Thermochemical Surface Treatment Research Articles

In situ HEXRD experimental study and prediction of microstructures and internal stresses during heat treatment of carburized and carbonitrided low-alloyed steels

Thermochemical surface treatments of steels, such as carburizing and carbonitriding are used to improve mechanical properties, especially resistance to fatigue. Carbonitriding can achieve excellent properties, but the origins remain to be understood, as it has been less studied than carburizing. Fundamental understanding has to be established regarding the formation of microstructures and internal stresses as well as their couplings, during the cooling which follows the enrichment treatment.For this purpose, an original experimental method is introduced to follow in situ by High-Energy X-Ray Diffraction the phase transformation kinetics as well as the evolutions of the internal stresses during cooling, inside laboratory scale, 3 mm-thick samples with C and/or N composition gradients. The usual trends are confirmed regarding carburizing: the carbon-enriched case is the last to transform and the surface ends up with compression residual stresses. Conversely, for carbonitriding, unusual evolutions of microstructure and internal stresses are observed. The presence of nitrogen reduces the hardenability in the enriched case. This modifies the chronology of the phase transformations and this leads to tensile residual stresses at the surface.A coupled thermal, mechanical and metallurgical model predicting the phase transformation kinetics and the evolutions of internal stresses is set up. It achieves quantitative predictions and it shows that, in the studied cases, the phase transformation plasticity strains are the main origin of the residual stresses.

Scratch Response of Hollow Cathode Radiofrequency Plasma-Nitrided and Sintered 316L Austenitic Stainless Steel

Low-temperature plasma nitriding is a thermochemical surface treatment that promotes surface hardening and wear resistance enhancement without compromising the corrosion resistance of sintered austenitic stainless steels. Hollow cathode radiofrequency (RF) plasma nitriding was conducted to evaluate the influence of the working pressure and nitriding time on the microstructure and thickness of the nitrided layers. A group of samples of sintered 316L austenitic stainless steel were plasma-nitrided at 400 °C for 4 h, varying the working pressure from 160 to 25 Pa, and the other group was treated at the same temperature, varying the nitriding time (2 h and 4 h) while keeping the pressure at 25 Pa. A higher pressure resulted in a thinner, non-homogeneous nitrided layer with an edge effect. Regardless of the nitriding duration, the lowest pressure (25 Pa) promoted the formation of a homogenously nitrided layer composed of nitrogen-expanded austenite that was free of iron or chromium nitride and harder and more scratching-wear-resistant than the soft steel substrate.

Aşınma ve Korozyona Dirençli Çelik Yüzeyler Oluşturmak Için Kullanılan Yüzey İşlemleri Üzerine Bir Derleme

Steel alloys are one of the most used engineering material classes due to their superior properties such as yield and tensile strength, good thermal conductivity, machinability, formability, ductility, magnetic properties, and recyclability. In addition to its advantages, steel suffers from two main factors that limit its use, namely wear and corrosion. Wear and corrosion, separately or in combination, cause a material loss in steel, resulting in increased costs in industrial production. However, with appropriate surface treatments, wear and corrosion of steels can be prevented or kept to a minimum. Corrosion and wear resistances provided by appropriate methods have the potential to reduce costs and also expand the set of suitable materials that designers can choose from. In this study, brief information about steel is given and then preventive applications against wear and corrosion of steel materials are examined. Definitions were made about surface treatments such as hot-dip coatings, electrochemical coatings, electroless coatings, thermochemical surface treatments, sol-gel coatings, chemical vapor deposition (CVD), thermal spray coatings, physical vapor deposition (PVD), and the effects of surface treatments on the wear and corrosion properties of steels were investigated. In addition, the effects of some process parameters of surface treatments and post-treatments such as heat treatment on corrosion and wear behavior are presented.

The Corrosion Behavior in Different Environments of Austenitic Stainless Steels Subjected to Thermochemical Surface Treatments at Low Temperatures: An Overview

Low-temperature thermochemical treatments are particularly suitable for use in the surface hardening of austenitic stainless steels without impairing their corrosion resistance. In fact, when using treatment media rich in nitrogen and/or carbon at relatively low temperatures (<450 °C for nitriding, <550 °C for carburizing), it is possible to inhibit the formation of chromium compounds and obtain modified surface layers that consist mainly of a supersaturated solid solution, known as expanded austenite or S-phase. It has been observed that this hard phase allows the enhancement of corrosion resistance in chloride-ion-containing solutions, while the results were contradictory for chloride-free acidic solutions. This overview aims to discuss the corrosion behavior of low-temperature-treated austenitic stainless steels, taking into account the different microstructures and phase compositions of the modified layers, as well as the different test environments and conditions. In particular, the corrosion behavior in both chloride-ion-containing solutions and chloride-free solutions (sulfuric acid, sulfate and borate solutions) is discussed. The analysis of the international literature presents evidence that the microstructure and phase composition of the modified layers have key roles in corrosion resistance, especially in sulfuric acid solutions.

