Solution Nitriding Research Articles

Influence of Nitrogen Introduced by Solution Nitriding on Microstructure and Room-temperature Mechanical Properties of Grade 91 Steel

Influence of Nitrogen Introduced by Solution Nitriding on Microstructure and Room-temperature Mechanical Properties of Grade 91 Steel

Isothermal aging treatment and its effect on the mechanical properties of Fe-Cr-Mn high-nitrogen austenitic steel

The high-temperature solution nitriding process is a suitable treatment for producing high-nitrogen steels by the diffusion of nitrogen from the surface to the center of steels under high nitrogen pressure at high temperatures. On the other hand, long-term solution nitriding at high temperatures can cause the formation of coarse-grained austenite. This study focused on the grain size, strength, and elongation properties of Fe-Cr-Mn high-nitrogen austenitic steels by isothermal aging. For this purpose, we produced high-nitrogen steels by three-step phase transformations: (i) high-temperature solution nitriding, (ii) isothermal aging, and (iii) reaustenitization. After solution nitriding, high-nitrogen austenitic steel was treated with isothermal aging to induce austenite decomposition. Supersaturated austenite (ϒ) transformed to less nitrogen-saturated austenite (ϒ′) and discontinuous cellular precipitation (DCP) during the isothermal aging treatment. Subsequently, the decomposed structure was reversed into austenite through reaustenitization. The results suggested that austenite grain refinement and increasing hardness were achieved by nucleating reversed austenite grains after reaustenitization treatment. On the other hand, a noticeable increase in elongation appeared after reaustenitization treatment for 30 min.

CALPHAD, preparation, and mechanical properties of quaternary solid solution nitrides with various nitrogen stoichiometry

High-entropy ceramics have gained increasing interest due to unique properties unattainable by traditional materials. Though a large number of high-entropy ceramics have emerged, there was still in lack of theoretical predictions of phase composition on multicomponent solid solutions, which was probably caused by the composition complexity. In this work, employing Thermo-Calc and TCHEA4 database, a series of equilibrium phase data was generated from quaternary solid solution nitrides formed by the five elements of Ti, V, Cr, Zr, and N, while five quaternary solid solution nitrides with various nitrogen stoichiometry were prepared by SPS utilizing commercially available transition metal and their nitrides at 1600 °C. The detection of phase composition during the experiment and thermodynamic calculation confirmed that an increase in nitrogen content was beneficial for the formation of a single-phase solid solution. Due to the variation of nitrogen stoichiometry, the concentrations of nitrogen-metal bonds changed accordingly, resulting in different relative densities and mechanical properties. On the basis of the experimental data, CALPHAD, with a maximum absolute error of only 8.31 %, was established, providing a powerful tool for predicting phase in the complex composition-temperature space of compositionally complex ceramics.

A facile synthesis of 2D bimetallic solid solution nitrides for robust stability in lithium-ion battery

Nanostructured transition metal nitrides (TMNs) demonstrate immense promise for lithium-ion battery applications owing to their outstanding conductivity and high theoretical capacity. However, challenges such as the expansion of volume, aggregation concerns, and rapid capacity decay still need to be effectively addressed. Herein, a facile synthesis of 2D nitrides, including TiVN, NbVN and TiNbN, is achieved through nitriding their corresponding MXenes. The distinct 2D layered structure enables them fast lithium-ion transport and inhibits volume variation, thereby improving rate performance and stability. Moreover, the incorporation of bimetallic elements in solid solution induces lattice distortion, creating lattice vacancies and increasing lithium storage capacity. Atomic-level interactions within solid solutions, confirming by the shifts of binding energy, contribute to improved structural stability and accelerated charge transfer. The theoretical calculation further proves that the bimetallic solid solution structure elevates the d-band center and increases the Li+ adsorption energy. Consequently, the synthesized TiVN, NbVN and TiNbN demonstrate remarkable lithium storage performance. Especially, NbVN with a porous morphology stands out by achieving a specific capacity of 216.6 mAh/g after 1,400 cycles at 1 A/g, significantly outperforming Nb4N5 and VN. This work may provide valuable insights for the efficient synthesis and electrochemical applications of two-dimensional solid-solution nitrides.

