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Effective case depth is an important factor and goal in gas carburizing, involving complicated procedures in the furnace and requiring precise control of many thermal parameters. Based upon diffusion theory and years of carburizing experience, this paper calculates the effective case depth governed by carburizing temperature, time, carbon content of steel, and carbon potential of atmosphere. In light of this analysis, carburizing factors at various temperatures and carbon potentials for steels with different carbon content were calculated to determine the necessary carburizing cycle time. This methodology provides simple (without computer simulation) and practical guidance of optimized gas carburizing and has been applied to plant production. It shows that measured, effective case depth of gear parts covering most of the industrial application range (0.020 inch to over 0.250 inch) was in good agreement with the calculation.
Often, the required hardness qualities of parts manufactured from steel can only be obtained through suitable heat treatment. In transmission manufacturing, the case hardening process is commonly used to produce parts with a hard and wear-resistant surface and an adequate toughness in the core. A tremendous potential for rationalization, which is only partially used, becomes available if the treatment time of the case hardening process is reduced. Low pressure carburizing (LPC) offers a reduction of treatment time in comparison to conventional gas carburizing because of the high carbon mass flow inherent to the process (Ref. 1).
Open any heat treating journal today and you’re certain to find multiple references (articles, technical papers and/or advertisements) promoting low-pressure carburizing (LPC). The uninformed might breeze by these references thinking it’s the next flash-in-the-pan, but unlike in the past, this time the process has legs.
A widespread weakness of gear drawings is the requirements called out for carburize heat treating operations. The use of heat treating specifications is a recommended solution to this problem. First of all, these specifications guide the designer to a proper callout. Secondly, they insure that certain metallurgical characteristics, and even to some extent processing, will be obtained to provide the required qualities in the hardened gear. A suggested structure of carburizing specifications is give.
Dana Corp. is developing a process that carburizes a straight bevel gear to a carbon content of 0.8% in 60 fewer minutes than atmosphere carburizing did with an identical straight bevel.
A carburized alloy steel gear has the greatest load-carrying capacity, but only if it is heat treated properly. For high quality carburizing, the case depth, case microstructure, and case hardness must be controlled carefully.
High demands for cost-effectiveness and improved product quality can be achieved via a new low pressure carburizing process with high pressure gas quenching. Up to 50% of the heat treatment time can be saved. Furthermore, the distortion of the gear parts could be reduced because of gas quenching, and grinding costs could be saved. This article gives an overview of the principles of the process technology and the required furnace technology. Also, some examples of practical applications are presented.
A study was conducted to isolate the influence of pre-rough machine processing on final dimensional distortion.
ALD-Holcroft Vacuum Technologies Co. will host a two-day technical symposium at the Henry Ford Museum in Dearborn, Michigan September 23– 24, 2008.
Heat treat alternative offers advantages over conventional methods.
Quality gear manufacturing depends on controlled tolerances and geometry. As a result, ferritic nitrocarburizing has become the heat treat process of choice for many gear manufacturers. The primary reasons for this are: 1. The process is performed at low temperatures, i.e. less than critical. 2. the quench methods increase fatigue strength by up to 125% without distorting. Ferritic nitrocarburizing is used in place of carburizing with conventional and induction hardening. 3. It establishes gradient base hardnesses, i.e. eliminates eggshell on TiN, TiAIN, CrC, etc. In addition, the process can also be applied to hobs, broaches, drills, and other cutting tools.
Carburized and hardened gears have optimum load-carrying capability. There are many alternative ways to produce a hard case on the gear surface. Also, selective direct hardening has some advantages in its ability to be used in the production line, and it is claimed that performance results equivalent to a carburized gear can be obtained. This article examines the alternative ways of carburizing, nitriding, and selective direct hardening, considering equipment, comparative costs, and other factors. The objective must be to obtain the desired quality at the lowest cost.
Acetylene with DMF solvent enables benefits of low-pressure vacuum carburizing.
This paper presents how low pressure carburizing and high pressure gas quenching processes are successfully applied on internal ring gears for a six-speed automatic transmission. The specific challenge in the heat treat process was to reduce distortion in such a way that subsequent machining operations are entirely eliminated.
This paper introduces new process developments in low-pressure carburizing and carbonitriding using either high-pressure gas quenching or interrupted gas quenching.
The complete Events section from January/February 2005, including coverage of a vacuum carburizing conference.
What gear material is suitable for high-temperature (350 – 550 degree C), high-vacuum, clean-environment use?
This paper presents the results of a study performed to measure the change in residual stress that results from the finish grinding of carburized gears. Residual stresses were measured in five gears using the x-ray diffraction equipment in the Large Specimen Residual Stress Facility at Oak Ridge National Laboratory.
Graded hardening technology has proven over the years to yield very good results when used in the heat treating of carburized gears. It is especially advantageous for smaller companies, subject to higher competitive pressures. Unfortunately, despite the fact that graded hardening is a very well-known method, its use has been limited. We strongly recommend this technology to all of those who need to produce gears with high metallurgical quality.
Suppliers are working hard to make sure their heat treating equipment is controllable, repeatable and efficient, and manufacturers continue to incorporate technology that gives heat treaters and their customers more information about what's going on inside the magic box.
Gears are designed to be manufactured, processed and used without failure throughout the design life of the gear. One of INFAC's objectives (*see p.24) is to help manufacture of gears to optimize performance and life. One way to achieve this is to identify failure mechanisms and then devise strategies to overcome them by modifying the manufacturing parameters.
The proper control of distortion after thermal treatment of powertrain components in the automotive industry is an important measure in ensuring high-quality parts and minimizing subsequent hard machining processes in order to reduce overall production costs.
The gear designer needs to know how to determine an appropriate case depth for a gear application in order to guarantee the required load capacity.
