Carburizing Process Control Boosts Productivity, Lowers Manufacturing Costs
High productivity and repeatable part quality are the benefits of integrating new heat-treating equipment into a cell manufacturing installation at ZF Passau GmbH, Germany. ZF needed a double-row, pusher carburizing furnace having the capability to carburize and oil quench crown ring gears and pinions and to carburize and atmosphere gas cool crown ring gears followed by reheating in an atmosphere rotary hearth furnace for final press oil quenching. A double-track pusher furnace (Ipsen International) not only meets these requirements, but also increases heat-treating flexibility, such as the capability to simultaneously produce two different case depths.
Most of the standard gear work at ZF is carburized and oil quenched. However, more critical, higher tolerance gear components that are sensitive to distortion are carburized, then press oil quenched or atmosphere gas quenched in special protective gas cooling chambers. The atmosphere gas quench operation significantly minimizes part distortion.
To improve overall production efficiency, the company implemented a cell-manufacturing concept by integrating the heat-treating equipment into the mainstream production line adjacent to its bevel-gear manufacturing center. The close proximity of the heat-treating operation to the upstream grinding operations reduces transportation and overall manufacturing time; gear-manufacturing equipment is connected to the heat treat equipment via a materials handling system.
Furnace capacity was defined based on available optimized charging information from ZF's master data schedules. The furnace was configured for an average cycle interval of 16.8 minutes, resulting in a gross throughput capacity of 2,350 lb/hr (1,070 kg / hour)-a significant improvement over the previous installation. In addition, the heat treatment cost per workpiece also is reduced compared with the previous furnace installation. Factors contributing to obtaining high-quality parts, higher productivity and a lower cost operation include:
- Higher efficiency furnace insulation and design.
- Reduction in endothermic gas consumption through bottom loading of the carburizing furnace.
- Faster heat up to carburizing temperature due to a preheat chamber and high-efficiency burner system.
- Flexible post-carburizing tray handling system; i.e., oil quench tank, atmosphere slow cool, rehardening in a rotary hardening furnace followed by a press quench or manual extraction for parts straightening.
- Higher carburizing-atmosphere uniformity and more consistent repeatable quench performance.
Total Process Control
The overall control concept is divided into four modules:
1. Energy management
2. Input/output control via the operator control panel
3. Furnace parameter control via the multichannel controller
4. Conti-Control furnace computer management system
Control inputs are digital signals (for end-position switches, cam geared switches, pressure switches, drives, valves, and heating control variable) and analog signals (for thermocouples, oxygen probes, pressures, dynamic control variables, and flow rates).
A Conti-Control computer management system (figure 1) allows a furnace operator to assume the responsibility for complete operation of the furnace rather than just load and unload the furnace and/or take instructions from a supervisor. Operator responsibilities include:
- Keeping the job order production log.
- Processing a job using either of the dual rows of the pusher furnace.
- Verifying the proper heat treat process, correcting part identification and quality certification and any job- or parts-specific documentation.
- Documenting process parameters
The Conti-Control system provides a complete graphical display of the furnace installation showing individual tray position with itemized parts information and tray movement throughout the furnace (figure 2). The set points and process values of each controlled carbon and temperature zone are shown. The actual process history from each tray can be graphed as if the tray was run in a batch furnace. Conti-Control also tracks all process variable information for all parts of the furnace, and the control system is configured to track post-processed parts quality information, such as surface and core hardness, carbon profiles, and resulting metallurgical information.
Process and EquipmentLoading
The parts and charge fixtures are loaded and unloaded by the operator with the help of a roller-top scissors lift table (figure 3).
Preloaded trays also can also be loaded directly into the preoxidation furnace or stored in a six-tray position storage buffer located in front of the entrance of the preoxidation furnace (figure 4).
A direct-fired preheat, or oxidation, furnace provides three important process enhancements:
- Incremental preheating minimizes distortion during heat-up and minimizes potential for sooting when parts are transferred to the carburizing furnace.
- Oxidation of the workpiece surface provides a consistent surface for uniform carburizing, thus avoiding soft spots.
- Preheating burns off cutting oils and cleaning solution residues (from a prewash).
The company determined that the best preoxidation temperature for its parts is 970F (520C). From the preoxidation furnace, the load feeds into a charge vestibule connected to the carburizing furnace charging area.
