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).

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.

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.

A method of predicting tempered hardness of mixed microstructures has been formulated, which uses the quenched hardness and steel chemistry as independent variables. This calculation is based upon a method first proposed in 1947 by Crafts and Lamont for mixed microstructures and modified using the 1977 chemistry-based, tempered martensite hardness calculation of Grange, Hribal, and Porter. Tempered hardness predictions were examined using Jominy end-quench bars tempered between 204°C (400°F) and 649°C (1200°F). The measured Jominy hardness after tempering was used to make adjustments to the Crafts and Lamont parameters used in the hybrid model. Both plain carbon (SAE 1045) and low alloy (SAE grades 8620, 4130, 4142, and 5160) were used to evaluate the chemistry-based hardness prediction. In combination with a ASTM A255 Jominy hardenability calculation, the proposed calculation can be used to predict the quenched and tempered hardness profile of a round bar based upon chemistry, quench severity, and tempering temperature.

Historically, steels with carbon contents above about 0.45% C that are quenched to attain hardness above about 53 HRC (560 HV) are prone to premature brittle fracture when stressed in uniaxial or cyclic tension. In this study, five laboratory melted steels containing nominally 0.56% C and no grain refining additions (Ti, Al, V, or Nb) were heat treated on a Gleeble 3500 simulator to emulate thermal heating and quenching cycles for induction hardening. Limiting the peak heating temperatures and times produced very fine grained austenite with final hardness above 60HRC (700HV). Fracture resistance measured by the peak breaking load in notched bend tests increased by up to 3 fold for the short low temperature heating cycles as compared to longer higher temperature cycles. Fracture surfaces showed trans-granular crack propagation for the short low temperature cycles as compared to inter-granular propagation for the longer higher temperature cycles.

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.

In the design of a downhole isolation tool for multi-stage fracturing in the oil and gas industry, a setting component, called slip, was used to set the tool in the casing prior to the hydraulic fracturing operation. The material of the slip is made of gray cast iron with surface hardening requirement. This study investigated the performance of slips treated by induction hardening versus flame hardening. The slip treated by induction hardening produced low hardness and insufficient affected layer. On the other hand, flame hardening generated satisfactory results of case hardening layer by 0.762 mm (0.030 in.) thickness with 50 HRC minimum hardness. The Type E graphite in the raw material was transformed to Type A in the flame hardening process, which is favored in the case hardened layer. The effect of different treatment processes on the affected layer and their microstructural response in gray cast iron was discussed in this study. The isolation tool using the slip treated by flame hardening, together with other proven components, showed successful performance of 82.7 MPa (12 ksi) pressure holding at 177 °C (350 °F) for high pressure and high temperature downhole applications.

The use of nitriding to improve a component’s resistance to wear, fatigue, and corrosion continues to increase across the industry. However, for nitrided components, no universally accepted definition of “case depth” is available to allow the comparison of different nitriding processes, cycles, and materials. This study documents currently published methods of specifying and determining case depth for nitrided components, and evaluates the reported case depth of multiple materials and cycles in an effort to determine an optimal and robust “universal” method of reporting case depth. After completing this exercise, it appears that the optimal “universal” method of specifying and reporting the case depth for a nitrided component is to report the depth at which a Vickers microhardness traverse crosses a threshold which is 50HV greater than the material hardness below the nitrided case.

The cooling behavior of neem and mineral oil was obtained using instrumented ISO 9950 Inconel probe. Flash, fire points and the viscosity of quenching media were measured. These oils were used to quench harden AISI 1045 and AISI 1090 grade steel probes of section diameters 25 and 50 mm. (The top and bottom faces of steel probes were coated with insulating paste to minimize end effects of heat transfer during quenching.) The measured temperature data in steel probes were used to estimate spatiotemporal heat flux by solving inverse heat conduction problem at the interface of the probe/quenching medium. The estimated heat flux transients, microstructure, and hardness measurements were found to be similar for both oils indicating the potential application of neem oil as quenchant for heat treatment of steels.