One-Piece-Flow: Case Hardening Technology for Lean Manufacturing
Today’s production philosophy for automotive and aerospace components usually relies on the traditional separation between soft machining, heat treatment and hard machining.
Traditionally, heat treatment is performed in a central hardening shop. There is no continuous flow of production parts between the different operations, such as soft machining, heat treatment, shot peening and hard machining. Instead, the parts are collected into batches and then moved from operation to operation. So, large numbers of production parts are stored in buffers or are in transit between the different operations. This discontinuous flow of production results in:
- Increased logistical and documentation efforts
- Increased turnaround times
- Increased production costs
New Production Philosophy
In order to establish a more effective and economic production system, the goal is to move away from batch-type logistics and toward “one-piece-flow” production (Fig.1).
The objective is to move single parts from operation to operation instead of moving batches of parts. This one-piece-flow (OPF) production system would realize a continuous flow of production parts and avoid huge efforts for storage and transportation of parts between operations.
Development of One-Piece-Flow Case Hardening Technology
For a rapid case hardening process that matches the cycle times of soft machining, it is necessary to dramatically accelerate the carburizing cycle. Developments to accelerate the case hardening process have been in place for many years. The established process of low-pressure carburizing (LPC) was successfully elevated to higher temperatures to reduce process times. However, this increase in process temperature called for the development of new micro-alloyed materials to avoid unwanted grain growth. Therefore, three fields – materials, process and equipment – were identified with the need for development.
Development of Materials
Conventional case hardening steels such as 5130H, 8625H, 20MnCr5 and 18CrNiMo7-6 can develop unwanted grain growth when treated at temperatures above 960°C (1760°F). Steel suppliers started to develop new micro-alloyed steel grades to prevent unwanted grain growth several years ago.
These micro-alloying elements form precipitates during the steel production process, which act as grain-boundary pinning particles, thereby inhibiting abnormal grain growth during heat treatment at high temperatures.
Several micro-alloyed steel grades have been developed and successfully tested jointly by steel suppliers and gear manufacturers.
Additionally, the material needs to have a sufficient and controlled hardenability to allow for quenching with moderate quench-intensities. Typical steel grades suited for that process are 5130H, 8625H, 20MnCr5-HH, 27MnCr5 or 18CrNiMo7-6.
Development of Process
LPC is a case hardening process that is performed in a vacuum level measuring only a few millibar of protective atmosphere.
LPC is usually combined with high-pressure gas quenching (HPGQ). During HPGQ the load is quenched using an inert-gas stream instead of a liquid quenching media. HPGQ provides clean components and offers significant potential for lower distortion values compared to oil quenching.
Carburizing is a diffusion-controlled process. The diffusion coefficient of carbon in steel is strongly dependent on temperature. This means that the carburizing step during LPC can be accelerated significantly if the carburizing takes place at elevated temperatures.
This high-temperature low-pressure carburizing (HT-LPC) process has been successfully developed and tested. Figure 2 shows the comparison between a conventional LPC process and the HT-LPC process. For a given case hardening depth (CHD) of 0.65 mm (0.025 inch), the cycle time is reduced from 180 minutes to 40 minutes.
Development of Equipment
The carburizing cycle can be dramatically accelerated by application of HT-LPC. Even with the accelerated process, however, it would be difficult to allow for a synchronous processing of individual part components – matching the cycle time from soft machining. Therefore, it was decided to accumulate enough parts to assemble a single layer of parts for case hardening, or so-called 2D batches.
These 2D batches can be heated up to carburizing temperature much quicker than large 3D batches consisting of several layers. Additionally, the single-layer concept offers a very homogenous (uniform) treatment in all process steps, such as austenitizing, carburizing and quenching.
Figures 3 and 8 show the new synchronized heat-treatment system (patent pending). Following the philosophy of one-piece-flow, the parts are:
- Taken one by one from the soft-machining unit
- Heat treated in time with the cycle time of soft machining (synchronized heat treatment)
- Passed down one by one to the hard-machining unit
There are six hot zones vertically arranged inside the housing. The pressure in all hot zones is identical, but each hot zone has its own temperature control and its own supply of process gases. Once the tray has entered a hot zone, it is heated rapidly from all sides and then carburized at 1050°C using acetylene.
After the carburizing is completed, the workload tray is transferred via elevator back to the quench cell. The system quenches with a maximum operating gas pressure of 6-bar nitrogen. After the quench sequence is complete, the parts are transferred to the tempering unit. Once tempered, the parts are singularized again and passed down individually to the hard-machining unit.
Gears with different geometries made of various steel grades and other automotive components were heat treated in the new unit. A tray of gear wheels made of 18CrNiMo76 was carburized using HT-LPC at 1050°C (1922°F). The treatment time was 38 minutes. Figure 4 shows the quality achieved on these gears. The targeted CHD of 0.6 mm (0.024 inch) and all other specified values were successfully met.
