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.

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.

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.

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.

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).

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).

Nitriding surface hardening is commonly used on steel components for high wear, fatigue and corrosion applications. Case hardening results from white layer formation and coherent alloy nitride precipitates in the diffusion zone. This paper evaluates the microstructure development in the nitrided case and its effects on the hardness in both the white layer and the substrate for two industry nitriding materials, Nitralloy 135M and AISI 4140. Computational thermodynamic calculations were used to identify the type and amount of stable alloy nitrides precipitation and helped explain the differences in the white layer hardness, degree of porosity at the surface, and the hardening effect within the substrate. Some initial insights toward designing nitriding alloys are shown.

Phase transformation and temper response of three martensitic alloys were investigated as an important portion of fundamental metallurgical information database related to heat treatment design for engine component applications. A limited metallographic evaluation has also been carried out with selected temper response run samples in this study. Basic descriptions on adequate hardening and tempering parameter design were provided in terms of optimizing the intended performance with these alloys.

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

The work presented in this paper addresses a data gap that continues to be a hinderance to users of precipitation modeling tools, particularly those based on Langer-Schwartz theory. Thermodynamic and kinetic data required for precipitation models can be obtained from CALPHAD databases, but interfacial energies between the bulk and precipitate phases are not available for many alloy systems. In this work, a number of matrix-precipitate interfacial energies have been determined for influential precipitates in alloys of industrial importance, for example, carbides in Grade 22 low-alloy steels, delta phase in Ni 625 and 718, S-phase in Al 2024, and Q’ and β’’ in Al 6111.

Rapid induction hardening of martensitic steel can attain the very high strength levels needed for light-weighting components subjected to high operating stresses. Specimens of martensitic 0.6% C steels were heat treated using a dilatometer to investigate the effects of heating rates of 5 to 500 °C/s to temperatures of 850 to 1050 °C on the transformation to austenite and subsequent transformation to martensite during quenching. Selected specimens were quenched after partial transformation to austenite to assess the initial cementite precipitate size formed in ferrite during heating. Other specimens were isothermally held at the austenitizing temperature to assess cementite dissolution rates. Higher heating rates increased the Ac1 and Ac3 temperatures, and lowered the Ms temperature. Alloy content and prior microstructure also influenced the transformation temperatures.

Microstructure refinement strategies for carburized steel were evaluated to assess their effect on the fatigue performance of case carburized components. Commercial 52100 steel samples were subjected to various treatments and analyzed to determine the micro-geometry of plate martensite and the size distribution of retained-austenite regions. Decreasing reheat temperature produced finer austenite grain size, while multiple reheating cycles helped narrow grain size distribution. The refinement of austenite grain size also led to a reduction in martensite plate size and finer distribution of retained austenite.

Hot Isostatic Pressing (HIP) is widely used today to eliminate internal defects in components to achieve improved material properties like ductility and fatigue. With today’s modern HIP systems there are possibilities to incorporate more process steps into the HIP process. These process steps can be stress relief, solutionizing, quenching, ageing, tempering etc. performed in the same equipment during the same cycle which makes a very effective process route. This presentation will focus on the possibilities to perform solutionizing and quench directly in the HIP system for a typical QT steels and evaluate how HIP quench compares to water and oil quench as well as how the austenite to perlite transformation react under pressure.

Several case studies are presented illustrating issues that may be encountered when developing induction heat treating processes. The relationship of how induction heat treating parameters affect the metallurgy of production parts is examined in the form of case studies. These include the importance of normalized versus anneal starting microstructure as it relates to the ability of pearlite to transform to martensite within the short induction hardening process window. The influence of a non-uniform microstructure with proeutectoid grain boundary ferrite is discussed as it relates to prior structure. A team approach to balancing design specification with manufacturing cost and sound metallurgical practice is covered for an AISI 1060 steel channel component with complex inductor design. Another case study addresses how evaluating hardness in the as-quenched versus tempered condition can provide additional detail relating to back tempering in tooth by tooth hardened gears. The final example is the influence of frequency of case depth formation for an AISI 4140 cross roller section.

Steels hardened by copper precipitation are the focus of many research programs. Most of this effort is devoted to development of low-carbon steels. Precipitation strengthening of ferrite is used for steel strengthening without losing the capability of deep drawing before the precipitation hardening. This article shows the results of precipitation strengthening in low alloyed steel containing 0.2% carbon. The steel composition is aimed at developing weldable high-strength steel for demanding structural applications. Copper precipitation was exploited to strengthen different types of microstructures. Quenching and ageing and isothermal austenite decomposition into bainite were used to develop copper precipitation. Mechanical properties and microstructure were compared. Tensile tests were performed and hardness was measured. Copper precipitation was documented by FEG SEM microscopy.

A finite element (FE) method was used to determine the important heat treating process parameters that impact the residual stress and distortion in steel. The FE model combines a commercially available heat treatment software DANTE to the finite element analysis software ABAQUS. A thermomechanical FE model was developed to model the evolution of microstructure, the volumetric changes associated with the kinetics of martensitic phase transformation and the formation and distribution of residual stress during quenching of steel. Alternative quenching parameters such as different steel grades, quenching orientation, immersion speed, quenching agent, quenching temperature, austenitizing temperature and part geometry were ranked based on their impact. The main purpose of this paper is to provide processing guidelines to control residual stress and distortion.

Blade curving due to quenching in the Japanese sword has been recognized by swordsmiths through the ages. In the late 1920s, Hattori noted that the sword curving is induced from not only martensitic transformation expansion in the near-edge region but also non-uniform elastic and plastic strains distributed in the section, based on his experimental results using cylindrical specimens. Our research for an updated explanation on the subject prepared Japanese sword (JS) type specimens made of the same steel and process as the Japanese sword, and model JS (MJS) type specimens with the almost same shape as the JS type specimens, which were machined from commercial carbon steel and austenite stainless steel bars. All specimens quenched by a swordsmith using the traditional way showed a usual curved shape with different curvatures. Curving, temperature, hardness, metallic structure and residual stress measurements for the specimens were performed to prepare their future simulation works.