Nitrided Steel Research Articles

Physico-chemical properties and wear resistance of nitrided steel 38KhMUA

The influence of regime (temperature, composition of gas mixture and its pressure) and energy characteristics (voltage, current density, specific discharge power) at hydrogen-free-nitriding in a glow discharge (HFNGD) on the structure, phase composition of nitrided layers is investigated. It is shown that due to the combination of regime and energy parameters of HFNGD it is possible to achieve physical and chemical indicators of nitrided layers set by operating conditions. The set of traditionally fixed values of regime parameters (temperature, gas mixture connection, pressure and saturation time) without taking into account energy characteristics (voltage, current density and specific discharge power) significantly reduces the technological capabilities of HFNGD. With controlled regulation of the energy characteristics of HFNGD, a significant reduction in the energy consumption of the nitriding process is achieved. It is established that the energy levels of the main subprocesses differ significantly: the formation of nitrides occurs at low energies, surface sputtering is realized at high voltage values, and nitrogen diffusion occurs at high current densities. In cases where the flow energy is insufficient, either a glow discharge may not occur at all, or at insufficient stress the nitride layer on the surface is not sprayed and it acts as a barrier that prevents the diffusion process into the inner layers of the metal, leading to low physicochemical indicators of nitrided layers. The priority in the formation of one or another phase (, , u, ), the quantitative ratio between them and the required performance properties of the metal, respectively, can be achieved only through an independent combination of energy and regime characteristics of HFNGD.

Quantification of Nitrogen in Soft Gas Nitriding Steels by SXES

Quantification of Nitrogen in Soft Gas Nitriding Steels by SXES

Computer modeling of nitrogen redistribution in technologies of complex ion nitriding of alloy steels

The work is devoted to modeling the redistribution of nitrogen in the parts made from alloy steel 9CrSi during isothermal annealing or holding for quenching after the operation of ionic nitriding. This integrated technology has a number of advantages over traditional nitriding methods. It is clear that assessment and prediction of the depth of nitrogen penetration are relevant for complex ion nitriding. Goal. Therefore, the purpose of the simulation was to study the diffusion redistribution of nitrogen in cylindrical punches made of 9CrSi steel at a temperature of 860 ° C, which had a finished nitrided layer with a thickness of 80 μm on the surface. Methodology. Externally, at a depth of up to 30 μm, the layer contained nitrides with a nitrogen concentration of 8.7% by mass, and deeper, up to 50 μm, - nitrogenous α-ferrite. Simulation was carried out in the COMSOL Multiphysics 5.5 software environment in the convection-diffusion module of classical differential equations. We used modern data on the diffusion coefficients in alloyed austenite and ε-nitride of steel. Within the framework of the initial conditions, for nitrogen, which forms solid solutions of interstitiality with carbon in phases, the mass percentages were recalculated into atomic ones with respect to the solid solution of substitution. The simulation results were programmatically returned to the usual atomic percentages. Results. In 3D modeling for up to 10 minutes, nitrogen redistribution from the end and a cylindrical 2 mm diameter of the lateral surface into the punch was assessed. It has been established that in the area of the cutting edge (end circumference) of the punch there is a geometric effect of increased nitrogen content during its redistribution. With a short holding time for hardening, this is an undesirable effect that must be taken into account by finishing machining. A more detailed simulation of the diffusion of nitrogen from the cylindrical surface of the punch into depth, taking into account the gradual transformation of the ε-nitride of the surface layer into austenite, was carried out in the 2D-simulation mode on the cross section. For this, the nitride layer was divided into ten sublayers, each 3 μm thick. The dependences of the nitrogen concentration in depth were obtained for different times of high-temperature holding up to 1 hour. Originality. Analysis of the data indicates that the nitride layer, despite its gradual dissolution in 3.25 min., decreases the average value of the concentration profile of nitrogen in austenite compared to the same initial conditions, but without nitrides. However, in this case, the depth of diffusion saturation, which gives a noticeable increase in microhardness, does not significantly decrease. Practical value. The simulation results correlate well with the microhardness data over the cross section of punches after complex ion nitriding and can be used for prediction in factory practice.

