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.

Vacuum carburizing 9310 gear steel followed by austenitizing, oil quench, cryogenic treatment, and tempering is known to impact the residual stress state of the material. Residual stress magnitude and depth distribution can have adverse effects on part distortion during intermediary and finish machining steps. This study provides residual stress measurement, microstructural, and mechanical property data for test samples undergoing a specific heat treat sequence. Test rings of 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. The rings along with metallurgical samples are characterized after each step in order to track residual stress and microstructural changes. The results presented here are particularly interesting because the highest compressive residual stresses appear after removal of copper masking, not after quenching as expected. Data can be used for future ICME models of the heat treat and subsequent machining steps. Analytical methods employed include X-ray diffraction, optical and electron microscopy, and hardness testing.

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.

Low pressure carburizing (LPC) in combination with high-pressure gas quenching (HPGQ) is a robust and versatile case hardening technology. This paper shows how recent advancements in LPC and HPGQ are being employed in the heat treatment of automotive and aerospace components. Significant progress has been made in areas such as fixturing, load densities, cycle times, distortion control, automation, traceability, and the integration of heat treatment into manufacturing lines. Practical applications are shown for both multiple- and single-layer treatment.

Two stainless steel parts used in automotive engines are carburized in the course of their production to achieve desired properties. To reduce costs and improve product quality, the gas carburizing process that had been used was replaced by low-pressure vacuum carburizing. The two parts are similar in composition except that one contains 0.25 wt% Mo and the other 0.4 wt% Mo. Both also contain around 17 wt% Cr and thus naturally form a Cr 2 O 3 passivation layer that provides corrosion resistance but also acts as a barrier to carbon. As a result, the parts are etched in a pickling solution prior to carburizing. In the initial assessment of the new carburizing and pretreatment process, engineers observed differences in the pitting and oxide regeneration behaviors of the two stainless steels. The paper describes how the engineers determined the cause of the pitting and the extent to which it could be controlled. Because of the tradeoffs involved, the engineers decided to make both parts from the same material and optimize process parameters accordingly.

Integral quench furnaces combine the benefits of low-pressure vacuum carburizing (LPC) with atmosphere oil quenching. This paper discusses key milestones in the development of integral quench furnaces and the advantages they provide in annealing, normalizing, and hardening applications.

Vacuum carburizing with high pressure gas quenching is increasingly employed to reduce near-surface intergranular oxidation and quenching distortion. It has also been shown to reduce processing times because it can be conducted at higher temperatures, up to 1100 °C. These temperatures, however, may cause austenite grain coarsening, making steel more susceptible to fatigue failure. This paper presents a study showing how microalloying carburizing steels with Mo and Nb improves resistance to austenite grain growth. The control of grain size is attributed to solute and precipitation effects.

The evolution of Low Pressure Vacuum Carburizing in the automotive industry is well embedded in assembly plants with continuous batch loading. This batch loading, which causes a need for high cost WIP (work in progress), can now be reduced with the Low Pressure Vacuum Carburizing furnace equipment being sized to fit into single piece flow line with small batches. This presentation will look into the recent integration of heat treatment for in-line machining cells and the overall influences for the customer to provide equipment for heat treating in-line. These details will be compared to batch or continuous batch heat treatment as we know it today in the automotive industry. High Pressure gas quenching will be illustrated in both in-line and continuous batch integration.