Investigating the Water Jet Erosion Performance of HVOF-Sprayed WC-10Co Coatings on 35CrMo Steel Utilizing Design of Experiments

To enhance the surface of a material with the desired qualities for diverse applications in service, a variety of thermal and thermo-chemical surface treatment processes are used. Due to the high-velocity impact inherent in the process, high-velocity oxy-fuel (HVOF) spray is now frequently employed in industrial applications for its ability to generate a high-quality coating with appropriate hardness and low oxide content. In this investigation, a high-velocity oxy-fuel (HVOF) thermal spraying process was utilized to coat WC-10Co powders on a 35CrMo steel substrate. A water jet erosion test was also used to examine the substrate and coated samples’ erosion behavior. The erosion rate was systematically investigated using water jet variables such as the angle of impingement, water jet velocity, standoff distance, and erodent discharge. For the development of multiple regression models, experiments were performed utilizing the central composite rotatable design and the response surface methodology. The angle of impingement had the most impact on the rate of coating erosion, leading to the water jet velocity, standoff distance, and erodent discharge.

The role of the carbon diffusion zone presence for the fracture toughness and wear resistance of a single-phase Fe2B layer produced on pure iron by gas boriding in N2–H2–B(CH3O)3 atmosphere

The aim of this work was to indicate the importance of the presence of the carbon diffusion zone on the fracture toughness of Fe2B layer. Single-phase Fe2B layer was produced as a result of gas boriding processes and simultaneous gas borocarburizing processes. The continuous gaseous processes were carried out at 1223 K for 2 h in N2–H2–B(CH3O)3 atmosphere. The temperature of trimethyl borate influenced its concentration in the gas atmosphere, therefore it was possible to carry out the two types of process: boriding and borocarburizing. As a result, in the case of the borided sample, the area directly of the Fe2B layer contained ferrite. Whereas, in the case of the borocarburized sample, pearlite grains appeared beneath the iron borides layer. Fracture toughness measurements were performed on the top surface of the specimens using a Vickers indenter under a load range from 0.981 N to 4.905 N. Both borided and borocarburized layers represent ceramic materials with a radial-median cracking mode. In the case of application of an indentation load lower than 3.924 N, an increase in indentation load from 0.981 N to 3.924 N resulted in increased fracture toughness. Whereas, a further increase in the load to 4.905 N was the reason for KC diminishing. In the case of high indentation load, the presence of a carbon diffusion zone in the gas borocarburized sample caused an increase in fracture toughness by about 50% compared to the gas-borided sample. The iron boride layers produced by gas boriding and simultaneous gas borocarburizing provided higher wear resistance in comparison to the pure iron without thermo-chemical surface treatment.

Thermochemical surface hardening of Ti-6Al-4V: On the role of temperature and treatment media

The present work addresses the surface hardening and heat treatment response of Ti-6Al-4V grades G5 and G23 on thermochemical surface treatment in different temperature regimes as well as in different gaseous and plasma-based media. Grades G5 and G23 were subjected to gaseous surface hardening using various gas compositions and temperature regimes. Two different series of gaseous surface treatments were carried out: 1) Carbo-oxidizing of G23 in mill-annealed condition was performed in CO gas at (high) temperatures ranging from 777 to 1027 °C. Treatment at 1027 °C resulted in the deepest case with the formation of a thin surface layer of titanium sub-oxides supported by a thick TiC x O 1-x layer as the dominant phase. Post-nitriding of the carbo-oxidized specimen raised the hardness of the TiC x O 1-x phase by the incorporation of nitrogen, yielding values up to ~2900 HV and multi-layered structures. 2) Carbo-oxidizing of G5 with a bi-modal microstructure in CO/CO 2 gas mixture at (intermediate) temperatures in the range 677–777 °C. Treatment at 777 °C resulted in a surface hardness of ~1900 HV and a ~60 μm-thick diffusion zone. In addition to gaseous surface hardening, also plasma-assisted methods were applied and compared to their gaseous counterparts. Intermediate- and low-temperature plasma treatments were carried out on G23: 3) Plasma (carbo-nitro)oxidizing was performed in a CO 2 and N 2 gas mixture at 750 °C and (carbo-)oxidizing in CO 2 gas at 650 °C. 4) Plasma nitriding was conducted in a N 2 /H 2 gas mixture at 750 °C and 850 °C. The widely different types of treatments applied in this work represent the diversity of the gaseous and plasma-based thermochemical methods that can be applied for surface engineering of titanium alloys . • Surface hardening of titanium is feasible in widely different temperature regimes. • Hard δ-TiC x O 1-x requires a high a C to form at high temperatures. • High p O 2 maintains a high hardness value in the expanded α at lower temperatures. • Plasma nitriding reduces the required temperature for the formation of hard nitrides. • Temperature is the key factor for surface hardening using CO 2 .

