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Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and...
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Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties

A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Metal Casting, Forming and Heat Treatment".

Deadline for manuscript submissions: closed (31 January 2023) | Viewed by 24532

Special Issue Editors

Dr. Shinichiro Adachi

E-Mail Website
Guest Editor
Osaka Research Institute of Industrial Science and Technology, Osaka 594-1157, Japan
Interests: plasma nitriding; plasma carburizing; thermal spraying; laser metal deposition
Special Issues, Collections and Topics in MDPI journals
Dr. Francesca Borgioli

E-Mail Website
Guest Editor
Department of Industrial Engineering (DIEF), Università di Firenze, via di S. Marta 3, 50139 Firenze, Italy
Interests: surface engineering; plasma nitriding; plasma carburizing; expanded austenite; expanded ferrite; expanded martensite
Special Issues, Collections and Topics in MDPI journals
Dr. Thomas Lindner

E-Mail Website
Guest Editor
Materials and Surface Engineering Group, Institute of Materials Science and Engineering, Chemnitz University of Technology, 09107 Chemnitz, Germany
Interests: thermal spraying; thermochemical treatment; heat treatment; spark plasma sintering; laser cladding; high-entropy alloys; stainless steel
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The formation of expanded austenite phase (S-phase) by low-temperature nitriding and carburizing of stainless steels was developed 35 years ago. Initially, this method was applied to austenitic stainless steels, but it has been extended to duplex stainless steels and martensitic precipitation-hardening stainless steels. In addition, those low-temperature treatments are also performed on a variety of other metallic materials. The discovery of important scientific findings and their practical application in industry have been achieved. In recent years, it has been combined with new processes such as thermal spray coating and is expected to contribute to the manufacturing of the next generation.

This Special Issue on “Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties” intends to cover original research and critical review articles on recent advances in all aspects of low-temperature nitriding and carburizing.

In particular, the topics of interest include, but are not limited to the following:

  • Fundamentals and new concepts
  • Material properties and metallurgical characterization
  • Application to novel stainless steel and metallic material alloys
  • Combined with other manufacturing processes
  • Industrial applications

Dr. Shinichiro Adachi
Prof. Dr. Francesca Borgioli
Dr. Thomas Lindner
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Metals is an international peer-reviewed open access monthly journal published by MDPI.

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Keywords

  • Low-temperature nitriding
  • Low-temperature carburizing
  • Expanded austenite (S-phase)
  • Metallic material properties
  • Metallurgical characterization

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Published Papers (13 papers)

