Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.

Techniques and Equipment Types to Harden Gears

Material science and thermodynamics are applied in heat treating to achieve mechanical performance in gears. The technique includes part design, fixturing, and process development. Different furnaces may offer unique advantages, like minimizing part distortion, while operating and maintenance costs vary greatly for hardening furnaces. The challenge is to understand which furnace type can most effectively process the gear design and material grade. Protective-atmosphere furnace solutions are well-suited for hardening of gears. The process techniques include gas or vacuum carburizing, carbonitriding, and neutral hardening in a carbon-based atmosphere or in a vacuum. This paper will discuss vacuum, controlled atmosphere, and hybrid furnace types highlighting available processes while sharing respective associated operation and maintenance costs. Batch integral quench (BIQ) furnaces will be the base case for comparison, as they comprise the largest installed base for gear heat treatment. While a discussion of when to consider continuous atmosphere furnace equipment by defining what is high production versus today’s BIQ furnace capacities for gear heat treatment.

Vacuum Carburizing in a Pit Furnace: A 21st Century Solution to Large Component Case Hardening

Low pressure carburizing (LPC) is a proven, robust case hardening process whose potential is only limited by the style and size of vacuum furnace. Today, LPC is typically used in horizontal vacuum furnaces where the opportunity to carburize large parts is limited. In this paper we present a new adaptation of the technology in large pit type vacuum furnaces, capable of opening to air at elevated temperature. This underscores the potential of LPC to carburize larger, more massive parts in a clean, effective and efficient process. The result is quality casehardened parts without the undesirable side effects of atmosphere gas carburizing such as the use of a flammable atmosphere, reduced CO and NOx emissions, no intergranular oxidation, and limited retort life. Another significant advantage is decreased process time. The case study presented here shows that eliminating furnace conditioning and increasing process temperature can significantly reduce cycle durations by nearly three times and cut utility costs in half. Under these conditions, a return on investment (ROI) is in the neighborhood of 1 – 2 years is possible, making LPC in a pit style furnace a cost-effective solution than traditional atmosphere gas carburizing technologies.

Heat Treating of EV Transmissions

As consumers embrace Electric Vehicle (EV) technology, the automotive industry is moving quickly into replacing internal combustion engines (ICE) and traditional transmissions. The change to electrically driven vehicles offers new challenges to the gear manufacturing world, and most importantly new specifications to heat treat these gears - specifically quieter gear sets and higher torque ratings. Today’s EVs have a much lower tolerance for noise from the gear set to power the vehicle; therefore, this continues the need for even quieter and stronger gears. This technical presentation will illustrate the heat treat and distortion specifications for these new gears, along with answering the “why” of selecting low pressure vacuum carburizing (LPC) for new programs around the world.

Effects of Molybendum on Hardening Properties of Stainless Steels by Low Temperature Vacuum Carburizing and Pre-Treatment

To develop low temperature vacuum carburizing for next generation of injector parts was the aim of this work. Concerned parts were stopper (SUS303) and position ring (1.4305) with current properties of carburized thickness of av. 22.7 μm and hardness of HV0.05 av. 935.New carburizing and pickling were developed to reduce the cost and improve product quality. Especially, both steels showed different pitting and oxide regeneration behavior during pickling due to the composition difference. In the case of SUS 303, MnS inclusions at the surface led to severe MnS pitting, in contrast, 1.4305 formed Mo-oxides on the surface during pickling, which the excessive pitting by MnS inclusion site was prevented, confirmed by OM, SEM-EDX and XPS. Furthermore, these oxides resolved easily by hydrogen during carburizing and subsequently enabled activated carburizing. In conclusion, 1.4305 showed excellent properties, hardness of 985 HV0.05, thickness of 23 μm incl. satisfaction of weldability.