Characterization of Chemically and Thermo-chemically Treated Water Reed and Mokolwane Palm Fibers

ABSTRACT Water reed (Phragmites spp.) and mokolwane palm (Hyphaene spp.) natural fibers, building materials indigenous to Botswana are potential reinforcement materials in manufacturing composites due to their desirable light weight, mechanical properties, and recyclable nature. Nevertheless, the surface modification mechanism and its effects on the quality characteristics of these fibers have not been explored. This study compares the impact of chemical and thermo-chemical surface treatments on the properties of the fibers. Furthermore, a suitable treatment method and fiber to produce natural fiber-reinforced polymer composites (NFRPCs) were identified. Thermo-chemical treatment is more effective in improving the thermal resistance and mechanical properties of the fibers relative to chemical treatment. Water reed fiber treated by 1.5 wt.% NaOH solution for 15 days followed by thermal treatment at 80°C for 24 hours is best for suitable for building insulation applications considering its quality characteristics (namely: tensile strength (76.41 MPa), CI (59.2%), CS (21.44 nm) and degradation temperature range of 288–598°C). Surface modification mechanism of the fibers by chemical treatment occurred via dissolution of hemicellulose which increased the interfibrillar region while the incorporation of thermal treatment further promoted the rupture of bonds existing between the cellulose and hemicellulose.

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

Austenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed.

Study of the Thermochemical Surface Treatment Effect on the Phase Precipitation and Degradation Behaviour of DSS and SDSS.

In this study, the effect of a plasma ion carburizing process to duplex and superduplex stainless steels (DSS and SDSS), at 925 °C for a long time, as thermochemical process influencing the microstructural evolution is presented. The objective is to analyse the diffusion elements’ influence on the precipitation of secondary phases after additional short thermal treatment. A comparison among the different treatments was performed after the resulting microstructures were analysed by Field Emission—Scanning Electron Microscope. Precipitation of secondary phases—sigma (σ), chi (χ), nitrides and carbides—seemed to occur during the treatments in a similar way for both steels (DSS and SDSS), although they showed a different morphology and precipitation mode. General corrosion behaviour of untreated and treated samples was investigated by potentiodynamic tests in order to prove their corrosion resistance. It was found that an improvement of the surface protection after the plasma carburizing process occurred.

Core microstructure-dependent bending fatigue behavior and crack growth of a case-hardened steel

Carburizing is a thermo-chemical surface treatment through which a very hard martensitic layer develops in the external surface (case) of steel components resulting in substantial improvement in the fatigue life. Nevertheless, the overall fatigue properties of carburized steel components are yet severely dependent on the microstructure which develops in the interior region (core). This paper deals with the effects of core microstructure on bending fatigue behavior and fatigue crack growth of carburized steel parts. V-notched steel specimens were fabricated and subjected to two case hardening cycles where, respectively, bainitic-martensitic and ferritic-bainitic-martensitic microstructures developed in the core regions supported by similar fully martensitic microstructures in the case. 4-point plane bending fatigue tests were conducted to study the fatigue behavior of the heat-treated specimens. Furthermore, the effects of the core microstructures on fatigue crack growth resistance were also investigated. Hardness measurements revealed that both batches of specimens have similar hardness properties on the exterior surfaces, in the case-hardened layers and also in the cores. Moreover, the results showed that the specimens with the bainitic-martensitic core microstructure provide a marginally better fatigue performance in the finite life regime as compared to the ferrite-containing counterparts. More noticeable difference was, however, observed in the corresponding endurance limits where the former demonstrated a higher magnitude than the latter. Besides, the bainitic-martensitic core microstructure resisted the fatigue crack propagation more effectively than the ferrite-containing specimens.

