The aim of the present research work was to investigate tribological performance and potential of Ni-based self-lubricating claddings for high temperature forming of lightweight alloys. Laser claddings included in this investigation were based on Ni-matrix with the incorporation of 5 wt% silver and 10 wt% MoS2 as solid lubricant precursors. Tribological evaluation and testing was performed by Load- Scanner to simulate hot forming process and results compared to high performance hot work tool steel. To simulate hot forming process of forging, wire drawing and extrusion, tests were done at room and elevated temperatures (150°C and 300°C) against typical light-weight alloys, including AISI 316L stainless steel, 6xxx series Al alloy and Ti6Al4V Ti alloy and results evaluated in terms of coefficient of friction vs. load, critical loads for galling initiation and volume of adhered work material. Results show that self-lubricated claddings with incorporated MoS2 and Ag as solid lubricants in general provide lower and more stable friction as well as improved galling resistance in high temperature forming of lightweight alloys. Positive effect of self-lubricating claddings intensifies with forming temperature, degree of plastic deformation and work material tendency to galling.

Martensitic stainless steels are an important group of steels for applications as knives, tools & molds and highly loaded parts in the food and plastics processing industry as well as for machinery components. Their typical hardening consists of quenching and (multiple) tempering (Q&T). As many of these steels contain at least smaller amounts of retained austenite (RA) after quenching, partitioning of carbon and nitrogen from the martensite into the RA can take place during tempering, changing it from Q&T to quenching & partitioning (Q&P). This contribution provides as systematic overview of such partitioning effects on the microstructure like the amount and stability of retained austenite as well as on subsequent effects on material properties such as hardness, toughness, strength and ductility. The various effects were investigated on several steel grades and cover also the effect of variation in heat treatment parameters like austenitizing temperature, quench rate, quenching temperature, number, duration and temperature of the tempering, respectively partitioning. The results clearly show that partitioning dominates over tempering effects at temperatures up to 500°C. Higher quenching temperatures can increase the RA-content similar to higher austenitizing temperatures. Lower quench rates can reduce it due to carbide (nitride) precipitation. Rising tempering (partitioning) temperatures up to 400°C enhances the austenite stabilization. Higher amounts of RA with reduced stability promotes transformation induced plasticity (TRIP), providing the possibility to optimized ductility and tensile strength but reduces yield strength. Increased amounts of RA with sufficient stability increases impact toughness at slightly reduced hardness. Increasing the tempering temperature above 500°C in contrast promotes, after a certain nucleation time, carbide and nitride precipitation, resulting in the elimination of the retained austenite and therefore a typical tempering condition.

Diamond-like carbon (DLC) coatings, which improve wear resistance and extend component service life, have gained considerable research attention as an approach for conserving limited resources. The DLC coating is a highly functional film with high hardness and excellent low-friction, wear-resistance, and corrosion-resistance properties; however, it has high residual stress and low adhesion between the substrate and the film. Existing studies have focused on using DLC containing metallic elements (Me-DLC) as an intermediate layer to minimize residual stress, thereby improving adhesion. Si-DLC is deposited using a mixture of hydrocarbon gases, such as methane (CH 4 ) and acetylene (C 2 H 2 ), and silicon gases, such as tetramethylsilane (TMS: Si(CH 3 ) 4 ), H, and Si, to form the DLC coating. The composition, hardness, Young’s modulus, and friction coefficient of the film can be controlled by changing the composition of the gas mixture. This study investigated the effect of the flow rate ratio of source gases (CH 4 and TMS; C 2 H 2 and TMS) on the properties of the DLC film when Si-DLC is deposited as an intermediate layer on austenitic stainless steel SUS304 using plasma-enhanced chemical vapor deposition. The coating time was adjusted to ensure that the thicknesses of the Si-DLC layer and DLC film were 1.0 and 0.2 μm, respectively, under both conditions. The results demonstrated that the durability of the DLC film improved and adhesion decreased with a decrease in the TMS ratio in the Si-DLC intermediate layer. Durability improved and adhesion decreased when C 2 H 2 was used as the source gas, as compared to when CH 4 was used.

