Low-pressure carburizing with acetylene (Ipsen's AvaC(tm), the acetylene-vacuum-carburizing process) and high-pressure (to 20 bar) nitrogen gas quenching have been implemented primarily in single-chamber (Fig. 1) and semicontinuous multiple-chamber furnaces (Fig. 2). An alternative to the single- and multiple-chamber furnaces is a continuous vacuum installation like the Ipsen mult-i-line system (Fig. 3), which is specially designed for high throughputs of identical or similar parts. To further meet the technological and commercial needs of a flexible heat treatment facility, Ipsen developed its mult-i-cell system, an expandable modular linked concept with stand-alone single-chamber modules (Fig. 4). Fully automated heat treatment centers for a wide variety of throughput rates and processes can be created around three basic modules, all designed on reliable, proven technology.
Mult-i-cell modular carburizing system
The modular design of the mult-i-cell system allows a variety of layouts to meet customers' specific needs. The basic configuration includes a high-temperature vacuum furnace module (VHTmod), high-pressure gas-quench module (QGP mod), heated vacuum handling module (VacMobil), buffer tank and central pump station. The sequence of steps in treating batches in a mult-i-cell system is the same as that for linked atmosphere systems including, load preparation on suitable batch holding positions, preprocess cleaning (e.g., in washers), vacuum carburizing (typically in the VHTmod (AvaC) module and/or vacuum carbonitriding in the VHTmod [AvaC(tm)-N] module, high-pressure gas quench in the QGP mod with nitrogen (helium optional) and post-quench treating (e.g., tempering in Ipsen's VDR or DL furnace).
The VHTmod consists of a cylindrical chamber and a door well housing (Fig. 5), which accommodates an electromechanically operated vacuum-tight door. A transformer system for controlling the heating is mounted on the body. The charge is heated inside the thick ceramic insulation-lined module by means of four-sided graphite rod heating. Each module has its own vacuum-pump system.
The module is also provided with the appropriate process-gas system. For vacuum carburizing with acetylene (AvaC), the process gas system consists of a gas train with an acetylene mass flow controller. The carburizing gas (acetylene) is evenly distributed to the heating chamber through a number of manifold gas-injection nozzles. Nitrogen gas is used for emergency purging and/or to support any partial-pressure control requirements in the module if needed.
For vacuum carbonitriding (AvaC-N), an ammonia gas train is incorporated into the process gas system. During vacuum carburizing, the load is carburized in a number of pulses with acetylene up to the solubility limit of austenite, and subsequent diffusion takes places in a vacuum or under partial nitrogen pressure to prevent any effusion of alloying elements.
In vacuum carbonitriding with acetylene and ammonia, carburizing pulses with acetylene alternate with diffusion/nitriding phases with ammonia. The partial nitrogen pressure is not required in vacuum carbonitriding as the diffusion phases are run with ammonia as the nitriding gas.
The VacMobil consists of an insulated chamber body with radiant-heating system, chassis and batch handling system. The chamber comprises a cylindrical housing, a door well housing with a vacuum-tight door and a second cylindrical housing flanged to the main body, which accommodates the batch charge/discharge system. The main chassis moves the VacMobil down the line of modules while a second chassis moves the chamber body forward toward the modules, docking it to them firmly. The heating system is used to counteract any possible heat losses from the batch during transfer. As a result the batches arrive at the quenching station at the desired preselected hardening temperature.
The batch transfer system has two functions: one is to pull batches from loading places or from modules into the mobile batch handling system and the other to push batches onto carrier places outside or inside other modules. The mobile handling system is evacuated as required after docking to any module. The vacuum generated in this way is sufficient to allow hot batches to be transferred between the modules totally under vacuum. A backfill system brings the mobile handling system back to atmospheric pressure when it docks with an atmospheric equipment. A precision position sensing system lines up the mobile batch handling system to a module, while the actual chamber is positioned for docking by limit switches.
A QGP gas-quench cell consists of a cylindrical container and a door well housing (Fig. 6) designed to withstand a maximum quenching pressure of 10 or 20 bar. A "through-principle" QGP module has a door at both ends of the module to allow continuous operation. A cooling gas-circulation impeller and copper-finned heat exchanger are mounted on either side of the batch for intensive re-cooling of the circulating cooling gas. A water-cooled gas circulating motor is mounted centrally on the chamber body.