Changes in Residual Stress State During Quench and Temper of Vacuum Carburized Gear Steel

Vacuum carburizing of 9310 gear steel followed by austenitizing, oil quench, cryogenic treatment, and tempering is known to impact residual stress state of the steel. Residual stress magnitude and depth distribution can have adverse effects on part distortion during intermediary and finish machining steps. The present research provides residual stress measurement, microstructural, and mechanical property data for samples taken along a specific heat treat sequence. Test rings of AISI 9310 steel are subjected to a representative gear manufacturing sequence that includes normalizing, rough machining, vacuum carburizing to 0.03”, austenitizing, quench, cryo-treatment, temper, and finish machining. Characterization of a test ring and a metallurgical sample after each manufacturing step allows tracking of residual stress and microstructural changes along the sequence. The results presented here are particularly interesting because the highest compressive residual stresses appear after removal of copper masking, not after quench as expected. Data can be used for future ICME models of the heat treat and subsequent machining steps. Analytical methods include X-Ray Diffraction, optical and electron microscopies, mechanical testing, and hardness testing.

Modeling and Simulation of Vacuum Low Pressure Carburizing Process in Gear Steel

A combination of simulation and experimental approaches to optimize the vacuum carburizing process is necessary to replace the costly experimental trial-and-error method in time and resources. In order to accurately predict the microstructure evolution and mechanical properties of the vacuum carburizing process, a multi-field multi-scale coupled model considering the interaction of temperature, diffusion, phase transformation, and stress was established. Meanwhile, the improved model is combined with the heat treatment software COSMAP to realize the simulation of the low-pressure vacuum carburizing process. The low-pressure vacuum carburizing process of 20CrMo gear steel was simulated by COSMAP and compared with the experimental results to verify the model. The results indicated that the model could quantitatively obtain the carbon concentration distribution, Fe-C phase fraction, and hardness distribution. It can be found that the carbon content gradually decreased from the surface to the center. The surface carbon concentration is relatively high only after the carburizing stage. With the increase in diffusion time, the surface carbon concentration decreases, and the carburized layer depth increases. The simulated surface carbon concentration results and experimental results are in good agreement. However, there is an error between calculations and observations for the depth of the carburized layer. The error between simulation and experiment of the depth of carburized layer is less than 6%. The simulated surface hardness is 34 HV lower than the experimental surface hardness. The error of surface hardness is less than 5%, which indicates that the simulation results are reliable. Furthermore, vacuum carburizing processes with different diffusion times were simulated to achieve the carburizing target under specific requirements. The results demonstrated that the optimum process parameters are a carburizing time of 42 min and a diffusion time of 105 min. This provides reference and guidance for the development and optimization of the vacuum carburizing process.

Impact of Ultrashort Laser Nanostructuring on Friction Properties of AISI 314 LVC

Laser irradiation yields a powerful tool to modify the symmetry and asymmetry features of materials surfaces. In this paper, femtosecond laser-induced periodic surface structures were applied on stainless steel AISI 314, specially hardened by a low-vacuum carburizing procedure. Symmetry modifications in the surface’s morphology and chemistry before and after the laser treatment were investigated by SEM and EDS, respectively. Coefficient of friction (COF) was observed in dry sliding condition by using block-on-ring sliding test. The results show that COF values are substantially lower after laser-induced periodic surface structures (LIPSS) surface treatment.

Microstructure, Hardness, and Tensile Properties of Vacuum Carburizing Gear Steel

We investigated the effects of the austenitizing temperature on the microstructure, hardness, and tensile properties of case-carburized steel after vacuum carburization at 930 °C and then re-austenitization at 820–900 °C followed by oil quenching and tempering. The results show that fractures occurred early with the increase in the austenitizing temperature, although all the carburized specimens showed a similar case hardness of 800 HV0.2 and case depth of 1.2 mm. The highest fracture stress of 1919 MPa was obtained for the experimental steel when the austenitizing temperature was 840 °C due to its fine microstructure and relatively high percentage of retained austenite transformed into martensite during the tensile tests. We also found that the stress–strain behavior of case-carburized specimens could be described by the area-weighted curves of the carburized case and the core in combination. The strain hardening exponent was about 0.4 and did not vary with the increase in the austenitizing temperature. We concluded that the optimum austenitizing temperature was around 840 °C for the experimental steel.