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Research

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16 pages, 6004 KiB  
Article
Comparison of Nitriding Behavior for Austenitic Stainless Steel 316Ti and Super Austenitic Stainless Steel 904L
by Stephan Mändl and Darina Manova
Metals 2024, 14(6), 659; https://doi.org/10.3390/met14060659 - 1 Jun 2024
Viewed by 567
Abstract
In situ X-ray diffraction (XRD) was used to compare nitrogen low-energy ion implantation (LEII) into austenitic stainless steel 316Ti and super austenitic stainless steel 904L. While the diffusion and layer growth were very similar, as derived from the decreasing intensity of the substrate [...] Read more.
In situ X-ray diffraction (XRD) was used to compare nitrogen low-energy ion implantation (LEII) into austenitic stainless steel 316Ti and super austenitic stainless steel 904L. While the diffusion and layer growth were very similar, as derived from the decreasing intensity of the substrate reflection, strong variations in the observed lattice expansion—as a function of orientation, the steel alloy, and nitriding temperature—were observed. Nevertheless, a similar resulting nitrogen content was measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Furthermore, for some conditions, the formation of a double layer with two distinct lattice expansions was observed, especially for steel 904L. Regarding the stability of expanded austenite, 316Ti had already decayed in CrN during nitriding at 500 °C, while no such effect was observed for 904L. Thus, the alloy composition has a strong influence only on the lattice expansion and the stability of expanded austenite—but not the diffusion and nitrogen content. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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15 pages, 11066 KiB  
Article
Identification of Expanded Austenite in Nitrogen-Implanted Ferritic Steel through In Situ Synchrotron X-ray Diffraction Analyses
by Bruna C. E. Schibicheski Kurelo, Carlos M. Lepienski, Willian R. de Oliveira, Gelson B. de Souza, Francisco C. Serbena, Rodrigo P. Cardoso, Julio C. K. das Neves and Paulo C. Borges
Metals 2023, 13(10), 1744; https://doi.org/10.3390/met13101744 - 14 Oct 2023
Cited by 1 | Viewed by 1133
Abstract
The existence and formation of expanded austenite in ferritic stainless steels remains a subject of debate. This research article aims to provide comprehensive insights into the formation and decomposition of expanded austenite through in situ structure analyses during thermal treatments of ferritic steels. [...] Read more.
The existence and formation of expanded austenite in ferritic stainless steels remains a subject of debate. This research article aims to provide comprehensive insights into the formation and decomposition of expanded austenite through in situ structure analyses during thermal treatments of ferritic steels. To achieve this objective, we employed the Plasma Immersion Ion Implantation (PIII) technique for nitriding in conjunction with in situ synchrotron X-ray diffraction (ISS-XRD) for microstructural analyses during the thermal treatment of the samples. The PIII was carried out at a low temperature (300–400 °C) to promote the formation of metastable phases. The ISS-XRD analyses were carried out at 450 °C, which is in the working temperature range of the ferritic steel UNS S44400, which has applications, for instance, in the coating of petroleum distillation towers. Nitrogen-expanded ferrite (αN) and nitrogen-expanded austenite (γN) metastable phases were formed by nitriding in the modified layers. The production of the αN or γN phase in a ferritic matrix during nitriding has a direct relationship with the nitrogen concentration attained on the treated surfaces, which depends on the ion fluence imposed during the PIII treatment. During the thermal evolution of crystallographic phase analyses by ISS-XRD, after nitriding, structure evolution occurs mainly by nitrogen diffusion. In the nitrided samples prepared under the highest ion fluences—longer treatment times and frequencies (PIII 300 °C 6 h and PIII 400 °C 3 h) containing a significant amount of γN—a transition from the γN phase to the α and CrN phases and the formation of oxides occurred. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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19 pages, 13061 KiB  
Article
Assessment of the Pitting, Crevice Corrosion, and Mechanical Properties of Low-Temperature Plasma-Nitrided Inconel Alloy 718
by Yamid Nuñez de la Rosa, Oriana Palma Calabokis, Vladimir Ballesteros-Ballesteros, Cristian Lozano Tafur and Paulo C. Borges
Metals 2023, 13(7), 1172; https://doi.org/10.3390/met13071172 - 23 Jun 2023
Cited by 3 | Viewed by 1450
Abstract
A comparative study on the mechanical properties, scratch resistance, and localized corrosion (pitting and crevice) of plasma-nitrided Inconel alloy 718 (UNS NO7718: IN 718) was carried out. Thermochemical treatment was performed at low temperatures (400 and 450 °C) for 4 h. The treatment [...] Read more.
A comparative study on the mechanical properties, scratch resistance, and localized corrosion (pitting and crevice) of plasma-nitrided Inconel alloy 718 (UNS NO7718: IN 718) was carried out. Thermochemical treatment was performed at low temperatures (400 and 450 °C) for 4 h. The treatment formed layers with thicknesses of 7.17 ± 0.89 µm (400 °C) and 7.96 ± 0.48 µm (450 °C). The XRD and nanohardness analyses indicated the formation of a hard layer composed of the expanded austenite phase (γN), CrN at 400 °C, and CrN + γ at 450 °C, with a maximum indentation hardness of 12 and 12.5 GPa, respectively, when compared to the 5 GPa substrate hardness. The scratching tests (2–8 N) showed that with increasing load, the nitrided surfaces had a transition from 100% microcutting to a combination of microplowing/cutting, with the presence of cracks. The critical load of the nitrided surfaces was 3 N for 400 °C and 4 N for 450 °C. The untreated condition maintained a crack-free combined mechanism regardless of the load. For the same load, the nitrided surfaces held lower coefficient of friction values and higher scratch resistance values, which were more pronounced at 450 °C. The linear polarization tests (3.56 wt.% NaCl) showed pitting corrosion in all samples, with the 450 °C condition being less resistant. Nitriding at 400 °C increased the crevice corrosion resistance of Inconel, while at 450 °C, it severely damaged it. Nitriding at 400 °C brought concomitant gains in hardness and scratch and crevice corrosion resistance when compared to the as-received IN 718. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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15 pages, 5449 KiB  
