatment process for parts which require wear resistance. The tempering softening resistance is improved by nitrogen infiltration. ANQ also improves the impact value and fatigue strength.

High-entropy alloys (HEA) are multinary alloys obtained by blending at least five metallic elements in compositions close to their isoatomic fractions (5–35 at%). Generally, HEAs are produced by arc melting and casting. However, the cast specimens undergo phase separation and have a non-uniform microstructure. In contrast to ingot metallurgy, powder metallurgy has several advantages such as the possibility of alloying metals with high melting points and large differences in melting points and specific gravity. Therefore, we investigated the preparation of HEAs by mechanical alloying (MA), which produces an alloy powder with a uniform microstructure, followed by consolidation by spark plasma sintering (SPS). In this study, CoCrFeNiTi HEA sintered after MA-SPS was subjected to direct current plasma nitriding with screen (S-DCPN) to evaluate the characteristics of the nitrided layer as a function of nitriding temperature. Ball milling with heptane in an argon atmosphere using pure powders of Co, Cr, Fe, Ni, and Ti as raw materials was performed for 50 h. Subsequently, sintered compacts were prepared by SPS and treated with S-DCPN at 673, 773, and 873 K for 15 h in 75% N 2 –25% H 2 at a gas pressure of 200 Pa. A screen made of austenitic stainless steel SUS316L was installed as an auxiliary cathode to ensure uniform heating and nitrogen supply during the plasma nitridation process. Then, X-ray diffraction test, cross-sectional microstructure observation, surface microstructure observation, cross-sectional hardness test, roughness test, glow discharge optical emission spectrometry, corrosion test, and wear test were performed on the nitrided samples. The corrosion test results demonstrated that corrosion resistance increased with decreasing nitriding temperature. Furthermore, the results of the roughness and wear tests confirmed that abrasive wear occurred on the specimens nitrided at 873 K.

In recent years, physical vapor deposition and chemical vapor deposition (CVD) methods have made significant advancements due to the growing demand for surface modification technologies. This study focuses on depositing diamond-like carbon (DLC) as a thin, hard film using plasma-enhanced CVD. DLC possesses properties such as high hardness, low friction, wear resistance, and chemical stability. However, a drawback is low adhesion caused by residual stress and differences in hardness between the film and the substrate material. Therefore, efforts are underway to improve adhesion by introducing a DLC intermediate layer containing metallic elements to reduce residual stress or by applying treatments to harden the substrate material, such as nitriding or carburizing. Active screen plasma nitriding (ASPN) is a nitriding method that eliminates edge effects and electrically insulates the sample during the process. However, during nitriding, deposits can cover the sample and slow down the nitriding rate. To address this, a nitriding method called "direct-current plasma nitriding with screen (S-DCPN)" has been developed. It involves applying a voltage to the sample and screen during ASPN to remove deposits via sputtering action, thereby increasing the nitriding rate. Although the duplex process of ASPN and DLC-coating deposition has been studied, there are limited reports on the duplex process with S-DCPN. This study investigates the effect of intermediate layer composition on mechanical properties by forming a nitrided layer on the surface of SUS304 through S-DCPN treatment, depositing a Si-DLC intermediate layer with varying compositions, and applying a DLC film on the top surface. The results demonstrate that the lower the Si ratio in the Si-DLC intermediate layer, the better the wear resistance. Furthermore, the study reveals that wear resistance and adhesion were improved compared to samples without S-DCPN treatment.

Surface modification involves the chemical or physical impartation of enhanced functionality to the surface of materials, and has become increasingly important in recent years. Nitriding is a surface modification method that hardens the surface of metallic materials by causing nitrogen to permeate and diffuse into the surface to form various nitrides or by supersaturating a solid solution of nitrogen in the metal. This is effective in improving the hardness, corrosion resistance, and wear resistance. Plasma nitriding, a type of nitriding process, has several advantages, such as low energy consumption, short processing time, and low environmental impact. In contrast, the conventional plasma nitriding method forms plasma on the surface of the treated material, which may cause phenomena that lead to defects in the treated material. Therefore, the directcurrent plasma nitriding with screen (S-DCPN) method reduces these problems because plasma is formed not only on the treated material but also on the surface of the screen. Stainless steel has excellent corrosion resistance; however, nitriding treatment above a certain temperature reduces the corrosion resistance owing to chromium nitride precipitation. In this study, the S-DCPN treatment, a type of plasma nitriding method, was applied to form a thick nitrided layer without reducing corrosion resistance. The S-DCPN treatment was performed using ferritic stainless steel SUS430 as the sample and austenitic stainless steel SUS304 as the screen material at treatment temperatures of 633 and 653 K, treatment times of 5 and 15 h, a gas pressure of 200 Pa, and a gas composition of 75% N 2 - 25% H 2 . Consequently, the α N phase with supersaturated nitrogen solid solution was identified under all conditions. Nitrogen diffusion and hardness increased with increasing treatment temperature and time. In the corrosion tests, corrosion resistance improved under all conditions.