High-speed thermo-cyclic nitriding carburized steels

The task of developing new methods of chemical and heat treatment (CHT), aimed at increasing wear resistance, mechanical properties and reducing length of process operations seems to be quite vital. The objective of the work is development of a method of complex surface strengthening of structural steels with application of the mechanism of additional self-strengthening at wear. Suggested is application of high-speed thermo-cyclic nitriding of carburized structural steels for formation of gradient meta-stable phase-structural modifications in saturated layers. Specimens of 18Cr2Ni4Мo steel underwent preliminary carburization in solid carburetor at 930 °С temperature for 12 hours it leading to obtaining a carbonized layer 1.2…1.3 mm in thickness. Then high-speed thermocyclic nitriding (HSTCN) was carried out in carbamide medium with CHT heating, according to heating-cooling mode 1000↔20 °С, the number of thermo cycles being 2 to 12. The following investigation methods were applied: metallographic, scanning electronic microscopy, durometric, wear testing, according to dry friction-sliding metal against metal mode. As a result of combined CHT with thermo-cycling a thin nitrited layer 0.07…0.14 mm thick is formed on the surface (following 2 HSTCN cycles) and up to 0. thick (after 12 HSTCN cycles) with increased nitrogen and carbon content. Further structure along the depth of the carbonized layer consists of high-carbon martensite, carbonitrides and increased amount of residual austenite (Аres). Along the depth of the saturated layer the microstructure is changed in gradient: the amount of Аres and carbonitrides is reduced, while the amount of quenching martensite is increased. Depending upon the number of HSTCN cycles relative wear resistance is extremely maximally changed with maximum (ε=4,1) at 7 cycles. Аres is meta-stable and undergoes deformation induced martensite transformation γres→αʹ in the process of wear testing (DIMTWT) in the thin surface layer, it bringing some additional contribution into deformational self-strengthening and improvement of relative wear resistance. Thus, improvement of wear resistance ensures additional amount on nitrides and carbonitrides in the preliminarily carbonized and then nitrited steel layer, as well as high-carbon martensite and increased amount of meta-stable Аres (45…70 %), that undergoes γres→αʹ DIMTWT. Thermocyclic character of additional nitriding ensures grain refinement, it promoting improvement in steel impact strength.

Influences of vacuum ion-nitriding on bending fatigue behaviors of 42CrMo4 steel: Experiment verification, numerical analysis and statistical approach

In this paper, the bending fatigue behaviors of 42CrMo4 nitrided steel and its residual compressive stress redistributive behaviors with different loading levels were investigated. Micro-structure analysis, hardness test and X-ray diffraction test were carried out to evaluate the properties of nitriding layer. By means of the staircase method, the bending fatigue limits of different stress ratios (R = 0.1, 0.3, 0.5, 0.7) were obtained and the polynomial formulas of fatigue limits were fitted based on the distributive characteristics of mean stress and stress amplitude. After the nitriding treatment, the fatigue limits and lives of samples can increase about 8% and 30%-50%, respectively. The redistributive behavior of surface compressive stress during the fatigue process is similar to the distribution characteristics of logarithmic function and has positive correlation with the loading level and the initial residual stress. The analysis of variance (ANOVA) result showed that both of stress amplitude and mean stress have profound influences on fatigue life and their weight ratio is about 4:1.

From the history of nitrided ferroalloys

The article considers research on the history of nitrided ferroalloys appearance and development of technologies for nitrogen-containing steels and ligatures. The most important advantages of nitrogen as an alloying element are its availability and almost unlimited reserves in nature. The technology of nitrogen extraction does not cause any harm to the environment and is not accompanied by the formation of waste. New technologies of nitrided ferroalloys and new compositions of nitrogen-containing ligatures emerged as a response to the creation of new grades of nitrogen-alloyed steels. At the same time, researchers in Europe, the United States, and the Soviet Union made the greatest contribution to the development of nitrided steel and ferroalloys technology. Nitrided ferrochrome emerged from the need for alloying stainless steels of various classes. Nitrided ferrovanadium was created for microalloying high-strength low-alloy steels. For nitrogen alloying of transformer steel, an alloying material based on silicon nitride was developed. Nitrogen-containing compositions based on manganese are universal alloying materials for a wide range of applications. Technologies of nitrided ferroalloys developed in the direction of creating compositions with the maximum nitrogen content with minimal consumption of material resources. Currently, technologies for direct introduction of nitrogen gas into liquid metal during out-of-furnace processing are being successfully developed. Alloying with its solid carriers remains a universal method for smelting nitrogen-containing steels. Nitrogen in nature occurs exclusively in a gaseous form, so for introduction to steel, it is necessary to fix it in the composition of a solid substance. At the same time, such a nitrogen-containing material must be compatible with the steel melt and technological in use. This problem is completely solved by the technology of self-propagating high-temperature synthesis (SHS), which allows obtaining composite ferroalloys based on nitrides, with properties that are unattainable for the furnace process.