New White Etch Cracking resistant martensitic stainless steel for bearing applications by high temperature solution nitriding

White Etch Cracking (WEC) is a severe and unpredictable failure mode that has been frequently observed in, among others, bearings for wind energy applications. In this work a novel, patented, WEC resistant bearing steel solution is developed through high temperature solution nitriding, followed by a custom heat treatment. Conventional medium carbon, 13 wt% Cr martensitic stainless steels were subjected to the novel process to obtain a nitrogen-rich case. A series of soft annealing, austenitization, cryogenic and tempering treatments converted the nitrogen-rich region into a fine-grained case containing nano-sized precipitates. Test bearings and tribometer specimens were treated with the same sequence of treatments. These specimens were tested under two different accelerated WEC failure promoting conditions. In contrast to standard through-hardened bearing steel the nitrided specimens showed no WEC formation, not even after prolonged testing.

Study on surface nitriding treatment of the sintered SUS430 ferritic stainless steel

Ferritic stainless steels with the advantages of high thermal conductivity, low expansion coefficient, excellent oxidation resistance, resistance to stress corrosion capacity, and low cost are widely applied. The parts are prepared by powder metallurgy with SUS430 ferritic stainless steel. Under an H2 protective atmosphere, the compactions are sintered at 1300 °C, and the density of the as-sintered sample achieves 7.4 g/cm3. Then, high-temperature solid solution nitriding treatment is carried out at 1110 °C under an N2/H2 mixed atmosphere. The nitride layers are formed on the surface of the samples. The thickness of nitride layers is about 130–150 μm, and its microhardness is 490 HV. Moreover, it suggests that gas nitriding treatment can effectively improve surface hardness and wear resistance. Still, the core matrix with ferritic microstructure maintains good plasticities and ductilities, and its hardness is about 126 HV.

Development of dual phase soft magnetic laminate for advanced electric machines

The development of dual phase soft magnetic laminate for advanced electric machines at General Electric is summarized, including alloy development, properties, manufacturing trials, prototype experience, potential applications, and future material development. Dual phase soft magnetic laminates provide the flexibility to tune magnetic/non-magnetic state at the desired locations, using solution nitriding treatment with protective mask/coating. This enabling material technology benefits electric machine performance by reducing flux leakage in the laminates. It also decouples the electromagnetic and mechanical design, which opens up the design space for advanced electric machines. Magnetic and mechanical properties of dual phase laminates were compared to commercial FeSi and FeCo soft magnetic laminates. Driven by the market needs for high power density and high-speed electric machines, a synchronous reluctance (SynRel) machine without PMs using dual phase soft magnetic laminates was assessed in machine electromagnetic performance as well as rotor stress analysis by finite element simulation. Successful prototype build and testing further retired manufacturing risks and advanced the technology maturity level. Future material development with higher mechanical strength, higher saturation magnetization, higher permeability, and lower loss are highly desired for ground and air transportation electrification.

S3P– Innovative Surface Treatment to Increase the Wear Resistance of Stainless Steel Components*

Abstract Austenitic and duplex stainless steels are used in a variety of industries due to their high corrosion resistance. However, this is countered by a high coefficient of friction and thus a limited wear behaviour, which restricts the technical application possibilities. Coatings or conventional hardening processes, such as solution nitriding above 1000 °C, offer a certain reduction of these wear phenomena, but are associated with the risk of delamination and/or loss of corrosion properties. In addition, the hardening effect is limited for austenitic and duplex grades. The low temperature surface hardening process below 500 °C offers the possibility to overcome these limitations. In this article the technology of this process is explained. In addition, research results show the positive influence of this treatment on the lifetime of components made of corrosion resistant steels. As a future outlook, possible properties and applications for the prospective increasing number of hydrogen applications are explained.

