Metal additive manufacturing is a molding method with a high degree of freedom because it can be created from high-strength materials using by CAD, etc. In recent years, there is a demand for metal additive manufacturing due to the demand for more complex mechanisms and shape in industrial products. However, the mechanical properties of metal additive manufacturing materials as metallic materials are not clear compared to metallic materials by melting method. In this study, two types of metal additive manufacturing (AM) materials with different lamination directions are carburized and heat treated to clarify the differences from general metallic materials and to clarify the causes. The carburized AM materials were confirmed to have a surface hardness of 550HV and a total carburization depth of 200 μm, but the amount of carburization differed depending on the orientation. In addition, when analyzed with a SEM, a metal structure was formed in an equiaxed crystal shape, and segregation of metal elements was observed.

It is well known that the maximum prior austenite grain size after carburizing heat treatment is approximately positively correlated with the maximum shear strain in the case of simple deformation of pre process as cold working treatment. On the other hand, it is generally known that the maximum shear strain and the maximum grain size do not correspond when complex cold working is performed, but the reason of these phenomena is not well known. Then, it is necessary to investigate the relationship between the applied strain during cold working with multiple steps and prior austenite grain size after heat treatment(GG). In this study, we used a processing method called HPT processing, which introduces shear strain by torsion deformation under applying high hydrostatic pressure to the top and bottom of a disk-shaped sample using a die, and investigated how GG changes due to the accumulation of dislocations by focusing on the strain amount | ± Δ ε| given in one pass controlled by a processing path called Cyclic-HPT (c-HPT) (4) and the total strain amount 𝛴| ± Δ ε| given to the sample by the accumulation of one pass. As a result, when finer strain is applied, the grain size does not necessarily become smaller, but rather there are boundary conditions that indicate the positive and negative grain size with respect to the number of strains. Similarly, for the grain size distribution, an increase and decrease in grain size was observed with respect to radial distance, so there are boundary conditions that indicate the positive and negative grain size with respect to distance. From these results, it is believed that this may be the mechanism for grain growth behavior in the case of cold working, which involves complex deformation.

This presentation will discuss the most common types of induction tooling failures and the best practices to improve the performance and longevity of inductor coils, bus bars quenches and related tooling. We will discuss the harsh environment of a typical induction machine installation and what can be done to reduce contamination, which is the leading cause of tooling failure. Robust tooling designs and how water cooling is essential to longevity shall be discussed. Cooling water temperature and how the water is presented and routed through the tooling components and the impact this has on performance and longevity shall be discussed. We will discuss the use of proper materials, fittings and hoses which are often overlooked and can be detrimental to a process if not correctly selected. We will cover the induction machine and how it is essential to have a proper earth ground and the importance of proper machine fixturing and alignment. We shall discuss the importance of scheduled machine maintenance, scheduled service and calibration. The presentation will summarize the most common types of failures, how maintenance is essential for longevity and the importance of high-quality robust tooling.

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.

Heat treatment of steels is a process of modifying the mechanical properties by solid-state phase transformations or microstructural changes through heating and cooling. The material volume changes with phase transformations, which is one of the main sources of distortion. The thermal stress also contributes to the distortion, and its effect increases with solidstate phase transformations, as the material stays in the plastic deformation field due to the TRIP effect. With the basic understanding described above, the sources of distortion from a quench hardening process can be categorized as: 1) nonuniform austenitizing transformation during heating, 2) nonuniform austenite decomposing transformations to ferrite, pearlite, bainite or martensite during quenching, 3) adding of carbon or nitrogen to the material, and forming carbides or nitrides during carburizing or nitriding, 4) coarsening of carbide in tempered martensite during tempering, 5) stress relaxation from the initial state, 6) thermal stress caused by temperature gradient, and 7) nonhomogeneous material conditions, etc. With the help of computer modeling, the causes of distortion by these sources are analyzed and quantified independently. In this article, a series of modeling case studies are used to simulate the specific heat treatment process steps. Solutions for controlling and reducing distortion are proposed and validated from the modeling aspect. A thinwalled part with various wall section thickness is used to demonstrate the effectiveness of stepped heating on distortion caused by austenitizing. A patented gas quenching process is used to demonstrate the controlling of distortion with martensitic transformation for high temperature tempering steels. The effect of adding carbon to austenite on size change during carburizing is quantified by modeling, and the distortion can be compensated by adjusting the heat treat part size.