Article
Electrochemical Corrosion Behavior of Ti-N-O Modified Layer on the TC4 Titanium Alloy Prepared by Hollow Cathodic Plasma Source Oxynitriding
by Jiwen Yan, Minghao Shao, Zelong Zhou, Zhehao Zhang, Xuening Yi, Mingjia Wang, Chengxu Wang, Dazhen Fang, Mufan Wang, Bing Xie, Yongyong He and Yang Li
Metals 2023, 13(6), 1083; https://doi.org/10.3390/met13061083 - 7 Jun 2023
Cited by 3 | Viewed by 1198
Abstract
TC4 alloy is widely used in dental implantation due to its excellent biocompatibility and low density. However, it is necessary to further improve the corrosion resistance and surface hardness of the titanium alloy to prevent surface damage that could result in the release [...] Read more.
TC4 alloy is widely used in dental implantation due to its excellent biocompatibility and low density. However, it is necessary to further improve the corrosion resistance and surface hardness of the titanium alloy to prevent surface damage that could result in the release of metal ions into the oral cavity, potentially affecting oral health. In this study, Ti-N-O layers were fabricated on the surface of TC4 alloy using a two-step hollow cathode plasma source oxynitriding technique. This resulted in the formation of TiN, Ti2N, TiO2, and nitrogen-stabilized α(N)-Ti phases on the TC4 alloy, forming a Ti-N-O modified layer. The microhardness of the samples treated with plasma oxynitriding (PNO) was found to be 300–400% higher than that of untreated (UN) samples. The experimental conditions were set at 520 °C, and the corrosion current density of the PNO sample was measured to be 7.65 × 10−8 A/cm2, which is two orders of magnitude lower than that of the UN sample. This indicates that the PNO-treated TC4 alloy exhibited significantly improved corrosion resistance in the artificial saliva solutions. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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12 pages, 11627 KiB  
Article
Mechanical and Magnetic Investigations of Balls Made of AISI 1010 and AISI 1085 Steels after Nitriding and Annealing
by Sławomir Maksymilian Kaczmarek, Jerzy Michalski, Tadeusz Frączek, Agata Dudek, Hubert Fuks and Grzegorz Leniec
Metals 2023, 13(6), 1060; https://doi.org/10.3390/met13061060 - 1 Jun 2023
Viewed by 1241
Abstract
This paper discusses the changes in the phase composition and magnetic properties of the AISI 1010 and AISI 1085 steels that were nitrided at 570 °C in an ammonia atmosphere for 5 h and that were then annealed at 520 °C in a [...] Read more.
This paper discusses the changes in the phase composition and magnetic properties of the AISI 1010 and AISI 1085 steels that were nitrided at 570 °C in an ammonia atmosphere for 5 h and that were then annealed at 520 °C in a N2/Ar atmosphere for 4 h. The test samples were made in the form of balls with diameters of less than 5 mm. The thickness of the obtained iron nitride layers was assessed through metallographic tests, while the phase composition was verified through X-ray tests. The magnetic properties were determined using ferromagnetic resonance (FMR) and superconducting quantum interference device (SQUID) techniques. Our research shows that, during the annealing of iron nitrides with a structure of ε + γ′, the ε phase decomposes first. As a result of this process, an increase in the content of the γ′ phase of the iron nitride is observed. When the ε phase is completely decomposed, the γ′ phase begins to decompose. The observed FMR signals did not come from isolated ions but from more magnetically complex systems, e.g., Fe–Fe pairs or iron clusters. Studies have shown that nitriding and annealing can be used to modify the magnetic properties of the tested steels. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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19 pages, 6095 KiB  
Article
Tribo-Mechanical Behavior of Films and Modified Layers Produced by Cathodic Cage and Glow Discharge Plasma Nitriding Techniques
by Bruna C. E. Schibicheski Kurelo, Gelson Biscaia De Souza, Silvio Luiz Rutz Da Silva, Carlos Maurício Lepienski, Clodomiro Alves Júnior, Rafael Fillus Chuproski and Giuseppe Pintaúde
Metals 2023, 13(2), 430; https://doi.org/10.3390/met13020430 - 19 Feb 2023
Cited by 2 | Viewed by 1702
Abstract
Two surface modification techniques, the glow discharge plasma nitriding (GDPN) and the cathodic cage plasma nitriding (CCPN), were compared regarding the mechanical and tribological behavior of layers produced on AISI 316 stainless-steel surfaces. The analyses were carried out at the micro/nanoscale using nanoindentation [...] Read more.
