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.

Induction surface hardening is a process often used in industrial applications to efficiently increase the lifetime of components. Recently, this process has been enhanced with the inductive short time austempering process, creating a martensitic-bainitic microstructure. It is well-known that in homogeneous mixed microstructures, an optimally adjusted volume fraction of bainite can significantly increase the lifetime of the components even further. Regarding inductive short time austempering, there is a lack of knowledge in characterizing and differentiating graded microstructures, which occur due to the temperature gradients within the process. Therefore, three methods were investigated: the analysis of the grayscale profile of metallographic sections, the hardness profile and the full width at half maximum (FWHM) profile from the intensity curve (rocking curve) of the X-ray diffraction pattern. These methods were initially applied to homogeneous structures and evaluated. The findings were then transferred to graded microstructures. Finally, the graded microstructures could be differentiated both via the hardness profile and the FWHM value, while the grayscale analysis only allowed qualitative statements to be made. It became evident that both the volume fractions and their structure are crucial for subsequent mechanical characterization. Since the martensitic microstructure is easier to identify, it serves as a reliable reference for evaluating the mixed microstructure. In summary, these findings offer the foundation for further characterization of graded martensitic-bainitic mixed microstructures.

Effective heat treatment is essential for optimizing the properties of steels in various applications. Understanding the evolution of steel microstructure during intrinsic or post-heat treatment, along with managing distortions and residual stresses, is crucial for ensuring component usability. In laser-based additive manufacturing, high temperature gradients and cooling rates induce residual stresses, impacting the heat-affected zones. However, there remains a gap in understanding how stress influences precipitation during heat treatment, particularly regarding transformation-induced plasticity (TRIP), where a stress triggers deformation during phase transformation. This study aims to investigate TRIP effects during the aging of maraging steels, commonly employed in laser-based powder bed fusion. During the experiments, the steels were continuously aged under varying compression stresses. By isolating TRIP strain from total strain, the study establishes a relationship between maximum TRIP strain after phase transformation and applied stress, defining specific TRIP constants for each steel. The presence of TRIP strain has been confirmed during short time continuous aging treatments, indicating its significance even in the initial stages of the heat treatment process. While the applied stress level does not affect hardness, significant differences in maximum hardness values after aging were observed among the investigated materials. Furthermore, a comparative analysis of different maraging steels revealed a positive correlation between the TRIP constant and the amount of precipitation, and consequently, hardness. These findings confirm the role of TRIP in precipitate formation in maraging steels and provide a foundation for further understanding and predicting post-heat treatment material states.

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.

Gas carburizing with quenching is one of the most useful heat treatment processes for steel parts. However, after quenching distortion is still occurs. The nitriding and nitrocarburizing are the surface hardening heat treatment methods with low distortion, but these methods require the long treating time to obtain a thick hardened layer. Austenitic nitriding and quenching (ANQ) solves these problems. In ANQ process, nitrogen is infiltrated into the steel parts in austenite phase, and they are quenched to harden. The ANQ process can also be applied to cheap low carbon steel such as the Cold Rolled Carbon Steel Sheet. In this study, the effect of ANQ on mechanical properties was examined. For infiltrating the nitrogen into the steel parts, the steel parts were heating to 750°C or higher in an ammonia atmosphere and heating to 750°C or higher in a nitrogen glow discharge. After the ANQ process, hardness profiles, structure, nitrogen and carbon concentration profiles were observed. Also, distortion, tribological properties, impact value and fatigue strength were examined. The effective case depth, which is treated by ANQ, is larger than the effective case depth of gas nitrocarburizing for same period of time. Distortion of ANQ is much smaller than that of gas carbonitriding, and it is almost equal with that of gas nitrocarburizing. The seizure load is same as with other surface hardening heat treatment processes. The wear loss of ANQ is a lower, in the amount of about 1/2 that of the carbonitrided specimen and 1/3 that of the gas nitrocarburized specimen. The ANQ is an effective heat treatment process for parts which require wear resistance. The tempering softening resistance is improved by nitrogen infiltration. ANQ also improves the impact value and fatigue strength.

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.

