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 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.

The automotive industry has searched for alternatives to reduce the weight of vehicles without neglecting the user’s safety by using new materials. Advanced high-strength steels of complex phases are used in structural applications requiring good performance and reducing the weight of vehicles. However, these steels have shown edge cracking, known as fissure, during processing, which has become a challenge for steelmakers and other companies that rely on them to manufacture structural components. Such defects can be associated with the interaction between the different microstructural constituents of the steel, such as various phases and precipitates generated during its processing to achieve the required mechanical properties. The present work presents the studies evaluate the effect that processing and chemical composition exerts on edge cracking in complex phase steels of grade 800 MPa produced by different steelmaking routes.

Gas nitriding and ferritic nitrocarburizing have seen tremendous growth. Today, it continues to accelerate as more uses are being found, especially in the growing electric vehicles (EV) sector. This success is due to the ability to control protective white layers consistent with the needs of an automotive engineer. Steels and cast irons are still the materials of choice for many applications and the nitrided layer is wellknown for its tribological features (some would say even more than three) which include wear resistance, lubricity, and a low coefficient of friction. Corrosion resistance in particular has become an important advantage and depends on white layer formation and quality. The white layer (known as the compound zone) consists of two iron nitrides, epsilon (Fe 2-3 [N]) and gamma prime (Fe 4 N). In addition, the epsilon layer can contain varying amounts of iron carbides and/or iron carbonitrides, Fe 2-3 [C]. This paper will focus mainly on the how’s and why’s of white layer: how to control its composition and properties; and how to minimize it, if required. Just as importantly, some applications of how the EV component engineers have found uses for this important steel treatment are discussed, including brake rotors.

Across all industries, material specifications are tightening beyond previously understood process capabilities. Slight shifts in material grade, microstructure, heat treatment, or alloy composition can significantly impact long term material integrity. This study examines the feasibility of noncontact, 100% inline magneto-inductive testing on materials and components to ensure material quality standards. To investigate the hypothesis that material grade, carbon content, density, and alloy composition can be accurately tested in real time during production, an experiment was conducted using magneto-inductive test instrumentation and an encircling coil. The results of the investigation confirmed that 100% of the material in a component could be thus tested, accurately, efficiently, and autonomously verifying that the specified material grade with the proper composition and properties had been used.

Carburization is a common method of hardening steel surfaces to be wear-resistant for a wide range of mechanical processes. One critical characteristic of the carburization process is the increase in carbon content that leads to the formation of martensite in the surface layer. Combustion and spark-OES are two common methods for determination of carbon in steels. However, these techniques do not effectively separate carbon from near surface contaminants, carburized layers, and base material composition. Careful consideration of glow discharge spectroscopy as a method of precisely characterizing carbon concentration in surface layers as part of a production process should be evaluated in terms of how the resulting data align with other common analytical and metallurgical measurements. When used together, glow discharge spectroscopy, optical microscopy, and microhardness testing are all useful, complementary techniques for characterizing the elemental composition, visually observable changes in material composition, and changes in surface hardness throughout the hardened case, respectively. Close agreement between related measurements can be used to support the use of each of these techniques as part of a strong quality program for heat treatment facilities.

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.

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.

AISI 8620 low carbon steel is widely used due to its relatively low cost and excellent case hardening properties. The nominal chemistry of AISI 8620 can have a large range, affecting the phase transformation timing and final hardness of a carburized case. Different vendors and different heats of steel can have different chemistries under the same AISI 8620 range which will change the result of a well-established heat treatment process. Modeling the effects of alloy element variation can save countless hours and scrap costs while providing assurance that mechanical requirements are met. The DANTE model was validated using data from a previous publication and was used to study the effect of chemistry variations on hardness and phase transformation timing. Finally, a model of high and low chemistries was executed to observe the changes in hardness, retained austenite and residual stress caused by alloy variation within the validated heat treatment process.

This paper discusses the growing use of automation in heat treating and some of the benefits that have been realized in early applications. It provides examples showing how articulated robots are used to load and unload parts on fixtures, how inline 3D cameras facilitate dimensional and distortion control, and how test coupons placed by robots at strategic locations throughout a load are weighed before and after heat treatment to determine if parts in different areas of the load are likely to be carburized to the same degree. It also includes an example of an automatically generated report and explains how binary codes on base trays can be used to automatically upload recipes for specific heat treatments.

The objective of this work is to develop the material and numerical models needed to simulate the carburizing process of an automotive gear. The paper discusses the factors that influence calculation time and accuracy and presents important equations and material property data. It describes how the simulation predicts local carbon content based on diffusion and how quenching computation provides information on stress states and residual stresses. It also explains how to account for the effects of grain growth, volume variation due to phase changes, and transformation plasticity.

Modeling of as-tempered hardness in steel is essential to understanding final properties of heat-treated components. Most of the tempering mathematical models derive a tempering parameter using Hollomon-Jaffe formulation. Some recent models incorporate chemical composition into the general Hollomon-Jaffe relationship. This paper compares model predictions with a substantial set of actual tempered Jominy End Quench bars and the hardness data from them. Improvements to the models and direction for future work are discussed.

High entropy alloys (HEA) are an exciting new class of alloys composed of several metallic elements with equiatomic or near-equiatomic composition to maximize configurational entropy, leading to desirable properties. However, during solidification, as in casting or welding processes, elements segregate, creating local regions of distinct composition. In conventional alloy systems, homogenization heat treatments are used to remove this segregation effect. This study examines the conditions of the heat treatment needed in HEA alloys. First, the solidification behavior of equiatomic alloy composition AlCoCrCuFeNi is modeled using the Scheil module within Thermo-Calc along with the TCHEA2 database. Energy dispersive spectroscopy (EDS) is performed across the dendrite arms of the as-melted HEA to compare with the Scheil calculations. The resulting dendritic and interdendritic compositions are used as inputs in Thermo- Calc to determine the stable phases as a function of temperature. Selected heat treatments are conducted on the as-melted HEA to compare with the calculation results.