Two surface modification techniques, the glow discharge plasma nitriding (GDPN) and the cathodic cage plasma nitriding (CCPN), were compared regarding the mechanical and tribological behavior of layers produced on AISI 316 stainless-steel surfaces. The analyses were carried out at the micro/nanoscale using nanoindentation and nanoscratch tests. The nitriding temperature (°C) and time (h) parameters were 350/6, 400/6, and 450/6. Morphology, structure, and microstructure were evaluated by X-ray diffraction, scanning electron and optical interferometry microscopies, and energy-dispersive X-ray spectroscopy. GDPN results in stratified modified surfaces, solidly integrated with the substrate, with a temperature-dependent composition comprising nitrides (γ’-Fe4N, ε-Fe2+xN, CrN) and N-solid solution (γN phase). The latter prevails for the low treatment temperatures. Hardness increases from ~2.5 GPa (bare surface) to ~15.5 GPa (450 °C). The scratch resistance of the GDPN-modified surfaces presents a strong correlation with the layer composition and thickness, with the result that the 400 °C condition exhibits the highest standards against microwear. In contrast, CCPN results in well-defined dual-layers for any of the temperatures. A top 0.3–0.8 µm-thick nitride film (most ε-phase), brittle and easily removable under scratch with loads as low as 63 mN, covers a γN-rich case with hardness of 10 GPa. The thickness of the underneath CCPN layer produced at 450 °C is similar to that from GDPN at 400 °C (3 µm); on the other hand, the average roughness is much lower, comparable to the reference surface (Ra ~10 nm), while the layer formation involves no chromium depletion. Moreover, edge effects are absent across the entire sample´s surface. In conclusion, among the studied conditions, the GDPN 400 °C disclosed the best tribo-mechanical performance, whereas CCPN resulted in superior surface finishing for application purposes. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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11 pages, 2866 KiB  
Article
Electronic Structure and Hardness of Mn3N2 Synthesized under High Temperature and High Pressure
by Shoufeng Zhang, Chao Zhou, Guiqian Sun, Xin Wang, Kuo Bao, Pinwen Zhu, Jinming Zhu, Zhaoqing Wang, Xingbin Zhao, Qiang Tao, Yufei Ge and Tian Cui
Metals 2022, 12(12), 2164; https://doi.org/10.3390/met12122164 - 16 Dec 2022
Viewed by 1573
Abstract
The hardness of materials is a complicated physical quantity, and the hardness models that are widely used do not function well for transition metal light element (TMLE) compounds. The overestimation of actual hardness is a common phenomenon in hardness models. In this work, [...] Read more.
The hardness of materials is a complicated physical quantity, and the hardness models that are widely used do not function well for transition metal light element (TMLE) compounds. The overestimation of actual hardness is a common phenomenon in hardness models. In this work, high-quality Mn3N2 bulk samples were synthesized under high temperature and high pressure (HTHP) to investigate this issue. The hardness of Mn3N2 was found to be 9.9 GPa, which was higher than the hardness predicted using Guo’s model of 7.01 GPa. Through the combination of the first-principle simulations and experimental analysis, it was found that the metal bonds, which are generally considered helpless to the hardness of crystals, are of importance when evaluating the hardness of TMLE compounds. Metal bonds were found to improve the hardness in TMLEs without strong covalent bonds. This work provides new considerations for the design and synthesis of high-hardness TMLE materials, which can be used to form wear-resistant coatings over the surfaces of typical alloy materials such as stainless steels. Moreover, our findings provide a basis for establishing a more comprehensive theoretical model of hardness in TMLEs, which will provide further insight to improve the hardness values of various alloys. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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14 pages, 5303 KiB  
Article
The Effect of Cathodic Cage Plasma TiN Deposition on Surface Properties of Conventional Plasma Nitrided AISI-M2 Steel
by Luiz Henrique Portela de Abreu, Muhammad Naeem, Renan Matos Monção, Thercio H. C. Costa, Juan C. Díaz-Guillén, Javed Iqbal and Rômulo Ribeiro Magalhães Sousa
Metals 2022, 12(6), 961; https://doi.org/10.3390/met12060961 - 2 Jun 2022
Cited by 6 | Viewed by 2059
Abstract
In this study, a combination of conventional plasma nitriding and cathodic cage plasma deposition (CCPD) at different temperatures (400 and 450 °C) is implemented to enhance the surface properties of AISI-M2 steel. This combination effectively improves the surface hardness and the formation of [...] Read more.
In this study, a combination of conventional plasma nitriding and cathodic cage plasma deposition (CCPD) at different temperatures (400 and 450 °C) is implemented to enhance the surface properties of AISI-M2 steel. This combination effectively improves the surface hardness and the formation of a favorable hardness gradient toward the core, which would benefit the load-bearing capacity of substrate. The duplex-treated samples exhibit iron nitrides Fe4N, Fe2−3N and titanium nitride TiN phases. The thickness of the hard-TiN layer is 1.35 and 2.37 μm, whereas the combined thickness of the hard film and diffusion layer is 87 and 124 μm, for treatment at 400 and 450 °C, respectively. The wear rate and friction coefficient are dramatically reduced by duplex treatment. The oxidative wear mechanism and adhesive wear mechanism are dominant for duplex-treated samples. This study suggests that the cathodic cage plasma deposition technique can attain a combination of hard film and diffusion layer. The plasma nitriding before CCPD is beneficial for attaining an adequate nitrogen diffusion layer thickness. The drawbacks of conventional TiN film deposition, such as “egg-shell” problems, can be removed. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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11 pages, 2686 KiB  
Article
Cold Gas Spraying of Solution-Hardened 316L Grade Stainless Steel Powder
by Thomas Lindner, Martin Löbel, Maximilian Grimm and Jochen Fiebig
Metals 2022, 12(1), 30; https://doi.org/10.3390/met12010030 - 24 Dec 2021
Cited by 4 | Viewed by 2180
Abstract
Austenitic steels are characterized by their outstanding corrosion resistance. They are therefore suitable for a wide range of surface protection requirements. The application potential of these stainless steels is often limited by their poor wear resistance. In the field of wrought alloys, interstitial [...] Read more.