Influence of ferrite-austenite distribution in 2205 duplex stainless steel on high-temperature solution nitriding behaviour

Duplex stainless steel 2205 was subjected to annealing treatments in the range of 1000–1200 °C to adjust the phase fractions of ferrite and austenite. Annealed steels were subsequently subjected to high-temperature solution nitriding, which transformed the surface-adjacent region into austenite under the influence of nitrogen ingress. Microstructural characterization of the pre-annealed state and the solution-nitrided cases were performed with X-ray diffractometry, light-optical microscopy, electron back-scatter diffraction and hardness indentation. The phase fractions of austenite and ferrite have a significant influence on the nitriding kinetics. A relatively high ferrite phase fraction results in finer grains in the nitrogen-stabilized austenite case. The developing austenite case and the evolution of the microstructure during solution nitriding are discussed in a computational thermodynamics and kinetics context. Slower grain growth in the austenite cases for higher ferrite phase fractions can be understood in terms of phase distribution, alloying element partitioning, and possibly, pinning effect by M2N nitrides.

Rapid Screening of the Mechanical Properties of 13 wt%Cr Steels with Uncharted Combinations of C and N Contents

Abstract Stainless steels containing both C and N have unique characteristics while requiring unconventional manufacturing methods. This latter feature also translates into a lack of systematic investigations into their properties. In this work, a series of 13 wt%Cr steels with various interstitial solid solutions of C and N was synthesized by high temperature solution nitriding, HTSN, of commercial 13 wt%Cr martensitic steels. Light optical microscopy was applied to reveal the microstructural features, which consisted of a combination of martensite and austenite in various ratios depending on the C and N content. The mechanical response was characterized by nanoindentation. Data analysis assumed a fixed hardening exponent and provided an estimate of the yield strength and Young’s modulus of the synthesized steels and their microstructural components. The comparison with literature data indicated that this estimate is sound for martensite, while the data for austenite suggests an underestimation of the hardening exponent for this phase. The investigation demonstrates the potential of HTSN for the synthesis of novel 13 wt%Cr steels alloyed with both C and N. Moreover, it suggests that the use of nanoindentation for extracting the mechanical properties is limited by the non-uniqueness of the method with regard to the hardening exponent.

Nitrogen‐Enhanced Temperature Dependence of Grain Refinement Strengthening in Austenitic Stainless Steel

Hall–Petch relationships at room temperature and lower temperatures are investigated for 316L steel (low‐N steel) and 0.43% nitrogen‐added 316L steel (high‐N steel) produced by solution nitriding. The yield stress is separated into thermal and athermal components, and the effects of nitrogen on the former are clarified. In the case of low‐N steel, the yield stress increases as the temperature decreases, and the degree of the increase is greater for fine‐grained steels. This is not only due to the increase in friction stress (including solid solution strengthening) but also results from the increase in grain refinement strengthening caused by the increase in the Hall–Petch coefficient at low temperatures. By contrast, in high‐N steel, the temperature dependence of the yield stress is more remarkable than in low‐N steel. However, the amount of increase in friction stress at low temperatures is not very different between the low‐N and high‐N steels. Instead, the increase in grain refinement strengthening caused by nitrogen is greatly enhanced at lower temperatures. This result suggests that the combination of nitrogen addition and grain refining is very effective at strengthening austenitic steels under low temperatures.

Thermochemical post-processing of additively manufactured austenitic stainless steel

The effects of different thermochemical post-processing treatments on the microstructure and properties of additively manufactured austenitic stainless steel were investigated. If the nitrogen content in the as-built condition is high, austenitization in vacuum causes a reduction in nitrogen content near the surface. This can be remedied by applying a small amount of nitrogen in the gas during austenitization. Using high temperature solution nitriding, the surface hardness could be effectively raised by deliberate nitrogen ingress, while maintaining a very fine structure inside the primary austenite grains. The excellent combination of strength and ductility of the as-built condition is accompanied by a low degree of work-hardening. This condition showed elastic and plastic anisotropy. The vertically built condition exhibits a lower strength and an early initiation of yielding as compared to the horizontally built conditions. High temperature treatments were able to efficiently reduce the mechanical anisotropy exhibited in the as-built condition. A combination of high- and low temperature surface hardening was investigated using nitriding, carburizing and nitrocarburizing. In all cases expanded austenite developed on the surface, which created a strong hardness increase.

