Plasmait GmbH has introduced a second generation plasma annealer designed for wire, rope and tubes made of stainless steel and nickel alloys and with cross-sections from 20mm2 down to the smallest fine wire sizes. With this new second generation concept Plasmait allowed for a radical increase of continuous annealing speeds of stainless steels and nickel alloys. In the fine wire sizes annealing in line with drawing or rolling is possible.
The slow speed of traditional tube furnace means that the annealing of stainless steel and nickel alloy wires generally involves a multi-line setup. Multi-line process is logistically demanding and involves multiple pay-offs and take-ups that can require substantial capital outlay and take large work shop space. Furthermore, slow annealing speed means that the drawing or rolling processes have to be performed separately, off-line from annealing, which adds to the complexity of process logistics.
The process speeds of the new plasma annealer are much higher than the process speeds of a traditional tube furnace. This allows a single line plasma annealing plant to substitute multiple lines of a tube annealer, whilst retaining the same output capacity. In smaller cross-sections and fine wire sizes it is also possible for plasma annealer to operate in-line with a drawing or a rolling machine.
The energy coupling in the plasma process is at least 10 times better than the energy coupling in the typical convection furnace, which is the reason for a compact design of the plasma chamber. Depending on application 70% to 85% of all the power used by the plasma annealer is converted into heat in the processed material, which makes the plasma heating much more energy efficient than any conventional tube furnace-a feature that has been gaining importance and is unlikely to change in years to come.
Rapid heating and reduced time of recrystallization results in fine grain size. The microscopic photo of the cross-section in Figure 2 indicates uniform crystal structure which was observed on a 0.5mm austenitic stainless steel wire that was plasma annealed at the speed of 6m/s. Small grain size with uniform crystal structure in the longitudinal and transversal direction improves material’s susceptibility to cold working and its resistance to surface cracking.
Annealing power is controlled instantaneously and with a high degree of accuracy via power supply. This gives the operator the ability to target mechanical properties with a great degree of accuracy and provides greater flexibility in new product development.
Ion bombardment or ion sputtering on the material surface results in removal of the upper surface layer, which makes up for an effective surface treatment. Dirty deposits, soaps, lubricants and fine oxides layer break under the ion bombardment in the plasma chamber. The debris and other cracked surface contamination are sucked out of the plasma chamber by the vacuum system and are filtered out through the exhaust installation. The dry surface cleaning and degreasing being performed simultaneously with plasma annealing is of particularly benefit to applications with demanding surface requirements in sectors such as medical, welding or aerospace to mention a few.
Figure 3 is a photo of a plasma annealer and its main components. The plasma module and the dwell section sit between two seals of the sealing system, which maintains protective atmosphere throughout the heating and cooling zone of the machine. The purging gas can include hydrogen, nitrogen, argon, helium or their mixtures. The gas mixture is chosen to suit the application, in particular material’s specific surface requirements. Direct water cooling can be utilized for some applications. The process is controlled with PLC and touch-screen HMI.
Unlike the traditional tube furnace, the plasma annealer can cold start production in few minutes and can be stopped imminently. This avoids the lengthy heating-up and cooling-down times and associated energy costs that are symptomatic for a conventional furnace.
The gas cooling section in the plasma annealer has a closed loop design to minimize purging gas consumption.
A photo of a plasma annealer for small to medium size cross-section wires is given in Figure 4. Such an annealer can be used as a stand-alone annealing plant in a combination with a take-up and pay-off or is installed in-line with a drawing machine or rolling mill.
The plasma annealer features a number of benefits when compared to the traditional tube furnace:
• Small grain size with uniform crystal structure in longitudinal and transversal direction, which results in consistent mechanical properties and makes plasma annealed material more susceptible to cold working;
• High production speed allows the plasma annealer to run in-line with some drawing, rolling or subsequent coating processes;
• Simultaneous dry chemical-free surface cleaning through ion sputtering (i.e. degreasing and oxide removal) results in superior surface finish, ready for subsequent coating;
• High energy efficiency means considerable energy savings and lower power connection;
• Increased uptime as well as no warming up and cooling down times;
• Low purging gas and maintenance costs compared to traditional tube furnace;
• Environment and operator friendly production;
• Compact design means short installation and commissioning times;
• Small machine footprint means reduction in required workshop space;
• Production flexibility through the ability for rapid manipulation of production parameters.
