Selective laser melting (SLM) is an additive manufacturing technique that can be used to make the near-net-shape metal parts. M2 is a high-speed steel widely used in cutting tools, which is due to its high hardness of this steel. Conventionally, the hardening heat treatment process, including quenching and tempering, is conducted to achieve the high hardness for M2 wrought parts. It was debated if the hardening is needed for additively manufactured M2 parts. In the present work, the M2 steel part is fabricated by SLM. It is found that the hardness of as-fabricated M2 SLM parts is much lower than the hardened M2 wrought parts. The characterization was conducted including X-ray diffraction (XRD), optical microscopy, Scanning Electron Microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) to investigate the microstructure evolution of as-fabricated, quenched, and tempered M2 SLM part. The M2 wrought part was heat-treated simultaneously with the SLM part for comparison. It was found the hardness of M2 SLM part after heat treatment is increased and comparable to the wrought part. Both quenched and tempered M2 SLM and wrought parts have the same microstructure, while the size of the carbides in the wrought part is larger than that in the SLM part.

Powder metallurgy (PM) is the fabrication process of compacting metal powders to shape and sintering these compacts to yield the final material’s properties. The PM compaction process allows for complex geometries to be formed that would normally lead to long and expensive machining processes from wrought steels. Special alloy selection can allow for hardening of the microstructure during the sintering procedure. The sinter hardened (SH) alloys exhibit good mechanical properties along with good hardenability and dimensional stability and may be a suitable replacement for wrought steels where low distortion from heat treatment or microstructural control is required. In this study, it was found for a complex geometry coupler application, a SH alloy could successfully replace an austenitizing heat treatment process with a low carbon steel. The low carbon steel was found to have micro heterogeneities from heat treatment that lead to premature failure in the application. Dimensional distortion and production variance were also of concern with the low carbon steel. The SH material demonstrated acceptable physical properties, hardness and microstructural uniformity to solve the concerns associated with processing of the low carbon steel coupler. Post processing optimization also added to the life performance of the coupler by tailoring the final microstructure to mating components.

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.

The cooling history of carburized heat-treated gears plays a significant role in developing microstructure, hardness, and residual stress in the tooth that influences the fatigue performance of the gear. Evaluating gear carburizing heat treatment should include a microstructure and hardened depth evaluation. This can be done on an actual part or with a test piece. The best practice for a test piece is to use a section size that closely approximates the cooling rate at the gear flank of the actual gear. This study furthers work already presented showing the correct test piece size that should be used for different gear modules (tooth thicknesses). Metallurgical comparisons between test pieces, actual gears, and FEA simulations are shown.

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 presents the results of a study on a new coating method for alloy steel. The coatings were synthesized on the surface of H21 die steel through a combination of thermal-chemical treatment (TCT) and electron beam processing (EBP). A paste containing boron and aluminum was applied to the test samples which were then heated to accelerate the diffusion process. After 2 h at 950 °C, the diffusion layers were found to be 120 μm thick, and after 2 h at 1050 °C, they were 580 μm thick. The subsequent EBP led to a complete transformation of the primary diffusion layer and an increase in thickness to 1.6 mm. XRD analysis showed significant differences in composition before and after EBP and the presence of tungsten and iron borides. It was also found that the distribution of microhardness and composition over the layer thickness had a more favorable profile after EBP.

In-envelope hybrid manufacturing systems comprised of directed energy deposition (DED) and machining provide flexibility for the fabrication of complex geometries with minimal setup changes. However, for these manufacturing set ups, the effects of deposition parameters such as laser power and scanning speed on the quality of the build remain relatively unexplored. An important aspect for developing components with reliable mechanical properties is a thorough understanding of DED thermodynamics during fabrication. Therefore, DED thermodynamics were defined based on the strengthening properties derived from the thermal gradient (G) and solidification rate (R) of the melt pool. Other factors influencing DED thermodynamics include substrate geometry and surface finish which are expected to affect cooling rates and adhesion, respectively. In this work, stainless steel 316L specimens were fabricated varying laser power intensity, scanning speed, and deposition substrate. The effect of these parameters on the microstructure of the sample components were analyzed. Microstructural evolution at various points within and between layers was studied and correlated to localized hardness. An increase in mechanical properties for fine, equiaxed grains demonstrates the Hall-Petch principle for strengthening of components.

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.

Low pressure carbonitriding (LPCN) has the potential to improve the impact and fatigue strength of steel components through the enrichment of nitrogen and the effect of carburizing at higher temperatures. The work described in this paper investigates the influence of boron on the LPCN response of 20MnCr5 steel and the effect of niobium on that of 8620. LPCN treatments were developed to achieve a surface hardness of ~700 HV and case depth of 0.65-0.75 mm in four alloys: 20MnCr5, 20MnCr5 + B, 8620, and 8620 + Nb. The hardness and case microstructure of treated and quenched test samples are correlated with bending fatigue measured in Brugger fatigue specimens, which simulate the root of a gear tooth.

The purpose of this work is to incorporate boriding and austempering treatments in a single thermal cycle and assess its effect on two high strength bainitic steels. The combined process, called boro-austempering, is a promising alternative to increase the surface wear resistance of advanced high strength steels as shown in the test results presented.

Gas nitriding is proving to be a viable low temperature case hardening process for stainless steels used in numerous applications. In this study, a comparison between austenitic (grade 304) and martensitic (grade 401) stainless steels shows how pre-oxidation temperature affects the thickness and porosity of the compound layer produced as well as hardness and nitriding diffusion depth. The results indicate that austenitic stainless steel would be the best choice for a part requiring wear resistance and strength, and that a standard rolled martensitic stainless steel would suffice if only a wear resistant surface is needed.

In this study, the creep properties of three titanium alloys were experimentally obtained at different applied stresses and at 683 K. X-ray diffraction and optical and electron microscopy were used to help characterize the microstructure before and after creep deformation and to show how changes in hardness correlate with the precipitation of α and ω phases in the β titanium matrix. The results of the study show that Ti-12Cr-1Fe-3Al is the most creep resistant followed by Ti-12Cr-3Al and Ti-12Cr.