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Billet Heating Systems Radyne In-Line Stainless Induction Annealing Systems
Market demand for better quality
finish has forced both the tube producers and equipment suppliers to
look for a better way of annealing stainless steel tube.
Induction heating is now the accepted technology for any high speed processing line.
In the case of dull annealing the time at temperature should be minimised to keep surface oxide formation to a minimum, with induction heating this is rarely a problem as the time at annealing temperature is measured in seconds before direct water quenching.
Bright annealing requires the use of a highly reducing gas atmosphere, hydrogen being the commonly used gas.
A further consideration of in-line tube annealing is the weld zone of the tube which is a continuous seam and becomes ferritic as a result of the welding process. It can be physically detected because of its response to a magnetic field.
As part of any in-line annealing process, it is essential the tube surface is cleaned thoroughly to ensure it is free of any grease, oil or other carbonaceous material before it enters the annealing system. Such residues will lead to surface carburisation and impair the corrosion resistance of the steel.
The annealing process also requires the quenching be rapid, particularly through the upper critical range to minimise carbide precipitation, with dull annealing this is not a problem but with bright annealing it must be carried out under the protective gas atmosphere which means the quench is indirect and slower.
PRACTICAL LIMITATIONS
Heating
For specific markets and applications the use of a quartz tube in the heating coil isolates the hydrogen gas atmosphere from the induction coil and creates a gas tight chamber, but at the same time is transparent to the induced currents from the heating coil. The down side to this approach is the size of the quartz tube relative to the product being heated and the resultant induction heating coil size which lowers the heating efficiency.
The alternative approach is a gas tight induction heating coil but it requires considerable care in manufacture with precision sealing of the enclosure and is generally more costly to produce than the quartz tube approach. It does however deliver a higher heating efficiency.
Quenching
In many if not all installations where the annealing is in-line with the drawing equipment or tube the mill it is not the heating that is the challenge, it is the quench. Where direct quenching using say water as the quenching medium is used for dull annealing there is no problem, however when an indirect quench has to be used such as with bright annealing of stainless steel because of the protective gas atmosphere the rate of heat transfer becomes a critical design element. The impact of indirect quenching is reflected in the overall length of the machine and can in some circumstances where space is limited, prevent the adoption of in-line bright annealing.
Gas System
For bright annealing systems there is one further limitation and that is the protective atmosphere gas required for the bright finish. The tube when annealed passes through a sealed system, so, providing the welding process is sound, mechanical seals at the entry and exit prevent contamination of the hydrogen gas atmosphere. This is important for a number of reasons:
First and foremost is safety, any entry of oxygen into the system and mixing with the hydrogen can create a hazardous situation.
Second, contamination can affect the surface finish of the tube but this is only on the outside of the tube.
For tubulars the inside surface of the tube is not exposed to the hydrogen atmosphere and must rely on a constant stream of argon gas to minimise oxidation not only during welding but through the heating and quenching phases of the annealing process. The practical limitation here is the volume of gas required to fill the tube which is open to the atmosphere at both ends of the mill line.
This limitation sets the upper diameter of tube that is practical to bright anneal in-line with the mill.
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.
Radyne 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 Radyne 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.
BRIGHT ANNEALING SYSTEM
These systems are by far creating the most interest in the market place primarily because they facilitate the production of better finish on the tube surface at much lower cost. Bright annealing of cut lengths of tube has been around for a long time using conveyor type furnaces and continuous in-line systems have been used by some manufacturers for ten years or more because they recognised the benefits would outweigh the costs.
In the last decade developments have taken place in the power supplies bringing to the line higher frequencies with higher operating efficiencies and the quench system technology has improved and will continue to improve with new materials and techniques.
The benefits of a bright in-line system are:
1. Wire, Rod, and Tube leaves the annealer with a bright outside surface requiring no further finishing.
2. Distortion is minimised. The wire, rod and tube is sized in line before it is cut to length thus enhancing the quality of tube produced.
3. 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:
3.1. Maintenance of refractories.
3.2. Maintenance of muffles or radiant tubes to contain the furnace atmosphere.
3.3. Maintenance of heating elements and connections.
3.4. Cost of furnace start-up time.
3.5. Cost of furnace idle time.
4. Work in progress is reduced.
5. Orders can be progressed faster and small orders filled more economically. (Subject to mill change over).
6. 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 6 metres x 0.75 metre plus the power supply and ancillary equipment which is usually mounted to one side.