Case hardening by carburizing is the most common heat treatment in mass production, which relies on atmosphere, or vacuum carburizing followed by oil or gas quenching and finally by tempering. Parts being heat-treated undergo the process in a configuration of a batch consists of hundreds or even thousands pieces. Under these circumstances, individual parts can’t help but be exposed to different process parameters in terms of temperature, atmosphere and quenching depends on their position within the batch. Parts near the outer portion of the load see a more rapid rise in temperature, are first exposed to the carburizing atmosphere and are more effectively quenched than parts located in the center of the batch. This can lead to significant variation from part to part and load to load; the resultant effective case depth deviation can be as high as 50%. Similarly, during quenching from hardening temperature distortion becomes highly unpredictable and unrepeatable. Modern industry demands greater precision and repeatability of results beyond those achievable by so-called traditional batch or continuous technologies and their associated equipment. Elimination of batches and focus on individual parts is the only true way to advance the industry. The article will introduce the first operational system for truly single-piece flow method for case hardening by low-pressure carburizing and hardening by high-pressure gas quench. The system treats each part individually and as such provides virtually identical process parameters, which results in extremely accurate and repeatable results. Quenching one part at a time in a specially design chamber, achieves more precise control and significantly reduces distortion so as to all make it possible to avoid post heat treatment hard machining operations. This single-piece flow heat treatment method is easily adapted into manufacturing and can be directly integrated into in-line manufacturing operations, working directly with machining centers. Materials handling and logistical issues are eliminated thus saving time and reducing unit cost. The results achieved on series of automotive gears will be reported and demonstrate incredible accuracy and repeatability, while significantly reducing distortion. Productivity and process costs prove the system to be highly competitive with other technologies. These proven advantages and savings.

Transmission-manufacturers constantly need to adapt their products and manufacturing technologies to meet future’s market and legislation requirements such as cost-efficiency, running-smoothness and drivetrain-agility. Components made of powder metal (“PM-components”) are established in today’s transmission industry as a cost efficient alternative even for high strength and high precision powertrain applications. The PM-material and the applied heat treatment processes have made significant improvements in recent years. One major step in the development was to combine the freedom in alloying-concepts of the PM-technology with the advantages of the Low Pressure Carburizing (LPC) heat treatment process. PM-components must be case-hardened to meet design-intent regarding wear resistance and strength. But when case hardening PM-components using a conventional atmospheric carburizing process, this can lead to serious overcarburizing and even massive carbide-formation. Another major challenge when using the conventional process is to clean PM-parts after the traditional oil-quenching process. Therefore, the process of Low Pressure Carburizing (LPC) combined with High Pressure Gas Quenching (HPGQ) was adapted to the special needs of serial production of PM-components. This heat treatment process offers significant benefits, such as: - no overcarburizing and excessive carbide-formation due to precise diffusion of carbon into the components - reproducible microstructures from part to part and from load to load - clean and shiny parts after quenching - superior control of distortion, - no intergranular oxidation, - better fatigue resistance and - the benefits of an environmentally friendly process. Over the past 25 years, Stackpole and ALD worked on powder metal technology and advanced heat treatment processes. Material, process and equipment have seen significant improvements over the last decades to offer true benefits. This presentation will give an insight into benefits and challenges of PM-components heat treated in low pressure with subsequent gas quenching. The paper refers to the industrial series production of components and it refers to R&D - case studies as well.

This presentation will discuss data on parts tested in Low Pressure Carburizing using oil and gas quenching. We will present data on metallurgy, distortion and load design to optimize each quenching media. As we know oil and gas quench respond differently, we will explore the evolution of high pressure gas quenching as it exist in today’s market. Low Pressure Carburizing has been growing among OEM’s and now Tier 2 suppliers as well as heat treaters in the Automotive and Aerospace markets. These details should help show the audience that they also can take advantage of the clean environment from Low Pressure Carburizing and just in time processing along with possible distortion control for all their parts currently being atmosphere carburized.

Carburizing grades of high strength steels, such as Ferrium C- 64 alloy, contain strong carbide forming elements such as chromium and molybdenum. Alloys with high amounts of strong carbide formers can form stable carbides during carburization that effectively block carbon diffusion and retard the carburization process. This is especially true for low pressure carburization. To achieve the desired case depth, the low pressure carburization process consists of a series of rapid boost and longer time diffusion cycles. One problem is how to determine an acceptable carburization schedule. This paper will discuss a methodology used to develop the data for Ferrium C-64 so that a proper low pressure carburizing schedule could be determined. Integral parts of this methodology are experiments to determine carbon diffusion rates, carbide formation kinetics, and carbide dissolution kinetics, and use of these data in computer software to simulate the process and to determine the proper schedule.