Results of steel 20Kh13 samples with combined hardened-structured surface erosion tests

The work presents the results of erosion tests and metallographic studies of samples from blade steel 20Kh13 with diffusion coating obtained by ion nitriding and samples from nitrided steel 20Kh13 with a relief previously formed on the surface. Studies of the samples resistance to high-speed water droplet erosion were carried out using the unique hydraulic impact rig «Erosion-M» of the NRU «MPEI». It was established that the use of nitriding and nitriding with preliminary surface structuring allows increasing the relative erosion resistance in the area with a maximum wear rate of at least 2 times, and in the area with a steady wear rate of at least 3 times. It was also found that structuring the surface layer before the nitriding process allows reducing the wear rate in the area with a maximum erosion rate compared to the nitrided surface by at least 10%.

Structure and corrosion behavior of ultra-thick nitrided layer produced by plasma nitriding of austenitic stainless steel

An ultra-thick nitrided layer (>80 μm) was formed by hot wire enhanced plasma-assisted nitriding of AISI 316L austenitic stainless steel under the higher pulsed bias to investigate the structure and the corrosion performance. The phase structure, the cross-sectional morphology, the element distribution, hardness and modulus, as well as the corrosion behavior of the nitrided layer were investigated by XRD, SEM, EDS, nano-indentation tester and electrochemical workstation, respectively. The results showed that the nitrided layer was divided into two sub-layers: the upper thick layer composed of N-expanded austenite and CrN, and the lower thin layer mainly composed of γN with a trace of CrN precipitate. Nitrided at −400 V, CrN and high N-expanded austenite formed in the upper layer, which exhibited a high hardness and an excellent corrosion resistance. As the bias increased to −500 V, more nitrogen atoms precipitated as CrN phase and low N-expanded austenite phase began to form, the thickness of nitrided layer reached a maximum of 84 μm. To −600 V, high N-expanded austenite almost completely transformed to low N-expanded austenite, accompanied by the increase of CrN, which caused the thickness and hardness of the nitrided layer to decrease, and the pitting corrosion occurred at −4.1 mV.

Fatigue damage assessment from cyclic spherical contact of borided and nitrided H13 steel

In this study, boride and nitride layers formed on H13 steel were assessed by cyclic spherical contacts. Boriding was conducted at 800 °C for 1 and 5 h, and nitriding at 580 °C for 1 h. The hardness and strain hardening exponents n of the layers were estimated by spherical indentations (5 µm radius). The standing contact fatigue (SCF) performance was investigated by cyclic spherical contacts (Al2O3, 3 mm diameter), first applying monotonic load tests from 300 to 1000 N to establish the critical loads considering the onset of certain cracks for each layer; later, between 102 and 105 cycles applying different subcritical loads. The results revealed strain hardening exponents of nB1h = 0.226, nB5h = 0.230 and nN1h = 0.241. In SCF, a critical load of 600 N in both layers was found. In cyclic tests, the thicker boride layer presented larger cracking and moderate spallations, attributed to its mechanical properties; the thinner layer exhibited a better SCF resistance. Regarding nitriding, well-defined circumferential cracks without spallations were observed.