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.

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.

Nickel-based Inconel 625 is widely used for both low and high-temperature applications. It has several applications in aerospace, marine, chemical, and petrochemical industries due to its high strength, corrosion resistance, good formability, and weldability. With the molten pool’s rapid solidification during laser powder bed fusion (LPBF), the resulting microstructures differ from those expected in equilibrium conditions. Residual stresses, microsegregation, anisotropy, undesirable phases, layered structure, and lower mechanical properties are the challenges that must be addressed before LPBF-ed Inconel 625 parts can be industrially implemented. Heat treatment of Inconel 625 after the LPBF process is widely discussed in the literature, and the proposed heat treatment processes do not address all the challenges mentioned above. For this reason, specific heat treatments should be designed to achieve desired mechanical properties. Five different high-temperature heat treatment procedures were developed and tested in recent work in comparison with the standard heat treatment for wrought alloy (AMS 5599), to study the effect of various heat treatment parameters on the type of precipitates, grain size, room, and elevated temperature mechanical properties, and to develop an elevated-temperature tensile curve between room temperature (RT) and 760°C of LPBF-ed Inconel 625. Four heat treatment procedures showed complete recrystallization and the formation of equiaxed grain size containing annealed twins and carbide precipitates. However, either eliminating the stress relief cycle or conducting it at a lower temperature resulted in microstructures having the same pool deposition morphology with grains containing dendritic microstructure and epitaxial grains. Two different grain sizes could be obtained, starting with the same as-built microstructure by controlling post-process heat treatment parameters. The first type, coarse grain size (ASTM grain size No. G 4.5), suitable for creep application, was achieved by applying hot isostatic pressing (HIP) followed by solution annealing. The second type, fine-grain size (ASTM grain size No. G 6), preferable for fatigue properties, was achieved by applying solution annealing followed by HIP. The mechanical properties at room and elevated temperature 540°C are higher than the available properties in the AMS 5599 for wrought Inconel 625 while maintaining a higher ductility above the average level found in the standards. It can be concluded that the performed heat treatment achieves higher mechanical properties. The values of ultimate tensile strength (UTS), yield strength (YS), elongation, and reduction of area percentages are similar in the XZ and XY orientations, revealing the presence of isotropic microstructure. The ultimate tensile strength values show an anomalous behavior as a function of the temperature. From the room temperature until around 500°C, there occurs a decrease in the yield strength and a slight increase up to 600°C, decreasing sharply at 700°C. An anomaly is also present in relation to the elongation, with a significant decrease in the elongation at temperatures after 600°C.

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.

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.

The effect of quench rate on the width of precipitate free zone was examined in aluminum alloys 7075 and 7050. It was determined that at quench rates greater than 60°C/sec., vacancy depletion dominated. At slower quench rates, it was determined that solute depletion dominated the precipitate free zone. The critical vacancy concentration for precipitation was established as a function of quench rate.

Prof. Tatsuo Inoue passed away on September 23, 2023, at the age of 83. He held a professorship at Kyoto University from 1983 to 2003 and made significant contributions to the theory of heat treatment simulation, which is now widely used. His theory was reported at an international conference in Linkoping, Sweden in 1984. Fundamental equations in his theory cover metallurgical coupling effects caused by changes due to phase transformation, temperature, and inelastic stress/strain as well as carbon diffusion during the carburizing process. Prof. Inoue designated these effects as “metallothermo- mechanical coupling”. Software applying his theory was presented at ASM International’s 1st International Conference on Quenching and the Control of Distortion in 1992, where its advanced nature was recognized. In 1994, Prof. Inoue published a paper on the application of heat treatment simulation to the quenching of Japanese swords, revealing changes in temperature, curving, microstructure, and stress/strain in their model during the traditional quenching process. In 2017, he published “The Science of Japanese Swords” with Sumihira Manabe, a swordsmith, to communicate his specific achievements to the general public.