Numerical model of controlled cooling in production of steel hot rolled bars was developed. By numerical model of controlled cooling is possible to predict a transient temperature field, microstructure evolution and hardness of rectangular steel bars during their cooling in cooling beds. The numerical model of transient temperature field is based on control volume method. The algorithm for prediction of hardness and microstructure distribution in steel bars is based on continues cooling transformation, (CCT) diagrams and real chemical composition. The numerical model and algorithm is completed to solve problems in controlled cooling of hot rolled bars in cooling beds. The controlled cooling are performed by special placement of hot rolled bars on cooling beds. Numerical model and computer program was experimentally verified by simulation of real industrial production of low alloyed steel bars. The verification of developed numerical model was performed by comparison of simulated hardness with experimentally evaluated results.

Dilatometry and transmission electron microscopy were used to characterize the effects of V content, Si content, tempering temperature and starting microstructure on the hardness and microstructural evolution of a 0.4 wt pct carbon steel after a simulated nitriding thermal cycle. When tempered at 500 °C, significant amounts of V are left in solution leading to precipitation during the nitride thermal cycle increasing the hardness and dilation strain. Increases in Si content also lead to higher core hardness after nitriding, but Si does not significantly increase dilation strain during nitriding. Bainite starting microstructures produced less dilation strain during nitriding compared to martensite starting microstructures when tempered at 500 °C.

Freight truck shipping is a massive industry and an important contributor to the US economy. Truck axle shafts for decades have been made from induction hardened carbon steel with 0.4% to 0.5% carbon. Associated metallurgical engineering of steel procurements, forging, processing, and applied machining, impacts axle shaft production and performance. This paper reviews metallurgical principles and controls currently applied to heavy truck axle shaft use and production in North America. Basic metallurgical engineering principles and controls, as historically and currently applied and specified, plus potential opportunities for increasing engineering value optimization, are reviewed. In particular, case depth, surface hardness, microstructure, grain size, chemical compositional interactions, procurement, processing, metallurgical evaluation, and overall engineering characterization and achievement targets are discussed.

During the liquid quenching process, there are three main phases between the solid and the liquid interface: film boiling where vapor blanket covers the entire solid structure, transition or nucleate boiling, and single phase convection. The type of the quenching media, the agitation, and the flow pattern of a quench tank have significant effect on the cooling behavior during these three phases, which will affect the cooling rate, phase transformation, stress evolution and shape change of the quenched components. In this paper, transient CFD analysis using AVL FIRE is coupled with heat treatment analysis using DANTE to simulate an oil quench hardening process of a test gear made of Pyrowear 53. The gear is carburized prior to quench hardening. During the coupling analyses, the heat flux between the gear and the oil calculated in the CFD model is applied to the solid heat treatment model, and the gear surface temperature predicted by the heat treatment model is passed back to the transient CFD model. The aforementioned CFD tool is capable of considering the entire quenching domains without considering phase transformations in the quenched components. In the present case the gear is treated with a finite element tool in combination with DANTE to account for the latent heat release, which slows down the cooling. The relations between carbon content, temperature field, phase transformation, internal stress, and shape change during quenching are explained from the heat treatment modeling results. The coupling of CFD and heat treatment analyses provides a more robust application of computer modeling in the heat treatment industry.

Wire rod of steel, produced by a hot rolling process, is cooled on a conveyor by action of fans sited under it. Mechanical properties of rod depend on chemical composition of steel and cooling rate experimented by the product. A mathematical model is developed in order to predict the thermal and metallurgical evolution of the steel rod on the conveyor. Convection and radiation losses are considered and the influence of the laying head is also discussed. Simulation results are compared with experimental measures obtained by an optical pyrometer in a real industrial process. Several product diameters and fan configurations are analyzed. Conclusions about the good accuracy of the model are shown. Thanks to the short computational time that the developed tool takes to predict the temperature of the wire rod along the conveyor, the tool can be potentially used in an on-line control system for the cooling process of wire rods.

Microalloying of medium carbon bar steels is a common practice for a number of traditional components; however, use of vanadium microalloyed steels is expanding into applications beyond their original designed use as controlled cooled forged and hot rolled products and into heat treated components. As a result, there is uncertainty regarding the influence of vanadium on the properties of heat treated components, specifically the effect of rapid heat treating such as induction hardening. In the current study, the torsional fatigue behavior of hot rolled and scan induction hardened 1045 and 10V45 bars are examined and evaluated at effective case depths of 25, 32, and 44% of the radius. Torsional fatigue tests were conducted at a stress ratio of 0.1 and shear stress amplitudes of 550, 600, and 650 MPa. Cycles to failure are compared to an empirical model, which accounts for case depth as well as carbon content.

The microstructure of 25Cr-35Ni-0.4C refractory steels consists of an austenitic matrix and eutectic carbides precipitated in the interdendritic regions. In-depth studies of the morphology and chemical composition of these carbides are extremely important for industry, since the microstructural components of these steels are responsible for their hot mechanical properties. In this context, the microstructural characterization of ASTM A297 Grade HP 40 steels modified with niobium and zirconium is using scanning electron microscopy, microanalysis and X-ray diffraction, and determination of the time to rupture at 1100ºC under a constant stress of 17 MPa is reported here.