Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

A gas quenching method was developed by DANTE Solutions, in conjunction with the U.S. Army Combat Capabilities Development Command Aviation & Missile Center (DEVCOM AvMC), to control distortion in difficult to quench geometries. This new method addresses the nonuniform cooling inherent in most gas quenching processes. A prototype unit was constructed and tested with the aim of controlling the martensite formation rate uniformity in the component being quenched. With the ability of the DANTE Controlled Gas Quenching (DCGQ) unit to control the temperature of the quench gas entering the quench chamber, thermal and phase transformation gradients are significantly reduced. This reduction in gradients yields a more uniform phase transformation, resulting in reduced and predictable distortion. Being able to minimize and predict distortion during gas quenching, post heat treatment finishing operations can be reduced or eliminated, and as such, fatigue performance can be improved. This paper will discuss the prototype unit performance. Mechanical testing and metallographic analysis were also performed on Ferrium C64 alloy steel coupons and will be discussed. The results obtained showed that the slower cooling rate provided by the prototype did not alter the microstructure, hardness, strength, ductility, toughness, or residual stress of the alloy.

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.

This paper presents the results of a study examining the cooling rates of two vacuum high-pressure gas quenching furnaces: a large 10-bar furnace equipped with a 600-hp blower motor and a smaller 10-bar furnace with a 300-hp motor. In comparing critical cooling temperatures for H13 in the 1850°F to 1300°F range, the furnace that is almost three times larger in volume (110 vs. 40 ft 3 of hot zone) cooled the same workload almost identically to smaller unit. The test results clearly show that gas flow, or velocity, is more meaningful than pressure when it comes to cooling rate.

Rapid induction hardening of martensitic steel can attain the very high strength levels needed for light-weighting components subjected to high operating stresses. Specimens of martensitic 0.6% C steels were heat treated using a dilatometer to investigate the effects of heating rates of 5 to 500 °C/s to temperatures of 850 to 1050 °C on the transformation to austenite and subsequent transformation to martensite during quenching. Selected specimens were quenched after partial transformation to austenite to assess the initial cementite precipitate size formed in ferrite during heating. Other specimens were isothermally held at the austenitizing temperature to assess cementite dissolution rates. Higher heating rates increased the Ac1 and Ac3 temperatures, and lowered the Ms temperature. Alloy content and prior microstructure also influenced the transformation temperatures.

This paper investigates the factors that influence quenching rates and temperature distributions in steel during dilatometry testing. In a prior study, the authors assessed the performance of the cooling system in a widely used dilatometer. The goal of the current work is to develop a cooling strategy that provides more uniform and possibly faster cooling in the same system. Several alternate quench concepts are analyzed, the most promising of which uses water-cooled tubes to deliver high velocity gas through a series of jets axially aligned with the test sample. The proposed cooling apparatus and its effect on the induction heating process are assessed using CFD, electromagnetic, and thermal analyses.

This paper presents a computational approach for assessing the potential for distortion when using high pressure gas to quench steel parts. It explains how to account for component geometry, heat transfer coefficient, gas temperature and velocity, heating and cooling rates, and phase transformations. The authors employ finite element modeling methods to determine local phase fraction and displacement in a Ferrium C64 disk for different quench pressures. Simulations at timed intervals show how distortion and phase fraction progress in different areas of the disk and along the edges of an off-center bore. The causes of distortion are examined and explained using the model, with insights into why the cooling rate has a nonlinear relation with distortion.

This paper describes the inner workings of a gas quenching chamber and assesses its potential for high-volume production of precision gears. The cooling manifold in the chamber surrounds the part, which sits on a rotating table. This ensures uniform flow of cooling gas across the top, bottom, and sides of the part and achieves uniform and repeatable quenching results. In addition, because the cooling nozzles can be adjusted to fit the geometry and size of the part, distortion can be effectively controlled.

This paper reviews several recent advancements in high pressure gas quenching technology along with the impact of new higher hardenability steels. With design upgrades and improved gas flow and heat removal, a wider variety of materials, part geometries, and load sizes can now be gas quenched.

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.

This paper will present the advantages and disadvantages of quenching media options like HPGQ (high pressure gas quenching), Oil and Press Quenching, Austempering (salt) for steel, ADI (austempered ductile iron) and aluminum to achieve certain targets relating to automotive component heat treating. Each heat treating/quenching process provides unique solutions for automobile designers and plant engineers. However, there likely is no single process or material that provides all of the answers that one would desire. Therefore, what process or combination of processes will satisfy the overall need? Detail will be discussed that outlines how OEM’s and heat treaters can and do take advantage of a particular hardening process.