Corrosion Resistance of Plasma Nitrided Austenitic Stainless Steel AISI 304 (X5CrNi18-10)

In this paper, austenitic stainless steel AISI 304 (X5CrNi 18-10) has been subjected to low-temperature plasma nitriding (LTPN) at 400°C (673 K) in 24H2:8N2 (l/h), and in 8H2:24N2 (l/h) marked as LTPN-R, and to high-temperature plasma nitriding (HTPN) at 550°C (823 K) in 24H2:8N2 (l/h) working atmosphere, for 15h. The microstructure, microhardness, and phase evaluation of the nitrided steel were studied. The PN led to significant microhardness increase from 206 ± 3 HV0.05 to 842 ± 27 HV0.05 (LTPN), 821 ± 18 HV0.05 (LTPN-R), resp. to 1246 ± 27 HV0.05 (HTPN). The corrosion properties were evaluated using the anodic potentiodynamic polarization tests in neutral 2.5% NaCl deaerated solution. The corrosion rate CR (mm.a-1) increased only slightly for LTPN procedures. But using the HTPN increased 43-times to untreated steel, due to reduced passivation ability by chromium depletion by formed CrN. Additionally, the pitting was evaluated, and the pitting coefficient was calculated.

Evolution of wear damage in gross sliding fretting of a nitrided high-carbon high-chromium steel

Fretting wear tests were performed to examine the evolution of fretting wear damage of the nitrided high-carbon high-chromium X210CrW12 steel against GCr15 steel ball, and to identify the role of nitrides (converted carbides) in fretting wear characteristics. The nitrided layer was characterized by optical microscope (OM), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), electron probe microanalyses (EPMA), X-ray diffraction (XRD), and microhardness analysis. The worn surfaces were evaluated by SEM, EDS and 3D optical profiler. The results indicated that the initial carbides in the nitrided layer undergone a transformation into nitrides during nitriding. The nitrided specimens showed excellent wear resistance compared to the steel balls, and their wear damage was mainly caused by the adhesive/abrasive action of oxidized particles. Coarse nitrides led to the rapid wear of steel balls, and acted as barriers against the grooving of oxidized particles. Higher loading conditions of 40 N and 80 N induced the generation of central smooth zone and W-shaped wear scar. The formation of the central smooth zone was very beneficial for reducing the wear rate and friction coefficient, thus the expected increase in wear volume with increasing applied load from 40 N to 80 N was not observed.

Вплив хімічного складу на теплостійкість конструкційної сталі

It is based of process analysis of structural steel operation at elevated temperatures and stresses, it is established that their workability depends with the chemical composition of steels and operation stress, time, temperature. The heat-resistant structural steels alloying is aimed for reducing the intensity of softening processes, that is, the redistribution of the alloying elements between the solid solution and the separation phases, their coagulation and changes of the crystal structure defects. The results of the study of chemical composition effect, temperature, stress and operation time on the creep and durability of structural steels are presented in the article. Analytical dependences of the relative elongation in the creep process and time at which the destruction of the chemical composition of heat-resistant structural steels and such operational factors as temperature, stress and holding are calculated. It is established that the creep and durability of heat-resistant structural steels, in the experiment conditions, are determined by their chemical composition in quantity 61–64 %, temperature 7–18 %, holding time 11 %, stress state 18–21 %. Nitrogen and vanadium alloying have been shown to more effectively reduce the softening degree heat-resistant steel during creep process than the molybdenum alloy, since the coagulation process of carbides and nitrides in steels with nitrogen and vanadium receives less development than carbides in steels alloyed by molybdenum.

Plasma Nitriding of Mirror-Finished Austenitic Stainless Steel

Plasma Nitriding of Mirror-Finished Austenitic Stainless Steel

Determination of the s‐phase formation coefficient of plasma nitrided austenitic steel

AbstractPlasma nitriding is an effective surface hardening treatment for austenitic stainless steels. During plasma nitriding, s‐phase formation takes place which is not only responsible for high hardness and wear resistance but also for good corrosion resistance. In order to estimate the thickness of the s‐phase for austenitic stainless steel in a plasma nitriding process, an empirical model is devised. A number of plasma nitriding processes of austenitic stainless steel (304 L) were carried out with varying treatment temperature from 360 °C to 450 °C and process duration ranging from 10 hours to 24 hours with constant pressure, voltage, pulse‐to‐pause‐ratio and gas mixture. A time‐temperature dependent s‐phase formation coefficient is determined by measuring the thickness of the s‐phase using a scanning electron microscope (SEM) and glow discharge optical emission spectroscopy (GDOES). The developed model is verified by three controlled experiments. This model fits the thickness of the s‐phase with an error of less than 6 %.