The purpose of this study is to clarify the mechanical properties of the expanded austenite (S phase) formed in austenitic stainless steel (ASS). A small thin rolled plate of SUS304 with 0.5 mm thickness was used as test sample. The test sample was nitrided by active screen plasma nitriding (ASPN) at low processing temperature of 400 °C and 450 °C during 4 h processing time. S phase was formed on the surface of the test sample. The surface hardness of ASPN sample was higher than that of untreated sample. Furthermore, tensile tests and fracture surface observations revealed that the tensile strength was also improved compared to untreated samples.

Gas nitriding and ferritic nitrocarburizing have seen tremendous growth. Today, it continues to accelerate as more uses are being found, especially in the growing electric vehicles (EV) sector. This success is due to the ability to control protective white layers consistent with the needs of an automotive engineer. Steels and cast irons are still the materials of choice for many applications and the nitrided layer is wellknown for its tribological features (some would say even more than three) which include wear resistance, lubricity, and a low coefficient of friction. Corrosion resistance in particular has become an important advantage and depends on white layer formation and quality. The white layer (known as the compound zone) consists of two iron nitrides, epsilon (Fe 2-3 [N]) and gamma prime (Fe 4 N). In addition, the epsilon layer can contain varying amounts of iron carbides and/or iron carbonitrides, Fe 2-3 [C]. This paper will focus mainly on the how’s and why’s of white layer: how to control its composition and properties; and how to minimize it, if required. Just as importantly, some applications of how the EV component engineers have found uses for this important steel treatment are discussed, including brake rotors.

Plasma nitriding is the low-nitriding potential process characteristic of its ability to nitride stainless steels and powder metal components without special preparations or unusual controls. This is possible thanks to its specific mechanism and presence of sputtering, the phenomenon which occurs throughout the entirety of the process. Typically, the plasma process produces a nitrided layer with the gamma prime-Fe4N compound zone on top of it. This is very important whenever a good bending fatigue property of the part is needed. The abovementioned materials can also be treated with conventional gas nitriding, but with special cycles requiring very sophisticated control. Mechanical masking, protection from direct contact of the glow discharge with a given surface, prevents hardening of the mechanical components in the areas, which should stay soft, such as the threads, small holes and others. The uniformity of nitriding large/long parts, such as shafts and extruder screws, allows economical treatment in module-type vessels. Easy doping of the plasma with hydrocarbons allows for forming a thicker compound zone of the ε-Fe2NxCy-type. This significantly improves tribological and anticorrosion properties. Enhancement of the wear properties for higher temperature applications is possible when doping plasma with silicon is applied. The plasma process can also be carried out at the temperature range 350-400° C to all types of stainless steels. Formation of expanded austenite at such a low temperature is possible when nitrogen or carbon is diffused. This is applied for stainless steels where their corrosion resistance must be supported or enhanced in their wear resistance applications. Examples of the best applications will be presented.

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.

Nitriding surface hardening is commonly used on steel components for high wear, fatigue and corrosion applications. Case hardening results from white layer formation and coherent alloy nitride precipitates in the diffusion zone. This paper evaluates the microstructure development in the nitrided case and its effects on the hardness in both the white layer and the substrate for two industry nitriding materials, Nitralloy 135M and AISI 4140. Computational thermodynamic calculations were used to identify the type and amount of stable alloy nitrides precipitation and helped explain the differences in the white layer hardness, degree of porosity at the surface, and the hardening effect within the substrate. Some initial insights toward designing nitriding alloys are shown.

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.

