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.

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.

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.

Carburizing and induction hardening are two surface heat treatments commonly used to increase wear resistance and fatigue performance of steel parts subject to cyclical torsional loading. It was originally hypothesized that performing an induction surface hardening heat treatment on parts previously carburized could provide further increased fatigue life, however initial torsional fatigue results from previous work indicated the opposite as the as-carburized conditions exhibited better torsional fatigue strength than the carburized plus induction surface hardened conditions. The aim of this work is to further elucidate these torsional fatigue results through metallography and material property characterization, namely non-martensitic transformation product (NTMP) analysis, prior austenite grain size (PAGS) analysis, and residual stress vs depth analysis using x-ray diffraction (XRD). A carburizing heat treatment with a case depth of 1.0 or 1.5 mm and an induction hardening heat treatment with a case depth of 0, 2.0, or 3.0 mm were applied to torsional fatigue specimens of 4121 steel modified with 0.84 wt pct Cr. The carburized samples without further induction processing, the 0 mm induction case depth, served as a baseline for comparison. The as-received microstructure of the alloy was a combination of polygonal ferrite and upper bainite with area fractions of approximately 27% and 73% respectively. The only conditions that exhibited NMTP were the as-carburized conditions. These conditions also exhibited larger average PAGS and higher magnitude compressive residual stresses at the surface compared to the carburized plus induction hardened conditions. The compressive residual stresses offer the best explanation for the trends observed in the torsional fatigue results as the conditions with NMTP present and larger PAGS exhibited the best torsional fatigue performance, which is opposite of what has been observed in literature.

Thermochemical treatments like carburizing and carbonitriding allow to improve the properties in low-alloyed steels, which depend mainly on the distributions of residual stresses and microstructures. As the fatigue properties depend mainly on the latter, a fundamental understanding must be established regarding their formation during the cooling after the enrichment treatment. This study introduces an experimental and simulation analysis of microstructure and internal stresses evolutions and their couplings. Influence of the carbon and nitrogen enrichments is highlighted. An original experimental technique is introduced to follow in situ by High-Energy XRD the phase transformation kinetics and the evolutions of the internal stresses during cooling, inside laboratory scale samples with C/N composition gradients. The usual trends are confirmed regarding the carburizing: the carbon-enriched case is the last to undergo phase transformations. Due to the phase transformation strains, the surface ends up with compression residual stresses, whereas the center is put in tension. Conversely, for carbonitriding, unusual profiles of microstructures and residual stresses are observed. The presence of nitrogen induces a drastic loss of hardenability in the enriched case. This modifies the chronology of the phase transformations and this leads to tensile residual stresses at the surface for the studied cooling conditions. In the nitrogen-enriched case, a fine microstructure is formed during cooling and retained austenite remains, leading to a lower hardness than in the martensite layer beneath. A coupled thermal, mechanical and metallurgical model predicting the phase transformation kinetics and the evolutions of internal stresses is set up. It takes account of the local carbon and nitrogen concentrations in the case. For carburizing, predictions are in good agreement with experiment. Simulations for carbonitriding achieve to predict the tensile stresses in the nitrogen-enriched case, which are due to the loss of hardenability. In both cases, residual stresses come mostly from phase transformation plasticity strains.

The objective of this work was conducted to investigate the influence of nickel (Ni) content and retained austenite on rolling-sliding contact fatigue (RSCF) life in carburized gear steel. In order to evaluate Ni and retained austenite effects, this study utilized carburized steel specimens of 4120 (0.13 wt pct Ni) and 4820 (3.38 wt pct Ni), which were subjected to RSCF testing. The specimens were gas carburized with a resulting case depth of approximately 1.3 mm, based on a hardness of 500 HV. The retained austenite was measured using x-ray diffraction at depths beneath the surface of 50, 250, 450, 650 μm. The 4120 specimens have a higher surface retained austenite content than the 4820. Specimens were surface ground to an average surface roughness of 0.2 μm to decrease the effect of as-carburized surface roughness on the fatigue life. The specimens underwent RSCF testing, with a surface contact stress of 2.5 GA and a slide to roll ratio of -20 pct, until a pit formed, as detected by an accelerometer. The pits that formed on the surface of the specimens were analysed with secondary electron microscopy, macrophotographs, and light optical microscopy. The pits that formed from the RSCF testing conditions were surface-initiated. The fatigue life of the 4820 specimens was higher than the fatigue life of the 4120 specimens, suggesting that the higher Ni level is beneficial to the fatigue life.

The proposition that compressive residual stresses are beneficial in improving the service life of components subject to rolling contact fatigue is well documented. However, the exact nature of the relationship between effective case depth (ECD) and the residual stress state is not well understood for components with deep case depth (>0.050inches, 1.27mm). It is expected that compressive residual stresses will gradually transition to tensile stresses as the case depth increases beyond a threshold value. In addition, the strain-induced transformation of retained austenite and its influence on the residual stress state of components resulting from service was explored. This study measured the residual stress state of components prepared with various ECD before and after simulated service with the goal of determining where the compressive to tensile transition occurs. Residual stress and retained austenite measurements were conducted using X-ray diffraction.