An intensive quenching (IQ) process is an environmentally friendly method of hardening steel parts. Digitally controlled, IQ employs highly agitated and directed water flow as the quenchant. An extremely high cooling rate applied uniformly over the entire part surface area induces high surface compressive stresses which prevents part distortion and cracking while forming a very fine microstructure. The fine microstructure results in better mechanical properties compared to properties imparted by conventional oil or polymer quenching. The improved mechanical properties enable engineers to design stronger steel parts for higher power density mechanical systems often using steels containing a less amount of alloying elements or using less expensive plain carbon steels. A broad and deep body of knowledge documents IQ’s ability to tailor a steel component’s microstructure to improve steel parts mechanical properties and performance. A sampling of data will be presented including surface and core hardness, tensile, yield and impact strength, elongation and reduction in area, residual surface compressive stresses for through hardened steels and the carburized grades. IQ systems can be readily “dropped in” to existing steel processing facilities or integrated into next generation heating and cooling systems through teamed relationships with equipment makers and part manufacturers seeking a sustainable future.

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.

Much more steel must be produced from scrap to meet emissions targets, and utilizing this growing resource is a sound economic strategy. However, the presence of contaminating elements restricts the applications in which end-of-life scrap can replace primary steel. The use of low alloyed quenching and tempering steel grade such as 39MnCrB6-2 to reach high mechanical characteristics (around 1000 MPa) obliges often to apply low tempering temperatures for which tempering embrittlement may be observed. In this paper, it is proposed to reduce the hold time and to increase the temperature during conventional tempering to (1) reduce the embrittlement because of segregation of elements like copper, (2) to change the fracture mechanism with finer martensite sub-grains and (3) to promote θ particles with smaller dimensions but higher density.

It is well known that distortion has and continues to present a challenge to the heat treater when hardening steel. However, recent advances in quenching technology are improving the opportunity for improved distortion control. 4 Dimension High-Pressure Gas Quenching (4DQ) is a unique gas quenching process that uses both quenching chamber design and part motion to minimize distortion during the quenching process. To understand 4DQ’s potential, the challenges of traditional batch quenching and press quenching techniques will be explored, emphasizing issues such as geometric distortion, residual thermal stresses, non-uniform microstructure transformation, safety, environmental, and handling concerns. In contrast, 4DQ is a process that enhances quenching uniformity and minimizes distortion by use of a specialized cooling chamber. Within the chamber it provides three-dimensional (3D) quenching by enveloping the part at specific areas with cooling gas while introducing the fourth dimension (4D) of part rotation during quenching that further optimizes quench uniformity. 4DQ gives the ability to “engineer” the quenching process by controlling quench pressure, gas velocity, gas manifold design, table rotation, table oscillation, and time-dependent gas flow. The system’s flexibility allows users to customize the quenching process for reduced distortion, repeatability, and precise accuracy. A case study on hypoid hears and coupling sleeves will demonstrate the effectiveness of the 4DQ system in minimizing distortion and achieving dimensional consistency. Results illustrate the system’s advantages over traditional quenching methods in terms of quality, repeatability, and cost-effectiveness. Considering the challenges of steel hardening processes, the 4DQ system has the potential to be a transformative solution for achieving enhanced quenching uniformity and reduced heat treatment distortion in manufacturing scenarios.

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.

Mold repair is a viable strategy for saving energy and reducing CO 2 emissions. Papers in the literature show that repairing a limited damaged area of the mold instead of producing a new one is becoming increasingly attractive, especially considering the latest European and international regulations introduced with the green deal. In this paper, the authors are pleased to present some preliminary results related to the repair of AISI H13 tool steel molds by Laser-Directed Energy Deposition. Steel blocks (20 x 55 x 100 mm3), previously tempered at 435±10 HV, were machined to reproduce the material removal of the damaged part of the mold. Subsequently, the region was repaired by L-DED using commercial H13 powder. The process parameters were optimized to obtain a defect-free welded area. Since the microstructure of the deposited tool steel consists of hard (730±10 HV) and brittle (7 J Charpy impact toughness) martensite, a series of post-process heat treatments were performed at different temperatures to restore a hardness compatible with that of the base steel. However, this goal was only partially achieved due to the different tempering behavior of L-DED-deposited and bulk H13 steel. In particular, the tempering temperature had to be limited to avoid softening of the base steel. In the best case, double tempering at 620 °C resulted in a toughness recovery of up to 42 J. Thermal fatigue tests showed better resistance to crack propagation after tempering, as evidenced by the shallower penetration depth compared to the as-built material.