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

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

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

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

Additively manufactured (AM) metals require a modified heat treatment to accommodate for slight differences in composition caused by powder atomization and cover gas used in the manufacturing process. 17-4PH stainless steel (17-4PH) is a precipitation hardening steel which hardens through the formation of Cu precipitates in a martensitic matrix during aging treatment. The powders used in Laser Powder Bed Fusion (LPBF) fabrication of 17-4PH are typically spray atomized using N 2 cover gas, which is associated with a certain amount of nitrogen uptake. Nitrogen is a potent austenite stabilizer and will lower the martensite start temperature of the steel. To counteract the effect of nitrogen, a sub-zero heat treatment can be introduced to promote a more complete transformation into martensite. In this work, the effect of nitrogen on the heat treatment response of 17-4PH is investigated through comparing standard wrought, nitrogen loaded wrought, and LPBF 17-4PH. In particular, the effect of introducing a subzero treatment is addressed. After quenching from the solutionizing step (austenitization) LPBF fabricated 17-4PH was cold-treated in different combinations of dry ice (-78 °C) and boiling nitrogen (-196 °C). Subsequently, these conditions were aged in the conventional way. The sub-zero treatments were compared with the conventional heat treatment procedure, which does not entail a sub-zero step. In addition, phase transformations (above room temperature) were monitored in-situ using dilatometry. Finally, hardness tests and XRD analysis were performed to characterize the final microstructure. It is demonstrated that sub-zero treatment can be an effective route to address the problems associated with the additional nitrogen present in LPBF 17-4PH fabricated parts.

Residual stresses are unavoidable in heat treatment and surface engineering and their presence can be advantageous or disastrous for the performance of components. Residual stresses cannot be measured directly, but are determined from strain measurements, either non-destructively from diffraction-based methods, or destructively from relaxation-based methods. In this presentation, three examples of stress determination from strain measurements showcase some of the possibilities. In the first example lattice strains are determined with energy dispersive analysis with synchrotron radiation in relation to the phase fraction during martensite formation in a soft martensitic stainless steel. The second example shows synchrotron lattice determination with energy dispersive analysis during in-situ tensile loading of super martensitic stainless steel containing reverted austenite. The third example concerns determination of residual stresses in internally oxidized bulk metallic glass with laboratory X-ray diffraction analysis of lattice strains and displacements by stress relaxation during incremental ring-core excavation of micron-scale columns with focused ion beam milling in an SEM.

This study investigates the heat treatment response and microstructure evolution of high-carbon steels for additive manufacturing. Moreover, the role of nitrogen as an interstitial alloying element is addressed. Stainless steel 440C, cold-work D2, hot-work H13, and T15 high-speed tool steel overspray powders from spray forming were investigated. The thermal behavior of these materials was examined using a thermal analyzer that combines calorimetry and thermogravimetry. Additionally, interstitial alloying with nitrogen was performed in-situ to understand its influence on thermal behavior. The (near-)equilibrium nitrogen solubility in 440C and D2 in contact with flowing N 2 gas was recorded as a function of temperature through the interval 1200 to 800 °C. The microstructure of the steel powders was characterized by light optical microscopy and X-ray diffraction. The potential of nitrogen alloying and the importance of optimized heat treatment protocols are emphasized with respect to high-carbon steels in additive manufacturing applications.

Advanced characterization techniques and modeling are used to get new insight on the microstructural evolutions occurring during the tempering of low-alloyed steels with initial martensitic microstructure. Tempering temperatures from 150°C to 600°C, are considered to make vary the metallurgical phenomena activated, form carbon segregation to defects to precipitation of different types of carbides (transition, cementite, alloyed). A large range of carbon compositions, from 0.1 to 0.7 wt.% are investigated, with the same main experimental technique: in situ HEXRD at synchrotron beamlines, with complementary post mortem fine-scale characterizations by TEM and 3D-APT. In the middle of this range (~0.3wt.%), the usual sequence is observed: successive precipitation of transition and cementite carbides. New observations concern the carbon concentrations outside this range. For high carbon concentrations (~0.6wt.%), the same sequence occurs but the martensite/ferrite matrix remains highly supersaturated in carbon compared to equilibrium, for a long time and even after the precipitation of cementite. For low carbon concentrations (~0.1wt.%) most of the carbon starts to segregate at defects (dislocations, lath boundaries). This enters in competition with the transition carbides which are almost fully hindered, whereas cementite precipitates afterwards. Two previous models from literature are combined to predict the concomitant kinetics of carbon segregation and precipitation. Segregation puts the transition carbides at a disadvantage with cementite and for this reason, the latter precipitates earlier than usually reported. The effects of nitrogen enrichment (up to ~0.4 wt.%N, context of carbonitriding thermochemical treatments) in austenite domain of stability (before the martensitic quench) are also investigated. In low-alloyed steel considered (23MnCrMo5), nitrides are formed upon enrichment (CrN, MnSiN 2 ). This has a strong impact on the precipitation sequence, compared to model systems previously investigated (Fe-N, Fe-C-N).

The increasing demand for accurate fatigue modeling of powder metallurgy components in automotive, aerospace, and medical industries necessitates improved knowledge of composition-microstructure interactions. Variations in feedstock composition and thermomechanical history can produce unique microstructures whose impact on fatigue performance has not been adequately quantified. When characterizing additively manufactured 316L that is within nominal standard chemistry limits, oxide and nitride species were observed preferentially in the specimen contour region. Thermodynamic simulations provide evidence of segregation of the low manganese and high nitrogen composition driving this precipitation of these phases. When present in the specimen, they promoted brittle fracture mechanisms during fatigue.