7. The existing batch or conveyor type annealing furnace can be scrapped and the floor space released for other uses.
8. Water cooling systems are not contaminated with scale thus maintenance costs are lowered.
9. Labor costs are usually lower as the need to move bundles of tube from one processing cell to another are eliminated.
Bright Annealing System
EQUIPMENT
Heating
The induction heating system for bright annealing is similar to that required for dull annealing in that the major components are the same but there the similarity ends.
The power supply is identical for both bright and dull annealing. Only the induction coil matching changes.
Earlier designs of the induction heating coil were sized to allow for a quartz tube to be inserted that isolated the refractory lined induction coil from the product passing through the centre of the quartz tube. The latest design utilizes an economical solution of a gas tight chamber providing an environment for annealing that is:
1. Impervious gas leakage at elevated temperatures.
2. It is not porous.
3. It can be readily sealed to the entry vestibule and dwell chambers by mechanical seals with expansion compensation.
4. It does not deteriorate with constant use.
From the heating coil the product tube passes through a dwell zone which is unheated but allows the recrystallization to complete and chromium carbides to be taken back into solution.
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.
The bright annealing system using induction heating technology offers any stainless steel wire, rod or tube manufacturer the opportunity of incorporating it into existing lines or specifying it for new lines. One has only to look at the development of high speed copper tube annealing lines by Radyne to see where it can go. These lines now operate at speeds up to 2000 ft./minute / 600 metres /minute.
Induction heating is now the accepted technology for any high speed processing line.
Radyne specializes in designing both dull and bright annealing systems for a wide variety of parts from engine parts to hand tools to wire and tube.
Bright and dull annealing of stainless steel wire, rod and
tube is not a new subject, what is new is the change in technology to
achieve the desired result. Most installations in the past
used off-line annealing facilities because the technology for a compact
installation in-line with the tube mill and welder did not
exist. For bright annealing it was a conveyorised furnace
using cracked ammonia for the process atmosphere and nitrogen for
purging, later bottled or bulk hydrogen proved to be a better
alternative as it removed the ever present problem of residual ammonia
in the annealing atmosphere.
For dull annealing batch furnaces many gas fired were used and still
are, quenching is by batch followed by pickling of the tubes to remove
the surface oxide and improve the surface finish.
Market demand for better quality finish has forced both the tube
manufacturers and equipment manufacturers to look for a better way of
annealing tube. Most of the production is in the austenitic
grades although some duplex steels and ferritic steels are annealed but
the volume is quite small compared to the austenitic grades in the 300
series.
Of course the other important factor is competition in the stainless
steel tube market globally which has brought to many manufacturers the
reality their existing plant is inadequate technically and commercially
Induction heating technology is now accepted as the technology of choice for any high speed processing line or lines requiring compact heating combined with rapid response. Examples of this technology adoption is hardening and tempering of carbon steel rod for spring wire and seam and full body annealing of carbon steel tube, quench and temper lines for pipe, steel bar and wire, copper tube annealing, steel strip coating both metallic and organic and stainless steel tube annealing. All of these examples have one common denominator – they are continuous and it the use of high speed induction heating technology that has made this possible.
METALLURGICAL REQUIREMENTS
Austenitic stainless steels are not hardenable but like most metals suffer from work hardening both during the manufacture of the tube and during later processes. Annealing which is also sometimes referred to as solution heat treatment not only recrystallises the structure but dissolves carbides back into the austenite. Annealing temperatures range from 1050oC to 1150oC although temperature can be altered according to the time at temperature and with induction heating this can be closely controlled. Time at temperature is not a critical issue, it can be too short and where the annealing is in-line this has to be carefully considered as there is a direct correlation between time and machine length.In the case of dull annealing the time at temperature should be minimised to keep surface oxide formation to a minimum, with induction heating this is rarely a problem as the time at annealing temperature is measured in seconds before direct water quenching.
Bright annealing requires the use of a highly reducing gas atmosphere, hydrogen being the commonly used gas.
A further consideration of in-line tube annealing is the weld zone of the tube which is a continuous seam and becomes ferritic as a result of the welding process. It can be physically detected because of its response to a magnetic field.
As part of any in-line annealing process, it is essential the tube surface is cleaned thoroughly to ensure it is free of any grease, oil or other carbonaceous material before it enters the annealing system. Such residues will lead to surface carburisation and impair the corrosion resistance of the steel.
The annealing process also requires the quenching be rapid, particularly through the upper critical range to minimise carbide precipitation, with dull annealing this is not a problem but with bright annealing it must be carried out under the protective gas atmosphere which means the quench is indirect and slower.