Effects of Thermochemical Surface Treatments on the Industrially Important Properties of X2CrNiMo 17-12-2 Austenitic Stainless Steel

Austenitic stainless steels have low corrosion resistance in applications where strong acids and vapors attack the surface, typically in food and chemical industries. This drawback can be improved by surface treatments. Salt bath, gaseous or plasma-based surface treatments are a diffusion process for improving the hardness of the surface layer of stainless steels without significantly affecting their corrosion resistance. Low temperature nitriding and carburizing process can form a diffusion zone or/and compound phase. The corrosion-wear resistance of austenitic stainless steels can also improve with low temperature plasma nitriding and carburizing. The effect of these treatments on hardness and corrosion resistance was investigated in this research. Optical microscopy and Vickers hardness test were used for the characterization of the surface and potentiodynamic tests were performed to determine the corrosion rate. The results show that the hardness of the kolsterised sample is higher compared to the plasma nitride one. Beside this property, the corrosion rate is similar, but pitting corrosion was observed on the surface, due to the Cr2N formation.

Hierarchical analysis of alloying element effects on gas nitriding rate of Fe alloys: A DFT, microkinetic and kMC study

Nitriding is the most widely employed thermochemical surface treatment to enhance the mechanical properties of steel. Specifically, gas nitriding, which is a low-temperature process for efficiently producing high-performance steels, has a disadvantage in that it consumes a large amount of time. To enhance the nitriding rate, we studied the surface alloying of iron (Fe) and its effect on ammonia (NH3) nitriding of Fe using a hierarchical protocol with density functional theory (DFT)-based microkinetics and real-time simulations. First, we considered the NH3 decomposition and nitrogen (N) diffusion mechanism on clean and alloyed (Fe-X) Fe (100) surfaces using DFT. In this study, the alloying elements including transition metals and period III to VI elements in the periodic table were considered for DFT-based computational screening. For the candidate Fe-X systems selected to improve the nitriding rate in the previous step, we calculated all the energy barriers for every elementary reaction step by varying the alloying elements and performed microkinetic analysis using those kinetic energy barriers to determine their influence on the nitriding rate. After adding consideration of thermodynamic factors, selected candidate alloys were subjected to detailed DFT calculations of the nitriding mechanism with N coverage, and based on these results, a kinetic Monte Carlo (kMC) simulation was performed to reconfirm the results under the actual nitriding process conditions. Through a hierarchical protocol, we performed a theoretical analysis and simulation of the effects of alloying elements on the nitriding rate that were not explained experimentally and suggested the best alloying element with the improved nitriding rate.

Effect of pre-existing VC carbides on nitriding and wear behavior of hot-work die steel

VC carbides are the common strengthening phases in the hot-work die steels containing V element, and gas nitriding is a popular thermo-chemical surface treatment, which has been using to improve thermal/mechanical fatigue and wear resistance of steels. In this study, the effects of VC precipitation on the hardness, thickness and wear resistance of the nitrided layer in AISI H13 hot-work die steel were systematically investigated. The results showed that a part of carbon atoms in the fine VC carbides were replaced by the nitrogen atoms during nitriding at 550 °C and some nitrogen were released to the matrix when the sample was cooled down to room temperature, so as to promote the hardness of the diffusion layer. Besides, VC precipitation promoted the diffusion of nitrogen by decreasing the C and V concentrations in the matrix, which decreased the diffusion activation energy of nitrogen in the matrix and made the diffusion layer thicker. Moreover, the thicker and harder nitride layer improved the wear resistance of the sample at a load of 100 N.

Analysis of boride layer thickness of borided AISI 430 by response surface methodology

The boriding process is a thermochemical surface treatment which can be applied to many iron and non-ferrous materials and improves the properties of the material such as hardness, wear resistance. In the present study, the layer thickness values of the boronized AISI 430 material were optimized using the Response Surface Methodology. Mathematical model was constructed using parameters such as temperature and time and the results were analyzed comparatively. As a result of the analysis, the optimum layer thickness value for AISI 430 material was obtained as 39.0183 µm for 1000 ºC and 5.9h and it was determined that the boriding temperature and time are effective on the boride layer formation process of AISI 430 material. Finally, the Response Surface Methodology and Face Centered Central Composite Design have been effectively applied to the boriding process.

Hierarchical Analysis of Alloying Element Effects on Gas Nitriding Rate of Fe Alloys: A DFT, Microkinetic and kMC Study