The influence of microstructure on hydrogen embrittlement of high strength steels for fastener applications is explored in this study. Space limiting applications in areas such as the automotive or agricultural industries provide a need for higher strength fasteners. Albeit, hydrogen embrittlement susceptibility typically increases with strength. Using a 9260 steel alloy, the influence of retained austenite volume fraction in a martensitic matrix was evaluated with microstructures generated via quenching and partitioning. X-ray diffraction and scanning electron microscopy were used to assess the influence of retained austenite in the matrix with different quenching parameters. The quench temperatures varied from 160 °C up to 220 °C, and a constant partitioning temperature of 290 °C was employed for all quench and partitioned conditions. The target hardness for all testing conditions was 52-54 HRC. Slow strain rate tensile testing was conducted with cathodic hydrogen pre-charging that introduced a hydrogen concentration of 1.0-1.5 ppm to evaluate hydrogen embrittlement susceptibility of these various microstructures. The retained austenite volume fraction and carbon content varied with the initial quench temperature. Additionally, the lowest initial quench temperature employed, which had the highest austenite carbon content, had the greatest hydrogen embrittlement resistance for a hydrogen concentration level of 1.0-1.5 ppm.

Press hardening steel (PHS) applications predominately use 22MnB5 AlSi coated in the automotive industry. This material has a limited supply chain. Increasing the tensile strength and bendability of the PHS material will enable light-weighting while maintaining crash protection. In this paper, a novel PHS is introduced, and properties are compared to 22MnB5. The new Coating Free PHS (CFPHS) steel, 25MnCr, has increased carbon, with chromium and silicon additions for oxidation resistance. Its ultimate tensile strength (UTS) of 1.7 GPa with bending angle above 55° at 1.4 mm thickness improves upon the 22MnB5 grade. This steel is not pre-coated, is oxidation resistant at high temperature, thus eliminating the need for AlSi or shot blasting post processing to maintain surface quality. Microstructural mechanisms used to enhance bendability and energy absorption are discussed for the novel steel. Performance evaluations such as: weldability, component level crush and intrusion testing and e-coat adhesion, are conducted on samples from industrial coils.

Retained austenite may be helpful or detrimental to the life of heat-treated components, but it can be difficult to accurately measure in manufactured steels. Commonly used visual sample investigations are subjective and often incorrect, magnetic measurements require part-specific calibration, and electron backscattering involves expensive equipment, intensive sample preparation, and long measurement times. Recent developments in X-ray diffractometry, however, provide measurements in minutes and can compensate for the influence of carbides in high-carbon steels as well as texture orientations in rolled sheet metals. This paper discusses the use of X-ray diffraction for measuring retained austenite and compares and contrasts it with other methods. It also provides a brief review of the formation of austenite and its effect on carburized gears, TRIP steels, and bearings.

During forging operations, strain can occur through three primary mechanisms: strain due to load applied through dies, strain due to thermal contraction, and strain due to creep. In materials behavior models, strain due to applied load and thermal contraction are directly considered and predictions are based on thermophysical properties and flow stress behaviors as inputs to the models. Strain due to creep after forging (during cooling) is often more difficult to predict and capture due to lack of materials data. In particular, data that capture the changing flow stress behavior during cooling (rather than from isothermal testing) are not commonly available. In this project, creep strain behavior during cooling was investigated by physical simulations using a Gleeble 3500. Standard cylinder-shaped Ti-6Al-4V samples with 10 mm diameter were heated to below β-transus temperature (1775°F) or above β-transus (1925°F), followed by constant cooling rates of 250°F/min and 1000°F/min with and without applied load during cooling to 1000°F. Total strain for the tests ranged from 2 – 6%. Characterization of prior microstructure and texture was carried out using XRD, optical microscopy, and SEM. The results provide insights on the relationship of flow stress behavior and microstructure as a function of temperature and cooling rate and are applicable to forging practices. These materials data can be used as input for future process modeling, potentially giving better prediction accuracy in industry applications.

The notion that compressive residual stresses can extend the service life of components subject to rolling contact fatigue is well documented. However, the exact nature of the relationship between effective case depth and the residual stress state is not well understood for components with case depths greater than 0.050 in. (1.27 mm). It is expected that compressive residual stresses gradually transition to tensile stresses as case depth increases beyond a threshold value. This study will measure the residual stress state of components with different case depths before and after simulated service in order to determine where the compressive to tensile transition occurs. It will also investigate the role of retained austenite and the effect of strain-induced transformation caused by rolling contact. Residual stress and retained austenite measurements will be conducted using X-ray diffraction.