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

Direct metal laser sintering (DMLS) is an established technology in metal additive manufacturing. This complex manufacturing process yields unique as-built material properties that influence mechanical performance and vary with different machine parameters. Part porosity and residual stresses, which lead to part failures, and grain structure, as it relates to mechanical properties and anisotropy of DMLS parts, require investigation for different print settings. This work presents results for density, residual stress, and microstructural inspections on designed test artifacts for the benchmarking of 3D metal printers. Results from printing artifacts on two separate DMLS printer models with default parameters show highly dense parts for both printers, with relative densities above 99.5%. Characterization of residual stress through cantilevered deflection specimens indicates similar resulting thermal stresses developed in both build processes, with deflection averages of 32.48% and 28.09% for the respective machines. Additionally, properties of the test artifact printed after adjusting default machine parameters for equal energy density are characterized.

The transient behavior of boiling phenomena during quenching of an AISI 304 stainless steel, conical-end, cylindrical probe in flowing water at 60 °C was studied. Two free-stream velocities (0.2 and 0.6 m/s) and two initial probe temperatures (850 and 950 °C) were investigated. From high-speed video recordings, undulations of the liquid vapor interface that appear periodically and propagate in the direction of the flow stream were observed during the vapor film stage. After the collapse of the vapor film, a wetting front is formed which consists of many small bubbles that coalesce rapidly in a small area while fewer and larger bubbles nucleate and grow below it. The initial temperature has a marginal effect on the size and half-life of the large bubbles. However, the water flow rate produces larger values of maximum diameter and half-life time for water flowing at 0.2 m/s than their equivalents for 0.6 m/s.

Two stainless steel parts used in automotive engines are carburized in the course of their production to achieve desired properties. To reduce costs and improve product quality, the gas carburizing process that had been used was replaced by low-pressure vacuum carburizing. The two parts are similar in composition except that one contains 0.25 wt% Mo and the other 0.4 wt% Mo. Both also contain around 17 wt% Cr and thus naturally form a Cr 2 O 3 passivation layer that provides corrosion resistance but also acts as a barrier to carbon. As a result, the parts are etched in a pickling solution prior to carburizing. In the initial assessment of the new carburizing and pretreatment process, engineers observed differences in the pitting and oxide regeneration behaviors of the two stainless steels. The paper describes how the engineers determined the cause of the pitting and the extent to which it could be controlled. Because of the tradeoffs involved, the engineers decided to make both parts from the same material and optimize process parameters accordingly.

Austenitic stainless steels are carburized or nitrided (i.e., surface hardened) at low temperatures in order to maintain their superior corrosion resistance. Treatment temperature must be low enough to prevent precipitation in the diffusion zone, yet high enough to allow sufficient diffusion depths to meet design specifications. At these temperatures, prior machining processes can have a significant effect not only on diffusion, but also the surface hardness and corrosion resistance achieved. This paper presents practical examples showing how cutting, grinding, honing, and polishing processes influence the results of low temperature surface hardening treatments for stainless steel parts. It also discusses the influence of surface deformation and finish.

Gas nitriding is proving to be a viable low temperature case hardening process for stainless steels used in numerous applications. In this study, a comparison between austenitic (grade 304) and martensitic (grade 401) stainless steels shows how pre-oxidation temperature affects the thickness and porosity of the compound layer produced as well as hardness and nitriding diffusion depth. The results indicate that austenitic stainless steel would be the best choice for a part requiring wear resistance and strength, and that a standard rolled martensitic stainless steel would suffice if only a wear resistant surface is needed.

Aluminide diffusion coatings enhance carburization and oxidation resistance of iron and nickel based alloys by formation of iron and nickel aluminides which extends the life of furnace alloys and fixtures. As a part of a large project in the Center for Heat Treating Excellence (CHTE), an aluminized coating on RA330 was studied by a hot dip process followed by diffusion heat treatment. Samples of RA330 steel were dipped in pure liquid aluminum at 700 °C for 10 minutes. After dipping, four samples were given an additional diffusion treatment. To predict the developed phases, computational analysis was used and the results were compared with the experimental data.

In this paper a novel nickel-base, alumina forming alloy, Nikrothal PM 58 is introduced. Similar to the previously developed, ferritic, iron-base alloy Kanthal APMT, the alloy bases its corrosion resistance on the formation of an adherent surface alumina layer. They both have high creep strength, due to a dispersion strengthened microstructure from the powder metallurgical processing route. This unique combination of properties enables application temperatures ranging from 1472 F (800°C) to 2372 F (1300°C) and new possibilities to design high temperature components like mesh belts, furnace rollers and muffles. Mechanical and corrosion properties for Nikrothal PM 58 at 2012 F (1100°C) and 2192 F (1200°C) are presented and compared with other commercial high temperature alloys.