Carbide free bainitic microstructures can be developed via different thermal processing routes, and the details affect the scale and morphology of the microstructural constituents. In this study, bainitic microstructures are formed by either a controlled cooling process or an austempering process to evaluate the relationship between microstructure and mechanical properties in a 0.2C - 2Mn - 1.5Si - 0.8Cr steel containing small amounts of Nb, Ti, B, and N, and the results are compared to a 4140 steel processed via quenching and tempering. The resulting microstructures are characterized with scanning electron microscopy. When compared to microstructures produced via austempering, microstructures produced with a controlled cool exhibit an increased variety of transformation products, specifically regarding size and distribution of martensite-austenite constituents within a lath-like bainitic ferrite matrix. Nanoindentation testing shows that different transformation products exhibit significantly different local hardness. In all (primarily) bainitic conditions tested for these materials, the martensite/austenite constituent exhibits the highest hardness, followed by the lath bainitic ferrite/retained austenite constituent. Granular bainite and coarse bainitic constituents exhibit the lowest relative hardness in the conditions where they are observed.

Decarburization of steel parts during heat treating results in a lower surface hardness, undesirable residual stress profiles, and poor part performance. Significant effort has been made towards preventing decarburization and determining the impact of annealing time and temperature on decarburization rate. Much of the published research has focused on medium carbon steels, ranging from 0.3wt% C to the eutectoid composition. The goal of the current research is to determine decarburization rates for steels with carbon concentrations above the eutectoid concentration. AISI 52100 steel was heated in air for 12, 24, and 36 hours at three temperature ranges (below A 1 , above A cm , and between A 1 and A cm ). Optical microscopy was used to determine the carbon concentration as a function of depth from the surface. The diffusion coefficients of carbon in austenite and ferrite plus cementite phase assemblages were calculated. These diffusion coefficients can be used in a finite difference simulation to predict decarburization at different temperatures and times.

Quenching and tempering (Q&T) allows a wide range of strength and toughness combinations to be produced in martensitic steels. Tempering is generally done to increase toughness, although embrittling mechanisms result in temperature ranges where strength and toughness may decrease simultaneously. Tempered martensite embrittlement (TME) represents one such mechanism, associated with the decomposition of retained austenite and precipitation of cementite during tempering, usually between 250 and 450 °C. The use of induction heating allows for time-temperature combinations, previously unobtainable by conventional methods, that have been shown to improve properties. The present work shows a beneficial effect of rapid tempering in alloy 1045, with an increase in energy absorption of about 50% when measured at room temperature via a three-point bending fracture test in the TME regime. Phase fraction measurements by Mössbauer spectroscopy showed that increased energy absorption was obtained despite essentially complete decomposition of retained austenite during tempering. Scanning electron microscopy (SEM) investigation of the carbide distribution showed refinement of the average carbide size of approximately 15% in the rapid tempered conditions. SEM characterization of the fracture surfaces of the rapid tempered three-point bend samples showed that, despite an increase in energy absorption in the TME regime, increased microscopic ductile fracture appearance was observed only at the highest test temperature.

Steel hardening is a long-standing practice that has accompanied human development over the last three millennia. For hardening, steel is heated to a high temperature to form austenite and subsequently cooled. During cooling, austenite transforms into various microstructural products, e.g. grain boundary ferrite, Widmanstätten ferrite, massive ferrite, pearlite, upper bainite, lower bainite,… and martensite. Martensite is the hardest of these products and is obtained when the applied cooling rate exceeds a critical value. This critical cooling rate for martensite formation is determined by the chemistry of the steel and is significantly reduced by increasing the content of alloying elements. Cooling from the austenite region by immersing the parts in water, generally provides this cooling condition. The transformation that leads to martensite is called martensitic and, unlike all other transformations that occur in steel, it does not involve the diffusion of atoms. Martensitic transformations begin when a characteristic temperature, the martensite start temperature Ms is reached during cooling. Ms is essentially determined by the chemical composition of the steel. Subsequently, martensitic transformations continue during further cooling below Ms. In contrast, no transformation occurs when the steel is held isothermally below Ms, indicating that the transformation is time independent, i.e. athermal. Consistently, martensitic transformations would not be suppressible, not even by applying the most rapid cooling possible.

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.