Dimensional Stability of Low Temperature Surface Hardened Stainless Steel Components*

Abstract Stainless steels are commonly used for high precision components, which often are exposed to corrosive media. However, their inferior tribological behaviour restrict the use of these materials in many technical applications. Thermochemical surface hardening is one way to overcome these weaknesses. Solution nitriding in the austenitic range above 1000 °C is mainly used for hardening martensitic and ferritic stainless grades. In austenitic and duplex stainless grades, however, the hardening effect is limited. Additionally, the high process temperatures combined with a necessary rapid cooling may lead to non-desired dimensional changes. Low temperature surface hardening processing below 500 °C here offers interesting alternatives for increasing the wear properties, while maintaining the corrosion resistance. This paper demonstrates the influence of high and low process temperatures of thermochemical surface hardening treatments on the tight dimensional tolerances of a rotationally symmetrical precision component made from cold worked AISI 304. Based on these results, current and new industrial applications, which benefit from low temperature surface hardening, will be discussed.

Effect of edge print parameters on microstructure and high temperature solution nitriding response of additively manufactured austenitic stainless steel

The as-built microstructure of edge printing parameters in the laser powder-bed fusion process is investigated as well as the effect of edge parameters on the response to high temperature solution nitriding. The results show that using edge parameters leads to a zone of elongated austenite grains with shallow melt pools. Subjecting parts printed with edge parameters to high temperature solution nitriding (and high temperatures in general) results in the formation of equi-axed grains with a high density of annealing twins. Conversely, bulk parameters create a pinning effect and preserve the fine misorientation structure inside the elongated structure from the as-built condition. The results show that a high temperature solution nitriding treatment is apt to post-processing of additively manufactured 316L.

Deformation mechanisms in meta-stable and nitrogen-stabilized austenitic stainless steel during severe surface deformation

AISI 304L stainless steel in austenitized and in solution nitrided condition was severely mechanically deformed by surface roller burnishing. High-temperature solution nitriding was applied to achieve a nitrogen-concentration depth profile, leading to a depth-gradient in the austenite stability. X-ray diffraction, electron microscopy and hardness indentation were applied for characterization of the graded microstructures obtained by combining a composition profile and a deformation profile. While severe plastic surface straining of an austenitized specimen leads to a deformation-induced transformation of austenite into martensite, the solution nitrided specimen remains austenitic upon deformation, even in the region where nanocrystallization occurs. The deformation mechanisms operable in the nitrogen-stabilized austenitic stainless steel, i.e. twinning or dislocation glide, depend on the combination of applied plastic strain/strain rate, and the nitrogen-concentration dependent stacking fault energy.

Influence of N content on structure and mechanical properties of multi-component Al-Cr-Nb-Y-Zr based thin films by reactive magnetron sputtering

Al-Cr-Nb-Y-Zr-N films have been deposited with reactive dc magnetron sputtering at various N2 flow ratios to achieve films with different nitrogen content, from purely metallic to fully nitrided films. The structure evolved from mainly amorphous with a minor crystalline intermetallic phase for the film without nitrogen, to nanocomposites with a cubic crystalline phase in an amorphous matrix for intermediate nitrogen content (15–41 at.% N), and at higher nitrogen content (46–51 at.% N) to crystalline solid solution nitrides with a NaCl-type structure. Partial elemental segregation on the nanoscale was found in all studied samples and the films exhibited different segregation behaviour depending on the nitrogen content, implying that the structural evolution on the nanoscale of films in this material system complex and highly composition-dependent. The hardness increased with increasing nitrogen content, reaching a maximum at about 30 GPa at for the nitride films with 50 at.% N. Deformation behaviour, studied by indentation measurements, of the nitride films was found to be ductile, where no sign of crack formation could be observed. This can be attributed to a metallic phase in the columnar boundaries caused by partial elemental segregation of mainly yttrium. Hence, films within in this material system, although the nanostructure is found to be relatively complex, show very promising mechanical properties and the structural complexity can be used as a guide for designing nitride materials that combine high hardness with ductility.