Low pressure carburizing (LPC) in a vacuum furnace is increasingly the preferred method of case hardening for aerospace gears, and acetylene is often one of the gases used in the process. Selective case hardening is common with gears, where certain sections of a part are “stopped off” or “masked” to prevent carburization at those locations. For aerospace parts, the masking used is typically copper electroplating. The low pressures and high temperatures used in LPC lead to copper evaporation, which contaminates the vacuum furnace hot zone and components. In a worst-case scenario, deposited copper can lead to short-circuiting of power feedthroughs. This study looks at the effect of vacuum and partial pressure gases on copper evaporation and its application in production processes.

Hardening and case hardening are among the most common types of heat treatment processes, which can be performed in either atmosphere or vacuum furnaces. These processes, followed by oil quenching, are carried out in batch sealed quench and continuous furnaces such as pusher, roller, or rotary hearth types. Atmosphere heat treatment technology and equipment was developed more than 60 years ago with little new product innovation or change since. However, in this time period the needs of manufacturing have changed dramatically, driven by global competitiveness and the drive for lower unit cost. As such the heat treatment solutions must be capable of achieving higher productivity (through shorter cycle times), increased flexibility (with respect to material and process/cycles) and meet higher product quality standards. In addition, today’s manufacturing requires absolute process reproducibility and integration with other manufacturing processes, all done using energy efficient and environmentally friendly equipment. The solution to this situation is modern vacuum furnace technology and vacuum equipment that easily adapts to stringent specifications and changing industry standards. In this discussion, two case studies of this technology are presented. The first includes a two-chamber sealed oil quench vacuum furnace to case harden a SAE 5120 component to a surface hardness of 61 HRC using a Low - Pressure Carburizing (LPC) process. The result was a 30% savings over traditional atmosphere carburizing integral quench furnace owned by a commercial heat treater. The second study involves the use of a three-chamber sealed oil quench vacuum furnace to case harden SAE 5115 steel automotive steering components to an effective case depth of 0.9 mm minimum and a minimum surface hardness of 60 HRC. Using LPC these parameters were easily achievable. By, using a three-chamber sealed oil quench furnace, the potential for up to 600 kg/hr throughput was demonstrated, while maintaining costs comparable to a traditional atmosphere style integral quench furnace. Together, both studies show that sealed oil quench vacuum furnaces can improve process time and quality over a traditional atmosphere integral quench furnace while maintaining the process costs needed to remain competitive.

For more than 20 years now Low Pressure Vacuum Carburizing (LPC) has taken over several industries as the main carburizing choice. These industries take advantage of LPC’s clean environment, versatility, “just in time” processing, along with possible distortion control that comes with gas quenching capabilities to process millions of parts each month. However, there are still hundreds of types of parts that can be converted to LPC with minimal effort. The authors will show the recent history and products currently being processed in LPC and how they were transitioned to LPC. Metallurgical results will be shown along with production loading scenarios. In addition, the process of a particular part showing timing and cost of each treatment process will be reviewed. These facts will show the audience the benefits and how they can take advantage of low pressure vacuum carburizing.

Heat treaters need an effective simulation tool to predict the carburization performance of a variety of steels. The tool is needed not only to predict the carbon profile but also to optimize the process in terms of the cycle time and the cost. CarbTool has been developed to meet these needs for gas and vacuum carburization. In this paper, CarbTool predictions were compared with industrial experimental results of four types of steels, heat treated by both gas and vacuum carburizing processes. Based on the excellent agreement of model predictions and experimental results, CarbTool may be used to predict the carbon concentration profile for a variety of alloys in both gas and vacuum carburizing processes.

The article describes the achievements and application of a new generation of HPGQ vacuum furnaces. Implementation of 25 bar quenching enables reaching hardening properties compared to the ones obtained with oil, while vacuum carburizing and nitriding additionally create a great potential for running different heat treatment and thermo-chemical processes as well as multiple processes combined in a single furnace cycle. Technical and technological aspects of the furnace exploitation are presented and operational costs reduction and energy saving are considered.