DULL ANNEALING SYSTEMS
 
This is often the stepping stone from off line annealing to in line annealing, it is compact requiring a standard induction heating system followed by a dwell zone and then a direct water quench.

Obviously the tube still needs to be pickled after annealing, but the benefits of using induction heating are:

1. The quantity of scale formed is very small and this results directly from the very short heating time, in many cases it is so light as to be considered simply a change in colour.

2. Pickling time is reduced resulting in higher productivity on a daily basis.

3. Acid useage is reduced as a direct result of reducing the oxide layer on the tube.

4. Tube distortion is minimised. The tube can be sized in line before it is cut to length thus enhancing the quality of tube produced.

5. Electrical energy is reduced when compared to the use of an electric resistance type furnace. If gas is the energy source for the annealing furnace other considerations must be taken into account such as:
5.1. Maintenance of refractories.
5.2. Furnace start-up time.
5.3. Furnace idle time.

6. Work in progress is reduced.

7. Orders can be progressed faster and small orders filled more economically. (Subject to mill change over).

8. Can be integrated into an existing line. A typical system for tube up to 80mm diameter and subject to line speed has a line foot print of approx 2 metres x 1metre plus the power supply and ancillary equipment.

9. The existing batch or conveyor type annealing furnace can be scrapped and the floor space released for other uses.

10. Costs associated with removal of spent acid and licences to discharge effluent into sewerage systems are eliminated.

EQUIPMENT
Heating

The induction heating system comprises three major items:

1. a power supply
2. an induction heating coil and interconnecting busbar
3. a closed loop water cooling system for the power electronics.

The power supplies for this type of application are solid state IGBT type having a series output circuit and variable ratio output isolation transformer for load impedance matching. The duty cycle is continuous and input power factor is 0.95 under all operating conditions.

One of the important requirements of a continuous line application is the accuracy of the output power regulation, the power supply has an output regulation accuracy of +/-1% of rated power with +/-10% line variance to ensure precise process control.

Since the wall thickness of most stainless steel tube up to 80mm diameter ranges from less than 1mm up to 3mm (this is the volume production), the frequency of the output is 30kHZ which delivers an excellent operational efficiency for the various combinations of wall thickness to outside diameter. The other consideration in selecting this frequency is the current carrying capacity of the IGBT’s is more conservative than at higher frequencies of 50kHZ.

At these frequencies the busbar length should be kept short to minimise power losses which means the power supply is close to the line and leads to a very compact installation.

Induction heating coils are made to suit each application but follow a standard design and manufacturing criteria, with dull annealing installations the coil coupling to the tube size range can be kept tight and hence the coil heating efficiency will be high. An important point to keep in mind with any induction heating application is the response of the power system to a load that is non magnetic ie materials such as copper, brass, aluminium and austenitic stainless steel to name the most common. Ferrous steel up to 720oC or what is called “curie” is magnetic and will heat quickly with very high coil efficiencies. Beyond this temperature it becomes non magnetic and responds similarly to the materials mentioned above with the result that the induced current must be higher to compensate for the lower coil efficiency. It is difficult to generalize on this point because of the many variables that affect the coil heating efficiency, but for a stainless steel tube mid range diameter say 50mm and wall of 2mm a coil efficiency of 68% is realistic.

Since the operating temperature is in the range 1050oC to 1100oC it is essential the inside of the coil be lined with refractory to reduce the radiant and conducted heat reaching the water cooled coil turns. This is one of the factors limiting how tight the induction coil can be wound for a particular application, the other is the range of tube

sizes that one coil can accommodate and this is dictated by the power supply and the ability to deliver the required power to a small tube in a relatively large coil.

Quench
For dull annealing the quench comprises a high volume discharge ring through which the tube passes and all is contained in a tank with the recirculating pump.

After leaving the quench the tube can be sized and cut to length ready for pickling.

 
Exit end of the induction coil and direct water quench for the Dull Annealing system.

Quench
From the dwell zone the product tube passes into the indirect gas quench which is an externally water cooled heat exchanger and uses solid carbon as the transfer medium from the product tube to the water to reduce the temperature from 1050oC to 120oC at exit. Each product size requires a change in the carbon blocks to achieve optimum cooling rates and this necessitates making space available at the exit end of the line to make the change with a minimum of inconvenience. A direct water quench is placed between the annealer and the sizing rolls to remove all heat from the product before it enters the sizing rolls. The quench is modular in construction and allows for easy removal of sections for maintenance, the design also allows for easy thread up and system sealing.

Gas System
To achieve a bright surface finish on the stainless steel during the annealing process requires a gas that is highly reducing. Hydrogen is the gas of choice. The down side to this is that when combined with oxygen in the form of air in the right proportions it is explosive. To overcome this an inert gas is used for purging and safety, the commercially available gases for this duty are nitrogen and argon and it is really a choice for the user based on cost and convenience. Most stainless steel tube plants have argon available for the tube welding process so it makes sense to use argon for purging and safety. Very few plants today use gas generators such as the ammonia dissociator combined with gas dryers to service bright annealing, most use bottled gas.

1. It is more convenient.
2. Gas generators and dryers are costly to purchase and maintain.
3. Induction annealing systems use very little gas provided the seals are maintained in good order.

It is important to note that the annealer must be completely sealed before hydrogen gas is introduced and this means the product tube must be passed through the annealer until it reaches the sizing rolls. At this point all seals are locked in place and the line started, when the welded tube exits the last seal of the quench chamber the operator starts the annealer. Inert gas is admitted to the system at a predetermined flow rate, the procedure is automatic from the time the operator initiates the start and cannot for safety reasons alter the purging times. Once hydrogen is admitted and the annealer cleared of the purge gas power is brought on to the induction heating coil and annealing proceeds.