Bottom-Loading Charge Chamber
Carburizing and metallurgical nonuniformity, distortion control and minimization of intergranular oxides are issues of concern with continuous pusher furnaces. These issues were addressed by selecting a furnace that charges loads from the bottom into a sealed charge chamber, which minimizes oxygen entering the first chamber of the carburizing furnace. The loads exit the preheat furnace and are fed onto an elevator that also serves as the bottom of the charge chamber.
A very small amount of protective gas escapes when the charge chamber is opened due to a small positive pressure of the protective gas in the furnace and chamber. The elevator door seals the charge chamber as the load elevator and load are raised into the chamber. The load is retained in the sealed charge chamber for a short time to allow the reduction of any air trapped in the charge through a chemical reaction between the air and endothermic gas. Only a small amount of endothermic/ atmosphere gas is lost throughout the loading process. The stability of the gas in the carburizing furnace chamber is maximized by using bottom loading and a sealed charge chamber.
In the bottom loading process, the loaded trays arrive at the charge chamber at a height of 31.5 in. (800 mm), and the furnace hearth is raised to a height of 70 in. (1,800 mm). The elevated hearth allows for pit-less installation.
Heat Up, Carburizing and Hardening
The two-row pusher furnace is divided into a heat up zone, two carburizing zones, and a separate diffusion section. The diffusion section is a single row chamber perpendicular to the main two-row carburizing chamber. Furnace atmosphere is controlled using oxygen probes. Zone-separation methods allow for maintaining different carbon potentials in each furnace zone. The compact furnace configuration has a high production capacity and is economical to run.
The quench tank includes dual elevators with under oil transfer connected to the end of the hardening zone. The system allows discharging a load from the quench without the use of a flame curtain, while still preventing air from entering the furnace (figure 5). A programmable quenching sequence ensures optimal oil quenching. Seven different quenching possibilities are possible via changes to oil bath agitation. After quenching, the charge exits through a gas-tight sealed door and is lowered back down to the 800 mm working level.
A special VarioClean post-washer simultaneously re-moves residual oil from two trays. Washing involves sequentially filling the washing chamber with different aqueous wash media, which are briefly vacuum boiled to enhance cleaning. Vacuum boiling reduces energy consumption in the wash process and also helps maintain the temperature of the wash fluid. After washing, clean workpieces are dried in the wash chamber under vacuum.
Tray Extraction Before Tempering
A pawl system transports the parts through the tempering furnace. Both single and double charges can be transported through the furnace, a feature that allows the furnace to be easily emptied. After extraction from the tempering furnace, charges can cool at either of two charge-cooling stations. Trays also can be extraction from the furnace prior to entering the tempering furnace because some parts may need straightening prior to stress relieving and/or tempering. This feature is critical to producing the high quality parts.
Charges requiring simple hardening can be extracted from the hardening zone and fed through a protective gas-cooling chamber. This chamber allows the parts to be slow cooled under a protective atmosphere of nitrogen. To minimize thermal shock to the parts as they enter the atmosphere cooling chamber, the first tray position is heated (figure 6). For safety purposes, the chamber is cooled using quench oil. The charges exit the cooling tunnel vertically downward to minimize the entry of oxygen to the tunnel.
After exiting the cooling-tunnel vestibule, the loaded trays are returned by roller track to the loading and unloading stations. Trays exiting the post-wash chamber also are returned along this track.
Charge tracking throughout the installation is particularly important, especially because charges can exit the hardening zone via the quench tank, cooling tunnel, and post-washer.
Gear production using this heat-treating set up meets all expectations of productivity and part quality. The furnace runs continuously 17 shifts per week in a four-shift operation. Gear materials are ZF 1: AISI 5115 (15CrNi6); and ZF7B: AISI 5120 (20MnCr5). Net parts loading per tray is 440 to 485 lb (200 to 220 kg), and gross weight per tray is 660 lb (300 kg). One of the main benefits of the dual-track furnace as mentioned previously is the ability to run two different case depths simultaneously. Different processing parameters in this example are:
Furnace Row 1
- 50 trays/21 hours
- Target case depth: 0.026 in. (0.66mm); hardness: 56 Rockwell C (610 Vickers)
- Acceptable case depth range: 0.016-0.028 in. (0.4-0.7 mm)
- Cycle time: 25 min
Furnace Row 2
- 25 trays/21 hours
- Target case depth: 0.033 in. (0.83 mm); hardness: 56 Rockwell C
- Acceptable case depth range: 0.028-0.039 in. (0.7-1 mm)
- Cycle time: 50 min
The small difference in the heat-up time (figure 7) for row 1 and row 2 has little effect on case hardening depths and cycle times.