TPT March 2013

Article

Volatile components of the residues evaporate and are filtered out through the exhaust pipe. Plasma treatment is not designed for removal of excessive amounts of dirt and soaps on the material surface. Excessive surface contamination has to be removed prior to plasma treatment with an appropriate conventional pre-cleaning system. Plasma surface treatment is only effective on a surface with minor contamination and should be regarded as a fine surface cleaning, effective only for removal of thin layers of surface deposits. Some wet drawn materials do not require conventional pre- cleaning. Wet lubricants tend to evaporate in low pressure, high temperature environments and are exhausted through the vacuum system before they reach the annealing zone. The effect of ion bombardment on the surface of the processed material manifests itself in a clean and degreased surface. Removal of a thin oxide layer is also achieved in many stainless steel, copper and copper alloy applications with appropriate production settings. A plasma treated surface without the oxide layer is highly susceptible to coating and would create a strong bond with polymers or metals.

The actual gas savings from using a plasma annealer instead of a traditional tube furnace will depend on specific application, production pattern and the cost of purging gas. No part of the plasma annealer touches the processed material, which minimises mechanical wear of annealer components and prevents contamination of the processed material surface. The maintenance of the plasma annealer involves regular changing of vacuum pump filters and oil in the vacuum pumps. Electrode and protective glass tube in the heating module have to be cleaned regularly. Replacing the glass tube takes a few minutes. The electrode tubes and glass tubes in the heating module of a plasma annealer are much shorter than the tubes in the traditional furnace and also less costly. Plasma annealing is started in a matter of minutes and does not require lengthy heating up and cooling down periods common for traditional tube furnaces. High speed annealing does not only bring operational savings. Rapid heating and reduced time of recrystallisation brings the benefits of small grain size. The photo in Figure 8 depicts a glowing 0.5mm stainless steel wire at the exit of plasma chamber. The wire was annealed at a speed of 6m/s. The microscopic photo in Figure 9 indicates homogeneous annealing across the cross-section of the wire and grain size below 10 micron. High speed plasma annealing features small grain size and delivers homogeneous crystal structure in the longitudinal and transversal direction, which in turn improves the material’s susceptibility to cold working. For example, plasma annealed materials that have been recrystallised and feature homogeneously small grain size in all directions and superior surface finish demonstrate better drawability and considerable lower drawing die wear rate. Annealed materials with small grain size will also better resist surface cracking during the bending process. Homogeneous grain size is also a benefit to producers of tubes for heating elements, where small radius bending is applied to the material. A plasma annealer allows the operator to manipulate annealing temperature via power input on a power supply instantaneously and with a high degree of accuracy. This gives the operator the ability to target mechanical properties with a great degree of accuracy. Development of new products with different mechanical specifications is therefore quicker and easier with a plasma annealer than with a traditional tube furnace. Traditional tube furnaces require time to adjust to different annealing temperatures, which makes new product development time consuming and costly. The speed and flexibility of new product development will offer an important advantage to producers of tubes for high tech applications in the medical, aerospace, automotive and precision mechanics fields. Plasma treatment facilitates simultaneous heat and surface treatment on the processed material. Dirty deposits, soaps, lubricants and oxide layers break under the ion bombardment in the plasma chamber or burn at high temperature. The ashes (carbon) are sucked out of the plasma chamber by the vacuum system and deposited in the vacuum pump filters. Comparison of finished product quality

Figure 8: Glowing SS wire with diameter of 0.5mm plasma annealed and cleaned at 6m/s

Figure 9: Cross-section of 0.5mm SS wire annealed in plasma at a speed of 6m/s

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March 2013