Application of technical nitrogen during nitriding or nitrocarburizing alloyed steels

In the present work technical nitrogen application for surface activation of alloyed steels with chrome, particularly stainless steel X20Cr13 during nitriding and carbonitriding was investigated .Hardness and microstructure of nitrided layers were examined. Possibility of using technical nitrogen containing 0,2%O2 for surface activation of X20Cr13 stainless steel was confirmed and activation conditions for investigated alloyed steels were determined.

Influences of Vanadium and Silicon on Case Hardness and Residual Stress of Nitrided Medium Carbon Steels

Medium carbon steels alloyed with two levels each of V and Si were heat treated to form tempered martensite and bainite, followed by nitriding to evaluate their effects on case hardness and residual stress. Microstructures were quantitatively characterized with Vickers microhardness testing, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), conventional transmission electron microscopy (CTEM), and scanning transmission electron microscopy (STEM). Measurements of subgrain size, cementite size, dislocation density, and volume fractions of nitride precipitates were combined in a strength model to predict the contributions of V and Si to maximum case hardness after nitriding. Higher V contents lead to increases in maximum case hardness by increasing the volume fraction of MX precipitates. Increases in Si content lead to increased maximum case hardness by increasing the volume fraction of amorphous (Si and Mn)-containing nitride precipitates; the presence of Mn in these precipitates has not been previously reported. Higher Si contents also lead to increases in solid solution strengthening because the majority of Si remains in solution in the ferrite matrix. Greater volume fractions of MX and (Si and Mn)-containing nitrides both lead to higher magnitudes of compressive residual stress. The results presented here demonstrate that alloying can improve the properties after nitriding, providing insight into advanced nitriding steel grades and potentially enabling a wider range of steel microstructures to be nitrided without a debit in performance.

Accelerated ion-plasma nitriding of austenitic steels in a low-frequency ferromagnetic enhanced induction discharge

In the present work, a possibility of the accelerated ion-plasma nitriding of austenitic stainless steels at elevated temperatures is discussed. Experimental data on the plasma-assisted nitriding process of AISI 304 stainless steel are presented in the temperature range of 400-500 °C, with the use of ferromagnetic enhanced induction discharge with a low-frequency and low-pressure as an external source of nitrogen ions and radicals.

Optimization of the ion-plasma nitriding of structural steel

Mathematical planning was used in order to determine the optimal mode for ion nitriding the structural steel 0.38C–2Cr–2Ni–Mo. This was justified by the requirement to preserve strictly limited functional parameters obtained as a result of diffusion saturation: hardness in the range of 450–650 HV in a layer 0.15–0.40 mm deep. According to of X-ray diffraction analysis in addition to reflections from the matrix phase (alloyed ferrite α-Fe), the reflexes of nitride compounds are recorded, namely, the γ′-phase with a FCC lattice (Fe4N) and the ε-phase with a HCP lattice (Fe3N) with a high nitrogen content. An analytical expression in the form of a linear function for the studied optimization parameters (microhardness and depth of nitration) was obtained by implementing factor planning and regression processing of the obtained data. This made it possible to assess the degree and direction of influence of the investigated factors on the optimization parameters under study.

CHTE Introduces Simulation Software for Gas Nitrating of Steels

Abstract NitrideTool software, developed by the the Center for Heat Treating Excellence (CHTE), simulates the gas nitriding of steels to understand nitriding potential, temperature, time, and surface condition.

Effects on corrosion resistance of low alloy steel after plasma nitriding at elevated temperature

In the present work plasma nitriding of low alloy steel at elevated temperature for different exposure time have been performed. With the view to enhance both the hardness and corrosion resistance of the steel plasma nitriding was performed at 500 °C for 8 and 10 h. Post nitrided steel contain Fe4N and ε-Fe2–3N phases as revealed by X-ray diffraction studies. A selected set of plasma nitided samples were exposed to potentiodynamic polarization in 3.5% NaCl electrolyte for the assessment of corrosion resistance property. It was found that the nitrided steel samples possess corrosion resistance much better than the one without nitriding.