A large number of manufactured parts have significant performance requirements at or near their surfaces. Therefore, the selection of a final surface treatment for a part is crucial for increased lifetime at optimum performance. Possible failure mechanisms, as well as the critical depth at which failure can occur are important criteria when choosing a suitable procedure. Figure 1 shows an overview of selected damage mechanisms and their typical penetration depths [1]. Nitriding and nitrocarburizing can economically produce surface zones having a layer thickness between a few µm and about 0.6 to 0.8 mm (0.023to 0.032 in.). The nitriding layer can contain a compound layer and a diffusion zone. The important characteristics are summarized in Table 1.
The low treatment temperatures during nitriding prevent phase changes from occurring in the part structure, which is a basic requirement for distortion-free thermal treatment. With controlled behavior of dimensional change during thermal treatment, parts can be finished to their final dimensions before treatment without the need for follow-on machining.
Surface treatment considerations
The ELTROPULS process produces special layers having very close tolerances [2]. The process provides important technical advantages, but questions of economy still must be addressed before making the selection of the final thermal treatment. For example, while nitrocarburizing of complex crankshaft geometries is expensive, it has reported as the most reliable solution [3]. In the experience of ELTRO GmbH, numerous applications in Formula 1 racing indicate that nitriding by itself using a pulsed plasma process works equally as well for high performance crankshafts.
Pulsed plasma nitriding advantages and disadvantages include:
Advantages
- Possible to run many jobs not possible without plasma (or which could be run, but only at higher temperatures)
- Can combine processes in a single installation (e.g., nitriding and coating)
- Can optimize overall performance specifications (e.g., surface hardness and wear and corrosion resistance)
- Shorter treatment times because of the accelerated mass transfer of the desired elements
- Very good reproducibility with close tolerances
- Selective treatments, which opens important new possibilities to technical designers producing composite structures
- Processes operate with low rates of utility consumption
- Environmental friendliness permits process integration with an existing manufacturing line; all process emissions are carried out of the plant through a vacuum pump exhaust
Disadvantages
- Vacuum processes are generally associated with high capital outlays
- With few exceptions, plasma processes require a well organized charging arrangement for the parts to be treated
- The depth of plasma penetration is generally less than 0.6 to 0.8 mm (0.02 to 0.03 in.), and treatment of bulk material is not possible
Operating economy can be significantly improved by using fully automatic loading and unloading systems, directly integrated with the other equipment in the manufacturing line. With the high productivity of integrated manufacturing systems, more capital-intensive investments can be readily justified.
Installation types
Plasma devices can be divided into continuous and batch type operating systems [4,5]. The most common systems permit batch operations, and are divided into bell furnaces and pit furnaces. With a bell furnace, the loading space is easily accessible from all sides and the parts can be manually loaded piece by piece, or by crane if fixtured in a large charge carrier. This vertical design allows optimal delivery of the thermal energy supporting the plasma (which does not exist in conventional furnaces) through the wall in a manner consistent with maximum temperature uniformity.
Long, slim parts are fixtured to hang to prevent distortion. Pit furnaces are most suitable for such parts. To conserve space, pit furnaces can normally be installed below floor level and operated and serviced from working platforms. Automated loading and unloading devices for these furnaces are available, but generally are more complex.
The operating sequence for both types of furnaces is the same: loading, pulsed plasma process (including warm up, thermochemical treatment, cool down) and unloading. A typical cost allocation for a plasma nitriding process is 60% capital investment, 35% direct labor and 5% expendable material.
Cost reductions can be readily achieved in the direct labor required for loading and unloading. Additional savings can be achieved by optimizing the degree of furnace use, and, in smaller measure, in the programmed times allowed for heating and cooling. The technical designer establishes the major portion of the process time based on the requirements of the layer structure. Modern installations permit heating and cooling in inert gas, so even several tons of workload can be heated and cooled in less than one hour. An improvement in wear characteristics often is achieved in many applications using accelerated cooling, as described in a camshaft application [6].
Substantial savings can be obtained by improving furnace use. For example, a lower cost double-bottom bell furnace (Fig. 2) is equipped with two bases so a finished load can be removed and a new load fixtured on the first base while the second base is used for plasma treatment. Upon completion of the treatment, the bell furnace moves to the newly loaded base, allowing unloading of the treated parts. Loading and unloading operations are decoupled from plasma treatment. In practice, a furnace with this design is constantly in use and the installation can be used in a more cost-effective manner.
Fully automatic systems with a complete complement of supporting subsystems integrated into the manufacturing line offer the best economy with highest quality.
Process integration into a manufacturing line
In an automated process, parts can be continuously placed into a manufacturing line and finally discharged using integrated handling and transport equipment. A properly designed system allows this to take place essentially without significant interruption to the line. Systems currently in operation handle parts commonly at 2 to 3 second intervals.
Parts are first transported through a washing machine and moved to an automated loading mechanism that fixtures them in a charge carrier. Relatively long furnace treatment times (4 to 12 hours) require storing charge carriers in a manner that makes them readily available for later treatment. Storage can be accomplished in two or three dimensions depending on part weight, the number of carriers to be stored and the available space. At the end of a treatment cycle in the plasma furnace, treated parts are unloaded in a manner that permits fixtured racks to be exchanged for treated racks, so reloading can take place in an efficient manner. Fig 3 shows a system for vacuum annealing and plasma nitriding up to 45,000 valves each per day (1.92 s/valve).
Valves are introduced into the manufacturing line over chutes and carried separated in three parallel rows through a five-step spray-wash machine using a neutral cleaner. Part handling is done using a robot with a three-position gripping arm. Calculation of total costs for an automated material handling system of this type depends on the particular case, but $0.05-0.10 per part is typical.
The quality of the treatment in such a system is excellent. Even for type X50CrMnNiNbN21.9 steel, the thickness of the diffusion zone reaches 13 +/-3 um. Figure 4 shows the spread in the thickness of the diffusion zone for a ferritic steel X45CrSi9.3 is about 24 +/-3 um [4].
Figure 5 shows an example of an integrated installation for treating balance beams and axles. The axles are supplied to the system in part carriers by way of a two-axle entry port. A transport system carries the parts into a dip-washing machine to remove residual emulsion from manufacturing operations. Subsequently, the axles arrive at the charge rack location where they are loaded using folding arm robots into an unloaded charging rack. A four wheel, fully steered rail crane installation takes over the transport of the hanging rack to and from the plasma devices. The treated parts have an average layer thickness of 12 µm, with a tolerance of +/-2um (Fig.6).
Summary
Plasma technologies such as pulsed plasma nitriding and nitrocarburizing offer important advantages to the user. The processes consistently reproduce well-defined layers that impart good wear and corrosion resistance. A plasma device can be integrated directly into the production line because there are no objectionable emissions from the process. Like all current production processes, plasma thermal treatments are subjected to continually rising costs, which can be reduced by increasing furnace availability with automatic double-bottom furnaces, by incorporating automatic loading and unloading systems that can handle substantial production quantities and by using high-speed cooling systems.