Deformation Mechanisms in Meta-Stable and Nitrogen-Stabilized Austenitic Stainless Steel During Severe Surface Deformation

AISI 304L stainless steel in austenitized and in solution nitrided condition was severely mechanically deformed by surface roller burnishing. High-temperature solution nitriding was applied to achieve a nitrogen-concentration depth profile, leading to a depth-gradient in the austenite stability. X-ray diffraction, electron microscopy and hardness indentation were applied for characterization of the graded microstructures obtained by combining a composition profile and a deformation profile. While severe plastic surface straining of an austenitized specimen leads to a deformation-induced transformation of austenite into martensite, the solution nitrided specimen remains austenitic upon deformation, even in the region where nanocrystallization occurs. The deformation mechanisms operable in the nitrogen-stabilized austenitic stainless steel, i.e. twinning or dislocation glide, depend on the combination of applied plastic strain/strain rate, and the nitrogen-concentration dependent stacking fault energy.

High Temperature Solution Nitriding of Stainless Steels; Current Status and Future Trends

AbstractThis contribution provides an overview of the concepts of high temperature solution nitriding (HTSN) and the state of the art of the technology. It shows how HTSN can be applied to different classes of stainless steels. New advances of HTSN are presented, in particular the combination of HTSN with low temperature surface hardening, LTSH, is addressed: HTSN is an excellent “pre-treatment” for LTSH for austenitic, duplex, ferritic and martensitic stainless steels. Examples of combined HTSN + LTSH treatments of duplex and martensitic stainless steels are shown.

Kinetics of Austenite Layer Growth on the Surface of Fe-23Cr-2.4Mo Ferritic Stainless Steel During Solution Nitriding

Kinetics of Austenite Layer Growth on the Surface of Fe-23Cr-2.4Mo Ferritic Stainless Steel During Solution Nitriding

Discontinuous precipitation in a nickel-free high nitrogen austenitic stainless steel on solution nitriding

Chromium-rich nitride precipitates in production of nickel-free austenitic stainless steel plates via pressurised solution nitriding of Fe–22.7Cr–2.4Mo ferritic stainless steel at 1473 K (1200 °C) under a nitrogen gas atmosphere was investigated. The microstructure, chemical and phase composition, morphology and crystallographic orientation between the resulted austenite and precipitates were investigated using optical microscopy, X-ray Diffraction (XRD), Scanning and Transmission Electron Microscopy (TEM) and Electron Back Scatter Diffraction (EBSD). On prolonged nitriding, Chromium-rich nitride precipitates were formed firstly close to the surface and later throughout the sample with austenitic structure. Chromium-rich nitride precipitates with a rod or strip-like morphology was developed by a discontinuous cellular precipitation mechanism. STEM-EDS analysis demonstrated partitioning of metallic elements between austenite and nitrides, with chromium contents of about 80 wt.% in the precipitates. XRD analysis indicated that the Chromium-rich nitride precipitates are hexagonal (Cr, Mo)2N. Based on the TEM studies, (Cr, Mo)2N precipitates presented a (1 1 1)γ//(0 0 2)(Cr, Mo)2N, γ//(Cr, Mo)2N orientation relationship with respect to the austenite matrix. EBSD studies revealed that the austenite in the regions that have transformed into austenite and (Cr, Mo)2N have no orientation relation to the untransformed austenite.