High-alloy steels, like Ferrium C64, are used in powertrain components due to their corrosion resistance and high temperature resistance properties. These steels undergo a tempering temperature that is well above traditional steel, and during this process alloy carbides or compounds form, increasing the materials hardness, mechanical strength, and high temperature resistance properties. In the early stages of tempering, softening occurs due to the formation and coarsening of iron carbide, followed by a hardening as the alloy elements combine to form nano-scale dispersoids. These alloy carbides block the path of dislocations in the grain, strengthening the material. At longer tempering times or high temperatures, the coarsening of these alloy carbides and compounds can cause softening. A predictive material model for the high-tempering response of steels is needed to ensure peak hardening properties are met. For a robust heat treatment model, the material response for every step of the process needs to be modeled. These material properties include austenitization rates and thermal expansion during heating, carbon diffusivity and saturation limits for carburization, phase transformation rates and thermal contraction rates per phase during cooling and quenching, deep-freeze kinetics for further martensitic transformation, tempering kinetics for formation of the tempered martensite phase, and carbide kinetics for formation, coarsening, and size. Additionally, mechanical properties of each phase as a function of carbon need to be defined to ensure the proper mechanical response during and after heat treatment. After the material model is developed it can be used to design and optimize the high-temperature tempering process for any part using the same material.

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.

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.

A physical model was developed to estimate the thermal history in hypoeutectoid steel under continuous cooling and forced convection conditions. The thermal histories were acquired at different cooling rates to emulate the forced-convection conditions in a controlled cooling conveyor and compared in every cooling condition with the microstructural evolution of the pearlite. Through Scanning Electron Microscopy (SEM) the pearlite interlamellar spacing was determined, as well as Vickers hardness and tensile tests, validated the effect of the cooling rate on microstructural parameters such as transformation temperature and pearlite interlamellar spacing. It was found that air velocity increased the undercooling rate and decreased the pearlite interlamellar spacing.

Carburizing is frequently utilized in the automotive industry in order to increase the surface hardness of a steel alloy while retaining toughness and ductility in the core. At elevated temperatures where some carburizing processes are performed, abnormal grain growth (AGG) can occur. During AGG, the microstructure undergoes bimodal grain growth with some grains growing exponentially faster than others. The growth of large austenite grains through AGG compromises the fatigue performance of carburized steels. AGG is further exacerbated by cold work introduced into the alloy prior to carburizing. Warm work is also sometimes utilized in part forming prior to carburizing. In this study, the effects of warm work on AGG were investigated. AISI 4121 and a modified AISI 4121 that contains Nb and Mo microalloying additions rather than Al for grain size control were warm worked in a range of 0-50% at a temperature of 900°C and then heated in a furnace for various lengths of time at a temperature of 930 °C to simulate a carburizing thermal history. The average prior austenite grain size (PAGS) tended to decrease as the degree of warm work increased, with the NbMo-modified alloy presenting a finer PAGS at all percentages of warm reduction and different lengths of time at the simulated carburization temperature. Specimens of the 50% warm reduced condition were also cold rolled at 5, 10, and 25% reductions, typical of cold sizing, prior to simulated carburization. The average PAGS of these CR samples was finer than their 0% CR counterparts, but the PAGS increased with CR in the modified alloy after 328 minutes of simulated carburization.

Quenched and tempered (Q&T) medium-C steels with various V and Mo additions were studied to understand the relationship between alloy carbide precipitation and hydrogen absorption and trapping behaviours. Heat treatments were selected in the temperature range favourable for V carbide formation, 500-600 °C, leading to higher hardness compared to similar V- and Mo-free alloys due to precipitation hardening. Heat-treated coupons were electrochemically charged to introduce hydrogen, and the bulk hydrogen concentration was measured using melt extraction analysis. Hardness and dislocation density were measured for each tempered condition to relate these properties to the hydrogen absorption and trapping behaviours of each material. Results indicate that dislocation density as well as V and Mo carbide precipitation increase the extent of hydrogen absorbed during charging and the amount of hydrogen remaining trapped after holding at ambient temperature for up to 168 h (1 week).