PRACTICAL LIMITATIONS
Heating
For specific markets and applications the use of a quartz tube in the heating coil isolates the hydrogen gas atmosphere from the induction coil and creates a gas tight chamber, but at the same time is transparent to the induced currents from the heating coil. The down side to this approach is the size of the quartz tube relative to the product being heated and the resultant induction heating coil size which lowers the heating efficiency.
The alternative approach is a gas tight induction heating coil but it requires considerable care in manufacture with precision sealing of the enclosure and is generally more costly to produce than the quartz tube approach. It does however deliver a higher heating efficiency.
Quenching
In many if not all installations where the annealing is in-line with the drawing equipment or tube the mill it is not the heating that is the challenge, it is the quench. Where direct quenching using say water as the quenching medium is used for dull annealing there is no problem, however when an indirect quench has to be used such as with bright annealing of stainless steel because of the protective gas atmosphere the rate of heat transfer becomes a critical design element. The impact of indirect quenching is reflected in the overall length of the machine and can in some circumstances where space is limited, prevent the adoption of in-line bright annealing.
Gas System
For bright annealing systems there is one further limitation and that is the protective atmosphere gas required for the bright finish. The tube when annealed passes through a sealed system, so, providing the welding process is sound, mechanical seals at the entry and exit prevent contamination of the hydrogen gas atmosphere. This is important for a number of reasons:
First and foremost is safety, any entry of oxygen into the system and mixing with the hydrogen can create a hazardous situation.
Second, contamination can affect the surface finish of the tube but this is only on the outside of the tube.
For tubulars the inside surface of the tube is not exposed to the hydrogen atmosphere and must rely on a constant stream of argon gas to minimise oxidation not only during welding but through the heating and quenching phases of the annealing process. The practical limitation here is the volume of gas required to fill the tube which is open to the atmosphere at both ends of the mill line.
This limitation sets the upper diameter of tube that is practical to bright anneal in-line with the mill.
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.
Radyne 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 Radyne 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.
BRIGHT ANNEALING SYSTEM
These systems are by far creating the most interest in the market place primarily because they facilitate the production of better finish on the tube surface at much lower cost. Bright annealing of cut lengths of tube has been around for a long time using conveyor type furnaces and continuous in-line systems have been used by some manufacturers for ten years or more because they recognised the benefits would outweigh the costs.
In the last decade developments have taken place in the power supplies bringing to the line higher frequencies with higher operating efficiencies and the quench system technology has improved and will continue to improve with new materials and techniques.
The benefits of a bright in-line system are:
1. Wire, Rod, and Tube leaves the annealer with a bright outside surface requiring no further finishing.
2. Distortion is minimised. The wire, rod and tube is sized in line before it is cut to length thus enhancing the quality of tube produced.
3. 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:
3.1. Maintenance of refractories.
3.2. Maintenance of muffles or radiant tubes to contain the furnace atmosphere.
3.3. Maintenance of heating elements and connections.
3.4. Cost of furnace start-up time.
3.5. Cost of furnace idle time.
4. Work in progress is reduced.
5. Orders can be progressed faster and small orders filled more economically. (Subject to mill change over).
6. 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 6 metres x 0.75 metre plus the power supply and ancillary equipment which is usually mounted to one side.
7. The existing batch or conveyor type annealing furnace can be scrapped and the floor space released for other uses.
8. Water cooling systems are not contaminated with scale thus maintenance costs are lowered.
9. Labor costs are usually lower as the need to move bundles of tube from one processing cell to another are eliminated.
Bright Annealing System
EQUIPMENT
Heating
The induction heating system for bright annealing is similar to that required for dull annealing in that the major components are the same but there the similarity ends.
The power supply is identical for both bright and dull annealing. Only the induction coil matching changes.
Earlier designs of the induction heating coil were sized to allow for a quartz tube to be inserted that isolated the refractory lined induction coil from the product passing through the centre of the quartz tube. The latest design utilizes an economical solution of a gas tight chamber providing an environment for annealing that is:
1. Impervious gas leakage at elevated temperatures.
2. It is not porous.
3. It can be readily sealed to the entry vestibule and dwell chambers by mechanical seals with expansion compensation.
4. It does not deteriorate with constant use.
From the heating coil the product tube passes through a dwell zone which is unheated but allows the recrystallization to complete and chromium carbides to be taken back into solution.
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.
The bright annealing system using induction heating technology offers any stainless steel wire, rod or tube manufacturer the opportunity of incorporating it into existing lines or specifying it for new lines. One has only to look at the development of high speed copper tube annealing lines by Radyne to see where it can go. These lines now operate at speeds up to 2000 ft./minute / 600 metres /minute.