Precise heat treatment plays an essential role in the production of quality carburized gears. Seemingly minor changes in the heat treating process can have significant effects on the quality, expense and production time of a gear, as we will demonstrate using a case study from one of our customer's gears.
The manufacturing process to produce a gear essentially consist of: material selection, blank preshaping, tooth shaping, heat treatment, and final shaping. Only by carefully integrating of the various operations into a complete manufacturing system can an optimum gear be obtained. The final application of the gear will determine what strength characteristics will be required which subsequently determine the material and heat treatments.
In a very general sense, increasing the hardness of a steel gear increases the strength of the gear. However, for each process there is a limit to its effectiveness. This article contains background information on each of the processes covered. In each section what is desired and what is achievable is discussed. Typical processes are presented along with comments on variables which affect the result. By reviewing the capabilities and processes, it is possible to determine the limits to each process.
The aim of our research is to clearly show the influence of defects on the bending fatigue strength of gear teeth. Carburized gears have many types of defects, such as non-martensitic layers, inclusions, tool marks, etc. It is well known that high strength gear teeth break from defects in their materials, so it’s important to know which defect limits the strength of a gear.
Carburized gears have higher strengths and longer lives compared with induction-hardened or quench-tempered gears. But in big module gears, carburizing heat-treatment becomes time-consuming and expensive and sometimes cannot achieve good hardness due to the big mass-effect. Also, it is not easy to reduce distortion of gears during heat treatment.
In recent years, improvements in the reliability of the vacuum carburizing process have allowed its benefits to be realized in high-volume, critical component manufacturing operations. The result: parts with enhanced hardness and mechanical properties.
The Instrumented Factory for Gears (INFAC) conducted a metallurgical experiment that examined the effects of carburizing process variables and types of cryogenic treatments in modifying the microstructure of the material. The initial experiment was designed so that, following the carburizing cycles, the same test coupons could be used in future experiment.
Geoffrey Parrish has updated and expanded his previous book: The Influence of Microstructure on the Properties of Case-Carburized Components. It now contains at least twice the material. References and bibliography include 449 citations.
Some years back, most spiral bevel gear sets were produced as cut, case hardened, and lapped. The case hardening process most frequently used was and is case carburizing. Many large gears were flame hardened, nitrided, or through hardened (hardness around 300 BHN) using medium carbon alloy steels, such as 4140, to avoid higher distortions related to the carburizing and hardening process.
For many years chromium has been a popular alloy for heat treatable steels because of its contribution to hardenability more than offsets its costs. As a consequence, it is specified in such high-tonnage steel grades as the 5100, 4100, and 8600 series; and, as a result, about 15% of the annual U.S. consumption of chromium is used in constructional alloy steels.
The heat treatment processing of powder metal (PM) materials like Astaloy requires four steps -- de-waxing, HT sintering, carburizing and surface hardening -- which are usually achieved in dedicated, atmospheric furnaces for sintering and heat treat, respectively, leading to intermediate handling operations and repeated heating and cooling cycles. This paper presents the concept of the multi-purpose batch vacuum furnace, one that is able to realize all of these steps in one unique cycle. The multiple benefits brought by this technology are summarized here, the main goal being to use this technology to manufacture high-load transmission gears in PM materials.
Induction hardening is becoming an increasingly popular alternative to thermochemical diffusion processes such as carburizing, and as it does so, manufacturers are on a never-ending quest to expand the scope of what's possible with the technology.
In the typical gear production facility, machining of gear teeth is followed by hear treatment to harden them. The hardening process often distorts the gear teeth, resulting in reduced and generally variable quality. Heat treating gears can involve many different types of operations, which all have the common purpose of producing a microstructure with certain optimum properties. Dual frequency induction hardening grew from the need to reduce cost while improving the accuracy (minimizing the distortion) of two selective hardening processes: single tooth induction and selective carburizing.
The October 2011 issue of Gear Technology featured the article “Low-Distortion Heat Treatment of Transmission Components,” which covered the combination of low-pressure carburizing and high pressure gas quenching in an automotive environment. Here, heat treating expert Dan Herring explains why oil quenching is an appropriate choice for many applications.
May 18-21. AGMA Annual Meeting, "The Changing World of Gears." Loews Ventana Canyon Resort, Tucson, AZ. July 12-14, 1989. ASM international Conference on Carburizing. Sheraton Hotel & Conference Center, Lakewood, CO. September 12-20, 1989. European Machine Tool Show, Hannover, West Germany.
News Items About carburizing
1 Solar Receives Nadcap Approval for Carburizing (April 3, 2013)
Solar Atmospheres recently received Nadcap approval for carburizing. The Souderton, PA plant can now better serve the aerospace market wi... Read News
2 Solar Atmospheres Offers Full Production Vacuum Carburizing Services (January 15, 2006)
Solar Atmospheres’ new Low Torr Vacuum Carburizing furnace and services are now operating at Solar’s Eastern Pennsylvania ... Read News
3 Solar Awarded Order for R&D Vacuum Carburizing Furnace (June 8, 2009)
A top loading model VTC-714 R&D furnace by Solar Atmospheres was ordered by a mid-Atlantic, aircraft industry manufacturer for proces... Read News
4 Vacuum Carburizing Symposium Registration Closes April 28th (April 14, 2014)
Ford Motor Company Conference and Exposition Center, Dearborn, Michigan. The ALD Holcroft Vacuum Carburizing Symposium will feature two days... Read News
5 ALD Announces Dates for Vacuum Carburizing Symposium (November 26, 2013)
ALD-Holcroft has announced the dates for the 8th Vacuum Carburizing Symposium, to be held in 2014 on May 6th and 7th. This two-day event ... Read News