The distortion of input shafts treated in the new SyncroTherm unit was analyzed as well. The input shaft is made of 16MnCr5 material, has a mass of 0.7 kg and is treated using a load size of 30 shafts per tray. The CHD after heat treat is specified as 0.5-0.8 mm, surface hardness is specified as 690-790 HV and core hardness is specified as 340-480 HV.
The carburizing temperature was varied from 960-1050°C (1760-1922°F). Two different ways of part orientation in the CFC fixture were tested – hanging and standing (Fig. 5).
Figure 6 shows the values for axial runout. Clearly the standing part orientation leads to much better results. When loading the shafts standing in the tray, the specification of axial runout after heat treatment (40 microns) was met successfully for all three analyzed carburizing temperatures.
Another study on a reaction internal gear from a 6-speed automatic transmission demonstrated the vast potential for distortion control as well. The standard deviation of helix-angle variation Vbf after heat treatment was reduced by 30% for the left flank and by 45% for the right flank when switching from multiple-layer to single-layer LPC treatment for identical carburizing temperature of 900°C (1652°F). When carburizing with single-layer treatment at 1050°C (1922°F), no increase in Vbf was observed.
In another study, sliding sleeves for trucks with an outer diameter of 175 mm (6.9 inches) were treated. For these parts made of 20MnCr5mod-material, it is important to note that the CHD was not defined at 550 HV but at 610 HV. It took only 48 minutes to reach CHD(610 HV) = 0.44 mm. Schueler et al performed a distortion study to compare the SyncroTherm unit with a conventional pusher furnace in combination with a hardening press.
Synchronized Vacuum Heat Treatment
An example of a U-shape manufacturing line is illustrated in Fig. 7. This line consists of several modular production segments. The green blanks enter the system from the bottom right. The first operation is the green-machining unit (hobbing) followed by the synchronized heat-treat unit and then the hard-machining units (hard turning and grinding). The finished gear components leave the system on the bottom left and are ready for assembly. Figure 8 shows the SyncroTherm system applied for serial production.
Conventional manufacturing concepts rely on the separation of different operations, such as soft machining, heat treatment and hard machining. This implies high logistical efforts, efforts for documentation and slow turnaround times, which ultimately lead to avoidable production costs.
With recent developments regarding materials, process and equipment, it is now possible to integrate heat treatment into the manufacturing line and to synchronize heat treatment with gear machining. Micro-alloyed steels have been developed that prevent grain growth even at high carburizing temperatures of up to 1050°C.
A new module called SyncroTherm® offers a rapid heat-treatment process that is fast enough to match the cycle times of soft and hard machining.
As a consequence of the fully integrated line following the one-piece-flow philosophy, the turnaround time of a typical gear wheel can be realistically reduced from a few days to less than four hours.
For more information: Contact Bill Gornicki, vice president sales & marketing, ALD-Holcroft Vacuum Technologies Co., Inc., 49630 Pontiac Trail, Wixom MI 48393; tel: 248-668-4130; fax: 248-668-2145; e-mail: WGornicki@ALD-Holcroft.com; web: www.ALD-Holcroft.com
- Sekine, K., One Piece Flow, Taylor and Francis 2005, ISBN-156327-325-X
- Heuer V., “Low-Pressure Carburizing,” ASM Handbook Volume 4A, 2013, pp. 581-590
- Hippenstiel, F., “Innovative Einsatzstähle als maßgeschneiderte Werkstofflösung zur Hochtemperaturaufkohlung von Getriebekomponenten,“ HTM 57(2002) 4, pp. 290-298
- Klenke, K., Kohlmann, R., Reinhold P., Schweinebraten, W., “Kornwachstumsverhalten des Einsatzstahles 20NiMoCr6-5 (VW4521+Nb) für Getriebeteile beim Hochtemperaturaufkohlen,“ HTM J. Heat Treatm. Mat. 63(2008) 5, pp. 265-275
- Heuer V., “Gas Quenching,” ASM Handbook Volume 4A, 2013, pp. 222-231
- Heuer, V, Leist, L., Schmitt, G., “Distortion control through synchronized vacuum heat treatment;” published at 5th International conference on Distortion Engineering (IDE 2015); 23.-25.9.2015 in Bremen, Germany
- Schueler, A., Kleff, J., Heuer, V., Schmitt, G., Leist, “Distortion of gears and sliding sleeves for truck gear boxes – a systematical analysis of different heat treatment topics;” published at 5th International conference on Distortion Engineering (IDE 2015); 23.-25.9.2015 in Bremen, Germany