Austenitic steels are characterized by their outstanding corrosion resistance. They are therefore suitable for a wide range of surface protection requirements. The application potential of these stainless steels is often limited by their poor wear resistance. In the field of wrought alloys, interstitial surface hardening has become established for simultaneously acting surface stresses. This approach also offers great potential for improvement in the field of coating technology. The hardening of powder feedstock materials promises an advantage in the treatment of large components and also as a repair technology. In this work, the surface hardening of AISI 316L powder and its processing by thermal spraying is presented. A partial formation of the metastable expanded austenitic phase was observed for the powder particles by low-temperature gas nitrocarburizing. The successful deposition was demonstrated by cold gas spraying. The amount of expanded austenitic phase within the coating structure strongly depends on the processing conditions. Microstructure, corrosion and wear behavior were studied. Process diagnostic methods were used to validate the results. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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18 pages, 10791 KiB  
Article
Rapid Alloy Surface Engineering through Closed-Vessel Reagent Pyrolysis
by Cyprian Illing, Zhe Ren, Anna Agaponova, Arthur Heuer and Frank Ernst
Metals 2021, 11(11), 1764; https://doi.org/10.3390/met11111764 - 2 Nov 2021
Cited by 3 | Viewed by 1540
Abstract
For rapid surface engineering of Cr-containing alloys by low-temperature nitrocarburization, we introduce a process based on pyrolysis of solid reagents, e.g., urea, performed in an evacuated closed vessel. Upon heating to temperatures high enough for rapid diffusion of interstitial solute, but low enough [...] Read more.
For rapid surface engineering of Cr-containing alloys by low-temperature nitrocarburization, we introduce a process based on pyrolysis of solid reagents, e.g., urea, performed in an evacuated closed vessel. Upon heating to temperatures high enough for rapid diffusion of interstitial solute, but low enough to avoid second-phase precipitation, the reagent is pyrolyzed to a gas atmosphere containing molecules that (i) activate the alloy surface by stripping away the passivating Cr2O3-rich surface film (diffusion barrier) and (ii) rapidly infuse carbon and nitrogen into the alloy. We demonstrate quantitatively that this method can generate a subsurface zone with concentrated carbon and nitrogen comparable to what can be accomplished by established (e.g., gas-phase- or plasma-based) methods, but with significantly reduced processing time. As another important difference to established gas-phase processing, the interaction of gas molecules with the alloy surface can have auto-catalytic effects by altering the gas composition in a way that accelerates solute infusion by providing a high activity of HNCO. The new method lends itself to rapid experimentation with a minimum of laboratory equipment. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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13 pages, 2391 KiB  
Article
Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing
by Hyunseok Cheon, Kyu-Sik Kim, Sunkwang Kim, Sung-Bo Heo, Jae-Hun Lim, Jun-Ho Kim and Seog-Young Yoon
Metals 2021, 11(11), 1762; https://doi.org/10.3390/met11111762 - 2 Nov 2021
Cited by 2 | Viewed by 1948
Abstract
The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the [...] Read more.
The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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13 pages, 5898 KiB  
Article
Formation and Properties of Nitrocarburizing S-Phase on AISI 316L Stainless Steel-Based WC Composite Layers by Low-Temperature Plasma Nitriding
by Shinichiro Adachi, Takuto Yamaguchi and Nobuhiro Ueda
Metals 2021, 11(10), 1538; https://doi.org/10.3390/met11101538 - 27 Sep 2021
Cited by 7 | Viewed by 2459
Abstract
Stainless steel-based WC composite layers fabricated by a laser cladding technique, have strong mechanical strength. However, the wear resistance of WC composite layers is not sufficient for use in severe friction and wear environments, and the corrosion resistance is significantly reduced by the [...] Read more.
Stainless steel-based WC composite layers fabricated by a laser cladding technique, have strong mechanical strength. However, the wear resistance of WC composite layers is not sufficient for use in severe friction and wear environments, and the corrosion resistance is significantly reduced by the formation of secondary carbides. Low-temperature plasma nitriding and carburizing of austenitic stainless steels, treated at temperatures of less than 450 °C, can produce a supersaturated solid solution of nitrogen or carbon, known as the S-phase. The combined treatment of nitriding and carburizing can form a nitrocarburizing S-phase, which is characterized by a thick layer and superior cross-sectional hardness distribution. During the laser cladding process, free carbon was produced by the decomposition of WC particles. To achieve excellent wear and corrosion resistance, we attempted to use this free carbon to form a nitrocarburizing S-phase on AISI 316 L stainless steel-based WC composite layers by plasma nitriding alone. As a result, the thick nitrocarburizing S-phase was formed. The Vickers hardness of the S-phase ranged from 1200 to 1400 HV, and the hardness depth distribution became smoother. The corrosion resistance was also improved through increasing the pitting resistance equivalent numbers due to the nitrogen that dissolved in the AISI 316 L steel matrix. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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Review