This paper reviews recent advances in the control of plasma ion nitriding processes and their effect on AR500 and 4140 steel and ductile and gray iron. The advanced discussed are primarily in the area of electrical power and gas flow control.

Controlled nitriding and ferritic nitrocarburizing can significantly improve the corrosion and wear resistance of carbon and low-alloy steels. The framework for maintaining these processes is based on standards, such as AMS 2759/10 and 2759/12A, that specify tolerances for control parameters. This work investigates the impact of admissible deviations in control parameters on the performance of treated alloy samples. The findings of the study demonstrate that although tolerances are allowed, precise control in specific furnace classes is necessary to consistently obtain superior results.

Nitriding is a surface hardening treatment used on steel components to improve their resistance to corrosion, fatigue, and wear. Iron nitrides at the nitrided steel surface form a compound layer known for its high hardness but also for its brittle nature. It is not uncommon for this layer to chip or break away during metallurgical sample preparation, making it difficult to accurately characterize the microstructure of the nitrided load. This paper presents the results of several studies that assess the effect of cutting and polishing operations along with polishing pressure, the use of foils, and Ni plating. A best practice procedure has been developed to prevent damage to nitrided samples and minimize uncertainty when evaluating part quality.

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.

Microstructural examination of a nitrided part is the most commonly used method for evaluating nitriding material and process performance. Microstructural evaluation also helps to validate that the process ran as intended and produced the desired nitrided case characteristics. However, sample preparation is often complicated by the partial or complete breakaway of the compound layer and may affect the accuracy of the conclusions made. A set of experiments was performed to evaluate the effect of two saw cutting methods, the use of metal foil for sample mounting, and the use of Ni plating before cutting. Microstructures of 12 experimental conditions were analyzed. Recommendations were made for the nitrided sample preparation best practice to analyze compound layer uniformity and thickness.

The use of nitriding to improve a component’s resistance to wear, fatigue, and corrosion continues to increase across the industry. However, for nitrided components, no universally accepted definition of “case depth” is available to allow the comparison of different nitriding processes, cycles, and materials. This study documents currently published methods of specifying and determining case depth for nitrided components, and evaluates the reported case depth of multiple materials and cycles in an effort to determine an optimal and robust “universal” method of reporting case depth. After completing this exercise, it appears that the optimal “universal” method of specifying and reporting the case depth for a nitrided component is to report the depth at which a Vickers microhardness traverse crosses a threshold which is 50HV greater than the material hardness below the nitrided case.

Numerous trademarked, standardized or proprietary nitriding and ferritic nitrocarburizing recipes and control methods are used in industrial furnaces. Surface Combustion has developed a new tool to predict the in-process composition of nitriding and ferritic nitrocarburizing atmospheres from these different processes. In addition, the activities and potentials of carbon, nitrogen and oxygen can be found, leading to prediction of the associated equilibrium phase on the part surface. The theory behind the tool and the application of the tool to control nitriding and ferritic nitrocarburizing atmospheres will be discussed. An overview of different equipment designs that can use the tool will also be presented.

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.

Dilatometry and transmission electron microscopy were used to characterize the effects of V content, Si content, tempering temperature and starting microstructure on the hardness and microstructural evolution of a 0.4 wt pct carbon steel after a simulated nitriding thermal cycle. When tempered at 500 °C, significant amounts of V are left in solution leading to precipitation during the nitride thermal cycle increasing the hardness and dilation strain. Increases in Si content also lead to higher core hardness after nitriding, but Si does not significantly increase dilation strain during nitriding. Bainite starting microstructures produced less dilation strain during nitriding compared to martensite starting microstructures when tempered at 500 °C.

This study deals with the fundamentals of intelligent computer-aided processes in thermal and thermochemical treatments. The aim of this study is the improvement of the conformity of the actual post-treatment properties with the assumed properties, thereby improving the repeatability of the process results. A detailed study was conducted involving low-pressure carburizing and low-pressure nitriding. The principal objective of the literature review was to better understand the cause-and-effect relationship in these processes and to develop a methodology of designing functional and effective processes of low-pressure thermal and thermochemical treatment, using effective computation methods. The paper contains a synthetic presentation of modeling methods, in particular of artificial intelligence methods; it also analyses the opportunities and threats associated with the methods.