The buffer tank supplies the quench module with a sufficient volume of gas at the correct pressure. Economical operation at 20 bar quenching pressure requires a buffer tank pressure of 30 bar. No additional compressor is required if liquid nitrogen evaporators are supplied with a 30 bar supply pressure.
The functions of the central pump station are to evacuate the door well housing and door airlocks when the mobile batch handling system is docked, to evacuate the system itself across the door airlock (when needed) and to evacuate the QGP gas-quench cell. To accomplish these functions, a vacuum pipe with valves controlled by the central control system connects the central pump stand to the door wells on the modules and to the chamber of the QGP module.
Plant automation is achieved essentially using two computerized systems. At furnace level, Vacu-Prof software manages the heat-treatment process in each individual module. Programs are executed and monitored centrally in each module's dedicated PLC. The ICTEAutoMag package is responsible for plant movements and coordinates the step sequences in different modules. ICTEAutoMag has a dedicated control cubicle with its own master PLC for controlling plant movements and other centrally administered duties (Fig. 7). The PLCs communicate with the software programs via a common bus to drive the individual modules. Vacu-Prof's multitasking capability allows running different heat treatment routines for different modules sequentially in a single computer.
ICTEAutoMag can run the entire plant unmanned. Defined entry and exit points plus an adequate number of storage locations allow work to be buffer stored so that, say, weekly production peaks can be processed at the weekend without the need for operator presence. The basic modular structure of the mult-i-cell system and the flexibility of the software allow its utilization in a wide variety of plant configurations. Two examples illustrate this capability:
- A mult-i-cell system with a VacMobil and 3 VHTmods plus one "return-principle" (single-door) QGPmod, 2 VDR vacuum tempering furnaces and load storage places (Fig. 8). The single-chamber vacuum tempering furnaces achieve even greater flexibility in this high-throughput layout.
- A mult-i-cell system with a VacMobil and 6 VHTmods plus one "through-principle" (double-door) QGPmod and a continuous tempering furnace with cooling station (Fig. 9).
The load sequence through a mult-i-cell system is (1) prepare load and transfer into the system; (2) transfer load via VacMobil to the pretreatment furnace (if installed); (3) load pretreatment (e.g., preheating); (4) transfer load to the VHTmod process module; (5) process load (e.g., vacuum carburizing); (6) hot transfer in the mobile handling system to the QGPmod; (7) quench load; (8) transfer to tempering furnace, to discharge station or (if a through QGPmod is installed) direct discharge and tempering in a continuous furnace or a parallel multiple chamber tempering line; (9) temper; and (10) transfer load to the discharge station.
Like the batch cycles themselves, VacMobil movements in mult-i-cell systems with a number of treatment modules are demand-oriented and priority-dependent. Control is provided by the ICTEAutoMag system.
The mult-i-cell plants are easily adapted to customer requirements. Possible processes using the basic concept and basic modules include:
- AvaC (vacuum carburizing with acetylene)
- AvaC-N (vacuum carbonitriding with acetylene and ammonia)
- Neutral heat treatment like hardening, annealing, etc.
Other processes can be planned.
The QGP quenching cell is designed to provide a high quench severity using nitrogen. Quenching pressure to 20 bar, gas flow rate to 30 m/s (100 ft/s) and very homogeneous flow of gas across the batch make for highly effective quenching. The quenching capability of the quench cell can be even further extended using helium as quench gas. The uniform gas flow allows work to be tightly packed into batches. The high intensity of the gas quench makes hardening thicker components possible (e.g., shafts and gears) as shown in the examples. Uniform gas flow and the very intense quenching have a major impact on a plant's capability.
The mult-i-cell modules are standalone. It is possible to isolate modules selectively using the ICTEAutoMag automation system to be able to perform maintenance work on them as required. Mult-i-cell installations can also be easily expanded. Individual modules can be set up and taken down without affecting the operation of the plant as a whole. Loading cycles permitting, the workload-handling system can be serviced while the plant is running. Generally, the principle of the mult-i-cell system, a fully automatic, interlinked vacuum furnace plant, can be compared to Ipsen's fully automatic, interlinked atmosphere carburizing plants. The open mult-i-cell system provides greater process integrity and enhanced plant availability by having movement-intensive components (those high-maintenance parts that are most prone to failure and wear) easily accessible for maintenance. By comparison, any malfunction in movement-intensive components in systems that do not have these components readily accessible can bring the entire system to a halt.