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39 pages, 5019 KiB  
Review
The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review
by Francesca Borgioli
Metals 2022, 12(2), 331; https://doi.org/10.3390/met12020331 - 14 Feb 2022
Cited by 38 | Viewed by 3905
Abstract
Low-temperature treatments have become a valuable method for improving the surface hardness of stainless steels, and thus their tribological properties, without impairing their corrosion resistance. By using treatment temperatures lower than those usually employed for nitriding or carburizing of low alloy steels or [...] Read more.
Low-temperature treatments have become a valuable method for improving the surface hardness of stainless steels, and thus their tribological properties, without impairing their corrosion resistance. By using treatment temperatures lower than those usually employed for nitriding or carburizing of low alloy steels or tool steels, it is possible to obtain a fairly fast (interstitial) diffusion of nitrogen and/or carbon atoms; on the contrary, the diffusion of substitutional atoms, as chromium atoms, has significantly slowed down, therefore the formation of chromium compounds is hindered, and corrosion resistance can be maintained. As a consequence, nitrogen and carbon atoms can be retained in solid solutions in an iron lattice well beyond their maximum solubility, and supersaturated solid solutions are produced. Depending on the iron lattice structure present in the stainless steel, the so-called “expanded austenite” or “S-phase”, “expanded ferrite”, and “expanded martensite” have been reported to be formed. This review summarizes the main studies on the characteristics and properties of these “expanded” phases and of the modified surface layers in which these phases form by using low-temperature treatments. A particular focus is on expanded martensite and expanded ferrite. Expanded austenite–S-phase is also discussed, with particular reference to the most recent studies. Full article
(This article belongs to the Special Issue Advances in Low-Temperature Nitriding and Carburizing of Stainless Steels and Metallic Materials: Formation and Properties)
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