Nitriding is the most widely employed thermochemical surface treatment to enhance the mechanical properties of steel. Specifically, gas nitriding, which is a low-temperature process for efficiently producing high-performance steels, has a disadvantage in that it consumes a large amount of time. To enhance the nitriding rate, we studied the surface alloying of iron (Fe) and its effect on ammonia nitriding of Fe using a hierarchical protocol with density functional theory (DFT)-based microkinetics and real-time simulations. First, we considered the NH3 decomposition and nitrogen (N) diffusion mechanism on clean and alloyed (Fe-X) Fe (100) surfaces using DFT. In this study, the alloying elements including transition metals and period III to VI elements in the periodic table were considered for DFT-based computational screening. For the candidate Fe-X systems selected to improve the nitriding rate in the previous step, we calculated all the energy barriers for every elementary reaction step by varying the alloying elements and performed microkinetic analysis using those kinetic energy barriers to determine their influence on the nitriding rate. After adding consideration of thermodynamic factors, selected candidate alloys were subjected to detailed DFT calculations of the nitriding mechanism with N coverage, and based on these results, a kinetic Monte Carlo (kMC) simulation was performed to reconfirm the results under the actual nitriding process conditions. Through a hierarchical protocol, we performed a theoretical analysis and simulation of the effects of alloying elements on the nitriding rate that were not explained experimentally and suggested the best alloying element with the improved nitriding rate.

Experimental Study on Wear Resistance of AISI 347 Treated With Salt Bath Nitriding and Gas Nitriding Processes-A Review

The surface treatments are very much essential for improvement in terms of wear resistance and increase in the service lifetime of stainless steels as they have poor wear resistance and easily corroded to the oxidation.This paper reviews the two thermo chemical surface treatments (salt bath nitriding and gas nitriding followed by the post oxidation) applied to AISI 347material.Morphological characterizations were carried out with the help of scanning electron microscope, X-Ray Diffractometer and Pin-on-disc wear testing machine. The results of the both the processes were compared and discussed. From the detailed study made by authors the results shown that with gas nitriding the wear resistance of the material is improved more compared with the salt bath nitriding.

Solid carbon active screen plasma nitrocarburizing of AISI 316L stainless steel: Influence of N2-H2 gas composition on structure and properties of expanded austenite

Plasma nitrocarburizing based on active screen technology using a carbon-fiber reinforced carbon active screen was applied in an industrial-scale unit for thermochemical surface treatment of austenitic stainless steel. This concept is based on the use of a solid-carbon-source for the generation of highly reactive process gases directly in the active screen plasma. In this work, plasma nitrocarburizing of AISI 316L stainless steel was performed by means of the variation of the precursor gases H2 and N2 in the range of 0% N2 up to 100% N2 without the use of any additional carbon bearing gas. For a set of H2-N2 gas mixtures, the resulting reaction gas was monitored using infrared laser absorption spectroscopy (IRLAS). The four main stable species HCN, CH4, NH3 and C2H2 were detected. A detailed analysis of the surface microstructure resulting for each specific H2-N2 precursor gas mixture was performed. This included glow discharge optical emission spectroscopy (GDOES), optical microscopy, micro hardness measurements, as well as X-ray diffraction analysis.The concentrations of hydrocarbons CH4 and C2H2 were most abundant in case of pure hydrogen plasma, and a carbon-expanded austenite layer with hardness values up to 600 HK0.01 and smooth hardness gradient resulted. The admixture of N2 to the precursor gas significantly increased the concentrations of HCN and NH3. Due to this, a duplex structure of nitrogen-expanded austenite γN and carbon-expanded austenite γC formed. With increasing content of N2 up to 50% in the precursor gas, the resulting layer thickness increased and the hardness reached values up to 1300 HK0.01. At strong nitrogen excess in the precursor gas, the nitrogen concentration in the expanded austenite significantly increased, the Fe2–3(N, C) phase was formed, and simultaneously the layer thickness decreased. Structure and properties of the expanded austenite layer significantly changed by only varying the H2-N2 ratio of the precursor gas mixture.

The Effects of Screen Sizes on the Surface Properties of Tepered Steel Treated by Active Screen Plasma Nitriding

Active screen plasma nitriding (ASPN) is a novel thermochemical surface treatment which has many specialities compared to the conventional direct current plasma nitriding (DCPN). The edge effect is eliminated, the hardness and layer thickness on the surface are homogeneously distributed. In this study, 42CrMo4 alloyed steel was treated with different screen sizes (screen diameter, hole size) active screen plasma nitriding. The parameters of treatment are similar in all cases (4 hours at 490°C and 2,2 torr). The nitrided samples were characterized by atomic force microscope (AFM) to analyse the roughness of the surface. The cross-section hardness of the samples and the thickness of the nitrided layers were also measured.

Optimization of gaseous nitriding of carbon iron-based alloy based on fatigue resistance modelling

Gaseous nitriding of steels is a well-established thermochemical surface treatments that increases the fatigue resistance of treated parts. The present work proposes a modelling of the fatigue life as a function of the applied stress. It takes into account the hardness and compressive residual stresses that developed during nitriding. The model is suitable to help optimizing nitriding parameters by reverse modelling.