Vacuum carburizing 9310 gear steel followed by austenitizing, oil quench, cryogenic treatment, and tempering is known to impact the residual stress state of the material. Residual stress magnitude and depth distribution can have adverse effects on part distortion during intermediary and finish machining steps. This study provides residual stress measurement, microstructural, and mechanical property data for test samples undergoing a specific heat treat sequence. Test rings of 9310 steel are subjected to a representative gear manufacturing sequence that includes normalizing, rough machining, vacuum carburizing to 0.03”, austenitizing, quench, cryo-treatment, temper, and finish machining. The rings along with metallurgical samples are characterized after each step in order to track residual stress and microstructural changes. The results presented here are particularly interesting because the highest compressive residual stresses appear after removal of copper masking, not after quenching as expected. Data can be used for future ICME models of the heat treat and subsequent machining steps. Analytical methods employed include X-ray diffraction, optical and electron microscopy, and hardness testing.

Plain carbon steel cylinders were heat treated and quenched in order to study the effects of heat treating on residual stresses and microstructure. Residual stress measurements were obtained via X-ray diffraction using the sin 2 Ψ method and microstructure was characterized based on the associated peak widths. Measurements were made both at the surface and through depth following electropolishing. Triaxial stress gradients were observed in all test samples with concomitant varying microstructural characteristics. The method used to measure residual stresses in this study is typical and recommended for general practice.

This paper provides an overview of the types of residual stress that engineers deal with and the tools and techniques available to measure it. It identifies the causes of manufacturing-induced residual stress along with their effects. It summarizes the practical aspects of a wide range of measurement methods, including hole drilling, layer removal, contour, lab XRD, and synchrotron analysis. It assesses current challenges and gaps in determining stress in relation to stress type, stress component, microstructure, specimen geometry, materials type, and measurement location.

This paper presents the preliminary results of experiments designed to mimic typical machining and thermal processing practices for aerospace titanium alloys. The most significant finding is that multiple side mill passes result in lower magnitude compressive stresses than a single side pass, which suggests that successive interactions with the milling tool serves to relieve residual stresses at the surface. The most likely mechanism for this is that Ti exhibits significant springback during machining, and multiple tool passes essentially remove the “springback” layer. Each successive removal of material allows stress relaxation in the remaining surface layer. By contrast, with only a single pass, surface residual stresses did not sufficiently relax.

In the present study unalloyed spheroidal graphite iron was compared with a sample alloyed with copper and nickel (Cu- Ni). Samples of those materials were treated in salt bath (a composite of sodium borate, aluminum, iron-niobium and/or iron-vanadium) at 1000 °C for 2 hours. This resulted in layers of niobium carbide (NbC), vanadium carbide (VC) and ternary carbide of niobium and vanadium (NbVC2). Characterization of these layers was obtained by optical microscopy, X-ray diffraction and microhardness Vickers. The microhardness and the thickness results were statistically analyzed by the two factors variance analysis method. The efficiency of the layered surface in increasing corrosion resistance was demonstrated by submitting it to potentiodynamic polarization corrosion tests.

To experimentally investigate the effect of tempering temperature and time on the structure and composition of martensite, AISI 52100 was austenized at 1000°C for 40 minutes and quenched in agitated water at 21°C. The as-quenched steel contained body-centered tetragonal (BCT) martensite with 22% retained austenite. These samples were tempered at 100°C, 200°C, and 300°C with different holding times and then were characterized by x-ray diffraction (XRD) to determine the effect on the structure of the martensite. It was found that the content of retained austenite did not change after tempering at 100°C. Retained austenite decomposed after tempering for 40 minutes at 300°C. The changes in crystal structures and lattice parameters for tempered martensite with different holding times and temperatures were measured. The effect of sample preparation on retained austenite and the structure of martensite and tempered martensite was evaluated. An effective technique for carbide extraction and collection in steel is introduced.

The use of aluminum alloys in automotive and other industries is due to their excellent combination of weight and strength, which is obtained by heat treating. The alloying elements are put in solution by holding the material at a high temperature, and cooling at a rate fast enough to allow them to remain in solution; strengthening is then produced by the precipitation of particles of different size, shape and nature. This work presents the results of the analyses of samples made from cast aluminum alloys hardened by either the precipitation of Mg 2 Si or Al 2 Cu and of an alloy that has both. The samples were solution treated at temperatures adequate for the different alloys and cooled by placing them in water close to boiling. Aging was conducted at two different temperatures (170 and 240°C) in all cases for times as long as 100 hrs. Changes due to aging were documented by microhardeness, microscopical examination and by X-ray diffraction. X-ray examination showed that the peak corresponding to the {311} position shifts the aging condition, indicating changes in the lattice parameter of aluminum, which depends on the type of particle that precipitates.