Gas quenching is drawing increasing attention within the heat treat industry. The heat transfer coefficient (HTC) for gas quenching can reach 2000 when using high pressure and high velocity nitrogen, helium, or mixtures of these gases. The HTC in water quenching is between 3000 and 4000. The lower HTC of gas quenching may result in workpieces with less distortion and residual stress after quenching. Compared to water, polymer, and oil quenching, gas quenching is environmentally friendly, and the surface of the part is clean after quenching.

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.

Austempering heat treatments of steels and cast irons are usually performed using salt bath quenching followed by isothermal transformation of austenite to bainite or ausferrite. High Pressure Gas Quenching (HPGQ) at 1-4 MPa gas pressures is increasingly used to replace oil quenching, but may also be used for austempering. However, to obtain sufficient heat transfer high gas speeds >25 m/s are required. Hot Isostatic Pressing (HIP) is widely used for densifying castings and powder-based materials. Recent equipment developments enable Uniform Rapid Quenching (URQ) under 200 MPa pressure and 0.3 m/s speed, providing uniform cooling. Superplastic conditions during austenitization and initially during URQ reduce residual stresses and eliminate internal porosity in castings and PM materials. Hardenability is increased due to stabilization of the close-packed austenite. The inherent freedom provided by HIP to select optimum levels and rates for temperatures and pressures has been shown to improve mechanical properties and reduce process duration.

The bainitizing potential for new forging steels and heat treatment gas quenching have both gained a growing interest in the automotive industry. The bainitizing process, particularly within the lower bainite range, aims at providing an improved ductility with strength above martensite level. Previous investigations demonstrated the ability of controlled gas quenching for bainitizing of a stepped shaft from the forging heat. In the present work, quenching and machining process steps have been combined to investigate the bainitizing potential of the specimen during machining at elevated temperatures (hot machining). Therefore, gas jet quenching has been experimentally evaluated and the derived heat transfer coefficient distributions have been implemented into heat treatment simulations. Bainitizing strategies featuring various quenching field configurations can be operated based on simulations and time-temperature-transformation diagrams for high-strength ductile bainite (HDB) steel grade. Selected strategies have been tested within a turning machine adapted for controlled heat treatment using a gas quenching field.

Gas quench, with advantages such as reducing distortion and residual stress, is developing rapidly with the intent to replace liquid quench. Medium and high hardenability steels are needed for gas quench, since the quenching power is lower compared to liquid quench 1 . The traditional Jominy end quench test and Grossmann test, designed for liquid quench steel hardenability, didn’t properly determine the hardenability of high alloyed steels. In order to determine gas quench steel hardenability, a new test is required. In this paper, a critical heat transfer coefficient (HTC) test based on the Grossmann test is proposed. Critical HTC, a concept like critical diameter, was successfully proved to describe the gas quench hardenability of steel. The critical HTC of AISI 4140 steel is 430 W/m 2 C and the critical HTC of AISI 52100 steel is 820 W/m 2 C, which reveals that the gas quench hardenability of 4140 is better than 52100. In the paper, the critical HTC test requirements are presented and discussed.

Hot isostatic pressing or HIP has been used for diffusion bonding, casting densification, and powder consolidation. Continuous advances in HIP equipment design have allowed increasingly rapid cooling, recently reaching a point where true high-pressure gas quenching is now possible within the HIP unit. This capability further enables the integration of a heat treat and HIP processing. Within the heat treat industry, high pressure gas quenching has been an area of significant development, however, where typical high pressure gas quenching equipment offers quench pressures up to 15 or 20 bar, common HIP pressures are 1000 bar or higher. The ability to quench from HIP pressures appears to offer heat treat options not previously available. This paper examines ultra-high pressure gas quenching (from 1500 bar) within the HIP unit from a heat treating point of view using AISI 4140 steel, a well characterized, medium hardenability alloy, comparing the properties and microstructure of ultra-high pressure gas/HIP quenched steel to conventional water and oil quenched results.

A recently introduced new vacuum furnace design allows the total integration of heat treatment into the manufacturing line. This compact unit can be implemented into the heart of the production chain and provides heat-treatment processes which can be fully synchronized with the green and hard machining-operations. When performing case hardening, the components are low pressure carburized at high temperatures (1050°C) followed by gas quenching. Using this technology, the components are treated in most cases in a single layer of parts (“2D-treatment”) which allows for an easy automated loading and unloading of the fixture-trays. Furthermore this “small batch – treatment” leads to an optimum in quality regarding quench homogeneity and distortion control. By using the small batch concept, a continuous flow of parts can be established. There is no need to wait until enough parts are collected to build a large batch with multiple layers (“3D-batch”). Therefore excessive costs are avoided for inventory, part storage and transportation within the plant. Typical components for this technology come from the automotive, aerospace and tool industry. The paper shows several new results from case hardening applications.